STUDY OF HIGH ENERGY OLIVINE PHOSPHATE CATHODE MATERIALS...

i

STUDY OF HIGH ENERGY OLIVINE

PHOSPHATE CATHODE MATERIALS DOPED

WITH TRANSITION METALS AND RARE

EARTHS FOR LITHIUM ION BATTERIES

A THESIS

Submitted by

KARTHICKPRABHU S (Reg No: 201115115)

In partial fulfillment for the award of the degree

Of

DOCTOR OF PHILOSOPHY

DEPARTMENT OF PHYSICS

KALASALINGAM UNIVERSITY

ANAND NAGAR

KRISHNANKOIL – 626 126

JULY 2014

ii

CERTIFICATE

This is to certify that all corrections and suggestions pointed out by the

Indian/Foreign Examiner(s) are incorporated in the Thesis titled “STUDY OF

HIGH ENERGY OLIVINE PHOSPHATE CATHODE MATERIALS

DOPED WITH TRANSITION METALS AND RARE EARTHS FOR

LITHIUM ION BATTERIES” submitted by Mr. KARTHICKPRABHU, S.

Dr. M.RAJA

JOINT SUPERVISOR

Assistant professor

Department of Physics

Kalasalingam University

Krishnankoil- 626 126

Dr. G. HIRANKUMAR

SUPERVISOR

Professor of Physics

PSN College of Engineering and

Technology

Melathediyoor - 627 152

Place: Krishnankovil Date : 24.01.2015

iii

KALASALINGAM UNIVERSITY

(KALASALINGAM ACADEMY OF RESEARCH AND EDUCATION)

KRISHNANKOIL – 626 126

BONAFIDE CERTIFICATE

Certify that this Thesis title “STUDY OF HIGH ENERGY OLIVINE

PHOSPHATE CATHODE MATERIALS DOPED WITH

TRANSITION METALS AND RARE EARTHS FOR LITHIUM ION

BATTERIES” is the bonafide work of Mr. KARTHICKPRABHU, S.

who carried out the research under my supervision. Certified further, that to

the best of my knowledge the work reported herein does not form part of

any other thesis or dissertation on the basis of which a degree or award was

conferred on an earlier occasion on this or any other scholar.

Dr. M.RAJA

JOINT SUPERVISOR

Assistant professor

Department of Physics

Kalasalingam University

Krishnankoil- 626 126

Dr. G. HIRANKUMAR

SUPERVISOR

Professor of Physics

PSN College of Engineering and

Technology

Melathediyoor - 627 152

iv

ABSTRACT

The intensive work is dedicated to the phosphate system, LiMPO4

(M = Fe, Ni, Co and Mn) with olivine-related structure as positive

electrodes for rechargeable lithium ion batteries owing to their non toxic,

cost effectiveness, environmental friendliness, electrochemical stability

even at over charge and better thermal stability during charging and

discharging compared with conventional cathode materials. The advantages

of utilizing olivine-type structure come from the following two reasons: (1)

these materials have relatively larger theoretical capacity, c.a. 170 mAh/g,

and higher voltage, over 5.1 V (in the case LiNiPO4) versus Li+/Li, than the

layered rocksalt-type LiCoO2, LiNiO2 and spinel LiMn2O4 now being

commercially used as 4 V positive electrode materials. (2) Recent efforts in

improving the electrolyte system make it possible to realize stable

charge/discharge reactions over 5.1 V. The present study is focused on the

synthesis, characterization and electrochemical characterizations of un-

doped and doped LiNiPO4. The transition metals Zn & Mn and rare earth

metals La & Nd are the dopants. The pure and doped samples are prepared

by polyol method using 1,2 propanediol as a polyol medium.

The structural and morphological characterizations of olivine type

pure, rare earth doped and transition metal doped LiNiPO4 samples

v

characterized by X-ray diffraction, particle size distribution and SEM

analysis. XRD analysis show that the doping of La3+ (up to 0.05 mol %)

and Nd3+ (up to 0.03 mol %) is more favorable because it does not collapse

the lattice structure of LiNiPO4. Insertion of transition metals also prevents

the lattice structure but slightly affects the lattice parameters within the

concentration range studied. Lattice parameters are computed from the

Rietveld refined analysis by TOPAS 3.0 software. Among all the doped

samples, La3+ doped LiNiPO4 shows smaller size particle distribution that

corresponds to shorter diffusive distance of Li-ions. SEM analysis reveals

that both pure and doped samples exhibit uniform morphology with sphere

like particles. This kind of morphology is more significant to achieve high

capacity and good cycleability.

Thermal and vibrational characterizations of pure, rare earth and

transition metal doped LiNiPO4 samples characterized by TG, FTIR and

Laser Raman studies. The weight loss of the precursor is observed up to

600°C then there is no reduction in the weight. Hence, the reaction

temperature is optimized at 650°C; 6 h. The vibrational properties of

LiNiPO4 and doped LiNiPO4 are studied by FTIR and Laser Raman studies.

FTIR analysis confirms that the dopant exactly occupy at Ni site of the

corresponding host lattice. From the Laser Raman analysis, rare-earth ions

and transition metal ions occupy at Ni site upon doping. Electrical

conductivity and dielectric studies of pure, rare earth and transition metal

vi

doped LiNiPO4 characterized by impedance spectroscopy. The electrical

conductivity of LiNiPO4 is found to be 4.36×10-6 at 300°C. Moreover,

doping of both rare earth (La3+ and Nd3+) and transition metal atoms (Zn2+

& Mn2+) improves the electrical conductivity of LiNiPO4. Among all

dopants, 0.05 mol% La3+ doped sample at ambient temperature shows the

improvement in conductivity by the factor of two order, an increase from

<10-9 to 10-7 S/cm at ambient temperature. So the doping of La3+ is

favorable in LiNiPO4 compared with all other systems. The magnitude of

dielectric constant value is found to be increased upon doping of both rare

earths and transition metals. Pristine, 0.05 mol% La doped, 0.03 mol% Nd,

LiNi0.85Zn0.15PO4 and LiNi0.9Mn0.1PO4 samples are subjected into

electrochemical characterizations such as electrochemical impedance,

cyclic voltametry and galvanostatic charge-discharge studies. The charge

transfer resistance value for pure LiNiPO4 (fresh cell) is 1302Ω. This value

gets decreased upon doping of both rare earth and transition metal atoms.

The charge-transfer resistance value is found to be low value (for fresh cell)

for 0.05 mol% La doped LiNiPO4 compared with other samples. After

cycling the Rct values are increased. Especially for pure sample, it is

increased suddenly even after 4th cycle. The abrupt increase of charge

transfer resistance of the LiNiPO4 is considered as the main reasons for the

capacity fading which is reflected in charge-discharge studies. From the

CV, it is seen that the 0.05 mol% La doped sample shows the redox couples

vii

in the range of and 4.3-4.5 V (anodic potential) 3.4-3.8 V (cathodic

potential). This indicates that the insertion/extraction process of Li-ions

through Ni2+/Ni3+ redox reactions become more reversible with cycling.

Similar results are also obtained for pure, Nd3+, Zn2+ and Mn2+ doped

samples. These results are deviated from our expectation in which the redox

potential of LiNiPO4 is 5.1 V vs Li. The first delivered capacity, i.e. 1.1

mA.h g-1 is far from the theoretical value of 167 mA h/g even if at a low

current rate of C/20 for pure LiNiPO4. Rare earth doped LiNiPO4 cathode

materials show a substantial increase of discharge capacity at C/20 rate,

especially for the La doped sample, which shows the capacity of about 30

mA.h g-1 than pure LiNiPO4 at C/20 rate. The discharge capacity of 0.03

mol% Nd doped LiNiPO4, Zn2+ and Mn2+ doped LiNiPO4 is found to be

6 mA.h g-1 and 4 mA.h g-1 and 2.5 mA.h g-1 respectively. The disappointing

discharge capacity of pure, Nd3+, Zn2+ and Mn2+ doped LiNiPO4

corresponds to the low intrinsic electronic conductivity and sluggish

kinetics of Li-ion transport.

viii

ACKNOWLEDGMENT

I would like to express my deep sense of gratitude and indebtedness to my

research supervisor Prof G. Hirankumar for his invaluable help, thought-

provoking guidance with friendly manner, encouraging attitude, highly

independent atmosphere and pleasant behavior throughout this study. I also

express my sincere thanks to my guide for selecting me as a JRF in BRNS project.

I thank my joint-supervisor Dr. M. Raja, Department of Physics,

Kalasalingam University, for the constant encouragement and support till the

completion of my work.

I thank Chairman, Chancellor, Vice-Chancellor, Registrar, Dean (R&D),

Kalasalingam University for their support rendered during the research work.

I acknowledge Board of Research in Nuclear Sciences (BRNS),

Government of India for providing financial support in the form of Junior

Research Fellow and Senior Research Fellow.

I express my sincere thanks to, Dr. S. Selvasekarapandian,

Prof. S. Asath Bahadur, Mr. A. Maheswaran, Mr. M. Muthuvinayagam, Dr.

N. Nallamuthu, Dr. S. Selvanayagam, and other staff members from the

department of physics and chemistry, Kalasalingam University, for their

encouragement during this course.

ix

I express my sincere thanks to Dr. P. Suyambu, Chairman, PSN Group of

Institutions. The Principal, Prof. Dr. X. Sahaya Shajan, Dean SOBES and

Research scholars, PSN College of Engineering and Technology, Tirunelveli, for

the opportunity extended to me to carry out research work in this reputed

institution.

I thank Prof. C. Sanjeeviraja, Former Head, Department of Physics,

Alagappa University for the XRD measurements of my samples. I thank my

seniors who helped me a lot.

I would like to thank faculty members from Department of physics, Ayya

Nadar Janaki Ammal College, Sivakasi, for their constant encouragement to start

my Ph.D.

My special thanks to Mrs. R. S. Daries Bella and her family, for their

constant support, care and encouragement throughout the period of my research

work.

I thank my friends for their encouragement and valuable suggestions.

I sincerely thank my parents Mr. S. Sivabalan, Mrs. Manickam Sivabalan

for their support and limitless sacrifice. I dedicate this work to my loveable

parents. I thank my loveable brothers Mr. S. Ragaganesh and Mr. S. Sivadinesh

for their love, inspiration and encouragement. Last but not least, I sincerely thank

to God (Pethanatchi) and all of my friends and my well wishers.

KARTHICKPRABHU S

x

TABLE OF CONTENTS

CHAPTER

NO

TITLE PAGE

NO

ABSTRACT iii

LIST OF TABLES xvii

LIST OF FIGURES xx

LIST OF SYMBOLS & ABBREVATIONS xxix

1 INTRODUCTION 1

1.1 INTRODUCTION 1

1.2 WHAT IS A BATTERY? 3

1.2.1 Role of Anode, cathode and

electrolyte in batteries and the

working principle of battery

3

1.3 TECHNICAL SPECIFICATIONS OF

BATTERY

4

1.4 CLASSIFICATIONS OF BATTERIES 8

1.4.1 Primary Batteries 8

1.4.2 Secondary Batteries 10

1.5 LITHIUM BASED BATTERIES 11

1.5.1 Why Lithium based batteries? 11

1.5.2 Lithium metal batteries 11

1.5.3 Lithium polymer battery 12

1.5.4 Lithium air batteries 12

1.5.5 Lithium-ion batteries 12

1.5.6 Interest of Lithium-ion batteries 14

1.6 WORKING PRINCIPLE OF LI-ION 15

xi

BATTERY

1.7 COMPONENTS OF LI-ION BATTERY 16

1.7.1 Anodes 17

1.7.2 Electrolyte 18

1.7.3 Separators 18

1.7.4 Cathode materials for Li-ion

batteries

19

1.8 LAYERED OXIDE CATHODE

MATERIALS

20

1.8.1 LiCoO2 22

1.8.2 LiNiO2 23

1.8.3 Other LiMO2 oxides 24

1.9 VANADIUM BASED CATHODES 25

1.10 SPINEL STRUCTURED CATHODES 26

1.11 OLIVINE STRUCTURED CATHODES 26

1.11.1 Research interest on LiNiPO4 28

1.12 GENERAL STRATEGIES TO

IMPROVE THE PERFORMANCE OF

THE CATHODES

33

1.12.1 Synthesis of nanostructured

materials

33

1.12.2 Coating with a conductive

medium

35

1.12.3 Doping 35

1.12.4 Literature review 35

1.13 OBJECTIVE AND RESEARCH

PROBLEM

39

xii

2 EXPERIMENTAL TECHNIQUES 41

2.1 INTRODUCTION 41

2.2 SYNTHESIS OF OLIVINE BASED

CATHODE MATERIALS

41

2.2.1 Solid state reaction method

(SSR)

42

2.2.2 Polyol method 44

2.2.3 Structure and properties of

polyol mediums

46

2.3 CHARACTERIZATION TECHNIQUES 47

2.3.1 Powder X-Ray Diffraction 48

2.3.2 Scanning Electron Microscopy 49

2.3.3 Particle size analysis 50

2.3.4 Thermogravimetry analysis

(TGA)

51

2.3.5 Infrared spectroscopy analysis 53

2.3.6 Laser Raman studies 55

2.3.7 Impedance spectroscopy studies 56

2.3.8 Electrode preparation and cell

assembly

60

2.3.9 Electrochemical impedance

spectroscopy

61

2.3.10 Cyclic voltametry studies 63

2.3.11 Galvanostatic charge-discharge

tests

63

xiii

3 STRUCTURAL AND MORPHOLOGICAL

STUDIES OF LiNiPO4: RE (RE=La & Nd),

LiNiMPO4 (M= Zn & Mn)

65

3.1 INTRODUCTION 65

3.2 STRUCTURAL INVESTIGATION OF

PURE LiNiPO4

66

3.2.1 XRD patterns of LiNiPO4

prepared by SSR

66


prepared by polyol method using

EG as a polyol medium

67


prepared by polyol method using

1,2 propanediol as a polyol

medium

67

3.3 EFFECT OF REFLUXING TIME ON

THE PREPARATION OF LiNiPO4

69

3.4 STRUCTURAL INVESTGATIONS OF

RARE EARTH DOPED LiNiPO4

70

3.5 STRUCTURAL INVESTIGATIONS OF

TRANSITION METAL DOPED

LiNiPO4

75

3.6 MORPHOLOGICAL STUDIES 80

3.6.1 SEM analysis of pure LiNiPO4 80

3.6.2 SEM analysis of rare earth (La3+

and Nd3+) doped LiNiPO4

80

3.6.3 SEM analysis of Transition

metal (Zn2+ and Mn2+) doped

LiNiPO4

81

xiv

3.7 PARTICLE SIZE DISTRIBUTION

ANALYSIS

83

3.7.1 Particle size distribution analysis

of pure and rare earth doped

LiNiPO4

83

3.7.2 Particle size distribution analysis

of Zn2+ and Mn2+ doped LiNiPO4

85

3.8 SUMMARY 87

4 THERMAL AND VIBRATIONAL

CHARACTERIZATION OF UNDOPED

AND DOPED LiNiPO4

89

4.1 INTRODUCTION 89

4.2 THERMAL STUDIES 90

4.2.1 Thermal studies of pure LiNiPO4 90

4.2.2 Thermal studies of rare earth

doped LiNiPO4

91

4.2.3 Thermal studies of transition

metals doped LiNiPO4

93

4.3 FTIR STUDIES 95

4.3.1 FTIR studies of pure LiNiPO4 95

4.3.2 FTIR studies of rare earth doped

LiNiPO4

96

4.3.3 FTIR studies of transition metals

doped LiNiPO4

99

4.4 LASER RAMAN ANALYSIS 101

4.4.1 Laser Raman analysis of pure 101

4.4.2 Laser Raman studies of rare

earth doped LiNiPO4

102

xv

4.4.3 Laser Raman studies of transition

metal doped LiNiPO4

107

4.5 SUMMARY 111

5 AC IMPEDANCE SPECTROSCOPIC

STUDIES ON PURE, RARE EARTH AND

TRANSITION METAL DOPED LiNiPO4

112

5.1 INTRODUCTION 112

5.2 IMPEDANCE STUDIES OF PURE

LiNiPO4

112

5.2.1 Conductance spectra analysis 113

5.2.2 Temperature dependent

conductivity analysis

116

5.2.3 Bode plot analysis for pure

LiNiPO4

118

5.2.4 Dielectric and modulus

spectroscopic analysis

120

5.3 IMPEDANCE ANALYSIS OF RARE

EARTH AND TRANSITION METAL

DOPED LiNiPO4

126

5.3.1 Conductance spectra analysis 126

5.3.2 Cole-Cole plot analysis 130

5.3.3 Concentration dependent of

conductivity

133

5.3.4 Dielectric and Modulus

spectroscopic analysis

134

5.4 SUMMARY 138

xvi

6 ELECTROCHEMICAL

CHARACTERIZATION ON PURE AND

DOPED LiNiPO4

140

6.1 INTRODUCTION 140

6.2 ELECTROCHEMICAL IMPEDANCE

SPECTROSCOPY

140

6.3 CYCLIC VOLTAMETRY STUDIES 146

6.4 GALVANOSTATIC CYCLING TESTS 156

6.5 SUMMARY 161

7 CONCLUSION 163

7.1 CONCLUSION OF THE WORK DONE 163

7.2 FUTURE PLAN OF WORK 167

REFERENCES 168

LIST OF PUBLICATIONS 196

CURRICULUM VITAE 199

xvii

LIST OF TABLES

Table 1.1. Some important primary batteries and their

characteristics

Table 1.2. Some important secondary batteries and their

characteristics

Table 1.3. Important Cathode materials for Li-ion batteries

Table 1.4. Physical and electrochemical properties of Olivine

metal phosphates

Table 1.5. Some important olivine Cathode materials for Li-ion

batteries

Table 2.1. Properties of Ethylene glycol

Table 2.2. Properties of 1,2 Propanediol

Table 2.3. Impedance parameters

Table 3.1. Crystallite size for LiNiPO4 calculated using Debye

Scherer formula

Table 3.2. Calculated lattice parameters for La3+ doped LiNiPO4

using TOPAS 3.0

Table 3.3. Calculated lattice parameters for Nd3+ doped LiNiPO4

using cell refinement software

Table 3.4. Lattice parameters for Zn2+ and Mn2+ doped LiNiPO4 by

TOPAS 3.0

xviii

Table 3.5. Particle size distribution of rare earth and transition

metal doped LiNiPO4

Table 4.1. TG analysis of La3+ doped LiNiPO4 at different

concentrations of Lanthanum

Table 4.2. TG analysis of Nd3+ doped LiNiPO4 at different

concentrations of Neodymium

Table 4.3. TG analysis of Zn2+ doped LiNiPO4 at different

concentrations of Zinc

Table 4.4. TG analysis of Mn2+ doped LiNiPO4 at different

concentrations of Manganese

Table 4.5. Vibrational analysis for La3+ doped LiNiPO4

Table 4.6. Vibrational analysis for Nd3+ doped LiNiPO4

Table 4.7. Vibrational analysis for LiNi1-xZnxPO4 (x=0.05, 0.10,

0.15, 0.20)

Table 4.8. Vibrational analysis for LiNi1-xMnxPO4 (x=0.05, 0.10,

0.15, 0.20)

Table 4.9. Laser Raman analysis of La3+ doped LiNiPO4

Table 4.10. Laser Raman analysis of Nd3+ doped LiNiPO4

Table 4.11. Laser Raman analysis of Zn2+ doped LiNiPO4

Table 4.12. Laser Raman analysis of Mn2+ doped LiNiPO4

Table 5.1. Calculated conductivity value for pure LiNiPO4 at

different temperatures

xix

Table 5.2. Activation energies of LiNiPO4 at different sintering

temperatures

Table 5.3. Conductivity values for La3+ and Nd3+ doped LiNiPO4 at

ambient temperature

Table 5.4. Conductivity and “n” values for Zn2+ and Mn2+ doped

LiNiPO4 at ambient temperature

Table 5.5. Conductivity of LiNiPO4 and doped LiNiPO4 found in

the literature

Table 5.6. ‘n’ values for rare earth doped LiNiPO4 calculated from

the dielectric loss spectra

Table 5.7. ‘n’ values for Transition metal doped LiNiPO4

calculated from the dielectric loss spectra

Table 6.1. Electrochemical impedance parameters

Table 6.2. Electrochemical performance of LiNiPO4 found in the

literature

Table 6.3. Cyclic voltametry peak analysis of pure LiNiPO4

Table 6.4. Cyclic voltametry peak analysis of 0.05 mol% La doped

LiNiPO4

Table 6.5 Cyclic voltametry peak analysis of 0.03 mol% Nd doped

LiNiPO4

Table 6.6 Cyclic voltametry peak analysis of LiNi0.85Zn0.15PO4

Table 6.7 Cyclic voltametry peak analysis of LiNi0.90Mn0.10PO4

xx

LIST OF FIGURES

Figure 1.1 : Comparison of different batteries based on specific

energy and volumetric energy

Figure 1.2 : Working Principle of Li-ion battery

Figure 1.3 : Graphical representation of Voltage vs. Capacity for

cathode and anode materials

Figure 1.4 : The schematic drawing of the crystal structure of LiNiPO4

in the ab-plane

Figure 1.5 : Simulated discharge curves of various cathode materials

Figure 1.6 : Simulated voltage versus energy curves for 18650-size

cells with selected Li-ion cathode materials

Figure 2.1 : Schematic diagram of solid state reaction method

Figure 2.2 : Schematic procedure adopted for polyol synthesis

Figure 2.3 : Structure of Ethylene glycol

Figure 2.4 : Structure of 1, 2 Propanediol

Figure 2.5 : Picture of TG analysis instrument

Figure 2.6 : Typical example for TG curve

Figure 2.7 : Schematic diagram of the FTIR spectrometer

Figure 2.8 : Energy level diagram of Raman Effect

Figure 2.9 : Picture of Impedance measurement instrument

xxi

Figure 2.10: Principle of impedance spectroscopy

Figure 2.11: A schematic diagram of the Swagelok cell

Figure 2.12: Picture of the Glove box

Figure 2.13: Computer controlled Biologic SP-300 electrochemical

work station

Figure 3.1 : XRD patterns of LiNiPO4 at 650°C for 2h (a) and 4h (b)

using SSR method

Figure 3.2 : XRD patterns of LiNiPO4 using ethylene glycol as a

Polyol medium

Figure 3.3 : XRD patterns of pure LiNiPO4 prepared at various

calcinations temperature and time using 1, 2 propanediol

as a polyol medium

Figure 3.4 : XRD pattern of pure LiNiPO4 using 1, 2 propanediol as a

polyol medium refluxed at 5 and 20 h and calcined at 650

C 4 h

Figure 3.5 : XRD patterns of La doped LiNiPO4 calcined at 650°C, 6h

(a) pure LiNiPO4 (b) 0.01 mol% (c) 0.03 mol% (d) 0.05

mol% (e) 0.07 mol% (f) 0.09 mol%

Figure 3.5.1: Refined XRD pattern of pure LiNiPO4

Figure 3.5.2: Refined XRD pattern of 0.01 mol% La doped LiNiPO4



xxii


Figure 3.6: XRD patterns of Nd doped LiNiPO4 calcined at 650°C, 6h

(a) Pure LiNiPO4 (b) 0.01 mol% (c) 0.03 mol% (d) 0.05

mol% (e) 0.07 mol% (f) 0.09 mol%

Figure 3.7: XRD patterns of (a) Pure LiNiPO4 (b) LiNi0.95Zn0.05PO4

(c) LiNi0.9Zn0.1PO4 (d) LiNi0.85Zn0.15PO4 (e)

LiNi0.8Zn0.2PO4

Figure 3.7.1: Refined XRD pattern of LiNi0.95Zn0.05PO4




Figure 3.8 : XRD patterns of (a) LiNi0.95Mn0.05PO4 (b) LiNi0.9Mn0.1PO4 (c)

LiNi0.85Mn0.15PO4 (d) LiNi0.8Mn0.2PO4

Figure 3.8.1: Refined XRD pattern of LiNi0.95Mn0.05PO4




Figure 3.9 : SEM images of a) as prepared LiNiPO4 b) Calcined at

650°C 4h

Figure 3.10 : SEM image of La3+ doped LiNiPO4 a) as prepared sample

b) calcined at 650°C 6h

xxiii

Figure 3.11: SEM image of Nd3+ doped LiNiPO4 a) as prepared sample


Figure 3.12: SEM image of Zn2+ doped LiNiPO4 a) as prepared sample


Figure 3.13: SEM image of Mn2+ doped LiNiPO4 a) as prepared sample


Figure 3.14 : Particle size distribution of Pure LiNiPO4

Figure 3.15 : Particle size distribution of 0.05 mol% La doped LiNiPO4

Figure 3.16 : Particle size distribution of 0.07 mol% Nd doped LiNiPO4

Figure 3.17 : Particle size distribution of LiNi0.85Zn0.15PO4

Figure 3.18 : Particle size distribution of LiNi0.90Mn0.10PO4

Figure 4.1 : Thermogravimetric studies of undoped LiNiPO4

Figure 4.2: Thermal studies of La3+ doped LiNiPO4 at different

concentrations

Figure 4.3: Thermal studies of Nd3+ doped LiNiPO4 at different

concentrations

Figure 4.4 : Thermal analysis of Zn2+ doped LiNiPO4

Figure 4.5 : Thermal analysis of Mn2+ doped LiNiPO4

Figure 4.6 : FTIR spectra of undoped LiNiPO4

Figure 4.7: FTIR spectra of La doped LiNiPO4 (a) 0.01 mol% (b)

0.03 mol% (c) 0.05 mol% (d) 0.07 mol% (e) 0.09 mol%

xxiv

Figure 4.8 : FTIR spectra of Nd3+ doped LiNiPO4 (a) 0.01 mol% (b)

0.03 mol% (c) 0.05 mol% (d) 0.07 mol% (e) 0.09 mol%

Figure 4.9 : FTIR spectra of Zn2+ doped LiNiPO4 (a) LiNi0.95Zn0.05PO4

(b) LiNi0.9Zn0.1PO4 (c) LiNi0.85Zn0.15PO4 (d)

LiNi0.80Zn0.20PO4

Figure 4.10: FTIR spectra of Mn2+ doped LiNiPO4 (a)

LiNi0.95Mn0.05PO4 (b) LiNi0.9Mn0.1PO4 (c)


Figure 4.11: Laser Raman spectrum of LiNiPO4

Figure 4.12: Laser Raman spectra of La doped LiNiPO4 (a) 0.01 mol%

(b) 0.03 mol% (c) 0.05 mol% (d) 0.07 mol% (e) 0.09

mol%

Figure 4.13: Laser Raman spectra of Nd3+ doped LiNiPO4 (a) 0.01

mol% (b) 0.03 mol% (c) 0.05 mol% (d) 0.07 mol% (e)

0.09 mol%

Figure 4.14: Deconvoluted Raman peak of La3+ doped LiNiPO4 (X-

axis (wave number) Y-axis (intensity) in the range of 920

cm-1 to 980 cm-1)

Figure 4.15: Deconvoluted Raman peak of Nd3+ doped LiNiPO4 (X-


cm-1 to 980 cm-1)

Figure 4.16: Laser Raman spectra of Zn2+ doped LiNiPO4 (a) Pure

LiNiPO4 (b) LiNi0.95Zn0.05PO4 (c) LiNi0.90Zn0.10PO4

(d)

LiNi0.85Zn0.15PO4 (e) LiNi0.80Zn0.20PO4

xxv

Figure 4.17: Laser Raman spectra of Mn2+ doped LiNiPO4 (a)

LiNi0.95Mn0.05PO4 (b) LiNi0.90Mn0.10PO4

(c)


Figure 4.18: Deconvoluted Raman peak of Zn2+ doped LiNiPO4 (X-


cm-1 to 980 cm-1)

Figure 4.19: Deconvoluted Raman peak of Mn2+ doped LiNiPO4 (X-


cm-1 to 980 cm-1)

Figure 5.1 : Frequency dependence conductivity of LiNiPO4 pellet

sintered at different temperatures (a) 550°C (b) 650°C (c)

750°C

Figure 5.2 : Temperature dependent conductivity of LiNiPO4 for

different sintering temperatures □ 550°C, ●650°C,

▲750°C

Figure 5.3 : Frequency dependence of a) Real and b) Imaginary part

of the impedance of LiNiPO4 (sintered at 650°C for 5 h) at

different temperatures

Figure 5.4 : Scaling behavior of Zꞌꞌ at different temperatures for pure

LiNiPO4

Figure 5.5 : Frequency dependent dielectric constant of LiNiPO4

(sintered at 650°C for 5 h) at different temperatures

Figure 5.6 : Scaling behavior of dielectric loss at different

temperatures of LiNiPO4 (sintered at 650°C for 5 h)

xxvi

Figure 5.7 : Variation of logarithmic of dielectric loss for LiNiPO4

with frequency at different temperatures (pellet sintered at

650°C, 5h)

Figure 5.8 : Real part of the modulus spectra for Pure LiNiPO4 at

different temperatures (LiNiPO4 pellet sintered at 650°C,

5h)

Figure 5.9 : Imaginary part of the modulus spectra for Pure LiNiPO4

at different temperatures (LiNiPO4 pellet sintered at

650°C, 5h)

Figure 5.10 : Frequency dependent conductivity spectra for Rare earth

doped LiNiPO4 at ambient temperature

Figure 5.11 : Frequency dependent conductivity spectra for Transition

metal doped LiNiPO4 at ambient temperature

Figure 5.12 : Cole-Cole Plot for Pure LiNiPO4 at ambient temperature

Figure 5.13 : Cole-Cole Plot for rare earth doped LiNiPO4 at ambient

temperature

Figure 5.14 : Cole-Cole Plot for Transition metal doped LiNiPO4 at

ambient temperature

Figure 5.15 : Concentration dependent conductivity spectra for a) rare

earth and b) transition metal doped LiNiPO4

Figure 5.16 : Frequency dependent dielectric constant of rare earth and

transition metal doped LiNiPO4

Figure 5.17 : Variation of logarithmic dielectric loss for rare earth and

xxvii


Figure 5.18 : Imaginary part of modulus spectra of rare earth and


Figure 6.1 : Electrochemical impedance spectra of pure LiNiPO4 after

and before cycling

Figure 6.2 : Electrochemical impedance spectra of 0.05 mol% La3+

doped LiNiPO4 before and after cycling

Figure 6.3 : Electrochemical impedance spectra of 0.03 mol% Nd3+

doped LiNiPO4 before and after cycling

Figure 6.4 : Electrochemical impedance spectra of LiNi0.85Zn0.15PO4

before and after cycling

Figure 6.5 : Electrochemical impedance spectra of LiNi0.90Mn0.10PO4

before and after cycling

Figure 6.6 : Cyclic voltametry curves of pure LiNiPO4

Figure 6.7 : Cyclic voltametry curves of 0.05 mol% La3+ doped

LiNiPO4

Figure 6.8 : Cyclic voltametry curves of 0.03 mol% Nd3+ doped

LiNiPO4

Figure 6.9 : Cyclic voltametry curves of LiNi0.85Zn0.15PO4

Figure 6.10 : Cyclic voltametry curves of LiNi0.80Mn0.10PO4

Figure 6.11 : Charge-discharge curves of pristine LiNiPO4 measured at

C/20

Figure 6.12 : Charge-discharge curves of 0.05 mol% La doped LiNiPO4

measured at C/20

xxviii

Figure 6.13 : Charge-discharge curves of 0.03 mol% Nd doped

LiNiPO4 measured at C/20

Figure 6.14 : Charge-discharge curves of LiNi0.85Zn0.15PO4 measured at

C/20

Figure 6.15 : Charge-discharge curves of LiNi0.9Mn0.1PO4 measured at

C/20

xxix

LIST OF SYMBOLS AND ABBREVIATIONS

CPE Constant Phase Element

CV Cyclic Voltametry

DEC Diethyl Carbonate

DMC Dimethlyl Carbonate

DME Dimethoxy Ethane

EIS Electrochemical Impedance Spectroscopy

EC Ethylene Carbonate

EG Ethylene Glycol

FCC Face Centered Cubic

FTIR Fourier Transform Infrared Spectroscopy

FWHM Full Width Half Maximum

HCP Hexagonal Closed Packing

HT-LiCoO2 High Temperature Lithium Cobalt Oxide

HEV Hybrid Electric Vehicles

JCPDS

Joint Committee on Powder Diffraction

Standards

LIB Lithium Ion Batteries

LT-LiCoO2 Low Temperature Lithium Cobalt Oxide

Ni-Cd Nickel Cadmium

Ni-MH Nickel Metal Hydride

NMP N-methyl-2-pyrrolidone

1D One Dimensional

OCV Open Circuit Voltage

Oxi. Oxidation

PHEV Plug-in Hybrid Electric Vehicles

PVDF Poly (Vinylidene fluoride)

xxx

PXRD Powder X-ray Diffraction

PC Propylene Carbonate

PLD Pulsed Laser Deposition

RE Rare Earth

Red. Reduction

Redox Reduction-Oxidation

SEM Scanning Electron Microscopy

SSR Solid State Reaction

SIC Super Ionic Conductor

TMS Tetra Methyl Sufolane

TGA Thermo Gravimetric Analysis

UDR Universal Dynamic Response

xxxi

SYMBOLS

Ea - Activation energy

Y - Admittance

ω - Angular frequency

k - Boltzmann’s Constant

Rb - Bulk resistance

Rct - Charge transfer resistance

Ich - Charging current

Δtch - Charging time

Vch - Charging voltage

I - Current

σdc - DC conductivity

ε' - Dielectric constant

ε'' - Dielectric loss

θ - Diffraction Angle

Idis - Discharging current

Δtdis - Discharging time

Vdis - Discharging voltage

F - Farady’s Constant

β - Full Width Half Maximum

ΔG - Gibbs free energy

h - Hours

Z - Imaginary part of Impedance

M'' - Imaginary part of complex modulus

σ - Electrical conductivity

mol% - Mole percentage

σo - Pre exponential factor

xxxii

Z - Real part of Impedance

M' - Real part of complex Modulus

Tanδ - Tangent loss

T - Temperature

t - Time

C0 - Vacuum Capacitance

V - Voltage

λ - Wavelength

1

CHAPTER 1

INTRODUCTION

1.1. INTRODUCTION

Energy and environmental based issues are becoming the major

areas of concern in the 21st century as these factors are directly linked to

technological development; therefore, the exploration of alternative energy

sources continues. With rapidly growing energy demands, there is a large

increase in demand for more efficient and nonrenewable energy resources.

At present, most of the research is focusing on fossil fuels for our energy

needs. Combustion of fossil fuels leads to the emission of greenhouse gases

into the atmosphere. Global warming is a direct consequence of the

accumulation of greenhouse gases. Internal combustion engines are a major

source of CO2 emission and hence alternative energy sources for

automotive propulsion applications is one of the prime focuses of research

throughout the world. The renewable sources such as solar energy and wind

energy are “green” sources of energy but these are intermittent sources. For

a continuous use, storage of energy is necessary. Batteries are

electrochemical storage devices with which the energy is stored in the form

of chemical potential difference and use it whenever and wherever it is

needed.

Rechargeable battery sources are one of the most promising energy

storage solutions for future automotive technology. Rechargeable batteries

are more portable than the resources such as flywheels, capacitors, biofuels,

2

solar cells, and fuel cells and provide quick energy storage and release of

energy [Cutler et al (2004), Kurzweil et al (2009), Ibrahim et al (2008)].

Among the various existing rechargeable energy storage systems,

Lithiumbased battery is one of the most attractive technologies to satisfy

our needs. Moreover, they are found to be the capable of storing energy due

to their lightweight, long lifetime and high operating voltage (~4V) with

high energy density range from 100-150 Wh/kg. Li-ion battery technology

is applied to both thin, light, and flexible portable electronic devices

[Moore et al (1999), Manning et al (2002)], electric vehicles and theyalso

find applications in space [Cuellar et al (2002), Jeevarajan et al (2002)].

The principal challenges of this area are safety, usage in larger Plug-in

Hybrid and all Electric Vehicles (PHEV) of larger driving range, faster

charging rates, and manufacturing at lower cost [Goodenough et al

(2001)].Figure 1.1 depicts the comparison of different batteries based on

specific energy and volumetric energy.

Figure 1.1: Comparison of different batteries based on specific energy and volumetric energy [Tarascon et al (2001)]

3

1.2. WHAT IS A BATTERY?

A battery converts chemical energy stored in its two electrodes in to

discharge electric current I=Idis at a voltage V=Vdis for a time Δt=Δtdis and a

rechargeable battery restores the chemical energy by the application of

charging current Ich at a voltage Vch over a time Δtch. The three primary

components of battery are anode, cathode and electrolyte.

1.2.1. Role of Anode, cathode and electrolyte in batteries and the working principle of battery

Designation of the anode and cathode in a rechargeable cell are

defined during the charge/discharge process occurring in the cell. The

anode always refers to the negative electrode and the cathode always refers

to the positive electrode, even though the reverse is actually true during

charging.

During discharge, the negative electrode (anode) is oxidized (loss of

electrons is oxidation) and it is the source of electrons, while the positive

electrode (cathode) is reduced (gain of electrons is reduction) and it is the

receiver of electrons. Each electrode depends upon the other electrode to

maintain a balance of flow of electrons. The number of electrons provided

by the anode must equal the number of electrons received by the cathode.

Electrode materials are often described by their capacity ratings (mAh/g),

from which the amount of each material required for the construction of a

balanced cell can be calculated.

During discharge, the number of electrons transferred in the external

electric circuit from the anode to the cathode equals the number of ions that

must be transferred by the cell’s internal electrolyte.

4

The electrolyte is ionically conductive, but electronically non-

conductive. The ionically conductive electrolyte completes the electro-

chemical circuit by carrying only ions between the active cathode and

anode materials. The electrode-electrolyte-electrode interfaces are where all

the real action occurs within the cell, and these two interfaces determine

much of the cells characteristics and features such as cell voltage, capacity,

power capability, cycle life, calendar life, self discharge, temperature

effects, and safety.

During charging, the anode and cathode reactions are reversed by

forcing electrons to flow opposite in direction than they flowed during

discharge. The charger must apply a voltage across the cells’ terminals that

are higher in potential than the open circuit cell voltage in order to generate

electron flow back into the anode from the cathode, electro-chemically

reversing the chemical reaction that took place during the discharge phase.

During charging the electrolyte must also reverse function and shuttle the

ions back from the cathode to the anode.

1.3. TECHNICAL SPECIFICATIONS OF BATTERY

Cell, Modules, and packs

A cell is the smallest form of a battery thatcomprises of anode,

electrolyte and cathode.A cell generally produces on the order of one to 5

volts. A module consists of several cells generally connected in either series

or in parallel. A battery pack is then assembled by connecting modules

together, again either inside in series or parallel to deliver the required

output energy.

5

Open circuit voltage

The open circuit voltage (OCV) is the voltage measured in the

absence of current. It is the function of the difference between the lithium

chemical potential of cathode (μLi(c)) and anode (μLi(a)) and is given by

the following equation

OCV= -ΔG/nF=(μLi(c)-μLi(a))/nF

Cut-off voltage

The minimum allowable voltage, it is the voltage that generally

defines the empty state of the battery.

Charge

It is the process when electrical energy applied to a cell or a battery

is converted into chemical energy within a cell or a battery

Discharge

It is the conversion of the chemical energy of a cell or a battery into

electrical energy and withdrawal of an electrical energy into a load.

C- and E- rates

In describing batteries, discharge current is often expressed as a C-

rate in order to normalize against battery capacity, which is often varying

differenttypes of batteries. A C-rate is a measure of the rate at which a

battery is discharged relative to its maximum capacity. A 1C rate means

6

that the discharge current will discharge the entire battery in 1 h. 0.1C

means transfer of 10% energy in 1 h or full transfer in 10h. 10C means full

transfer in 6 mins. Similarly, E-rate describes the discharge power. A 1E

rate is the discharge power to discharge the entire battery in 1h.

Capacity

The coulometric capacity, the total Amp-hours available when the

battery is discharged at a certain discharge current (Specified as a C-rate)

from 100 percent state- of- charge to cut-off voltage.

Capacity of a battery= discharge current (in Amps)×discharge time (in

hours)

Theoretical specific capacity

The specific charge/discharge capacity (mAh g-1) (the number of

moles of Li) can be calculated from the active material weight and its

molecular weight. The specific capacity of an electrode material is

calculated assuming that all the Li+ ions (and e-) per formula unit of the

material contribute in the electrochemical reaction and is given by the

following equation.

Specific theoretical capacity (mAh g-1)= (F×nLi/M×3600)×1000

Where, F=Farady constant (96,496 colombs) per gram equivalent,

nLi=number of Li+ ions and electrons involving per formula unit of the

compound (eg.,nLi=1 for LiMn2O4) and M=Molecular weight.

7

Specific energy

The nominal battery energy per unit mass sometimes referred to as

the gravimetric energy density. Specific energy is a characteristic of the

battery chemistry and packaging.

Energy density

The nominal battery energy per unit volume sometimes referred to

as the volumetric energy density. The energy density is often expressed as

watt-hour/litre (Wh/litre).

Energy density= voltage ×capacity

Specific power

The maximum available power per unit mass. Specific power is a

characteristic of the battery chemistry and packaging. It determines the

battery weight required to achieve a given performance target.

Power density

It is the maximum rate at which energy is discharged or delivered

per unit mass or per unit volume.

Power density=energy density/unit time

Higher power density can be achieved by

i. Using electrodes of smaller particle sizes

ii. Higher surface area

8

iii. Larger fraction of porosity and thinner electrode

Columbic efficiency

It is the difference in charge and discharge capacities at any cycle

number given in %. Higher the columbic efficiency better is the battery

performance.

Cycle

It refers to complete battery discharge and a full recharge.

Cycle life

The total number of cycles a battery is capable of producing before it

fails.

1.4. CLASSIFICATION OF BATTERIES

Electrochemical cells and batteries are identified as primary (non-

rechargeable) or secondary (rechargeable), depending on their capability of

being electrically recharged.

1.4.1. Primary Batteries

Primary batteries are not capable of being easily or effectively

recharged electrically and, hence, are discharged once and discarded. The

primary battery is a convenient, usually inexpensive, lightweight source of

packaged power for portable electronic and electric devices, lighting,

photographic equipment, toys, memory backup, and a host of other

applications, giving freedom from utility power. The general advantages of

9

primary batteries are good shelf life, high energy density, maintenance, and

ease of use. Ex: Alkaline Manganese Battery, Lithium primary battery,

Primary Zinc-air battery etc. Table 1.1 represents some important primary

batteries and their characteristics.

Table 1.1: Some important primary batteries and their characteristics:

System

Characteristics

Applications Voltage (V)

Energy density mWh/g

Power W/Kg

Zinc-carbon Zinc/MnO2

0.7 65 Low Flash light, toys, portable radios, novelties instruments

Magnesium (Mg/MnO2)

1.6 100 Moderate Military receiver-transmitters, aircraft emergency transmitters

Mercury (Zinc/HgO)

1.35 105 Moderate Hearing aids, medical devices, photography, detectors

Mercad (Cd/HgO)

0.9 55 Moderate Special applications requiring operation under extreme

temperature conditions and long life

Alkaline (Zinc/alkalaine/

MnO2)

1.5 145 Moderate Most popular primary-battery: used in a variety of portable battery operated equipments

Silver/Zinc (Zn/Ag2O)

1.5 135 (button type)

Moderate Hearing aids, photography, electric watches, space

applications (larger sizes)

Zinc/air (Zn/O2) 1.5 370

(button type)

Low Special applications, pagers, medical devices, portable

electronics

Lithium/SO2 3.0 260 High Moderate production, mainly

in military Lithium/SOCl2 3.65 380 Medium Special applications Lithium/MnO2 3.0 230 Moderate Special applications

Lithium/ FeS2 1.5 260 Medium-

high In production for special

applications

10

1.4.2. Secondary Batteries

These batteries can be recharged electrically, after discharge, to their

original condition by passing current through them in the opposite direction

to that of the discharge current. Secondary batteries are characterized (in

addition to their ability to be recharged) by high power density, high

discharge rate, flat discharge curves, and good low-temperature

performance. Generally, secondary batteries can be categorized to several

types, such as Ni-Cd, Lead acid batteries, Ni-MH and Li-ion batteries.

Table 1.2 represents some important secondary batteries and their

characteristics.

Table 1.2: Some important secondary batteries and their characteristics:

System

Characteristics

Applications Voltage (V)

Energy density mWh/g

Power W/Kg

Lead acid 2.1 30-40 180 Automotive and traction

applications Nickel-iron 1.2 50 100 Railway vehicles

Nickel-cadmium

1.2 40-60 150 Motorised equipment,

Power tools, Two way radios

Nickel-hydrogen

1.5 75 220 Aerospace, used to power

the Hubble Space Telescope

Nickel-Metal

hydride 1.2 30-80

250-1000

Cameras, Pagers, Medical instruments and equipment

Nickel-Zinc 1.7 60 900 Traction applications,

Electric Bicycles, Scooters

Lithium-air 2.7 2000 400 automotive power supplies,

mobile devices

Lithium-ion 3.6 150-250 1800 Mobile phones, laptops,

digital cameras etc., Lithium-ion

polymer 3.7 130-200 3000+

portable media player and tablet computers

11

1.5. LITHIUM BASEDBATTERIES

1.5.1. Why Lithium based batteries?

Pioneer work with the lithium battery began in 1912 by G.N. Lewis,

but it was not until the early 1970s that the first non-rechargeable lithium

batteries became commercially available. Lithium is the lightest of all

metals, has the greatest electrochemical potential and provides the largest

specific energy per weight. The theoretical specific capacity of lithium is

3860 mAh/g and the redox potential is -3.01 V versus standard hydrogen

electrode. These features have attracted battery investigators. Rechargeable

batteries with lithium metal as anode (negative electrodes) could provide

extraordinarily high energy densities compared with other batteries. There

are several types of lithium based batteries available which is given by

1. Lithium metal battery (primary lithium battery)

2. Lithium polymer battery

3. Lithium air battery

4. Lithium-ion battery

1.5.2. Lithium metal batteries

Lithium metal batteries are generally primary (non-rechargeable)

batteries that have lithium metal or lithium compounds as an anode.

Lithium metal batteries are generally used to power devices such as

watches, calculators, cameras and data loggers.

12

1.5.3. Lithium polymer battery:

Their first design included a dry solid polymer electrolyte that

resembled a plastic film. Therefore, this type of battery can result in credit

card thin designs while still holding relatively good battery life. The

advantages of these batteries are very lightweight, safety, high voltage (3.6

V) and higher cycle life (300-400 cycles).However, it has a worse energy

density than lithium-ion batteries

1.5.4. Lithium air batteries:

Li ions react directly with oxygen from air dissolved in the

electrolyte, resulting in the formation of lithium-oxygen compounds. In

rechargeable non-aqueous Li-O2 batteries, Li+ ions are formed from the

oxidation of metallic Li at the anode and travel through the electrolyte to

the cathode during discharge. At the cathode, oxygen is simultaneously

reduced in the presence of these Li+ ions to form a solid, insoluble Li2O2

phase, while electrons in the external circuit perform electrical work. This

process is reversed during charge with the decomposition of Li2O2,

evolution of molecular oxygen and plating of Li at the anode.

1.5.5. Lithium-ion batteries:

Lithium-ion batteries are one of the great successes of modern

materials electrochemistry. Lithium ion secondary batteries with non-

aqueous electrolytes were successfully developed and introduced into

market for the first time in 1991 by Sony Corporation. The outstanding

properties of LIBs are as follows

13

i. High operating voltage

ii. High gravimetric and volumetric energy densities,

iii. No memory effect

iv. Low self discharge rate (less than 10% per month)

v. Operation over a wide temperature range

These features have given a huge boost to LIBs and the

number of cells produced is increased from year to year. In Japan, as far as

the production amount is concerned, LIBs surpass the other small sized

rechargeable batteries for consumer use. LIBs are now essential for mobile

gears such as cellular phones, notebook personal computers, personal

digital assistants and portable audio-visual equipment [Schalkwijk et al

(2002)]. Lithium-ion batteries comprise of cells that employ lithium

intercalation compounds as the positive and negative materials. As a battery

is cycled, lithium ions (Li+) exchange between the positive and negative

electrodes. They are also referred to as rocking chair batteries as the lithium

ions "rock" back and forth between the positive and negative electrodes as

the cell is charged and discharged. The positive electrode material is

typically a metal oxide with a layered structure, such as lithium cobalt

oxide (LiCoO2), or a material with a tunneled structure, such as lithium

manganese oxide (LiMn2O4), on a current collector of aluminum foil. The

negative electrode material is typically a graphitic carbon, lithium, also a

layered material, on a copper current collector or metallic lithium. In the

charge/ discharge process, lithium ions are inserted or extracted from

interstitial space between atomic layers within the active materials.

14

1.5.6. Interest of Lithium-ion batteries:

o High operating voltage: A single cell has an average

operating potential of approx. 3.6 V, three times the

operating voltage of both Ni-Cd and Ni-MH batteries and

about twice that of sealed Pb-acid batteries.

o Compact, lightweight, and high energy density: the energy

density is about 1.5 times and specific energy is about twice

that of high-capacity Ni-Cd batteries.

o Fast charging potential; batteries can be charged to about 80-

90% of full capacity in one hour.

o High discharge rate: up to 3C is attainable.

o Wide range of operating temperature: from –20°C to +60°C.

o Superior cycle life: service life of a battery exceeds 500

cycles.

o Excellent safety: United States Department of Transportation,

Dangerous Materials Division has declared Li-ion batteries

exempt from dangerous materials regulations.

o Low self-discharge: only 8-12% per month.

o Long shelf-life: no reconditioning required up to

approximately 5 years (Ni-Cd: 3months; Ni-MH: 1 month).

o No memory-effect: can be recharged at any time.

15

o Non-polluting: does not use toxic heavy metals such as Pb,

Cd or Hg.

1.6. WORKING PRINCIPLE OF LI-ION BATTERY:

A Li-ion battery usually refers to a secondary battery in which

energy is stored chemically through red-ox reactions that employ lithium

intercalation between the positive (cathode) and the negative (anode)

electrodes.

To understand this process, one can consider an example of a

standard LiCoO2 battery in which LiCoO2 acts as a cathode and carbon as

anode. During charging, lithium ions move from cathode to anode and

electrons are removed from cathode by an external field and then are

transferred to anode. During discharge, anode supplies the ions to

electrolyte and electrons to the external circuit where the ions intercalate

into cathode and electrons from the external circuit for charge

compensation. This red-ox reaction is shown below.

The positive electrode half-reaction is

LiCoO2 -->Li1-xCoO2 + xLi+ + xe-

The negative electrode half-reaction is

xLi+ + xe- +6C-->LixC6

Overall cell reaction during charging

LiCoO2 +6C-->Li1-xCoO2+LixC6

The reverse reaction takes place during discharging. The working

principle of Li-ion battery is given in figure 1.2.

16

Figure 1.2: Working Principle of Li-ion battery

1.7. COMPONENTS OF LI-ION BATTERY:

A Li-ion battery consists of a positive electrode (cathode), negative

electrode (anode) and an electrolyte

Figure 1.3: Graphical representation of Voltage vs. Capacity for cathode and anode materials [Tarascon et al (2001)]

17

In addition to these major active components, a separator between

the cathode and anode compartments and current collectors attached to the

electrodes are also involved. A brief discussion about these components

and recent research to improve their performance are discussed in the

following sections. The figure 1.3 represents the cell voltage vs capacity of

different electrode materials [Tarascon et al (2001)]

1.7.1. Anodes:

Anode materials are more amenable to chemical modifications to

improve capacity and other material properties of interest to battery

applications. High capacity anodes are used to free up the internal cell

volume for use by the cathode (which has inherently lower capacity and is

not as modifiable). The cyclability of batteries is also known to depend on

the prudent selection of the anode material.

Carbon is used for anode in the previous generation of Li-ion cells

[Imanishi et al (1998), Winter et al (1998), Levi et al (2008), Endo et al

(2000), Rahul Mukherjee et al (2012)]. The anode electrode is usually made

up of graphite and iscoated on thin copper foil. A single lithium ion can

easily be inserted into each hexagon in the graphite’s molecular structure.

The theoretical capacity of graphite is found to be 372mAh/g. To improve

the capacity, researchers are trying to identify other compounds

likematerials alloys, and other intermetallic compounds. The best carbons

in current research intercalate 2.5 Li ions and achieve capacities as high as

750 mAh/g. Carbon-coated copper-tin alloys could provide 460 mAh/g and

stable cyclic performance ever after 40 cycles [Sheng Liu et al(2009)].

Silicon has an extremely high capacity of 4199mAh/g, corresponding with

a composition of Si5Li22 (Daniel, 2009). In addition, a Li4Ti5O12 lithium cell

18

discharged at C/12 delivered 155mAh/g. However, cycling behavior is

poor, and capacity fading is still not yet understood. Recently, researchers

use lithium metal as anode for its high energy density of 3860 mAh/g.

1.7.2. Electrolyte

The main task of the electrolyte in lithium ion battery is to

continuously carry lithium ion from anode to cathode during charge and

back during discharge. The basic requirements of a suitableelectrolyte are

high ionic conductivity, low melting and high boiling points, chemicaland

electrochemical stability and safety. Liquid electrolyte normally consists of

lithium salts,such as LiPF6, LiPF3(C2F5)3 (Lithium floroalkylphosphate)

[Oesten et al (2002), Gnanaraj et al (2003)], LiBC4O8 (chelating boronate

salt) [Barthel et al (1998)] and LiN(SO2CF3)2dissolved in organic solvents,

such as ethylenecarbonate (EC), dimethyl carbonate (DMC), and diethyl

carbonate (DEC).

1.7.3. Separators

In liquid electrolyte batteries, separators are placed between cathode

and anode inorder to prevent physical contact. Separator should allow the

lithium ion while hinder the electrons through it. The basic requirements of

a suitableseparator are sufficient porosity (typically 40%), chemical

stability,mechanical strength to resist the assembly process, and appropriate

melting point for safety concern.

19

1.7.4. Cathode materials for Li-ion batteries:

The cathode electrode consists of current collector (15μm thick Al),

and the cathode material in the form of slurry spread onto Aluminum foil.

The cathode electrode is one of the key components in the battery pack. It

dominates around 48.8% of the total raw materials cost. Moreover, it

affects the energy capacity, voltage, cycle life, safety and other things.

Key requirements for a material to be used as a cathode for Li-ion Batteries

are as follows,

1. The material should be quickly reducible/oxidizable ion, for

example a transition metal

2. The material react with lithium in a reversible manner

(a) This dictates an intercalation-type reaction in which the host

structure essentially does not change as lithium is added.

3. The material react with lithium with a high free energy of

reaction

(a) High capacity, preferably at least one lithium per transition

metal

(b) High voltage, preferably around 4V (as limited by stability of

electrolyte)

(c) This leads to high energy storage.

20

4. The material reacts with lithium very rapidly both on insertion

and removal.

(a) This leads to high power density, which is needed to replace

the Ni/Cd battery

5. The material must be a good mixed conductor (i.e. both ionic and

electronic).

(a) This allows for the easy addition or removal of electrons

during the electrochemical reaction

(b) This allows the reaction at all contact points between the

cathode active material and the electrolyte.

(c) This minimizes the need for inactive conductive diluents,

which take away from the overall energy density.

6. The material must be stable, i.e., not to change structure or

otherwise degrade, to over discharge and over charge.

7. The material must be of low cost

8. The material should be environmentally benign.

The following sections explicate some of important commercially

available cathode materials.

1.8. LAYERED OXIDE CATHODE MATERIALS:

The structure of two dimensional lithium transition metal oxides

with the general formula LiMO2 (M=V, Cr, Fe, Co and Ni) can be regarded

21

as a distorted rock salt superstructure [Patil et al (2008), Kobayashi et al

(1969), Liu et al (2006)]. These materials form α-NaFeO2-type structure

and these oxides are thermodynamically stable only in the intercalated state

of LiMO2. Among various layered oxide materials, LiCoO2, LiNiO2, and

mixed oxides Li (Ni, Co, Mn) O2 have been considered as important

cathode materials.Table 1.3 depicts the important layered oxide cathode

materials used in the Li-ion batteries.

Table 1.3:Important Cathode materials for Li-ion batteries

Cathode Voltage V vs Li

Counter electrode

Electrolyte Synthesis Ref

LiCoO2

3.8-4.1 Graphi-te 1 mole LiClO4,

EC/DME (1:1)

Li2Co3+CoCO3

850°C, air

Ohzuku et al

(1992)

LiMn2O4 4.1-4.3 Li LiClO4

PC/DMC (1:1)

SSR Naoto et al (2009)

LiNi0.8Co0.2O2

3.6-3.8 Li 1M LiPF6 in

EC:DMC 2:1

Pechini type Nikolowski et al (2007)

LiNiAl0.1V0.9

O4

4.7-4.8 Li 1M LiAsF6 EC+DMC

(1:1)

Starch Assisted

Combustion method

Kalyani et al

(2005)

LiNi1/3Co1/3Mn1/3O2

37-3.9 Li 1M LiPF6 EC/DEC

(1:1)

Carbonate precipitation

method

Xizheng Liu et al (2013)

Li2FeSiO4

2.5 Li LiPF6 (EC/DEC/DMC)(1:1:

1)

SSR Bing Huang et al (2012)

Li[Li0.131Ni0.30

4Mn0.565]O2

3.8-4.5 Li 1M LiPF6 (EC/DMC)

Precursor Controlled

Xingde Xiang et

22

synthesis al (2013)

LiNi0.5Co0.5O2

2.5-4.1 Natural graphite

1M LiClO4 EC/DME

(1:1)

SSR Ohzuku et al

(1992)

LiV2O4

1-3.5 LiMn2O4 1M LiClO4 /PC

Li0.5VO2, Vacuum

Picciotto et al

(1985)

Li1.1Mn0.1Ni0.9

O2

3-4.2 Li 1M LiClO4 EC/PC 1:1

LiOH+NiO+ MnO

Rossen et al

(1992)

LiMn0.1Ni0.9O2

3-4.2 Li 1M LiPF6 PC/DEC

1:4

SSR Yoshio et al

(1995)

LiNi0.7Co0.3O2

3-4 Li 1M LiClO4/ PC

SSR Delmas et al

(1993)

LiFe0.2Ni0.8O2 2-4.2 Li 1M LiClO4

EC/PC (1:1)

Nitrates Reimers et al

(1993)

Vanadium oxide

3-3.4 Li 1M LiPF6 EC+DEC

(1:1)

Electrostatic Spray

deposition

Yi-sun et al

(2013)

LiNi0.8Co0.2O2

3.3-3.8 Li 1M LiClO4 +PC

PLD Baskaran et al (2009)

1.8.1. LiCoO2

LiCoO2is considered as one of the commercial cathode material for

Lithium ion batteries. It can be divided in to two types such as LT-LiCoO2

(Low temperature LiCoO2) and HT-LiCoO2 (High temperature LiCoO2).

The structure of HT-LiCoO2 is found to be a layered rock salt structure

where the cations (both Lithium and Co ions) occupy alternate layers in

octahedral sites between the cubic close-packed oxygen (111) planes. This

ordering results in a rhombohedral structure with 3 symmetry. On the

23

other hand, the LT-LiCoO2 is related to a spinel-type structure (cubic

structure) with symmetry 3 [Akimoto et al, (1998), Santiago et al

(2003)]. The theoretical capacity is reported as 274 mAh g-1 [Koksbang et

al (1996)]. The theoretical discharge voltage and the diffusivity of LiCoO2

are computed as 3.6V vs Li+, 10-8 cm2s-1 respectively [Howard et al

(2007)]. When the concentration of Li decreases from 0.45 to 0.3, the cut

off voltage is increased from 4.3V to 4.5V and it leads to the large

anisotropic volume change of near 3% due to the phase transition between

hexagonal and monoclinic phases [Jang et al (2002), Amatucci et al

(1996)]. Further, Cobalt dissolution, structural changes, and oxidative

decomposition of the electrolyte produce a dramatic increase in the capacity

fade at higher potentials which are the severe drawbacks of this material

[Nazri et al (2004)]. Although, LiCoO2 is widely popularized cathode

material, the OCV vs Li+ is limited.It is also found that half of the lithium

can only be removed and reinserted reversibly thereby reducing the

practical capacity to130 mAh g-1[Balaji et al (2009)]. Substitution of

foreign elements such as Mn, Fe, Nienhance the capacity and capacity

retention during cycling [Wu et al (2009), Liu et al (2009), Gross et al

(2005)]. Coating of metal oxides could reduce the reaction of LiCoO2 with

electrolyte [Gu et al (2007)]. However, higher price and toxicity of Cobalt

element in LiCoO2 boost up extensive studies to find alternative to LiCoO2.

1.8.2. LiNiO2

Lithium nickel oxide (LiNiO2) is cheaper and less toxic than

LiCoO2. LiNiO2 has a rhombohedral structure with trigonal symmetry

(space group 3 ) comprising of two interpenetrating close-packed FCC

sub-lattices: one consists of oxygen anions, and the other consists of Li and

Ni cations on alternating (111) planes. The working voltage and theoretical

24

capacity of LiNiO2 are found to be 3.7 V, 275 mAh g-1 respectively. The

specific capacity is computed as 200 Ah/Kg during the first charge which is

higher than that of commercial LiCoO2. Even though, LiNiO2has

advantages over LiCoO2, LiNiO2 suffers from some structural and chemical

stability on charge state and encounter the problem of forming higher state

of Ni3+during the synthesis, easily decompose in to Lithium deficient

compound at higher heat treatment.

Doping the elements such as Mg, Al, Ti, Co and Mn could improve

the reversible capacity, cyclability and rate capability [Reimers et al (1993),

Gao et al (1998), Kim et al (2001), Kweon et al (1999), Uchida et al (1999),

Nikolowski et al (2007)]. Addition of calcium, arsenium and indium

improve the cell performance of LiNiO2. It is also found that the solid

solution, LiCo0.5Ni0.5O2 is a promising electrode material for LiB’s [Li et al

(2006), Kim et al (2006)].

1.8.3. Other LiMO2 oxides

LiMnO2[Thackeray et al (1999), Davidson et al (1995)] provided

researchers with considerable success as viable metal oxide cathodes, with

capacities in excess of 250 mA h g-1 and stable cycle ability up to 4.4 V

[Armstrong et al (1996)]. LiCrO2 [Jones et al (1994), Delmas et al (1992)],

LiVO2 [Delmas et al (1993)] and LiCuO2 [Kang et al (1995)]are considered

as cathode material for Li-ion batteries but suffers from lower reversible

practical capacities. Theoretical capacity of LixCrO2 is found to be 295mAh

g-1 by assuming 1Li per CrO2 unit can be cycled but very low reversible

capacity about 60mAh/g is obtained by experimentally. Moreover, the

cycling performance of this material is unacceptable. Another member of

25

this family named as LiFeO2 has rock salt structure does not insert/de-insert

the Li-ions.

1.9. VANADIUM BASED CATHODES:

Vanadium based cathode materials with layered structures like

lithium trivanadate (LiV3O8) and vanadium pentoxide (V2O5) are

considered as the potential cathode materials for Li-ion batteries due to

their low cost, high energy density, higher theoretical capacity and less

toxic but it possesses low working voltage ~3V vs Li+ [Sakunthala et al

(2010), Daliang et al (2008), Kannan et al, (2003), Vijayakumar et al

(2003), Li et al (2006), Wang et al (2006), Liu et al (2009) ]. Li1+xV3O8is

regarded as a cathode material for Li-ion batteries for past 20 years. This

material crystallizes in to monoclinic structure. The structure is made up of

(V3O8)(1+x)- layers in the b-c planes, stacked one above the other along a-

axis, which in turn, consists of two basic structural units, VO6 octahedra

and VO5 distorted trigonal bipyramids interconnected to each other by

cornor-sharing oxygen atoms to form V-O layers. Between the V-O layers,

there are different octahedral and tetrahedral sites for Li-ions and these

layers are held together by weak van-der walls forces. The immobile

lithium ions play a pinning role between the layers and keep the layers

strongly connected. The excess lithium corresponding to the amount x is

accommodated at the tetrahedral sites between the layers and take part in

charge/discharge process [Kawakita et al (2000), Kawakita et al (1998),].

Nanocrystalline Li3V2(PO4)3/C (4.6 V) delivers the appreciable specific

capacity of 174mAh/g and better capacity retention [Nathiya et al (2012)].

Though this materials having some positive factors, limited long term

cycling stability is a major problem of such electrode materials.

26

1.10. SPINEL STRUCTURED CATHODES:

LiMn2O4 with spinel structure (space-group: Fd3m) has attracted as

a cathode material for Li-ion batteries due to their lower cost and lower

toxicity than commercialized LiCoO2[Thackaray et al (1993)]. LiMn2O4 is

safer than LiCoO2 [Pasquier et al (2009), Belharouak et al (2007)] but has a

lower capacity [Ritchie et al (2004)] (110mAh/g).Capacity loss is observed

due to dissolution of manganese (LiMn2O4) in electrolyte [Jang et al

(1996)] or changes in particle morphology, Jahn-Teller Effect, oxygen

deficiency [Bohua Denga et al (2008), Masaki Yoshio et al (2006)] and

crystallinity.

Partial substitution of metal atoms at Mn site is one of the effective

methods to reduce the capacity loss [Kim et al (2003), Amine et al (1996),

Ito et al (2002), Ito et al (2003)]. For example, substitution of Al, Mg, Cr,

Co, Ni and Zn at Mn site exhibit higher thermodynamic and structural

stability compared with undoped LiMn2O4 and structural stability results

better cathode performance. LiFexMn2-xO4 [Patil et al, (2008)] spinel is

considered as a 5V cathode materials for Li-ion battery. Doping of cobalt in

LiMn2O4 (LiCoyMn2-yO4) shows higher conductivity compared with pure

LiMn2O4 because of the increment of diffusion coefficient of Li+. It is

found that doping of Sulfur and Zinc on LiMn2O4 improves the reversible

capacity and it does not show the Jahn-Teller effect.

1.11. OLIVINE STRUCTURED CATHODES:

Inspired by the success of LiCoO2’s commercial developments,

researchers have extended their interests to olivine-structured

orthophosphates LiMPO4 (M=Fe, Mn, Co, Ni) which are seen as alternative

27

electrode materials for next generation of rechargeable lithium-ion

batteries.Intensive work is dedicated to a phosphate system, LiMPO4 (M =

Fe, Ni, Co and Mn) [Arun patil et al (2008), Balakrishnan et al (2006),

Wolfenstine et al (2005), Wilmont et al (2007), Padhi et al (1997)] with

olivine-related structure as positive electrodes for rechargeable lithium ion

batteries. The advantages of utilizing olivine-type structure come from the

following two reasons: (1) these materials have relatively larger theoretical

capacity, c.a. 170 mAh/g, and higher voltage, over 5.1 V (in the case

LiNiPO4) versus Li+/Li, than the layered rocksalt-type LiCoO2, LiNiO2 and

spinel LiMn2O4, now being commercially used as 4 V positive electrode

materials, and (2) recent efforts in improving the electrolyte make it

possible to realize stable charge/discharge reactions over 5.1 V.

In this structure, P ions reside in tetrahedral sites and form compact

PO4 polyanion units. It was generally recognized that these polyanion units

formed their valence and conduction band at the region far from the Fermi

level where electronic exchange mainly occurs because of its closed shell

electronic configuration. (For example, main group of metal oxides showed

insulating behavior with large band gap.) In addition, these PO4 polyanion

units form strong covalent bonding. As a result, the valence electrons of

transition metals tend to be isolated from those of polyanions, leading to the

fact that the electronic exchange arising from Li removal/uptake mainly

occurs at transition metal ions. This simple electronic description of

LiMPO4 gives us a variety of knowledge on electrochemical behavior in

this system.

Olivine phosphate materials are also interested for lithium ion

batteries owing to their non toxic, cost effectiveness, environmental

friendly, stable even at over charge and better thermal stability during

28

charging and discharging compared with conventional materials

[Wolfenstine et al (2005), Wilmont (2007), Lim et al (2010), Ramana et al

(2006), Wolfenstine et al (2004)]. Iron and Mn based olivine intercalants

are low energy producers compared to Ni and Co compounds. Its key

applications are for some stationary uses such as energy storage systems for

grid stabilization and for some specific mobile applications such as buses

where volume and weight are not a major issue.Charging and discharging

mechanisms of LiMPO4 are shown in the followingequations

→ 1

→ 1

1.11.1. Research interest on LiNiPO4

Current and upcoming Fe based olivine intercalants are low-energy

producers compared to Mn, Co and Ni compounds. Their energy density is

low to compete with layered oxides in applications where volumetric or

gravimetric energy density is more important. If we replace the transition

metal Fe2+ with Mn2+, Co2+ and Ni2+, the OCV ofthe LiMPO4battery rises

from 3.5 V for LiFePO4 to 4.1, 4.8, and 5.1 V vs Li+ for LiMnPO4,

LiCoPO4 and LiNiPO4 respectively. LiNiPO4 has the potential for greater

energy output (>10.7 Wh in 18,650 batteries) compared with commercial

layered cathode materials (up to 9.9 Wh). LiNiPO4 can in theory deliver

34% greater energy per dollar of cell material cost than

LiAl0.05Co0.15Ni0.8O2. So the demand for a maximum voltage and specific

energy leads to the use of Co or Ni in olivine structure cathode materials

[Hautier et al (2011)].

29

The structure of olivine phosphate cathode is presented in figure 1.4.

LiNiPO4 crystallizes in an ordered olivine structure with the space group

Pnma [Julien et al (2012)]. The olivine structure can be considered as a

hexagonal analogue of the spinel structure and is normally described as a

slightly distorted hexagonal close packing (HCP) of oxygen atoms. Li+ or

Ni2+ cations are situated in half of the octahedral sites and P5+ cations in 1/8

of the tetrahedral sites. The NiO6 octahedra share four corners in the bc

plane being cross-linked along the a-axis by the PO4 groups. Li-ions are

located in rows, running along a, of edge-shared LiO6 octahedra. The

olivine structure builds up a 3D network of perpendicular tunnels along the

[010] and [001] direction, occupied by Li+ ions. This network is of great

importance for Li-ion mobility and qualifies the olivine as a potential

cathode material. Computational models and first principle calculations on

LiMPO4 are shown that in the olivine structure, the lowest Li+ migration

energy is found for the pathway along the [010] channel. Figure indicates

the 1D lithium ion mobility along the b-axis during the charge-discharge

process. The main advantage of LiMPO4 is that the strong P-O covalent

bonds in the PO43- polyanion stabilize the oxygen when fully charged and

avoid O2 release at high states of charge, making LiNiPO4 an excellent,

stable, and safe material.

Figure 1.4: The schematic drawing of the crystal structure of LiNiPO4 in the ab-plane. [Julien et al (2012)]

30

Table 1.4 [Wilmont et al (2007)] lists physical and electrochemical

properties of existing olivine cathodes. LiFePO4 is the only commercial

olivine, and must be calcined in an oxygen-free environment while particle

surfaces are modified to incorporate a conductive layer of carbon

[Wolfenstine et al (2005)]or Fe3P [Sung-Yoon Chung et al (2002)]. This

difficult synthesis has slowed process scale-up, thus restricting wide-spread

evaluation and acceptance by the battery industry. LiMnPO4 suffers from

anisotropic Jahn-Teller distortion with delithiation [Subramaniya Herle et al

(2004)], which reduces conductivity and results in rapid capacity fade with

cycling. Though LiCoPO4 and LiNiPO4 work at high voltage vs Li+, their

applications are limited since it has poor electric conductivity, low lithium

ion diffusion and poor rate capability.If diffusivity is the measure of

cathode capability, the listings in Table 1.4indicate LiNiPO4 would be the

optimum intercalate.

Table 1.4: Physical and electrochemical properties of Olivine metal phosphates

Cathode material

Discharge voltage V vs

Li0

Theoretical capacity (mAh/g)

True density (g/cm3)

Diffusivity (cm2/s)

LiFePO4 3.4 170 3.60 10-8

LiMnPO4 4.1 171 3.43 10-7

LiCoPO4 4.8 167 3.70 10-9

LiNiPO4 5.1 167 3.89 10-5

31

Figure 1.5: Simulated discharge curves of various cathode materials [Wilmont et al (2007)].

Fig. 1.5 and 1.6 [Wilmont et al (2007)] visually compare the

capacity and energy curves, respectively, for the seven cathode materials.

From the figures, it is noted that LiNiPO4 shows the maximum voltage with

high energy density.

Figure 1.6: Simulated voltage versus energy curves for 18650-size cells with selected Li-ion cathode materials [Wilmont et al (2007)].

32

Though LiNiPO4 having the positive factors, it suffers from slow

lithium ion diffusion (by experimentally), low electronic and ionic

conductivity and rate capability [Wolfenstine et al (2004)]. The electronic

conductivity of LiNiPO4 is found to be 10-14 S cm-1 even at 100ºC

[Ressouli et al (2003)], 9.34×10-9 S cm-1 at ambient temperature [Prabu et

al (2012)], 10-8 at 400°C [Moreno et al (2001)] and 1.41x 10-7 at 130° C

[Vijayan et al (2014)]. The lower electrical conductivity of LiNiPO4 causes

miserable Li+ intercalation/deintercalation. Wolfenstine et al have found

that the solid solutions of LiNiPO4-LiCoPO4 do not have adequate

conductivity to observe Ni3+/Ni2+ potential [Wolfenstine et al (2004)].

These factors could be improved by reducing the size of the particle using

different synthesis methods [She-Huang Wu et al (2005), Bramnik et al

(2005), Delacourt et al (2006), Kang et al (2009), Sides et al (2005), Fey et

al (2009), Kim et al (2006), Prabu et al (2011)], metal doping on lattice site

[Wolfenstine et al (2005), Sung-Yoon Chung et al (2002)] carbon coating

on the particle surface and lattice doping [Wolfenstine et al (2005),

Subramaniya Herle et al (2004), Huang et al (2001), Doeff et al (2003),

Kim et al (2008), Myung et al (2004),], material prepared from carbon

containing precursors [Subramaniya Herle et al (2004)] and rare earth

doping [LUO Shaohua et al (2010), George Ting-kuo Fey et al (1997)].

Kim et al have prepared nanostructured LiMnPO4 by the Polyol method,

and found that the electronic conductivity and electrochemical performance

are improved [Kim et al (2007)]. Table 1.5 lists the different olivine

cathodes present in the literature.

33

1.12. GENERAL STRATEGIES TO IMPROVE THE

PERFORMANCE OF THE CATHODES

Since last decade, some efforts are being made to beat the problems

inherent with LiNiPO4. These include particle size reduction, lattice doping

and improvement of electronic contact between particles by coatings.

1.12.1. Synthesis of nanostructured materials

A smaller particle size is favorable to enhance the electrochemical

performance owing to the shortening of both lithium-ion diffusion and

electron diffusion lengths within the particle. Nano-sizing should therefore

enhance the rate capability, and in addition reduce structural degradation

and strain during cycling. Nanostructures, such as nanorods, nanowires and

nanosheets, are interesting, as they can efficiently transport charge carriers

while maintaining a large surface to volume ratio, enhancing the contact

with the electrolyte and the reaction kinetics.

Table 1.5: Some important olivine Cathode materials for Li-ion batteries

Cathode Voltage V vs Li

Counter

electrode

Electrolyte

Synthesis Ref

Li1.05Fe0.997Cu0.003PO4

3.3-3.5 Li 1M LiPF6 EC/DMC

(1:1)

SSR Heo et al (2009)

LiFe1-

xCoxPO4 (x=0.02, 0.04,

0.08,0.1)


(1:1)

Citrate gel method

Shanmugaraj et

al (2008)

LiCoPO4/C 4.5-4.8 Li 1M LiPF6 EC/DMC

Spray pyrolysis

Doan et al (2011)

34

Cu doped LiMnPO4


(1:1)

Hydrothermal Jiangfeng Ni et al

(2011) Nd doped LiMn2O4


(1:1)

Co-precipitation

Balaji et al (2012)

LiFePO4 3.3-3.5 Li 1M LiPF6 EC/DMC

(1:1)

Polyol Madhav singh et al (2011)

Vanadium doped

LiFePO4

3.3-3.6 Li LiPF6 DMC+EM

C+EC 1:1:1

Carbothermal reduction method

Ning Hua et al (2010)

LiNiPO4 -0.1to -0.3

Tin metal powd

er

Aqueous LiOH

SSR Minakshi et al (2011)

LiFe1-

xCoxPO4 (Solid

solution)

3.3-4.8 Li LiPF6 EC/DMC

(1:1)

SSR Deyu wang et al (2005)

LiNiyPO4 (y=0.8-1)

Composite

5.1 Li 1M Li-FAP

EC/DMC (1:1)

Rheological phase method

Lucangelo

Dimesso et al

(2012) LiNi0.9Mg0.1P

O4 5.1 Li 1M Li-

FAP EC/DMC

(1:1)

Rheological phase method

Lucangelo

Dimesso et al

(2012) Li3V2(PO4)3

/C 3.6-4.2 Li - Combustion

method Nathiya

et al (2012)

LiCoPO4 4.8 Li 1M LiPF6 EC/DMC

(1:1)

Combination of spray

pyrolysis and Wet ball milling

The Nam Long

Doan et al (2011)

35

1.12.2. Coating with a conductive medium

Coating with a conductive medium, like carbon, organic polymers or

nickel phosphide, is common practice in battery science. It is targeted to

improve the electronic conductivity mainly by increasing the electrical

contact between the active materials particles or between the active material

and conductive agent.

1.12.3. Doping

Defects are defined as slight deviation in the crystal lattice from its

perfect atomic arrangement. The electrochemical performance of cathode

materials can be improved by doping, but the interpretation of doping

effects can be complicated by the interrelations between doping and

microstructure and morphology, since the microstructure formed can be

affected by the dopant additions [Fergus et al (2010)].

1.12.4. Literature review:

Rare earth sources are ecofriendly, abundant and usage of this

material in the lithium ion batteries has much significance. Doping of rare

earth metals with phospho olivine compounds shows high electronic

conductivity [LUO Shaohua et al (2010)] and considerable electro chemical

stability during charging and discharging. Doping of rare earth to LiNiVO4

can improve the electrical conductivity of 2-3 orders [George Ting-kuo Fey

et al (1997)]. Doping of RE (Lanthanum and Yttrium) on LiCoO2 cathode

material can improve the performance of charge-discharge capacity and

potential plateau [Wei et al (2003)]. Jiezi Hu et al have shown that doping

of transition metal and rare earth on LiFePO4 enhance the conductivity by1-

36

3 orders. The conductivity of LiNiPO4 isincreased by one order due to the

addition of 1 mol% europium [Prabu et al (2011)]. It is reported that the

rare earth doping is an effective method for improving the electrochemical

properties of LiFePO4 [Luo Shanhua et al (2010)]. The cycle performance

of the LiMn2O4 cathode material is improved by substituting the rare earth

ions (RE=Nd and Ce)in the Mn site [Peng et al (2000)]. The capacity

retention of LiMn2O4 is increased due to doping of Nd3+ [Balaji et al

(2012)]. Theelectrochemical performance of LiFePO4 and LiCoO2is

improved by addition of La3+ [Cho et al (2008), Ghosh et al (2009)]. It is

seen that the addition of Lanthanum in LiCoO2 forms the Li+ conducting

phase (La2Li0.5Co0.5O4) which improves the ion transfers across the

electrode-electrolyte interface. Such contact resistance is particularly

important in solid-solid contacts and oxide additions can improve the

contact between cathode particles or between the electrode and electrolyte

materials. Furthermore, the ionic conductivity of LiFePO4 is enhanced with

the addition of CeO2 [Liu et al (2009)]. The electronic conductivity for

LiFePO4/C is increased due to the doping of La3+ ions [Kuei-Feng Hsu et al

(2004)]. Tian Yanwen et al suggested that the yttrium ion doping in

LiFePO4 improves its ionic conductivity in comparison with the relative

low ionic conductivity observed in the high valence metal ions doped

sample [Tian Yanwen et al (2008)]. Moreover, doping effect of yttrium ion

enhances the conductivity of Li3V2(PO4)3 [Zhong Shengkui et al (2009)]. In

addition to that, doping of metal atoms can also improve the cathode

performance.

Wide range of transition metal ions with range of valences (M1+ to

M5+) is successfully doped both in M1 site (Lithium site) [Yamada et al

(2006)] and M2 site (Metal site) [Bhuvaneswari et al (2011), Zaghib et al

(2008), LIN Zhi-Ping et al (2009)] of Olivine (LiMPO4 (M=Fe, Mn, Co,

37

Ni)) cathode. The substitution of both isovalent and supervalent cations on

Li & M site without affecting the olivine structure enhance the overall

performance of the cathode materials. Formation of defects and cation

valences help in reducing the grain size and increasing the Li-ion migration

channel size while giving high Li+ mobility [Kandhasamy et al (2012)].

Herle et al have found that pure and zirconium doped LiNiPO4 shows

higher conductivity than pristine LiNiPO4 and the increment in the

conductivity value is mainly associated with the formation of Ni3P as nano

phase impurity [Subramaniya Herle et al (2004)]. But, for the good

performance of cathodes, it requires a high electrically conducting single

phase pure compound without any impurity.

The electrical conductivity of LiCoPO4is improved by 2-3 orders by

carbon coating on the particle surface [Wolfenstine et al (2005)]. The cell

voltage is improved by doping of Co and Ni cations in LiMPO4 (M=Mn,

Fe). The electrical conductivity of Li3V2 (PO4)3is improved by coating of

RuO2 [Ren et al (2009)]. Magnesium doped LiFePO4 (LiMg0.05Fe0.95PO4)

retains more than 89% of the capacity even after 60 cycles [Arumugam et al

(2009)]. Doping an appropriate amount of Ti4+ can improve the

electrochemical performance of LiFePO4 and Ti doped sample shows

excellent specific capacity of 138.5 mAh/g [Wang et al (2006)]. Li ion

diffusivity of LiCoPO4 was enhanced due to the doping of Fe [Don-wook

Han et al (2009)]. Wang et al have observed that the addition of vanadium

to LiCoPO4 improves the electrical conductivity around 5.3 times [Wang et

al (2010)]. Zinc doped LiCoPO4 possesses higher electrical conductivity

compared with pure LiCoPO4 [Karthickprabhu et al (2014)]. It is also found

that the addition of transition metal atoms such as Cr3+ [Aklalouch et al

(2008), Yi et al (2009)], Zn2+ [Shenouda et al (2009), Eom et al (2008)],

Ti4+ [Willcox et al (2009), Tang et al (2009)], Zr2+[Takahashi et al (2008),

38

Sivaprakash et al (2009)], Al3+ [Ju et al (2008), Shizuka et al (2008)],

Mn2+ [Yang et al (2008)] and Mg2+ [Xiuqin Ou et al (2008)]improves the

electrochemical properties of cathode materials such as LiMn2O4&

LiMn1.5Ni0.5O4 (Improvement of capacity retention), LiFePO4& LiCoO2

(Stabilize the crystal structure and reduces the reaction between electrode

and electrolyte), Li (Ni1/3Co1/3Mn1/3)O2& LiNi0.8Co0.2O2 (stabilizing crystal

structure), LiCoO2& LiNi0.8Co0.2O2 (Stabilize the layered crystal structure),

LiCoO2 & LiFePO4 (Improves the capacity,), and LiFePO4 (electronic

conductivity , improve the performance of phosphate electrodes)

respectively. LiFe1/4Mn1/4Co1/4Ni1/4PO4 solid solution delivers the high

electrochemical activity with three redox couples corresponds to Fe, Mn,

Co [Wang et al (2008)].

Graphite carbon foams coated on LiNiyPO4 (y=0.8, 1.0) shows redox

couple in the range of 5.1 and 5.2V and the specific capacity of this

composite increases by reducing the nickel content in LiNiPO4 [Lucangelo

Dimesso et al (2012)]. The electronic conductivity of LiNiPO4 is found to

be increased upon 10% doping of Cu2+, Mg2+, Zn2+ and Al3+ [Lakshmi

vijayan et al (2014), Karthickprabhu et al (2014)]. Further, the specific

capacity has also improved by Mg2+ doped graphite carbon foams LiNi1-

yMgyPO4 composites [Lucangelo Dimesso et al (2012)]. The electrical

conductivity of LiNiPO4 is increased by doping of Zn2+ by polyol method

[Karthickprabhu et al (2014)]. Doping of Zn2+ on LiFePO4 provides more

space for Li-ion intercalation/de-intercalation [Atef Shenouda et al (2009)].

Addition of Zn2+ to LiFePO4 causes improvement in the electronic

conductivity and Li-ion diffusion coefficient of the material [Liu et al

(2006)].

39

1.13. OBJECTIVE AND RESEARCH PROBLEM

The current range of commercially produced cathode materials for

advanced lithium-ion batteries is poorly suited to today’s energy supply

demands. The serious issues like release of O2, toxicity, low voltage of

commercially available cathode make us to think an alternate. From the

above literature studies, the research problem of the present work is focused

on LiNiPO4 that is having high voltage, high energy and high capacity. Due

to the low ionic and electronic conductivity, absence of redox couple and

electrochemical reversibility, LiNiPO4 is not yet commercialized. So the

present work aims to,

Prepare quality pristine LiNiPO4, rare earth doped LiNiPO4 and

transition metal doped LiNiPO4 cathode materials by polyol method,

Prepared cathodes are,

Pristine LiNiPO4

LiNiPO4: x mol% La (x=0.01, 0.03, 0.05, 0.07, 0.09)

LiNiPO4: x mol% Nd (x=0.01, 0.03, 0.05, 0.07, 0.09)

LiNi1-xZnxPO4 (x= 0.05, 0.10, 0.15, 0.20)

LiNi1-xMnxPO4 (x= 0.05, 0.10, 0.15, 0.20)

Study structural, morphological characterizations of pristine and

doped LiNiPO4

Study vibrational, thermal characterizations of doped and undoped

LiNiPO4

Improve the Electrical conductivity and electrochemical

performance of LiNiPO4 by doping.

Fabricate a model battery with Swagelok cell and find its

performance.

40

The reason for choosing La & Nd is the fact that both lanthanides are

having stable oxidation state compared with other in their series. Moreover,

the selection of transition metals is on the basis of ionic radius. It is also

expected that the electrochemical performance of LiNiPO4 will be

improved by rare earth and transition metals doping. The present study is

focused on the synthesis of submicron size cathode materials, their

characterizations and electrochemical studies of pure LiNiPO4, rare earth

and transition metal doped LiNiPO4 at different concentrations.

41

CHAPTER 2

EXPERIMENTAL TECHNIQUES

2.1. INTRODUCTION

This chapter focuses on the synthesis of pure LiNiPO4, rare earth

and transition metal doped LiNiPO4 and different characterizations of the

prepared cathode materials employed in the present investigations to find

out a suitable composition for electrochemical applications.

2.2. SYNTHESIS OF OLIVINE BASED CATHODE MATERIALS

Synthesis of olivine phosphate cathode materials is a major subject

of interest to researchers since synthesis method controls the size and shape

of the particles. There are several methods adopted for synthesizing these

high voltage Olivine compounds such as conventional high temperature

solid state reaction (high temperature heat treatment) [Okada et al (2001),

Mi et al (2005), Wang et al (2007), Deniard et al (2004), Penazzi et al

(2004), Masanobu Nakayama et al (2004), Bramnik et al (2005), Bramnik

et al (2004)], co-precipitation [Yang et al (2008)], sol-gel preparation

[Gaberscek et al (2005)], carbothermal method [Jingjing et al (2011)], high

energy ball mill method [Rabanal et al (2006)], microwave heating

synthesis method [Li et al (2009)], Electrostatic spray deposition [Shui et al

(2006)], pulsed laser deposition, modified mechanical activation process,

hydrothermal method [Jiajun Chen et al (2007)], Microwave assisted sol-

gel process, Pechini method [Ehrenberg et al (2009)], Microwave irradiated

42

solvothermal method [Vadivel murugan et al (2009)] and polyol method

[Karthickprabhu et al (2013)] etc.,

Synthesis methods of polyanion-based olivine type cathode materials

are selected by considering two different viewpoints. The first point of view

is related to how are precursors of lithium, metals (M= Fe, Mn, Co, Ni) and

phosphate (PO4)3- reacted to each other into intermediate products (ii) how

is energy transferred to convert the precursors or the intermediate to a

desired crystalline structure. These two different points of views are

dependent to each other because the reaction of precursor can fix the type

of energy transfer.

From the first point of view, the synthesis methods can be

categorized into solid state and solution based synthesis [Hyun-kong Song

et al (2010)]. The solid state synthesis is usually followed by preheating

and annealing at high temperature like 600-800°C. Various solvents can be

used in the solution-based synthesis such as aqueous or organic solvents,

polyol or ionic liquids, which define the energy transfer method and

corresponding process temperature. Various thermal methods are adopted:

Conventional high temperature annealing, hydro/solvothermal methods and

reflux around the boiling point of solvent. The conventional annealing and

the reflux methods are operated at atmospheric pressure while high pressure

should be kept for hydro/solvothermal methods by using autoclaves.

2.2.1. Solid State Reaction method (SSR)

Solid state reaction method is most conventional method used to

synthesize polyanion cathode materials in lithium ion batteries [Bramnik et

al (2005), Penazzi et al (2004), Shigeto Okada et al (2001)] because of its

simple procedure and easy scale up. Solid state reactions are simple to

43

perform and starting materials are often readily available at low cost. The

reactions should be held at clean environment. The procedure usually

includes

i. Grinding and/or milling the precursors

ii. Preheat treatment 350°C for 4-5 hrs

iii. Re-grinding the precursors

iv. Calcined at above 600°C

From the SSR method, bulk particles with micrometer size were obtained

because of the high thermal energy

Figure 2.1: Schematic diagram of solid state reaction method

Stoichiometric amount of Lithium acetate + Nickel acetate + Ammonium di

hydrogen phosphate

Mixing

Preheating 350°C, 4h

Grinding 20 minutes

Calcination at required temperature

Solid State Reaction method

44

2.2.2. Polyol method

Among the different sample preparation methods, polyol method is

on the best, very simplest and effective method to produce nano structured

olivine cathode materials [San Moon et al (2011), Deyu Wang et al (2009),

Madhav Singh et al (2011), Shihui Jiao et al (2010), Jinsub Lim et al

(2011), Lim et al (2010), Choi et al (2010)]. The polyalcohol media such as

tetraethylene glycol, diethylene glycol, ethylene glycol, 1, 2 propanediol

can be used based on their boiling point. This method is similar to the

solvothermal case except the reaction is controlled under refluxed at

atmospheric pressure. It is also known that, this method usually yields

nanoparticles because the medium acts both as solvent to dissolve the

precursors and a stabilizer to inhibit particle growth. Another advantage is

that more highly crystalline materials are obtained compared to other

methods. Polyol method also requires low reaction temperature to form the

desired product. The schematic procedure adopted for polyol synthesis is

given in the figure 2.2.

LiNiPO4:X RE (RE = La & Nd) (X = 0, 0.01 mol%, 0.03 mol%,

0.05 mol%, 0.07 mol%, 0.09 mol%) and LiNi1-xMxPO4 (M=Zn, Mn and

x=0.05, 0.10, 0.15, 0.20)were prepared by Polyol method. In this method,

Lithium acetate (CH3COOLi.2H2O, Himedia, 99.9% Purity), Nickel acetate

Ni (O(C=O) CH3)2.4H2O, Himedia, 99.9% Purity), Ortho Phosphoric acid

(H3PO4, Merck), Lanthanum acetate (Himedia, 99.9% Purity) Neodymium

acetate (Himedia, 99.9% Purity), Zinc acetate (Himedia, 99.9% Purity),

Manganese acetate (Himedia, 99.9% Purity) were taken as the starting

materials. 1, 2 propanediol and ethylene glycol were used as polyol

medium.

45

Figure 2.2: Schematic procedure adopted for polyol synthesis

Polyol Process

Lithium acetate

CH3COOLi.2H2O

Nickel acetate

Ni(O(C=O)CH3)2.4H2O

Phosphoric acid

Dissolved in the polyol medium separately and to maintain the stoichiometric (Ethylene glycol and 1,2 propanediol)

Heated the solutions at 220°C for 5 hrs under refluxing

Greenish yellow color Precipitates were obtained

Centrifuged and washed with acetone

Particles are dried in oven at 150°C

Required amount of Rare earth and Transition metal

Calcination at required temperature

46

2.2.3. Structure and properties of polyol mediums

Ethylene glycol

Figure 2.3: Structure of Ethylene glycol

Table 2.1: Properties of Ethylene glycol

Molecular formula C2H6O2

Molar mass 62.07 g mol−1

Density 1.1132 g/cm³

Melting point −12.9 °C

Boiling point 197.3 °C

Solubility in water Soluble

1,2 Propanediol

Figure 2.4: Structure of 1, 2 Propanediol

47

Table 2.2: Properties of 1,2 Propanediol

Melting point -60 °C Boiling point 187 °C(lit.)

density 1.036 g/mL at 25 °C(lit.)Water Solubility Soluble in water

The stoichiometric amounts of the precursors were taken and

dissolved in the polyol medium separately. Then all the solutions were

transferred into a round bottom flask equipped with condensed unit. The

reaction mixture was heated at constant temperature of 240°C for 20 h and

5 h under constant refluxing. Initially, the refluxing time was maintained

for 20 h then it was optimized to 5 h. It is discussed in detail in the chapter

3. The greenish yellow colour precipitate was obtained after long refluxing

time. The precipitate was separated by centrifugation and washed with

acetone several times to remove the remaining organic residues. Then

powders were dried in vacuum oven at 150°C for 15 hours to remove the

physically adsorbed water. To get the crystalline nature of LiNiPO4: (La,

Nd, Zn and Mn), the prepared powders were calcined at different

temperatures for different calcinations time.

2.3. CHARACTERIZATION TECHNIQUES

Materials properties like structure, morphology, thermal and

electrical are studied in details in order to understand the chemistry and

physics of materials. These experimental characterization techniques also

provide useful guideline on the suitability of the material for possible

device applications. In the present case, it is plan to study the structural,

vibrational, morphological, electrical and electrochemical properties of the

material using following characterization.

48

Powder XRD analysis

Scanning Electron Microscopy

Particle size distribution analysis

Thermogravimetry analysis (TGA)

FTIR analysis

Laser Raman analysis

Electrical conduction studies using complex Impedance

Spectroscopy

Electrode preparation and cell assembly

Electrochemical impedance spectroscopy

cyclic voltametry studies

Galvanostatic charge-discharge studies

2.3.1 POWDER X-RAY DIFFRACTION (PXRD)

X-ray diffraction is a very well established method used for material

studies. Many researchers have used the X-ray diffraction to study whether

a material is amorphous or crystalline and to determine the crystallite size

of the sample [Sammes et al (1997), Guinier et al (1994)]. When X-rays hit

a crystalline target, the phenomenon of diffraction occur and producing

interference fringes. This arises from a scattering process by which the X-

rays are scattered by the target material without change in wavelength (also

called Bragg scattering). Diffraction occurs only when certain conditions

are satisfied according to Bragg’s law [Cullity et al (2001), Jenkins et al

(1996)]. The Bragg’s condition is given by

nλ = 2d sinθ ------ (2.1)

49

Where n is an integer

λ is the wavelength of the X-rays

d is the interplanar spacing generating the diffraction and

θ is the diffraction angle

λ and d are measured in the same units, usually angstroms.

X-ray Diffraction pattern is acquired in the form drawn between

intensity (au) and 2ϴ (degrees). As the XRD pattern is the characteristic of

the material, the structure of an unknown compound can be identified by

comparing it with the data available in the Joint Committee on Powder

Diffraction Standards (JCPDS). It also provides the information about the

crystallographic structure, lattice parameters, planar spacing, crystallite size

etc. TOPAS-3 software program is used to find the lattice parameters.

X-ray diffraction was conducted on the entire sample using a

XPERT-PRO-X ray diffractometer PW 3050/60 (with Cu Kα radiation) at

2θ with a step of 0.05° in the range of 10° to 80° at 25°C.

2.3.2. SCANNING ELECTRON MICROSCOPY

The scanning electron microscopy (SEM) is a versatile tool to

characterize microstructures of the samples. SEM makes use of point to

point scanning of the solid surface, which produces a clear image of

specimens, and provides information about their size which lies in the range

of micro-meter. In a typical SEM, an electron beam is emitted from an

electron gun fitted with a tungsten filament cathode. When the primary

electron beam interacts with the sample, the electron loses energy by

repeated random scattering. The energy exchange between the electron

beam and the sample results in the reflection of high energy electrons by

elastic scattering, emission of secondary electrons by inelastic scattering

50

and the emission of electromagnetic radiation, each of which can be

detected by specialized detectors [Guozhonq Cao et al (2004)].

In the present study, morphology of the prepared powders are

identified from the JEOL-6390 type computer controlled Scanning Electron

Microscope

2.3.3. PARTICLE SIZE ANALYSIS

The most important physical property of particulate sample is

particle size. Particle size measurement is routinely carried out across a

wide range of industries and is often a critical parameter in the manufacture

of products. Particle size has a direct influence on materials properties such

as reactivity, stability in suspension, appearance, flowability, viscosity and

packing density.

A particle size distribution can be represented in different ways with

respect to the weighting of individual particles. The weighting mechanism

will depend upon the measuring principle being used.

1. Number weighted distributions

2. Volume weighted distributions

3. Intensity weighted distributions

Number weighted distributions

A counting technique such as image analysis gives a number

weighted distribution where each particle is given equal weighting

irrespective of its size.

51

Volume weighted distributions

Static light scattering techniques such as laser diffraction will give a

volume weighted distribution. Here the contribution of each particle in the

distribution relates to the volume of that particle (equivalent to massif the

density is uniform), i.e. the relative contribution will be proportional to

(size)3.

Intensity weighted distributions

Dynamic light scattering techniques provide an intensity weighted

distribution, where the contribution of each particle in the distribution

relates to the intensity of light scattered by the particle. For example, using

the Rayleigh approximation, the relative contribution for very small

particles will be proportional to (size) 6.

In the present study particle size is measured on the basis of intensity

weighted distributions by Malvern instrument at 25°C.

2.3.4. THERMOGRAVIMETRY ANALYSIS (TGA)

Thermo gravimetric analysis (TGA) is an analytical technique used

to determine a material’s thermal stability and its fraction of volatile

components by monitoring the weight change that occurs when specimen is

heated. The measurement is normally carried out in air or in an inert

atmosphere, such as Nitrogen or Argon, and the weight is recorded as a

function of temperature. In addition to weight changes, some instruments

record the temperature difference between the specimen and one or more

reference pans (differential thermal analysis, or DTA). Figure 2.5 shows the

52

TG instrument, which was used in thepresent investigation.The typical

example of the TG spectrum is shown in figure 2.6.

Figure 2.5: Picture of TG analysis instrument

Figure 2.6: Typical example for TG curve

53

Applications

TGA includes measurement of a material’s thermal stability and its

composition. Typical applications include determination of:

a. Filler content of polymer resins

b. Residual solvent content

c. Carbon black content

d. Decomposition temperature

e. Moisture content of organic and inorganic materials

f. Plasticizer content of polymers

g. Oxidative stability

h. Performance of stabilizers

i. Low molecular weight monomers in polymers

In the present work, the thermal analysis has been done by

SHIMADZU Thermo Gravimetric Analyzer in the temperature range of

30ºC to 1000ºC with a step of 10°C/mins to identify the sample formation

temperature in air atmosphere (pure LiNiPO4) and inert atmosphere (doped

materials).

2.3.5. INFRARED SPECTROSCOPY ANALYSIS

FTIR is a powerful and broadly applicable spectroscopy method

implemented to identify the chemical functional groups. FTIR spectrometer

is an analytical instrument used to study the materials in both liquid and

solid phase. Over the years, it becomes one of the most important tools for

both qualitative and quantitative characterization of organic materials

[Hirankumar et al (2004), Ramya et al (2008)].

54

For application and instrumentation point of view, the infrared

region has been subdivided into three parts.

1. Near infrared region 13,000 – 4000 cm-1 (0.78 – 2.5 μm)

2. Middle infrared region 4000 – 200 cm-1 (2.5 - 50 μm)

3. Far infrared region 200 – 10 cm-1 (50 – 1000 μm)

Figure 2.7:Schematic diagram of the FTIR spectrometer

The change in dipole moment of the molecules in the materials

causes the absorption of infrared radiation. This change in dipole moment is

due to the vibrational and rotational motion of the atoms in the molecules,

which are called as heteronuclear molecules. When there is no change in

the dipole moment of molecule during vibrational and rotational motion,

DETECTOR AMPLIFIER

COMPUTER

WITH

PLOTTER

Sample

Referen

ce

Drive

Mylar

Beam Reflector

55

the molecules will be infrared inactive and commonly called symmetric

molecules. The amount of absorption energy and corresponding spectral

positions of the FTIR spectrum are characteristic for different materials.

Hence, infrared spectrum is essentially a fingerprint of the different groups

present in the sample.The block diagram of the FTIR spectrometer is shown

in Figure 2.7.

In the present investigation, the infrared spectra of all the cathode

materials were recorded on a JASCO FT/IR-4100 Fourier transform

infrared spectrometer in the wave number region between 1200 cm-1 and

350cm-1.

2.3.6. LASER RAMAN STUDIES

Raman spectroscopy is a spectroscopic technique based on inelastic

scattering of monochromatic light, usually from a laser source. Inelastic

scattering means that the frequency of photons in monochromatic light

changes upon interaction with a sample. Photons of the laser light are

absorbed by the sample and then reemitted. Frequency of the reemitted

photons is shifted up or down in comparison with original monochromatic

frequency, which is called the Raman Effect. This shift provides

information about vibrational, rotational and other low frequency

transitions modes in molecules [Anderson (1973), Long (1977), Masahiro

Kitajima et al (1997)]. Raman spectroscopy can be used to study solid,

liquid and gaseous samples. The energy diagram of Raman effect is given

in figure 2.8.

56

Figure 2.8: Energy level diagram of Raman Effect

When intense beam of monochromatic radiation of wavenumber i is

incident on a substance like molecule or crystal, most of it is transmitted

without change, but, in addition, some scattering of the radiation occurs.

The frequency of the scattered light may have the same wavenumber with

the incident light known as Rayleigh scattering. If the molecule gains

energy, the scattered photon will have frequency (ʋ0-ʋm) called stokes line.

On the other hand if it loses energy the scattered photon will have

frequency (ʋ0+ʋm) called anti-stokes line.

In the present study, Raman spectra were recorded by Laser

Confocal Raman Spectroscopy with RENISHAW via Raman microscope

using the 715 nm line of Argon laser source. The commercial software

PEAKFIT is used to fit the Raman spectra from which the peak position

and full width at half maximum (FWHM) of the Raman peaksare obtained.

2.3.7. IMPEDANCE SPECTROSCOPY STUDIES

Impedance spectroscopy is used to study the electrical properties of

SIC materials, where the response of a system to an applied sinusoidally

57

varying alternating voltage is recorded as a function of frequencies.

Impedance analysis of ionic solids identifies the basic process such as, Ion

transport, bulk conduction, grain boundary conduction, electrode-

electrolyte interface process in the measured frequency domain. It is a non-

destructive technique and also can provide the dynamic properties to

understand the microscopic nature of the SIC materials [Ross macdonald et

al (1987), Jonscher (1974), Jonscher (1975), Muralidharan et al

(2004)].Figure 2.9 shows the HIOKI-LCR 3532-50 instrument, which is

used for the investigation of impedance spectroscopy for prepared samples.

Figure 2.9: Picture of Impedance measurement instrument

A monochromatic signal exp , involving the single

frequency 2 is applied to a cell and the resulting steady state

current exp is measured. Here, is the phase difference

between the voltage and the current; it is zero for purely resistive behavior.

Figure 2.10shows the schematic principle of impedance spectroscopy.

58

Figure 2.10: Principle of impedance spectroscopy

According to ohm’s law, impedance of the circuit (Z) at any

frequency (ω) is given by,

∗ exp ---------------- (2.2)

exp

∗ ′ where Z'=cosφ and Z

√ 1

Zꞌ and Zꞌꞌ are real and imaginary parts of the impedance. The phase

difference ( ) is represented by

tan′′

′) --------------------------- (2.3)

The Impedance spectroscopy is used to measure the behavior of the

materials in terms of the complex impedance (I*), complex admittance

(Y*), complex permittivity (ɛ*) and complex modulus (M*) [Jonscher

(1977)]. The relationship between these parameters is listed in table 2.3.

59

The pressure of about 6000Kg cm-2 was applied to the prepared

powders to form the pellet of about 1-1.5mm thickness and diameter of 10

mm. For ac impedance measurement, silver was deposited on both sides of

the pellets which acted as blocking electrodes.

Table 2.3: Impedance parameters

Ext.var Z (impedance) Y

(admittance)

C (capciatance)

= ′ " ′| |

"

| |

′′ ′

"

| |

"

| |

′| |

"

| |

′ " ′′| |

′| |

′ ′′

" ′ ′′

| |

′| |

′ ′′

′| |

"

| |

′′| |

′| |

′′ ′

′| |

′′| |

′ "

60

2.3.8. ELECTRODE PREPARATION AND CELL ASSEMBLY

The electrodes for lithium-ion batteries tests were fabricated by

mixing the 75wt% active material, 15 wt% activated carbon and 10 wt%

polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP)

homogenously. The slurry was uniformly pasted onto Al foil (for cathode)

and dried in an oven at 80-100 °C overnight under vacuum. Electrodes are

kept in an argon filled glove box.

For testing the electrochemical properties of the as-prepared

electrodes, Swagelok cell was used. The cells contained the as-prepared

electrode as the working electrode, lithium foil as the counter and reference

electrode, a porous polypropylene as the separator, and 0.5mole Lithium

nanoflouro 1,2 butane sulfonate in a 2:1 mixture of propylene carbonate

(PC) and dimethaxy ethane(DME) as the electrolyte. A schematic diagram

of the Swagelok cell is shown in figure 2.11.The cells were assembled in an

argon filled glove box (figure 2.12) (MIKROUNA) with O2 and H2O levels

less than 0.1 ppm.

Figure 2.11: A schematic diagram of the Swagelok cell

61

Figure 2.12:Picture of the Glove box

2.3.9. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

(EIS)

The measurement of impedance of electrochemical interface is

applied to many fields including studies of metal corrosion behavior,

development of solid state electrolytes, charge state prediction of batteries,

and material characterization of porous electrodes.

In the electrochemical impedance measurement, the test cell is

considered as a parallel circuit which consists of a capacitance (Cp) and an

ohmic resistance (Rp). The relationship between current and applied

voltage of a circuit can be characterized by the amplitudes of current, the

voltage, and the phase shift. These quantities can also be represented as a

complex number, thus the impedance response is usually characterized by a

real part-imaginary couple. The data from impedance measurement is

plotted as a complex plane with the frequency as a parameter.

62

Figure 2.13: Computer controlled Biologic SP-300 electrochemical work

station

The measurement of impedance is often carried with a frequency

response analyzer. The capacitance and large time constant involved in

electrochemical process dictate that the measurement of impedance has to

be applied in a very wide frequency range (e.g. from 100 kHz to 1 mHz).

The impedance changes between its high-frequency limit and low-

frequency limit. The high-frequency limit refers to the contact resistance,

and the low-frequency limit refers to charge transfer resistance.

The electrochemical impedance measurements are conducted with

Biologic Electrochemical Workstations SP-300 (figure 2.13) on assembled

batteries at open potential. The frequency range was between 7 MHz and 1

Hz with the amplitude of 10 mV.

63

2.3.10. CYCLIC VOLTAMETRY STUDIES

Cyclic voltametry (CV) is become an important and widely used for

electro analytical techniques in many areas of chemistry. It is rarely used

for quantitative determinations, but it is widely used for the study of redox

processes, for understanding reaction intermediates, and for obtaining

stability of reaction products. This technique is based on varying the

applied potential at a working electrode in both forward and reverse

directions (at slow scan rate) while monitoring the current. For example,

the initial scan could be in the negative direction to the switching potential.

At that point, the scan would be reversed and run in the positive direction.

Depending on the analysis, one full cycle, a partial cycle, or a series of

cycles can be performed. The rate of change of potential (mV/s) is referred

to as the scan rate.

Cyclic voltametry studies werecarried out by a Biologic

electrochemical workstation SP-300 (shown in figure 2.13) in the scan

range of 2.5V-5.3V.

2.3.11. GALVANOSTATIC CHARGE-DISCHARGE TESTS

Generally, the capacity of the electrode material was calculated by

galvanostatic charge and discharge testing. The measurement was

conducted under a constant current density. The charge/discharge capacities

(Q) can be calculated using the following formula

Q = I × t------- (2.4)

Where Iis current density and t is the charge/discharge time.

64

The cut-off voltage for galvanostatic testing voltage cut-offs was

fixed 2.5-5.2 V for cathode materials vs Li+. In lithium-ion battery testing,

the C-rate performance was used to evaluate the capacity of the electrode at

different charge/discharge current densities. Charge/discharge the cell at

C/n rate means completely charge/discharge the cell within n hour. In the

doctoral work, the galvanostatic charge-discharge measurements were

collected on a computer-controlled Bio-logic (SP-300) battery testing

system which is presented in figure 2.13.

65

CHAPTER 3

STRUCTURAL AND MORPHOLOGICAL STUDIES OF LiNiPO4: RE (RE=La & Nd) LiNiMPO4 (M= Zn & Mn)

3.1. INTRODUCTION

The crystalline and phase identification of pristine cathode materials

is typically measured using X-ray diffraction. The X-ray diffraction studies

are also used to verify/identify whether the structure of the said compound

is formed or not. The scanning electron microscopy is invaluable to

investigate particle size, particle agglomeration and morphology. This

chapter briefly discusses the structural and morphological characterizations

of pure, rare earth and transition metal doped LiNiPO4. XRD patterns are

refined through TOPAS software and the lattice parameters are calculated.

Particle size distribution analysis is also performed in order to find the

range of the particle size.

Undoped LiNiPO4 is prepared by both solid state reaction and polyol

method. In the case of polyol method, ethylene glycol and 1, 2 propanediol

are used as the polyol medium. The prepared samples are calcined at

different calcination temperature and different time. All the prepared

cathode materials are subjected in to XRD analysis to find out the

formation of the sample.

66

3.2. STRUCTURAL INVESTIGATION OF PURE LiNiPO4

3.2.1. XRD patterns of LiNiPO4 prepared by SSR

Figure 3.1 shows the XRD patterns of the LiNiPO4samples prepared

by solid state reaction method at 650°C for 2h & 4h. It reveals that there is

no prominent diffraction peaks observed of LiNiPO4. So this temperature is

not suitable to obtain the pure single phase LiNiPO4. To prepare high

crystallinepure single phase LiNiPO4, it requireshigher temperature.

Figure 3.1: XRD patterns of LiNiPO4at 650°C for 2h (a) and 4h (b) using SSR method

67

3.2.2. XRD patterns of LiNiPO4 prepared by polyol method using EG

as a polyol medium

Figure 3.2exhibits the XRD patterns of pure LiNiPO4 using ethylene

glycol as a polyol medium. It is clear that no characteristic diffraction peaks

of LiNiPO4areobserved. It may be due to the reason that EG is not a good

polyol medium for this type of olivine cathode and/or insufficient

calcination temperature and/or time.

Figure 3.2: XRD patterns of LiNiPO4using ethylene glycol as a Polyol medium

3.2.3. XRD patterns of LiNiPO4 prepared by polyol method using 1,2

propanediol as a polyol medium:

Figure 3.3 depicts the XRD spectra of LiNiPO4 using 1, 2

propanediol as a polyol medium at different calcination temperatures for

different times. The samples calcined at temperatures such as 450ºC and

550ºC for 2 hdo not showany prominent diffraction peaks. It may be due to

68

the insufficient calcination temperature or calcination time. The sample

calcined at 650ºC for 2 h shows the diffraction peaks and matches well with

the JCPDS data (81-1528). To improve the crystallinity, the calcination

time is increased to 4 h at 650 °C and the XRD pattern of such calcined

sample is presented in figure (3.3-e). From this analysis, it is confirmed that

650ºC and 4h issufficientcalcination temperature and time to prepare single

phase pure crystalline LiNiPO4of orthorhombic structure. The Miller

indices (hkl) of the main diffraction peaks are compared with the JCPDS

data (file number 81-1528). From the XRD analysis, LiNiPO4 prepared by

using 1,2 propanediol as a Polyol medium shows better crystalline nature

than the ethylene glycol Polyol medium [Karthickprabhu et al (2013)].

Figure 3.3: XRD patterns of pure LiNiPO4 prepared at various calcinations temperature and timeusing 1, 2 propanediol as a polyol medium

The crystallite size “d” is calculated using the Debye Scherer

formula

Cos

d2/1

9.0 , where λ is the X-ray (Cu Kα ray) wavelength, 2/1 the

corrected width of main diffraction peak at half height and θ is the Bragg’s

69

diffraction angle. The following (hkl) values (311), (111), (121) and (101)

are used to calculate the crystallite size “d”. The observed “d” values for

such main diffraction peaks are presented in the Table 3.1. They are in the

range of 55-66 nm [Karthickprabhu et al (2013)].In addition, the crystallite

size increases with an increase of calcination time but the size is in the

order of nanometer.

Table 3.1: Crystallite size for LiNiPO4 calculated using Debye Scherer formula.

(hkl)

Intensity of the main diffraction peak in %

Crystallite size in nm

Calcined at 650°C for 2h

Calcined at 650°C for 4h

(Calcined at 650°C for 2h)

(Calcined at 650°C for

4h) (311) 100 100 35 55.9 (111) 90.04 94.2 28 55.52 (121) 62.30 39.53 42 66 (101) 40.64 40.25 31 56.2

3.3. EFFECT OF REFLUXING TIME ON THE PREPARATION OF

LiNiPO4

Initially, refluxing time was maintained for 20h during the

preparation of LiNiPO4. It is aimed to check the effect of refluxing time on

the preparation of LiNiPO4. So the refluxing time is reduced to 5h while

maintaining the same procedure. Figure 3.4 shows the XRD patterns for

LiNiPO4 prepared by polyol method using 1, 2 propanediol as a polyol

medium refluxed for 5h and 20 h. The crystalline nature of the sample

refluxed for 5h is comparable with the sample refluxed for 20h. So it is

decided to prepare samples with maintaining 5 h reflux time and calcination

temperature of 650°C, 4 h.

70

Figure 3.4: XRD pattern of pure LiNiPO4 using 1, 2 propanediol as a polyol medium refluxed at 5 and 20 h and calcined at 650 C 4 h.

3.4. STRUCTURAL INVESTIGATIONS OF RARE EARTH DOPED

LiNiPO4

Figures 3.5 and 3.6 depicts XRD patterns of lanthanum and

neodymium doped LiNiPO4 with different concentrations of rare earths

respectively. The observed diffraction peaks are in full agreement with the

ordered LiNiPO4 olivine structure indexed to the orthorhombic Pnmaspace

group. All diffraction peaks of LiNiPO4, miller indices and ˝d˝ spacing

values are well matched with the data of JCPDS card number 01-081-1528.

It is observed that the main diffraction peaks ((311), (111)) are shifted to

higher 2θ side upon doping. The less intense impurity peak is observed at

2θ = 33.6° only in the case of LiNiPO4doped with 0.09 mol% La owing to

the formation Li3PO4.

71

Figure 3.5:XRD patterns ofLa doped LiNiPO4 calcined at 650°C, 6h(a) pure LiNiPO4 (b) 0.01 mol% (c) 0.03mol% (d) 0.05 mol% (e) 0.07 mol% (f) 0.09 mol%

Figure 3.6: XRD patterns ofNd doped LiNiPO4 calcined at 650°C, 6h(a) pure LiNiPO4 (b) 0.01 mol% (c) 0.03mol% (d) 0.05 mol% (e) 0.07 mol% (f) 0.09 mol%

72

The lattice parameters were calculated from the TOPAS software

version3 and presented in table 3.2. The slight changes in lattice parameters

may be due to the effect of dopant that occupies the host lattice. Doping

effect will sometimes increase or decrease the lattice parameters depending

on the dopant ion and amount of doping [Atef et al (2009), Sun et al (2009),

LIN Zhi-Ping et al (2009), Yurong et al (2011), Zhongli et al (2008)]. The

refined XRD data for all La doped samples are shown in figure 3.5.1 -

3.5.5.From this analysis, it is concluded that the doping of La3+ (up to 0.07

mol %) may be favorable and do not collapse the lattice structure of

LiNiPO4.

For Nd doped LiNiPO4 samples, no impurity peaks are observed at

low concentrations of Nd (0.01 and 0.03 mol %). At higher concentrations

of Nd, the impurity peak at 2θ = 29.7° is observed which corresponds to

NiPO4 (JCPDS 00-049-1082). The lattice parameters are calculated from

the cell refinement software and presented in Table 3.3. Hence, Nd doping

upto 0.03 mol% does not destruct the lattice structure of LiNiPO4.

Table 3.2: Calculated lattice parameters for La3+ doped LiNiPO4 using TOPAS 3.0

Sample a (Å) b (Å) c (Å)

Pure LiNiPO4 10.052 5.858 4.685

LiNiPO4:0.01mol% La 10.080 5.854 4.695

LiNiPO4:0.03mol% La 10.080 5.854 4.694

LiNiPO4:0.05mol% La 10.070 5.852 4.688

LiNiPO4:0.07mol% La 10.066 5.849 4.688

73

Table 3.3: Calculated lattice parameters for Nd3+ doped LiNiPO4 using cell refinement software

Concentration of Nd

(in mol%) a (Å) b (Å) c (Å)

Pure LiNiPO4 10.052 5.858 4.685

0.01 10.049 5.851 4.678

0.03 10.069 5.850 4.693

0.05 10.063 5.853 4.689

0.07 10.072 5.845 4.693

0.09 10.050 5.837 4.679

Figure 3.5.1: Refined XRD pattern of pure LiNiPO4


74


Figure 3.5.4: Refined XRD pattern of 0.05mol% La doped LiNiPO4


75

3.5. STRUCTURAL INVESTIGATIONS OF TRANSITION METAL

DOPED LiNiPO4

Figure 3.7 and 3.8 represents the X-ray diffraction patterns of

LiNiPO4doped with different concentrations of Zn2+ and Mn2+respectively.

Figure 3.7: XRD patterns of (a) Pure LiNiPO4 (b) LiNi0.95Zn0.05PO4 (c) LiNi0.9Zn0.1PO4 (d) LiNi0.85Zn0.15PO4 (e) LiNi0.8Zn0.2PO4

The doped LiNiPO4 samples might have the olivine structure

indexed by orthorhombic pnma space group as evidence by the observed

XRD spectra. The XRD peaks of the doped samples exhibit rightward shift

in the axis of 2θ when compared to the undoped sample, indicating the

influence on the lattice parameters due to doping. Similar results are also

obtained for Zn2+ doped LiFePO4and Mn2+ doped LiCoPO4 [Liu et al

(2006), Kishore et al (2005)].

76

Figure 3.8: XRD patterns of (a) LiNi0.95Mn0.05PO4 (b) LiNi0.9Mn0.1PO4 (c)


On the other hand, change in lattice parameters due to insertion of

metals may be explained as follow as: Ionic radius of Zn2+ (88pm) and

Mn2+ (82pm) almost comparable with the radius of Ni2+ (85pm) which

causes the shifting of peaks in the XRD pattern and change in lattice

parameters. Lattice parameters are obtained from the XRD data by Rietveld

fitting using TOPAS (version 3.0) software. The calculated lattice

parameters are listed in table 3.4. From the analysis, insertion of transition

metals does not collapse the lattice structure of LiNiPO4. The XRD

refinement for both Zn and Mn doped samples are presented in figure 3.7.1-

3.7.4 (Zn2+ doped samples) and 3.8.1-3.8.4 (Mn2+ doped samples).

77




78




79



Table 3.4: Lattice parameters for Zn2+ and Mn2+ doped LiNiPO4 by TOPAS 3.0

Samples a (Å) b (Å) c (Å) Pure LiNiPO4 10.052 5.858 4.685

LiNi0.95Zn0.05PO4 10.037 5.854 4.674 LiNi0.90Zn0.10PO4 10.045 5.863 4.681 LiNi0.85Zn0.15PO4 10.045 5.865 4.682 LiNi0.80Zn0.20PO4 10.054 5.867 4.683 LiNi0.95Mn0.05PO4 10.047 5.857 4.681 LiNi0.90Mn0.10PO4 10.052 5.863 4.679 LiNi0.85Mn0.15PO4 10.064 5.865 4.686 LiNi0.80Mn0.20PO4 10.070 5.877 4.686

80

3.6. MORPHOLOGICAL STUDIES

3.6.1. SEM analysis of LiNiPO4

The morphology of pure LiNiPO4 (before calcination) and calcined at

650ºC for 4 h are analysed by SEM images and is presented in figures3.9

(a) and (b) respectively.

Figure 3.9: SEM images of a) as prepared LiNiPO4 b) Calcined at 650°C 4h

The as prepared sample shows the agglomerated morphology which

may be due to the presence of polyol medium. Figure 3.9 (b)reveals the

particles with spherical and chain like structure. This uniform morphology

may lead to the good electrochemical performance of cathode material.

3.6.2. SEM analysis of rare earth (La3+ and Nd3+) doped LiNiPO4

Figures 3.10 (a) and 3.11(a) show SEM image of La doped LiNiPO4

and Nd doped LiNiPO4 respectively before calcinations. Both the samples

show the agglomerated morphology which may be due to the presence of

polyol medium. Figures 3.10 (b) and 3.11 (b) show SEM pictures of

samples calcined at 650°C for 6 h. The samples exhibit homogeneous

81

particles with sphere like structure anduniform distribution of fine particles

with little agglomeration.The average particle size of prepared samples is

found as in submicron size.

Figure 3.10:SEM image of La3+ doped LiNiPO4a) as prepared sample b) calcined at 650°C 6h.

Figure 3.11:SEM image of Nd3+doped LiNiPO4a) as prepared sample b) calcined at 650°C 6h

3.6.3. SEM analysis of Transition metal (Zn2+ and Mn2+) doped

LiNiPO4

The SEM photographs of LiNi0.85Zn0.15PO4 and LiNi0.90Mn0.10PO4

(before and after calcination) are presented in figure 3.12 & 3.13. From the

figure 3.12 (a) and 3.13 (a), the as prepared samples show aggregates and

82

particles are not in uniform size. This may be due to presence of polyol

medium.

Figure 3.12: SEM image of Zn2+doped LiNiPO4a) as prepared sample b)

calcined at 650°C 6h

Figure 3.13:SEM image of Mn2+doped LiNiPO4a) as prepared sample b)

calcined at 650°C 6h

The figures 3.12 (b) and 3.12 (b) show the image of calcined

samples. It shows homogeneous particle distribution and relatively small

spherical particles with similar shape irrespective of doping. Such kind of

morphology is very important to achieve high capacity and good

cycleability of the cathode materials.

83

3.7. PARTICLE SIZE DISTRIBUTION ANALYSIS

3.7.1. Particle size distribution analysis of pure and rare earth doped

LiNiPO4

Figure 3.14 shows the particle size distribution pattern of pure

LiNiPO4. Most of the particles are distributed in the range of 200 - 400 nm.

This result is consistent with the SEM analysis in which the particle size in

the range of submicron size (~300 nm).The average particle size is

measured to be 330 nm. In case of RE doped samples, the particle size

distribution range is measured as 180-250 nm, 250-290 nm for La doped

and Nd doped LiNiPO4samples respectively which is presented in figure

3.15 and 3.16. It is also evidenced that doping of RE metal with LiNiPO4

causes the reduction in the particle size of LiNiPO4. Similar results are also

observed for La doped LiFePO4 [LUO Shaohua et al (2010)].The average

diameter (Z average d in nm) of the particles is found to be 188 nm and 252

nm for La doped and Nd doped LiNiPO4 samples respectively. Further it is

already reported that the smaller sized particles provide the shorter

diffusive distance for Li ions. Hence, the RE doped LiNiPO4 particles may

yield higher conductivity. In this view, the doping of La3+ may be favorable

compare to Nd3+ due to its smaller size.

84

Figure 3.14: Particle size distribution of Pure LiNiPO4

Figure 3.15: Particle size distribution of 0.05 mol% La doped LiNiPO4

85

Figure 3.16: Particle size distribution of 0.07 mol% Nd doped LiNiPO4

3.7.2. Particle size distribution analysis of Zn2+ and Mn2+ doped

LiNiPO4

Figure 3.17 & 3.18 shows the particle size distribution of Zn& Mn

doped LiNiPO4. The average diameter (Z average “d”) of the particles is

found to be 249 nm and 270 nm for Zn doped and Mn doped LiNiPO4

respectively. Once again the particle size gets reduced due to insertion of

metal atoms compared with pure LiNiPO4. The particle size of Zn doped

sample is found to be low as compared with Mn doped system. This

corresponds to the shorter diffusive distance of Li ions and hence higher

conductivity is expected. The particle size distribution ranges for all

samples are listed in tables 3.5.From the table 3.5, 56% of particles of

undoped LiNiPO4 lie in the range of 200 – 300 nm. Higher number

particles are found in the region of 100 – 200 nm upon doping of RE and

Metal except Mn. So, it is expected that La, Nd and Zn doped LiNiPO4 will

86

show better electrical and electrochemical performance than Mn. It is also

inferred from the particle size distribution data that the La doped sample

possesses more number of nano particles among all the samples.

Figure 3.17: Particle size distribution of LiNi0.85Zn0.15PO4

Figure 3.18: Particle size distribution of LiNi0.90Mn0.10PO4

87

Table 3.5: Particle size distribution of rare earth and transition metal doped

LiNiPO4

Distribution range

(nm)

% of distribution of particles out of 100

Pure

LNP

0.05 mol% La

doped

LNP

0.07 mol%

Nd doped LNP

LiNi0.85Zn0.15PO4

LiNi0.90Mn0.10PO4

10-50 0 0.6 0 0 0 50-100 0 8.7 0 1.2 0 100-200 13.4 33.1 28.5 25.9 19.8 200-300 56.6 31.6 36.1 23.2 57.8 300-400 26.2 16 18.9 16.1 20.3 400-500 3.7 5.1 6.3 7.7 2.1 500-600 0.2 3.1 4.2 7.1 0 600-700 0 1.4 2.3 6.1 0 700 and above

0 0.4 1.4 12.6 0

3.8. SUMMARY

Olivine type pure, rareearth and transition metal doped LiNiPO4

samples were characterized by X-ray diffraction, particle size distribution

and SEM analysis. XRD analysis showed that the doping of La3+ (up to

0.07 mol %) and Nd3+(up to 0.03 mol %)are more favorable becausetheydo

not collapse the lattice structure of LiNiPO4.Insertion of transition metals is

also prevent the lattice structure but slightly affects the lattice parameters

within the concentration range studied which is confirmed from the

Rietveld refined analysis by TOPAS 3.0. Among all the doped samples La

doped LiNiPO4 shows nearly 40% of nano size particle distribution

corresponds to shorter diffusive distance favorable for Li-ions. SEM

88

analysis reveals that the both pure and doped samples exhibit uniform

morphology with sphere like particles. This kind of morphology is more

significance to achieve high capacity and good cycleability inthe cathode

materials.

89

CHAPTER 4

THERMAL AND VIBRATIONAL CHARACTERIZATION OFUNDOPED AND DOPED LiNiPO4

4.1. INTRODUCTION

This chapter describes the thermal and vibrational studies on pure

and doped LiNiPO4 by TGA, FTIR and Raman spectroscopy.

Thermogravimetric analysis is used to determine changes in the sample

weight, which may result from chemical or physical transformations, as a

function of temperature or time. The local structure and chemical bonding

of the samples are identified from the FTIR spectroscopy. It probes the

structure of LiNiPO4 at the molecular scale, according to the vibrational

modes of the ions, primarily associated to the motion of nickel and

phosphate. Raman spectroscopy is an excellent analytical technique for

probing the vibrational and structural properties of electrode materials. This

technique is especially useful for detecting phosphate-based cathodes since

the intramolecular vibrational modes of the PO43-anions yield rich and

intense bands in the Raman spectrum, which are sensitive to the presence of

Li ions. Raman spectroscopy can provide the local structural and

compositional differences of the pure and doped LiNiPO4 from the

perspective of the PO43-anions.

90

4.2. THERMAL STUDIES

4.2.1.Thermal studies of pure LiNiPO4

Figure 4.1: Thermogravimetric studies of undoped LiNiPO4

Figure 4.1 shows TGA curves for LiNiPO4 prepared by the polyol

method using 1, 2 propanediol as a polyol medium before calcination at air

atmosphere. The result shows that the weight lossis observed in four

regions from ambient temperature to 8000C. The first weight loss region

corresponds to the evaporation of water (until 130ºC, nearly 7% mass

change) and the second region is due to removal of propanediol at 206-

320ºC (6.3% mass change) [Karthickprabhu et al (2013)]. Third region

(320ºC-410ºC, 4.3% mass change) ensures the elimination of other organic

residue and acetate ions present in the LiNiPO4. Final region (above 650ºC)

ensures the formation of LiNiPO4. It is consistent with XRD analysis where

the formation of phase pure LiNiPO4 occurs at 650ºC. At higher

temperature (above 650°C), a small increase in weight loss is also

observed. This may be due to the reaction of the sample with surrounding

atmosphere.

91

4.2.2.Thermal studies of rare earth doped LiNiPO4

Figures 4.2 & 4.3 show TG curve for LiNiPO4: La and LiNiPO4: Nd

precursor recorded under nitrogen atmosphere. The TG curve is separated

into three regions. The detailed analysis is listed in the Tables 4.1 and 4.2.

Figure 4.2: Thermal studies of La3+ doped LiNiPO4 at different concentrations.

Table 4.1: TG analysis of La3+ doped LiNiPO4 at different concentrations of Lanthanum

Samples Weight loss region Remarks 0.03 mol% La doped LiNiPO4 0.05 mol% La doped LiNiPO4 0.07 mol% La doped LiNiPO4

Room temperature-130°C

Removal of lattice water

130°C-301°C Removal of organic residues and propanediol present in the precursor

301°C-531°C Elimination of acetates

92

Moreover, there is no weight loss observed beyond the region III which

indicates that crystallization of doped LiNiPO4 may occur.

Figure 4.3: Thermal studies of Nd3+ doped LiNiPO4 at different concentrations.

Table 4.2: TG analysis of Nd3+ doped LiNiPO4 at different concentrations of Neodymium

Samples Weight loss region Remarks 0.05 mol% La doped LiNiPO4 0.07 mol% La doped LiNiPO4 0.09 mol% La doped LiNiPO4


Removal of water

120°C-301°C Removal of organic residues and propanediol present in the precursor


93

4.2.3.Thermal studies of transition metal doped LiNiPO4

The thermal property of Zn2+ and Mn2+ doped LiNiPO4 precursors

isstudied using thermo-gravimetric analysis. Figures 4.4 and 4.5 show the

TG curves of Zn and Mn doped LiNiPO4respectively.The detailed weight

loss regions are presented in the Tables 4.4 and 4.5.

Figure 4.4: Thermal analysis of Zn2+ doped LiNiPO4

Table 4.3: TG analysis of Zn2+ doped LiNiPO4 at different concentrations of Zinc.

Samples Weight loss region Remarks LiNi0.9Zn0.1PO4 LiNi0.85Zn0.15PO4


Removal of water

124°C-290°C Removal of propanediol and organic residues present in the precursor


94

Figure 4.5: Thermal analysis of Mn2+ doped LiNiPO4

Table 4.4: TG analysis of Mn2+ doped LiNiPO4 at different concentrations of Manganese

Sample Weight loss region Remarks LiNi0.95Mn0.05PO4 LiNi0.9Mn0.1PO4


Removal of hydrated molecules and propanediol

128°C-301°C Removal of organic residues


From the TG analysis, there is no weight loss observed above the

600°C for all the samples. So the calcination temperature is fixed at 650°C

for the sample formation and is consistent with the XRD analysis.

95

4.3. FTIR STUDIES

4.3.1. FTIR studies of pure LiNiPO4

The variations in the local structure and chemical bonding of the

samples upon doping are identified from FTIR spectroscopy. Figure 4.6

shows FTIR spectra ofpure LiNiPO4. The detailed vibrational analysis is

presented in the table 4.5. The band observed at 469 cm-1 is attributed to the

Li-O vibrations. The stretching mode of NiO6 distorted octahedra is

observed at652 cm-1[Julien et al (2004)]. The higher frequency vibrational

band located around 1163 cm-1, 1097 cm-1, 1063 cm-1 and 961 cm-1 are

imputedto the intramoleculer stretching (ʋ1 and ʋ3) motions of each

tetrahedral PO43- anion that is correlated to those of other PO4

3- ions in the

unit cell. The vibrational bands are very well in agreement with the bands

observed in literatures. [Salah et al (2005), (2006)]

Figure 4.6: FTIR spectra of undoped LiNiPO4

96

4.3.2. FTIR STUDIES OF RARE EARTH DOPED LiNiPO4

Figures 4.7 and 4.8 show the FTIR spectra for La3+ and Nd3+ doped

LiNiPO4 at ambient temperaturerespectively. All the vibrational bands are

shifted upon doping of rare earth atoms. For La3+ doped sample, the band

observed at 652 cm-1corresponds to NiO6 stretching is shifted to lower

wavenumber side such as 645, 645, 647, 648 and 650 for 0.01 mol%, 0.03

mol%, 0.05 mol%, 0.07 mol% and 0.09 mol% La3+ doped LiNiPO4

respectively. Further, intramolecular stretching vibrational band for PO43-

anion is shifted to higher wavenumber side such as 961cm-1, 981cm-1,

974cm-1, 980cm-1, 979cm-1 for 0.01mol%, 0.03 mol%, 0.05 mol%, 0.07

mol% and 0.09 mol % La doped LiNiPO4 respectively.

Figure 4.7: FTIR spectra of La3+ doped LiNiPO4 (a) 0.01 mol% (b) 0.03mol% (c) 0.05 mol% (d) 0.07 mol% (e) 0.09 mol%

97

Figure 4.8: FTIR spectra of Nd3+ doped LiNiPO4 (a) 0.01 mol% (b) 0.03mol% (c) 0.05 mol% (d) 0.07 mol% (e) 0.09 mol%

In contrast toLa3+ doped samples, Nd3+ doped LiNiPO4 show no

appreciable shift corresponding to NiO6 stretching mode of vibration. The

vibrational band frequency for PO43- anion (intramolecular stretching) is

increased from 961cm-1 (pure LiNiPO4) to 972 cm-1, 976cm-1, 969cm-1,

967cm-1 and 967cm-1 for 0.01 mol%, 0.03 mol%, 0.05 mol%, 0.07mol%

and 0.09 mol% Nd doped samples respectively. The FWHM of the peak

(NiO6 band) increases due to the doping of both rare earth atoms.From this

analysis, it is observed that dopant affect the band position but do not cause

any dramatic change in the characteristics band vibration. It indicates that

dopants do not destruct the structure of LiNiPO4 and this result is consistent

with the XRD results in which the lattice parameters are slightly affected

upon doping of rare earth atoms and does not destruct the lattice structure

of LiNiPO4. The detailed vibrational analysis is presented in tables 4.5 and

4.6.

98

Table 4.5: Vibrational analysis for La3+ doped LiNiPO4

Vibrational frequency (ν in cm-1)

AssignmentsPure LiNiPO4

0.01 mol%

La doped

0.03 mol%

La doped

0.05 mol%

La doped

0.07 mol%

La doped

0.09 mol%

La doped

350 ---- 363 370 373 367 External

mode

399 ---- ----- ----- ----- ----- External

mode 469 ----- 473 465 474 ----- ν (Li-O) 538 ----- 538 538 540 542 ν (Ni-O) 548 547 547 ----- ----- ---- ν2 (PO4

3-) 582 583 583 584 582 584 ν4 (PO4

3-) 652 645 645 647 648 650 ν4 (PO4

3-) 748 ---- ---- 742 ---- ---- ν4 (PO4

3-) 890 890 891 888 889 889 ν4 (PO4

3-) 961 961 981 974 980 979 ν1 (PO4) 1063 1063 ---- 1069 ---- ---- ν3 (PO4) 1097 ---- 1089 ---- 1089 1090 ν3 (PO4)

Table 4.6: Vibrational analysis for Nd3+ doped LiNiPO4


AssignmentsPure LiNiPO4

0.01 mol%

Nd doped

0.03 mol%

Nd doped

0.05 mol%

Nd doped

0.07 mol%

Nd doped

0.09 mol%

Nd doped

350 ---- 363 370 373 367 External

mode

399 372 384 395 396 395 External

mode 469 471 473 473 471 472 ν (Li-O) 538 539 541 545 541 543 ν (Ni-O) 582 582 583 579 581 579 ν4 (PO4

3-) 652 651 650 653 652 652 ν4 (PO4

3-) 748 ---- ---- 742 743 742 ν4 (PO4

3-) 890 889 889 ---- 889 890 ν4 (PO4

3-) 961 972 976 969 967 967 ν1 (PO4) 1063 1068 ---- 1067 1066 1067 ν3 (PO4) 1097 ---- 1092 1096 1096 1097 ν3 (PO4)

99

4.3.3. FTIR STUDIES OF TRANSITION METALS DOPED LiNiPO4

FTIR spectra for the transition metals doped LiNiPO4 are shown in

figures 4.9 & 4.10. The fundamental vibrational band at 961 cm-1

corresponding to (PO4)3- is shifted upon doping of both transition metal

atoms (Zn2+ and Mn2+). Further it is noted that the band situated in the

range 200-500 cm-1 corresponds to translational motions of Li & Ni cations.

An increase in the wavenumber of most of the dominant bands

corresponding to ν1, ν3 and ν4 modes with the substitution of Zn and Mn

point out the both the P-O bonding and M-O (Ni or Li) bonding

strengthened as a result of substitution of Ni by Zn & Mn

[Devarajshanmukaraj et al (2004)]. The detailed vibrational frequencies are

listed in the tables 4.7 and 4.8. From this analysis, the doping of transition

metal atoms easily occupies the Ni site.

Figure 4.9: FTIR spectra of Zn2+ doped LiNiPO4 (a) LiNi0.95Zn0.05PO4 (b) LiNi0.9Zn0.1PO4 (c) LiNi0.85Zn0.15PO4 (d) LiNi0.8Zn0.2PO4

100

Figure 4.10: FTIR spectra of Mn2+ doped LiNiPO4 (a) LiNi0.95Mn0.05PO4 (b) LiNi0.9Mn0.1PO4 (c) LiNi0.85Mn0.15PO4 (d) LiNi0.8Mn0.2PO4

Table 4.7: Vibrational analysis for LiNi1-xZnxPO4 (x=0.05, 0.10, 0.15, 0.20)

Vibrational frequency (ν in cm-1) Assignments Pure

LiNiPO4 5% Zn 10% Zn

15% Zn

20% Zn

350 374 ----- 362 370 External mode 399 394 382 394 ------ External mode 469 470 465 476 475 ν (Li-O) 538 542 542 544 545 ν (Ni-O) 582 581 581 586 583 ν4 (PO4

3-) 652 653 651 651 652 ν4 (PO4

3-) 748 743 742 ---- ----- ν4 (PO4

3-) 890 889 891 ----- ------ ν4 (PO4

3-) 961 969 969 981 981 ν1 (PO4) 1063 1066 1064 1081 1083 ν3 (PO4) 1097 1098 1100 ----- ----- ν3 (PO4)

101

Table 4.8: Vibrational analysis for LiNi1-xMnxPO4 (x=0.05, 0.10, 0.15,

0.20)


LiNiPO4 5% Mn

10% Mn

15% Mn 20% Mn

350 374 382 362 370 External mode 399 394 ------ 394 ------ External mode 469 470 465 476 475 ν (Li-O) 538 542 542 544 545 ν (Ni-O) 582 581 581 586 583 ν4 (PO4

3-) 652 653 651 651 652 ν4 (PO4

3-) 748 743 742 ---- ----- ν4 (PO4

3-) 890 891 ----- ----- ------ ν4 (PO4

3-) 961 969 969 981 981 ν1 (PO4) 1063 1066 1064 1081 1083 ν3 (PO4) 1097 1098 1100 ----- ----- ν3 (PO4)

4.4. LASER RAMAN ANALYSIS

4.4.1. Laser Raman analysis of pure LiNiPO4

Figure 4.11 shows the Laser Raman spectrum of pure LiNiPO4 at

ambient temperature. Raman spectrum is generally classified in to two

regions such as above and below 400 cm-1. The bands obtained above

400cm-1 impute to the strong bands due to the internal vibrational modes

whereas the bands below 400 cm-1 corresponds to the external lattice

modes. The internal vibrations are vibrations which are caused by the

motion of atoms which induces deformation or change in the volume of

PO43-. External vibrations explain the translational motion of the mass

center of PO43-, Li+ and Ni2+. The very sharp and strongest peak is observed

at 945cm-1 (ν1) represents the P-O symmetric stretching vibrations of

102

(PO4)3- anion. This is the fundamental peak in the Olivine family

[Markevich et al (2011)]. The high wave number bands at (1007cm-1,

1067cm-1) are originating from the ν3 mode which belongs to the

asymmetric stretching mode of PO43- anion. Further, the bands at 498 cm-1

(ν2) and 635 cm-1 (ν4) attributed to symmetric and asymmetric bending

vibration mode of PO43- anion.

Figure 4.11: Laser Raman spectrum of pure LiNiPO4

4.4.2. Laser Raman studies of rare earth doped LiNiPO4

Figures 4.12 and 4.13 illustrate the Laser Raman spectra for La3+ and

Nd3+ doped LiNiPO4 at ambient temperature. All the vibrational bands are

slightly shifted upon doping of rare earth atoms. The new vibrational peak

(966 cm-1) is observed near the characteristic peak that imputes the removal

of Li+ from LiNiPO4. [Lemos et al (2006), Jing Wu et al (2013)]. The

detailed vibrational analysis is presented in Tables 4.9 and 4.10.

103

Figure 4.12:Laser Raman spectra of La doped LiNiPO4 (a) 0.01 mol% (b) 0.03mol% (c) 0.05 mol% (d) 0.07 mol% (e) 0.09 mol%

Figure 4.13: Laser Raman spectra of Nd3+ doped LiNiPO4 (a) 0.01 mol% (b) 0.03 mol% (c) 0.05 mol% (d) 0.07 mol% (e) 0.09 mol%

104

It will be interesting to find whether the dopant occupied at Li or

Metalsite. The crystal structure of LiNiPO4 is made up of two types of

polyhedra, MO6 octahedral unit that corner shared and cross linked with the

PO4 tetrahedral units. Sharing the vertex of this octahedra and PO4

tetrahedral makes a 3d network and the presence of M-O-P band in their

structure. In addition, PO4 polyanion units form strong covalent bond so the

dopant cannot substitute P in the PO4 unit. The characteristic peaks are

fitted by using PEAKFIT software with Lorentz fitting. The fitted data is

presented in figures 4.14 and 4.15. The FWHM of the peak(945 cm-1)

increases upon doping of rare earth atoms. So the dopant occupies the

corresponding host lattice may be either Ni2+ or Li+site.

Table 4.9:Laser Raman analysis of La3+doped LiNiPO4


LiNiPO4 0.01

mol% 0.03

mol% 0.05

mol% 0.07

mol% 0.09

mol% 115 114 114 109 115 115 Lattice mode 169 165 163 167 165 167 Lattice mode 258 273 271 273 273 ---- Lattice mode 396 --- ---- 396 --- ---- Lattice mode 498 ---- ---- ---- ---- 504 ν2 (PO4) 412 413 412 412 412 412 ν4 (PO4) 635 637 635 635 637 635 ν2 (PO4) + ν4NiO6

945 946 945 946 946 945 ν1 (PO4) --- 966 966 966 966 966 ν1 (PO4)

1007 1010 1008 1011 1010 1006 ν3 (PO4) --- 1048 ---- 1050 1049 1051 ν3 (PO4)

1067 1070 1068 ---- ---- ---- ν3 (PO4)

The emergence of new peak is positioned at 966cm-1 impute the

delithiation of Lithium from LiNiPO4 [Lemos et al (2006), Jing Wu et al

(2013)]. Moreover, the vibrational peak for NiO6 is also shifted upon

doping of both rare earth atoms. The FWHM of the corresponding peak (Ni

105

site) is also increased upon doping. Therefore, nickel site is disturbed due to

doping of La3+ and Nd3+. From this analysis it is concluded that the doping

of rare earth atoms affects Ni2+ site. To maintain the charge neutrality, one

Li+ions comes out from the lattice site due to the doping of one rare earth

atom and produce the vacant site in Li+.

Table 4.10:Laser Raman analysis of Nd3+doped LiNiPO4


Assignments Pure LiNiPO4

0.01 mol%

0.03 mol%

0.05 mol%

0.07 mol%

0.09 mol%

115 --- --- --- ---- --- Lattice mode 169 --- --- --- ---- --- Lattice mode 258 242 240 242 242 242 Lattice mode ---- 307 309 305 307 307 Lattice mode 396 ---- 396 396 398 396 Lattice mode 412 --- --- 410 410 412 ν4 (PO4) 635 --- --- 634 --- 634 ν2 (PO4) + ν4NiO6 945 945 947 949 945 949 ν1 (PO4) --- 966 966 966 966 966 ν1 (PO4)

1007 ---- ---- ---- --- --- ν3 (PO4) 1067 --- --- ---- ---- --- ν3 (PO4)

From the figure 4.14, it is observed that the intensity of the

vibrational peak at 966 cm-1 is increased up to 0.05 mol% of dopant

concentration. Further increase of dopant concentration diminishes the

intensity of the peak at 966 cm-1. Hence it is clear that more number of free

lithium ions which is due to the effect of dopant in order to maintain charge

neutrality are available. But, in the case of Nd doped sample a very less

intense peak at 966 cm-1 is observed. The lesser intensity of the peak at

966 cm-1 for Nd3+ doped sample compared with La3+ doped sample may be

due to the formation of impurity available in the Nd3+ doped system. This

106

data is consistent with the results of XRD which shows the Nd3+ doped

system has high level of impurity.

Figure 4.14: Deconvoluted Raman peak of La3+ doped LiNiPO4 (X-axis (wave number) Y-axis (intensity)in the range of 920 cm-1to 980 cm-1)

Figure 4.15: Deconvoluted Raman peak of Nd3+ doped LiNiPO4 (X-axis (wave number) Y-axis (intensity)in the range of 920 cm-1to 980 cm-1)

107

4.4.3. LASER RAMAN STUDIES OF TRANSITION METAL

DOPED LiNiPO4

Figures 4.16 and 4.17 show the Laser Raman spectra for Zn2+ and

Mn2+ doped LiNiPO4 at ambient temperature. All the vibrational bands are

listed in the Tables 4.11 and 4.12. Zn2+ and Mn2+ doped samples prefers

Ni2+ site since each 2+ valance transition metal atoms easily occupying the

Ni2+ site. Moreover the ionic radius of the transition metals (Zn2+=88 pm,

Mn2+=82 pm and Ni2+=83 pm) is comparable with the Ni2+. So the dopant

prefers the Ni2+ site. The deconvolution is also done for characteristic peak

in olivine family (945 cm-1) which is presented in figures 4.18 and 4.19.

The FWHM of the peak increases upon doping of metal atoms. The

characteristic vibrational peak (945 cm-1) for LiNiPO4 is also shifted due to

doping of Mn2+ ions. It is concluded that the dopant occupies the

corresponding host lattice (Ni2+).

The ionic radius of Li+, Ni2+, Zn2+ and Mn2+ are found to be 90 pm,

83 pm, 88 pm and 82 pm respectively. The ionic radius of Mn2+ and Ni2+

are almost equal, so the Mn2+ resides in the Ni2+ site. This is the one of the

evidence for the Mn2+ resides in the Ni2+ site and does not prefers Li+ site.

Further Zn2+ is also preferred Ni2+ site based on the ionic radius

comparison. When the concentration of the transition metal increases

(LiNi0.80Zn0.20PO4and LiNi0.80Mn0.20PO4), the new peak with small intensity

is observed at 966 cm-1. This may be due to the saturation limit of the

dopant in the Ni2+ site and hence it prefers Li site. The higher ionic radius

of Zn2+ offers mobility of Lithium ions faster which makes to expect the

higher electrical conductivity compared with Mn2+ doped sample. From the

laser Raman analysis, transition metal atomsas dopant prefer Ni2+ site.

108

Figure 4.16: Laser Raman spectra of Zn2+ doped LiNiPO4 (a) Pure LiNiPO4 (b) LiNi0.95Zn0.05PO4(c) LiNi0.90Zn0.10PO4(d) LiNi0.85Zn0.15PO4(e) LiNi0.80Zn0.20PO4

Table 4.11: Laser Raman analysis of Zn2+doped LiNiPO4


LiNiPO4 5% Zn 10% Zn

15% Zn

20% Zn

258 272 272 272 --- Lattice mode 396 --- ---- 396 --- Lattice mode 498 ---- ---- ---- ---- ν2 (PO4) 412 413 412 412 412 ν4 (PO4) 635 637 635 635 637 ν2 (PO4) + ν4NiO6 945 946 945 946 946 ν1 (PO4) --- 966 966 966 966 ν1 (PO4)

1007 1010 1008 1011 1010 ν3 (PO4) --- 1048 ---- 1050 1049 ν3 (PO4)

1067 1070 1068 ---- ---- ν3 (PO4)

109

Figure 4.17: Laser Raman spectra of Mn2+ doped LiNiPO4 (a) LiNi0.95Mn0.05PO4(b) LiNi0.90Mn0.10PO4(c) LiNi0.85Mn0.15PO4(d) LiNi0.80Mn0.20PO4

Table 4.12: Laser Raman analysis of Mn2+ doped LiNiPO4


LiNiPO4 5% Mn

10% Mn

15% Mn

20% Mn

258 --- ---- ---- ---- ---- 396 --- --- --- --- ---- 498 ---- ---- --- --- ---- 412 412 ---- --- --- ν4 (PO4) --- 571 589 589 588 ν2 (PO4) 635 634 635 635 635 ν2 (PO4) + ν4NiO6 945 943 945 943 945 ν1 (PO4) 1007 1004 1006 1006 1005 ν3 (PO4) 1067 1065 1067 --- 1063 ν3 (PO4)

110

Figure 4.18: Deconvoluted Raman peak of Zn2+ doped LiNiPO4 (X-axis (wave number) Y-axis (intensity) in the range of 920 cm-1to 980 cm-1)

Figure 4.19: Deconvoluted Raman peak of Mn2+ doped LiNiPO4 (X-axis (wave number) Y-axis (intensity)in the range of 920 cm-1to 980 cm-1)

111

4.5. SUMMARY

Thermal properties of as prepared samples are characterized by TG

analysis. It confirmed that the weight loss of the precursor is up to 600°C

then there is no reduction in the weight. Hence the reaction temperature is

optimized at 650°C; 6 h. The vibrational properties of Olivine type Lithium

nickel phosphate, rare-earth and transition metal doped LiNiPO4 are studied

by FTIR and Laser Raman studies. FTIR analysis confirms the dopant

exactly occupiesNi2+ site of the corresponding host lattice. Moreover, the

shift of vibrational bands results in change of lattice parameters which is

consistent with XRD results. Rare-earth ions and transition metal ions are

occupying Ni site upon doping with LiNiPO4which is proved from Laser

Raman analysis. Emergence of new peak in the case of RE doped systems

denotes the removal of lithium ion from LiNiPO4to maintain charge

neutrality.

112

CHAPTER 5

AC IMPEDANCE SPECTROSCOPIC STUDIES ON PURE, RARE EARTH AND TRANSITION METAL DOPED LiNiPO4

5.1. INTRODUCTION

Impedance spectroscopic analysis is a well-suitable technique to

study the dielectric response as a function of frequency and temperature and

obtain a wealth of information on ionic and electronically conducting solids

[Ross Macdonald et al (1987), Mccann et al (1982)]. The electrical

response of the sample is demonstrated by resistive and capacitive

behaviors that are attributed mainly to bulk grains, the grain boundaries or

the defects present at the sample-electrode interface in frequency domain. It

is a non-destructive technique and provide the dynamical properties to

understand the microscopic nature of the SIC (Super Ion Conducting)

materials. This chapter thoroughly discusses ac impedance spectroscopic

analysis of pure, rare earth and transition metal doped LiNiPO4.

5.2. IMPEDANCE STUDIES OF PURE LiNiPO4

The undoped samples are prepared in the form of pellet and subjected

in to different temperatures to optimize the sintering temperature. Sintering

is the process to remove any pores/gases between the grains and harden the

material which makes the sample to get better contact between the grain

113

boundaries. In the present work, the pure LiNiPO4 is subjected in to three

different sintering temperatures such as 550°C, 650°C and 750°C.

5.2.1. Conductance Spectra analysis

Figure 5.1 shows the frequency dependent of ac conductivity of

LiNiPO4pellet sintered at different sintering temperatures and it shows the

behavior of ionic material. This spectrum exhibits two regions: the

frequency independent plateau in the low frequency region and is attributed

to the dc part of the conductivity. The ac conductivity shows the high

frequency dispersion region that is typical of UDR (Universal Dielectric

Response) and the electrical network response [Bowen et al (2006)].

The low frequency plateau region occurs due to the hopping motion

of Li ions. i.e. the long range transport of Li-ions in the response of applied

field. The extrapolation of this plateau region to Y-axis gives the dc

conductivity of the material. The dc conductivity value isalso calculated by

Jonscher’s power law from the nonlinear curve fit.

The dc conductivity of pure LiNiPO4 is found to be 4.36×10-6 at

300ºC (650°C sintered sample) which is greater than the previously

reported values by Prabu et al [Prabhu et al (2011)] and Ressouli et al

[Ressouli et al (2003)]. The increase in conductivity in the high frequency

region is attributed to correlated forward-backward motions of ions

[Mariappan et al (2004), Ashok Kumar Baral et al (2009)].

The frequency dependence ofelectrical conductivity ofsolid

electrolytes is explained by a simple expressiongiven by Jonscher's power

law. The power law relates the frequency dependentconductivity or the so-

114

called universal dynamic response (UDR) of ionicconductivity and

frequency by [Jonscher A.K. (1977)]

ndc A )( ---------- (5.1)

Figure 5.1: Frequency dependence conductivity of LiNiPO4 pellet sinteredat different temperatures (a) 550°C (b) 650°C (c) 750°C

Here is the dc conductivity. The calculated conductivity values

are listed in the table 5.1.A and n are temperature dependent parameters. It

is also evident that dc conductivity becomes dominant at higher

temperatures. This increase in conductivity at higher temperature is due to

the mobile ions which acquire more thermal energy and can easily cross the

potential barriers [pal et al (2009)]. The exponent ‘n’ represents the degree

115

of interaction between mobiles ions and the environments surrounding

them [Hannachi et al (2010), Chen et al (2009)].According to Funke [Funke

et al (1993)], ‘n’ might have physical meaning and it is given by

timerelaxationSite

ratebackhopn ---------------------- (5.2)

Where, back hop rate is the backward motion of an ion to its initial

site due to either bad site or by coulomb repulsive interaction between

mobile ions. The site relaxation time is the time required for an ion to come

to rest when site potential is minimum to the position of the ion that is

caused by a rearrangement of neighboring ions. When the value of ‘n’ is

less than 1, the hopping motion involved is a translational motion with a

long hop but if the value of ‘n’ is greater than 1, it corresponds to the well

localized and/or reorientational motion of the ion wherein the well localized

means hopping of mobile ion occurs only in the vicinity of its initial site

and reorientational motion signifies dipole orientation with respect to the

external electric field [Oueslati et al (2010)].

Further, ‘n’ value measures the degree of order of the system. If n<1,

it is said to be ordered system and if n>1 means, the disorder of the system

increases. The computed n values are presented in table 5.1. In this case, the

‘n’ values ranges from 1.2-1.4 which is greater than one and it signifies the

well localized and/or the reorientational hopping motion of charge carriers.

Further it is seen that the “n” value decrease gradually with increase of

temperature for 650°C sintered sample. It may be due to formation of free

sites for Li ion migration, which in turn reduces the backhop rate and hence

the value of “n” decreases. The uniform variation of “n” value is not

observed for the pellet sintered at 550°C and 750°C which indicates that the

presence of unequal potential barriers.

116

Table 5.1: Calculated conductivity value for pure LiNiPO4 at different temperatures

Temperature In 0C

5500C sintered sample

6500C sintered sample S cm-1

7500C sintered sample S cm-1

Conductivity S cm-1

n Conductivity

S cm-1 n

Conductivity S cm-1

n

200 ----- ----- 3.36×10-7 ----- 8.92×10-8 -----250 3.10×10-9 1.26 1.37×10-6 1.28 2.04×10-7 1.02300 8.89×10-9 1.31 4.36×10-6 1.28 2.21×10-7 0.92350 7.32×10-8 1.39 9.90×10-6 1.27 8.95×10-6 0.87400 4.27×10-7 1.41 1.52×10-5 1.26 2.46×10-5 1.22425 1.01×10-6 1.44 1.88×10-5 1.26 ----- 1.06

5.2.2. Temperature dependent conductivity analysis

Figure 5.2 shows the temperature dependent dc conductivity of

LiNiPO4 pellet sintered at different temperatures. This plot is drawn

between 1/T Vs log σdc. The dc conductivity values are extracted from the

conductance spectra (Fig 5.1) and listed in the table 5.1. It is found that the

conductivity increases with increase of temperature. The regression value

of the linear fit plot is close to unity, indicating that the temperature

dependent of dc conductivity obeys the Arrhenius relation which can be

written as,

)(

0 exp KT

Ea

T

--------- (5.3)

Here 0 is the pre exponential factor and Ea is the activation energy for

conduction. From the slope value, the activation energy is calculated. The

activation energy for LiNiPO4 sintered at 650ºC is found to be low

117

compared with the pellets sintered at 550ºC and 750ºC. So the conductivity

value for the pellet sintered at 650ºC is high compared with the pellets

sintered at 550ºC and 750ºC. The calculated values of activation energies

are tabulated in the Table 5.2.

Figure 5.2: Temperature dependent conductivity of LiNiPO4 for different sintering temperatures □ 550°C, ●650°C, ▲750°C.

Table 5.2: Activation energies of LiNiPO4 at different sintering temperatures

Sintering

temperature

Activation energy

in eV

5500C 1.02

6500C 0.54

7500C 0.97

118

5.2.3. Bode plot analysis for pure LiNiPO4

Figures 5.3 (a) and 5.3(b) show the frequency dependent real and

imaginary parts of the impedance at different temperatures. From the figure

5.3 (a), it is observed that the magnitude of real part of impedance (Z')

appeaars to be constant at low frequency region and is due to long range

motion of ions in the material. As the frequency increases, the magnitude of

Z' starts to decrease at particular frequency and is called as cross over

frequency. This low value of Z' at high frequency is caused due to the short

range motion of the ions. The trend of the plot drawn between Z' vs. log ω

may be due to the parallel combination of resistance and capacitance

present in the system. It is noticed that the magnitude of Z' decreases as the

temperature increases. Same behavior is also observed for the samples

sintered at 550°C and 750°C. Similar results are already observed [Oueslati

et al (2010)].

From the figure 5.3 (b), the relaxation peak is identified at each

temperature and it is broad in nature which is due to distribution of

relaxation times present in the material. The FWHM of the peak increases

as the temperature increases.The peak maximum shifts to higher frequency

side as the temperature increases [AloDutta et al (2008)].

The inset figure 5.3 (b) shows the inverse of temperature versus

logarithmic of peak frequency corresponding to Z''Max. All the peaks are

fitted by single lorentian peak fit and the peak frequency values are

computed. The peak frequency shifted as the temperature increases and

follows the Arrhenius law of conduction. From the Arrhenius plot, the

activation energy is found to be 0.50 eV in the temperature range of 250ºC

to 400ºC. The activation energy calculated from relaxation is comparable to

119

the activation energy determined from the conductivity analysis within the

experimental errors. From this analysis, the conduction is explained by the

thermally activated mechanism and the free charge carriers are responsible

for conduction.[Nishant Kumar et al (2010), Oueslati et al (2010)]. i.e., the

enthalpy of free charge carrier formation is negligible in this system.

Figure 5.3:Frequency dependence of (a) Real and (b) Imaginary part of the impedance of LiNiPO4 (sintered at 650°C for 5 h) at different temperatures.

To make sure whether the relaxation mechanism is temperature

dependent or not, the scaling was done and the plot is drawn between

Z/ZMax and log (ω/ωmax) and is shown in figure 5.4. The entire data of

imaginary part of impedance can collapse into the single master curve and

indicates that the relaxation mechanism is temperature independent [Bajpai

et al (2011), Lata Agrawal et al (2009) Ved prakash et al (2008)].

120

Figure 5.4: Scaling behavior of Z at different temperatures for pure LiNiPO4

5.2.4. Dielectric and modulus spectroscopic analysis

The dielectric and electric modulus studies are done for the LiNiPO4

pellet sintered at 650°C, 5h because it shows higher conductivity compared

with other samples.The complex permittivity * is calculated using the

impedance data by the following equation [Bahgat et al (1998), Checa et al

(2009)]

* *

*

0

1' "

j C Z

----------- (5.4)

Where ' and " are the real and imaginary part of dielectric

permittivity. *Z is the complex impedance of the system and 0C is the

vacuum capacitance of the measuring cell.

121

Figure 5.5: Frequency dependent dielectric constant of LiNiPO4 (sintered at 650°C for 5 h) at different temperatures.

Figure 5.5 shows the variation of dielectric constant (ε') with respect

to frequency at different temperatures for pure LiNiPO4 pellet sintered at

650°C. It is seen that the dielectric constant decreases with increase of

frequency and attains constant value at higher frequencies for all samples. It

is also noted that as the temperature increases the magnitude of ε' also

increases in the lower frequency region. The higher value of ε' in the low

frequency region may be due to the presence of blocking electrodes which

are unable to permit to the transfer of mobile ions as a result ions are

accumulated near the silver electrodes and produce the bulk polarization

effect in the material [Alo dutta et al (2008), (2011), Mahato et al (2011)].

Due to the high periodic reversal of the field at higher frequencies, there is

no charge accumulation takes place at the electrode surfaces and hence ε'

decreases at higher frequencies [Lata Agrawal et al (2009)].

Figure 5.6 shows the scaling behavior of dielectric loss of

LiNiPO4 (sintered at 650ºC for 5 h) at different temperatures. The imaginary

122

part of the dielectric spectra have been scaled to each frequency by ωMax

and each ε'' by εMax'' respectively. The merged nature of all spectra at

different temperatures in to a single master curve reveals that the same

relaxation mechanism exists in the range of temperatures studied. The

relaxation mechanism is independent of temperature [Jayaseelan et al

(2004)].

Figure 5.6: Scaling behavior of dielectric loss at different temperatures of

LiNiPO4 (sintered at 6500C for 5 h).

The plot of log ε versus log ω is shown in figure 5.7 using the

below equation (5.5).

log)1()log()"log( nA -------- (5.5)

123

Figure 5.7: Variation of logarithmic of dielectric loss for LiNiPO4 with frequency at different temperatures (pellet sintered at 650°C, 5h).

The slope of the straight line is found to be in the range of -0.94 to -

0.99. Hence logε versus log ω plot with a slope value near to -1 indicates

that the dc conduction contribution is predominant in the present sample.

In ionically conducting materials, at low frequencies there is an

unavoidable electrode polarization effects and the dielectric constant is

rather high. This often yields a large experimental error during the

separation of dc conductivity from the total conductivity. However, the data

were converted into electric modulus where the contributions from the

electrode effects are minimized.The complex electric modulus spectrum

represents the measure of the distribution of ion energies or configurations

in the structure and it also describes the electrical relaxation and

microscopic properties. Moreover, it is frequently used to analyze the

dynamic relaxation of charge carriers. The electric modulus M* is expressed

in the complex modulus formalism [Ben Amor et al (2009)]

124

M*=1/εr =M+iM --------- (5.6)

Here in M and M are the real and imaginary components of the

modulus spectrum. The variation of M and imaginary part of M of

electrical modulus (for LiNiPO4 sintered at 650°C) is depicted in figure 5.8

and 5.9. At low frequencies, electric modulus approaches zero which might

be due to the large value of capacitance associated with the electrode

polarization effect [Himanshu et al (2010)]. M shows a slightly

asymmetric peak at each temperature. The peak maximum is shifted

towards higher frequencies with increasing temperature. As a result, the

relaxation time decreases [Chakraborty et al (2011)]. The presence of such

relaxation peak in the M plots indicates that the samples are ionic

conductors. The frequency region below MMax determines the range in

which charge carriers are mobile on long distances. The frequency above

MMax, the charge carriers are confined to a potential well, being mobile on

shorter distances.

Figure 5.8: Real part of the modulus spectra for Pure LiNiPO4 at different temperatures (LiNiPO4 pellet sintered at 650°C, 5h)

125

Figure 5.9: Imaginary part of the modulus spectra for Pure LiNiPO4 at different temperatures (LiNiPO4 pellet sintered at 650°C, 5h)

126

5.3. IMPEDANCE ANALYSIS OF RARE EARTH AND

TRANSITION METAL DOPED LiNiPO4

This section deals with the impedance analysis of rare earth (La and

Nd) and transition metal doped LiNiPO4. The use of a dopant or additive to

a material of interest is a widely used method of increasing its electrical

conductivity. The selection of lanthanum (La3+) and Nd3+ as dopants in this

work is based on the concept of that both the lanthanides form M3+ (M= La

& Nd) ions and the most stable tripositive ions. Moreover, the selection of

transition metals is also obeying the same procedure but they are most

stable dipositive valence ions.

5.3.1. Conductance spectra analysis

Figure 5.10 and 5.11 represents the frequency dependent

conductance spectra for rare earth doped and transition metal doped

LiNiPO4 at ambient temperature. All the spectra having the trend like pure

LiNiPO4 which is discussed in the section 5.2. From the figures 5.10 and

5.11, it is found that 0.05mol% La, 0.07mol% Nddoped LiNiPO4,

LiNi0.85Zn0.15PO4 and LiNi0.90Mn0.10PO4 possesseshigher dc conductivity

value compared with all other concentrations studied. The calculated

conductivity (extracted from conductance spectra) values are listed in Table

5.3 and 5.4. Among the all dopant, 0.05 mol% La doped LiNiPO4 shows

higher conductivity which is found to be 3.05×10-7 S cm-1 at ambient

temperature. Table 5.5 depicts the conductivity values for LiNiPO4 found in

literature.

The power law relates (equation 5.1) the frequency dependent

conductivity or the so-called universal dynamic response (UDR) of ionic

127

conductivity and frequency by [Jonscher A.K (1977)]. From the equation

5.1, the exponent ‘n’ represents the degree of interaction between mobiles

ions and the environments surrounding them. [Hannachi et al (2010), Chen

et al (2009)]The physical meaning of “n” value is already discussed in the

section 5.1. Among the La doped systems, 0.05 mol% La doped sample has

lower n value compared with other samples which signifies the backhop

rate less than that of forward hopping. The same mechanism is also

applicable for 0.07 mol% Nd doped LiNiPO4, LiNi0.85Zn0.15PO4 and

LiNi0.90Mn0.10PO4 samples.

Table 5.3: Conductivity values for La3+ and Nd3+ doped LiNiPO4 at ambient temperature

Concentration in mol%

La3+ doped LiNiPO4 Nd3+ doped LiNiPO4

Conductivity (S cm-1)

n Conductivity

(S cm-1) n

Pure LiNiPO4 Above 10-9 ----- ---------- ------- 0.01 5.60×10-8 1.39 4.74×10-9 1.92 0.03 8.40×10-8 0.96 4.95×10-9 1.84 0.05 3.05×10-7 0.92 1.15×10-8 1.76 0.07 1.08×10-7 1.21 1.54×10-8 1.67 0.09 3.13×10-8 1.55 7.75×10-9 1.99

Table 5.4: Conductivity and “n” values for Zn2+ and Mn2+ doped LiNiPO4

at ambient temperature

Sample Conductivity

(S cm-1) n

LiNi0.95Zn0.05PO4 3.04×10-8 1.25 LiNi0.9Zn0.1PO4 8.74×10-8 1.10

LiNi0.85Zn0.15PO4 1.09×10-7 1.03 LiNi0.8Zn0.2PO4 2.81×10-8 1.17

LiNi0.95Mn0.05PO4 2.21×10-8 1.74 LiNi0.9Mn0.1PO4 5.45×10-8 1.59

LiNi0.85Mn0.15PO4 3.19×10-8 1.73 LiNi0.8Mn0.2PO4 3.60×10-9 1.99

128

Table 5.5: Conductivity of LiNiPO4 and doped LiNiPO4 found in the literature

Sample Method Conductivity (S cm-1)

Ref

LiNiPO4 polyol Method

< 10-9 present work

LiNiPO4:0.05%La polyol Method

3.04 x 10-7 present work

LiNi0.85Zn0.15PO4 polyol method

1.09 x 10-7 Karthickprabhu et al (2014)

LiNi0.9Mn0.1PO4 polyol method

2.81x 10-8 present work

LiNiPO4:0.07%Nd polyol Method

1.54 x 10-8 present work

LiNiPO4 Li0.99Eu0.01NiPO4

Pechini Method Pechini Method

9.34×10-9

7.02×10-8 [Prabhu et al (2012)] [Prabhu et al (2012)]

LiNiPO4 SSR 10-4 [Herle et al (2004)]

LiNiPO4 SSR 10-14 at 100°C Le Bacg et al (2005)

LiNiPO4 SSR 10-8 at 400°C

Ressouli et al (2003) Moreno et al (2001)

LiNiPO4 SSR 1.41x 10-7at 130° C

Vijayan et al (2014)

LiNi0.9Mg0.1PO4 Solution combustion

1.57x 10-7at 130° C


LiNi0.9Cu0.1PO4 Solution combustion

7.82x 10-8at 130° C


LiNiPO4 SSR 1.19 x 10-8 at 300° C

Karthickprabhu et al (2014)

LiNi0.85Zn0.15PO4 SSR 4.20 x 10-8 at 300° C


LiNi0.925Al0.05PO4 SSR 2.32 x 10-8 at 300° C


The enhancement of the conductivity of LiNiPO4 upon doping can

be explained as follows; substitution of rare earth or transition metal

129

produce structural defects (vacancies)in the lattice site which creates the

pathway wider that facilitates Li-ion mobility and the dopant can reduce the

interaction force between Li+ and Li+.This result is consistent with the laser

Raman analysis (as discussed in chapter 4), in which the presence of new

vibrational band at 966 cm-1 represents the delithiation of lithium from

LiNiPO4. Due to formation of these lithium ion vacancies, conductivity of

LiNiPO4 is improved upon doping. Moreover, the intensity of the

vibrational peak positioned at 966 cm-1 is lesser for Nd3+ doped sample

compared with La3+ doped system. It may be due to the presence of

impurity peak (evidenced from the XRD analysis) in the Nd3+ doped

samples which may mask the production of Li-ion vacancies hence the

electrical conductivity of Nd3+ doped sample is lesser than La3+ doped

sample. Moreover, the higher ionic radius of La3+ provides more path way

to the mobility of Li ions which support higher electrical conductivity. The

higher lattice volume is observed for La3+ doped sample than Nd3+ doped

system. This increment in the lattice volume grants easier intercalation/de-

intercalation of Li-ions which is inferred from XRD analysis.

Figure 5.10: Frequency dependent conductivity spectra for Rare earth doped LiNiPO4 at ambient temperature

130

Figure 5.11: Frequency dependent conductivity spectra for Transition metal doped LiNiPO4 at ambient temperature

5.3.2. Cole-Cole plot analysis

Figure 5.12:Cole-Cole Plot for Pure LiNiPO4 at ambient temperature.

131

Figures 5.12, 5.13 and 5.14 show the cole-cole plot of pure, rare

earth and transition metal doped LiNiPO4 for different concentrations of

dopant at ambient temperature. The bulk resistance can be retrieved from

the intercept of high frequency semicircle on the x-axis. It is found that

0.05mol% La, 0.07mol% Nddoped LiNiPO4, LiNi0.85Zn0.15PO4 and

LiNi0.9Mn0.1PO4 possesses low bulk resistance value compared with pure

LiNiPO4 and all other concentrations studied. The semicircle alone is

observed for LiNiPO4: xmol%Nd, LiNi1-xZnxPO4and LiNi1-

xMnxPO4samples whereas the sample LiNiPO4: xmol %La shows spike at

low frequency region along with the depressed semicircle in the high

frequency region in the frequency range studied. The bulk resistance (Rb)

and capacitance values are calculated from the Z view software and the

corresponding equivalent circuits are also provided in inset figure.

Figure 5.13: Cole-Cole Plot for rare earth doped LiNiPO4 at ambient temperature

The equivalent circuit represents the presence of parallel

combination of resistance (R1) and constant phase element (CPE1) along

with CPE2 (due to double layer capacitance) in series combination in the

case of La doped sample whereas parallel combination of resistance (R1)

and CPE1 along with resistance in series. Migration of Li-ions in the bulk

132

LiNiPO4 is represented by resistor and the immobile Li-ions becomes

polarized by applying the ac field, which can be represented by a

capacitor.The ionic migration and bulk polarization are physically parallel,

and, therefore, the semicircle at high frequency can be observed. The value

of capacitance isin the order of pico farads which indicate that the ion

conduction process occurs through the bulk of the material. It is also noted

that the centre of the semicircle is lying down below the real axis for all

samples i.e., the depressed semicircle which ensures that the distribution of

relaxation times and it deviates from the ideal Debye behavior [Anantha et

al (2005), Ben rhaiem et al (2008), Sheoran et al (2009)].

Figure 5.14: Cole-Cole Plot for Transition metal doped LiNiPO4at ambient

temperature

From the bulk resistance (Rb) of the sample, the conductivity value

has been calculated using the following relation.

A

L

Rbb

1 --------------------- (5.7)

133

Herein σb is the bulk conductivity of the sample and L&A are thickness and

area of the sample respectively. The conductivity values are calculated from

the above equation. It is greatly matched with the conductivity calculated

from the conductance spectra.

5.3.3. Concentration dependent of conductivity

The variation of dc conductivity with respect to rare earth

concentrations and transition metal concentrations is shown in figure 5.15.

For both the systems, the dc conductivity value increases upon increase of

rare earths, transition metal concentration and a maxima is reached. Further

increase of both concentrations causes decrease of conductivity. The

enhancement in the conductivity of the RE and metal doped LiNiPO4 is due

to the effect of introducing the impurity bands in the electronic structure of

pristine LiNiPO4. The doping of La3+, Nd3+, Zn2+ and Mn2+ in the structure

of LiNiPO4 provides the vacant site which favors higher ion hoping rate

and hence enhancement of the conductivity. Similar results have also been

observed for rare earth and metal doped LiFePO4[Jiezi Hu et al (2009)].

Further, it is noted that, doping with cations (La3+, Nd3+, Zn2+ and

Mn2+) on LiNiPO4 could decrease the grain size due to cation vacancies

created during charge compensation and it may increases the size of

channel for Li+ diffusion. So, the electrical conductivity gets increased

[Sathiyaraj kandhasamy et al (2012)]. This result is consistent with the

particle size distribution analysis (discussed in chapter 3) in which the

particle size gets decreased because of insertion of both rare earth and

transition metal atoms. It is also found that the 0.05mol% La3+ doped

LiNiPO4, 0.07mol% Nd3+ doped LiNiPO4, LiNi0.85Zn0.15PO4 and

LiNi0.9Mn0.1PO4samples possess higher conductivity compared with other

134

concentrations. The decrease of conductivity at higher concentration of rare

earth and transition metals may be due to decrease of availability space for

the movement of ion with the excess amount of rare earth and hence

blocked the migration of lithium ions that contribute to the conductivity

[Fey et al (1997)].

Figure 5.15: Concentration dependent conductivity spectra for a) rare earth and b) transition metal doped LiNiPO4

5.3.4. Dielectric and modulus spectroscopic analysis

Figure 5.16 represents the magnitude of dielectric constant spectra for

rare earth and transition metal doped LiNiPO4 various concentrations at

ambient temperature. The magnitude of dielectric constant is high for 0.05

135

mol% La doped, 0.07 mol% Nd doped, LiNi0.85Zn0.15PO4 and

LiNi0.9Mn0.1PO4 among all samples.

Figure 5.16: Frequency dependent dielectric constant of rare earth and


This can be explained on the basis of the availability of maximum

number of free charge carriers due to doping within the material for motion

which contribute to the conductivity. It was already reported that the

dielectric loss is directly related with the conductivity [Hiroshi Yamamura

et al (2007)] of the material which is suggested by the equation ε" (ω) = σ

(ω) / ωε0. Further it is seen that the dielectric constant is high for 0.05

mol% La doped LiNiPO4 compared with other samples like 0.07 mol% Nd

doped, LiNi0.85Zn0.15PO4 and LiNi0.9Mn0.1PO4. So the doping of La may be

136

favorable in LiNiPO4 which is also confirmed through conductivity

analysis.

Figure 5.17 represents the variation of logarithmic dielectric loss for

rareearth and metal doped LiNiPO4 at different concentrations. The plot is

fitted with linear fit data with regression 0.99 and ‘n’ values are calculated

by using the equation 5.5 [Vijayakumar et al (2003)]

Figure 5.17: Variation of logarithmic dielectric loss for rare earth and transition metal doped LiNiPO4

The calculated n values are provided in the Table 5.6 and 5.7. The

contribution of dc conduction mechanism is predominant when the slope

value of the plot is -1 or the ‘n’value approaches to zero.From the table, it

is noticed that the value of ‘n’ is approaching zero which signifies d.c.

conduction contribution is predominant in these systems. Among the rare

earth doped systems, La doped LiNiPO4 is having the ‘n’ value very nearer

137

to zero in particular 0.05mol% La doped sample that also shows higher dc

conductivity. In the similar way, Zn doped LiNiPO4 is having the n value

near to zero in particular LiNi0.85Zn0.15PO4 compared with other

concentrations and Mn doped system. From the analysis, it is concluded

that the doping of La3+ in LiNiPO4 is favorable for the enhancement of the

conductivity of LiNiPO4 compared with other dopants.

Table 5.6: ‘n’ values for rare earth doped LiNiPO4 calculated from the dielectric loss spectra

Concentration in mol% La doped LiNiPO4 Nd doped LiNiPO4 0.01 0.09 0.48 0.03 0.07 0.47 0.05 0.06 0.30 0.07 0.10 0.18 0.09 0.24 0.39

Table 5.7: ‘n’ values for Transition metal doped LiNiPO4 calculated from the dielectric loss spectra

Concentration of Zn and Mn

Zn doped LiNiPO4 Mn doped LiNiPO4

0.05 0.21 0.18 0.10 0.14 0.14 0.15 0.08 0.41 0.20 0.18 0.40

Figure 5.18 shows the frequency dependence of imaginary part of

modulus spectrum for pure, La, Nd, Zn and Mn doped LiNiPO4 at ambient

temperature. This spectrum shows the relaxation peak for each sample. The

maximum of M'' peak has been observed at higher frequency for high

conducting sample compared with other samples. The emergence of more

than one peak at low frequency region may be due to presence of different

138

relaxation mechanisms. Appearance of more than one peak would be due to

the “islands of mobility” or clusters present in the disordered materials due

to doping [Diaz-calleja et al (2004)].It is also noted that the broadening of

these peak denotes the non-Debye type of relaxation.

Figure 5.18: Imaginary part of modulus spectra of rare earth and transition metal doped LiNiPO4

5.4. SUMMARY

Impedance spectral studies of pure, rare earth and transition metal

doped LiNiPO4 characterized by Hioki LCR Hitester-3532-50. The

sintering temperature of undoped and doped LiNiPO4 is optimized at

650°C; 5h with the help of activation energy calculations. The conductivity

values of pure and doped LiNiPO4are higher than the previously reported

139

values. The conductivity of LiNiPO4is increased significantly with the

incorporation of Lanthanides (La3+& Nd3+) and transition metals (Zn2+&

Mn2+). Among the all dopants, 0.05 mol% La3+ doped sample at ambient

temperature showed the best improvement in conductivity by the factor of

two orders, an increase from <10-9 to 10-7 S/cm. So, the doping of La3+ is

favorable in LiNiPO4 compared with all other systems. The magnitude of

dielectric constant value is found to be increased upon doping of both rare

earths and transition metals. The dielectric loss spectra confirm the dc

conduction mechanism is predominant for all samples. Modulus formalism

ensures that the non-Debye type relaxation present in the all prepared

samples.

140

CHAPTER 6

ELECTROCHEMICAL CHARACTERIZATION ON PURE AND DOPED LiNiPO4

6.1. INTRODUCTION

Pristine LiNiPO4, 0.05 mol% La doped LiNiPO4, 0.03 mol% Nd

doped LiNiPO4, LiNi0.85Zn0.15PO4 and LiNi0.9Mn0.1PO4 samples (optimized

samples) are subjected into electrochemical characterizations such as

electrochemical impedance, cyclic voltametry and galvanostatic charge-

discharge studies. Electrochemical impedance spectroscopy is a useful

technique for studying the electrode kinetics of cathode materials [Shaju et

al (2004), Reddy et al (2007)]. Cyclic voltametry study is used to

investigate the oxidation –reduction reactions of Ni2+/Ni3+ redox couple.

The performance of the prepared cathode materials is identified by

galvanostatic charge-discharge studies. In the present investigation,

electrochemical impedance spectraare recorded with Li-metal foil is used as

a counter electrode and reference electrode. The prepared cathode materials

are serving as a working electrode.

6.2. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

Generally, in the electrochemical impedance plot, the high frequency

semicircle represents the impedance due to a solid electrolyte interface

141

formed on the surface of the electrode and liquid electrolyte, and the

intermediate frequency semicircle is related to the charge-transfer

resistance in the electrode/electrolyte interface [Kim et al (2011)]. A

straight line in the low frequency region is attributed to the Warburg

impedance arising from the diffusion of Li-ions.

Nyquist plots recorded individually for pure LiNiPO4, 0.05 mol% La

doped LiNiPO4, 0.03 mol% Nd doped, LiNi0.85Zn0.15PO4 and

LiNi0.90Mn0.10PO4 investigated under two conditions, viz., impedance of as

fabricated cell and impedance of the cell after cycles are furnished in figure

6.1, 6.2, 6.3, 6.4 and 6.5 respectively at ambient temperature.

Figure 6.1: Electrochemical impedance spectra of pure LiNiPO4 after and

before cycling

142

(Before Cycling)

(After Cycling)

Figure 6.2: Electrochemical impedance spectra of 0.05 mol% La3+ doped LiNiPO4 before and after cycling

143

Figure 6.3: Electrochemical impedance spectra of 0.03 mol% Nd3+ doped LiNiPO4 before and after cycling

Figure 6.4: Electrochemical impedance spectra of LiNi0.85Zn0.15PO4 before and after cycling

144

Figure 6.5: Electrochemical impedance spectra of LiNi0.90Mn0.10PO4 before and after cycling

All profiles exhibit a semicircle in the high frequency region and a

straight line in the low frequency region other than pure LiNiPO4. The

numerical value of the diameter of the semicircle on the Zreal axis is

approximately equal to the charge transfer resistance (Rct) (internal

resistance), therefore, it can be seen that there is a marked decrease in Rct

after doping. The straight line is attributed to the diffusion of the lithium

ion into the bulk of the electrode material or so-called Warburg diffusion.

The EIS data analysis using the equivalent circuit fitting could be

used for the qualitative estimation of the contribution of different

components of the electrochemical system presented in this work into the

interface development and changes in the electrochemical performance of

the cathode materials. Although this kind of estimation is not precise

enough, useful information could be derived for the system for further

145

performance enhancement [Zhang et al (2013)]. The impedance data are

fitted by Z-view software. In the equivalent circuit diagram, R1 represents

the resistivity of the electrolyte used in the prepared cell [Aurbach et al

(2008), Bakenov et al (2010), Zhang et al (2009)]. The Rct arises due to the

resistance at the interface between the electrode and electrolyte (internal

resistance). A fresh cell of pure LiNiPO4 (OCV-2.753 V) shows a single

semicircle in the high frequency region, followed by Warburg region. The

charge transfer resistance is calculated as 1302Ω for pristine LiNiPO4.

Further, the charge transfer resistance (fresh cell) values for 0.05 mol% La

doped, 0.03 mol% Nd doped, LiNi0.85Zn0.15PO4 and LiNi0.90Mn0.10PO4 are

found as 45Ω, 266Ω, 57Ω and 1212Ω respectively. The charge transfer

resistance value gets decreased upon doping of both rare earth and

transition metal atoms. The calculated charge transfer-resistance and CPE

values (before and after cycling) are listed in the table 6.1. It is also evident

that, from table 6.1, the charge-transfer resistance is low value (for fresh

cell) for 0.05 mol% La doped LiNiPO4 compared with other samples. This

result is consistent with the impedance analysis (chapter 5) in which the

sample posses higher electrical conductivity compared with other samples.

This higher electrical conductivity may help to improve the cell

performance of La3+ doped LiNiPO4. After cycling the Rct values are

increased. Especially for pure sample, it is increased suddenly even after 4th

cycle. This may be due to the presence of sluggish kinetic behavior in the

pure and doped LiNiPO4. The abrupt increase of charge transfer resistance

of the LiNiPO4 is considered as the main reason for the capacity fading

which is expected to reflect in charge-discharge studies [Li et al (2009)].

Moreover, CPE represents the surface film capacitance of the electrode

material. The CPEs reflect the charge accumulation upon cycling and its

value is determined by the double layer capacitance at the boundaries. The

CPE values are not similar for all the samples which implies that the

146

formation of interfacial layers on the electrode surface is different for

different sample.

Table 6.1: Electrochemical impedance parameters

Fresh cell Samples OCV

(V) No. of cycles

Rct (Ω) CPE (surface film capacitance)

(μf) Pure LiNiPO4 2.95 ---- 1302 13

LiNiPO4: 0.05 mol% La doped

3.69 ---- 45 8.5

LiNiPO4:0.03 mol% Nd doped

2.36 --- 266 11

LiNi0.85Zn0.15PO4 2.34 --- 57 11 LiNi0.90Mn0.10PO4 1.99 --- 1212 230

After cycling Pure LiNiPO4 3.70 5 4358 4 LiNiPO4: 0.05

mol% La doped 3.70 7 2389 3.6

LiNiPO4:0.03 mol% Nd doped

3.54 5 606 4

LiNi0.85Zn0.15PO4 3.56 6 ---- ---- LiNi0.90Mn0.10PO4 3.28 5 1388 14

6.3. CYCLIC VOLTAMETRY STUDIES

To investigate oxidation –reduction reactions of Ni2+/Ni3+, cyclic

voltametry test is undertaken. The working electrode is scanned between

2.5-5.3 V at a scan rate of 0.5mV/sec. On each occasion, the potential is

started at open circuit voltage (Eocv) moving initially in the anodic direction

to 5.3 V and then reversing it to lower cutoff potential. The prepared

cathode materials are served as a working electrode and Li metal foil as the

counter and reference electrode.

147

Figure 6.6 shows the cyclic voltammogram for pure LiNiPO4

prepared by polyol method. There is no significant oxidation and reduction

peak identified in the first cycle. The material shows one anodic peak at

4.29 V and one distinct cathodic peak at 3.37 V subsequent cycles (4th, 5th

and 6th cycles) signifying a reversible one-stage process of intercalating and

de-intercalating lithium from and into LiNiPO4. The oxidation and

reduction potential is listed in Table 6.3. Similar results are also observed

[Gangulibabu et al (2009)]for LiNiPO4 by CAM sol-gel method under air

atmosphere. The redox potential is found to be 3.8 V and 3.5V for

oxidation and reduction respectively. LiNiPO4 prepared under air and argon

atmosphere do not show any redox couple in CV analysis. The material

synthesized from both argon atmosphere and carbon coating shows the

redox couple at 5.3 V (anodic) and 5.1 V (cathodic) [Wolfenstine et al

(2005)]. The role of carbon is still not fully understood. Despite that, their

data underline that some kind of carbon support is required for LiNiPO4 to

exhibit lithium insertion/deinsertion [Rommel et al (2014)]. But, in the

present study show that polyol method is one of the best methods to

produce the phase pure LiNiPO4 which also shows the redox couples.

Several groups were not successful in de-intercalation of Li from

LiNiPO4, synthesized by a solid state route, using different electrolytes and

anodes. Okada et al [Okada et al (2001)] did not observe a voltage plateau

in the discharge profile of LiNiPO4, even after initial charging up to 5.2 V.

Moreover, β-LiNiPO4 (high pressure form of LiNiPO4) does not show the

electrochemical activity using LiPF6/ EC+DME as an electrolyte.

Tetramethylene sulfone (TMS) is used by Wolfenstine groups because of

its high oxidative stability of around 5.8 V vs Li [Wolfenstine et al (2004)].

Though TMS shows higher oxidative stability, it does not produce

significant reduction or oxidation peaks in LiNiPO4. The detailed

148

electrochemical properties for LiNiPO4 by other researchers are provided in

the table 6.2. In the present work, the redox potential of LiNiPO4 is

observed using high oxidative stability (~6 V) electrolyte Lithium

nanofluoro 1,2-butanesulfonate dissolved in PC+DME (2:1).

Table 6.2:Electrochemical performance of LiNiPO4 found in the literature.

Product Electrochemical characteristics/ other properties

Reference

LiNiPO4 Scan range [2.5-4.5 V] Li/LiClO4/PC+DME

No electrochemical performance

Padhi et al (1997)

LiNiPO4 Scan range [3.0-5.5 V] Li/LiPF6/EC+DMC


Li et al (2006)

LiNiPO4 Scan range [2.0-5.5 V] Li/LiPF6/EC+DMC


Garcia-Moreno et al

(2001)

LiNiPO4 Scan range [3.5-6 V], Li/LiPF6/TMS No electrochemical performance

Wolfenstine et al (2004)

LiNi0.8Co0.2PO4 Scan range [3.5-5.8 V]

Li/LiPF6/TMS No electrochemical performance


Li/LiPF6/TMS Red. 4.7V, Oxi. 5.2 V


Li/LiPF6/TMS Red. 4.7V, Oxi. 5.2 V

LiNiPO4 Scan range [3.5-6 V]

Li/LiPF6/TMS No electrochemical performance


149

LiNiPO4 Scan range [0.2-0.3 V] Hg/HgO/Sn/LiOH

Red. 231 mV, Oxi. 94 mV, 45% reversible

Minakshi et al (2011)

LiNiPO4 Scan range [0.2-0.3 V] Hg/HgO/Zn/LiOH, very reversible


Li (Mg0.5Ni0.5)PO4+ Li3PO4

Scan range [0.2-0.3 V] Hg/HgO/Zn/LiOH

Oxi+red+shoulder peak very reversible

LiCo0.6Ni0.4PO4 Scan range [3-5.2 V] Li/LiPF6/EC+DMC


Bramnik et al (2009)

LiNiPO4 LiNi0.5Co0.5PO4

Scan range [0.2-0.3 V] Hg/HgO/Zn/LiOH.H2O

Red. 231 mV, Oxi. 98 mV, 30% reversible


LiNiPO4+Ni3P Scan range [3.5-6 V] Li/LiPF6/TMS



LiNiPO4/C+Ni3P Scan range [3.5-6 V] Li/LiPF6/TMS

Oxi.5.3 V, Red.5.1 V LiNiPO4 Scan range [2.5-6 V]

Li/LiPF6/EMS + DMS No electrochemical performance

Wang et al (2010)

LiNi0.8Fe0.2PO4/C First cycle: Oxi.5.2 V, No red. Peak 2nd cycle: No peaks

LiNiPO4/C Scan range [0-4.2 V] Li4Ti5O12/LiPF6/EC + DEC


Jingjing et al (2011)

Li0.98Zr0.02NiPO4/C Scan range [0-4.2 V] Li4Ti5O12/LiPF6/EC + DEC

No oxidation peak, red.3.5 V Li0.98Fe0.02NiPO4/C Scan range [0-4.2 V]

Li4Ti5O12/LiPF6/EC + DEC No oxidation peak, red.3.5 V

LiNiPO4 Scan range [2.5-5.3 V] Li/LiPF6/PC+DMC


Ruffo et al (2005)

150

LiNiPO4 Scan range [2.8-4.8 V] Li/LiPF6/EC:PC

Red. 3.82V, Oxi. 3.4 V

Gangulibabu et al (2009)

Layer of LiNiPO4 on the graphite

foams

Scan range [3.5-5.4 V] Li/Li-FAP/EC+DMC Red. 4.9 V, Oxi. 5.2 V

Discharge capacity 86 mAh g-1 Fade capacity of 8 mAh g-1 at 10th

cycle

Dimesso et al (2012)

LiNiPO4 Scan range [0.2 V-0.4 V] Hg/HgO/Zn/LiOH

Two anodic peaks (-95 mV, -92 mV) Two cathodic peaks (-282 mV, -272

mV)

Kandhasamy et al (2012)

LiNiPO4 Scan range [2.5-5.3 V] Li/LiNF1,2butanesulfonate/PC+DME

Oxi. 4.3 V, red. 3.3 V

Present work

LiNiPO4: 0.05 mol% La doped

Scan range [2.5-5.3 V] Li/LiNF1,

2butanesulfonate/PC+DME Oxi. 4.5 V, red. 3.8 V

Present work

LiNiPO4: 0.03 mol% Nd doped

Scan range [2.5-5.3 V] Li/LiNF1,

2butanesulfonate/PC+DME Oxi. 4.9 V, shoulder peak at 5.07 V,

red. 3.74V

Present work

LiNi0.85Zn0.15PO4 Scan range [2.5-5.3 V] Li/LiNF 1,2-

butanesulfonate/PC+DME Oxi. 4.4 V, red. 3.46V

Present work

LiNi0.90Mn0.10PO4 Scan range [2.5-5.3 V] Li/LiNF1, 2

butanesulfonate/PC+DME Oxi. 4.19 V, red. 3.31V

Present work

151

Figure 6.6: Cyclic voltametry curves of pure LiNiPO4

Table 6.3:Cyclic voltametry peak analysis of pure LiNiPO4

Cycle number Anodic peak (V)

Cathodic peak (V)

Reversibility %

1 No peak No peak ---- 2 4.39 3.41 77 3 4.39 3.37 76 4 4.29 3.29 76 5 4.55 3.46 76

Figures 6.7 and 6.8 show the cyclic voltammograms for 0.05 mol%

La doped and 0.03 mol% Nd doped LiNiPO4 cathodes vs Li+respectively.

The oxidation and reduction potentials for the doped samples are provided

in the tables 6.4 and 6.5.

152

Figure 6.7: Cyclic voltametry curves of 0.05 mol% La3+ doped LiNiPO4

Figure 6.8: Cyclic voltametry curves of 0.03 mol% Nd3+ doped LiNiPO4

153

Table 6.4: Cyclic voltametry peak analysis of 0.05 mol% La doped LiNiPO4


Cathodic peak (V)

Reversibility %

1 4.34 3.86, 3.42 89 2 4.34 3.82, 3.46 88 3 4.36 3.78, 3.45 87 4 4.51 3.75, 3.46 83 5 4.52 3.75, 3.46 84 6 4.51 3.75, 3.46 83

Table 6.5: Cyclic voltametry peak analysis of 0.03 mol% Nd doped LiNiPO4


Cathodic peak (V)

Reversibility %

1 4.76 3.86 81 2 4.26 3.82 89 3 4.27 3.78 88 4 4.5 3.75 83 5 4.5 3.46 76

It is noted from the table 6.4 that the 0.05 mol% La doped sample

shows the redox couples in the range of 4.3-4.5 V (anodic potential) and

3.4 & 3.7 V (cathodic potential). This indicates that the insertion/extraction

process of Li-ions through Ni2+/Ni3+ redox reactions become more

reversible with cycling [Xingde Xiang et al (2013)]. The CV response also

infers that the voltage difference between the Li+ intercalation-de-

intercalation is less (~0.6 V), thus accounting for the high degree of

reversibility in Li+ intercalation-deintercalation process [Kalyani et al

(2005)]. This result is compared with the XRD analysis (discussed in

chapter 3) in which the higher lattice volume is observed for La3+ doped

LiNiPO4 thanother samples. The higher lattice volume supports the easier

154

intercalation/ de-intercalation of Lithium ions in LiNiPO4. Similar behavior

is also observed for Nd doped LiNiPO4 in which the cathodic peak

observed in the range of 3.6-3.7 V and the anodic peak observed at ~4.3 V.

The small shoulder peaks at 5.07 V and 5.2 V during fourth and fifth

cycles. The two peaks in the reduction process are observed for both La3+

and Nd3+ doped samples. This indicates the two step reduction process.

[Julien et al (2013)].

Figure 6.9 & 6.10 shows the CV curves for LiNi0.85Zn0.15PO4 and

LiNi0.90Mn0.10PO4 in the scan range of 2.5 V-5.3 V at 0.5mV/sec.

Figure 6.9: Cyclic voltametry curves of LiNi0.85Zn0.15PO4

In the CV spectra, there is no distinct anodic and cathodic peak

during 1st cycle. Further cycling causes well defined redox peaks in the CV

features which are tabulated in table 6.6 and 6.7. Presence of both anodic

and cathodic peak reveals that the easier insertion/extraction of Li-ions in

the LiNiPO4. From the CV analysis, the degree of reversibility is found to

155

be higher for rare earth doped system compared with pure and transition

metal doped LiNiPO4.

Figure 6.10: Cyclic voltametry curves of LiNi0.80Mn0.10PO4

Table 6.6: Cyclic voltametry peak analysis of LiNi0.85Zn0.15PO4


Cathodic peak (V)

Reversibility %

1 No peak 3.5 ---- 2 No peak 3.64 ---- 3 4.01 3.64 90 4 4.47 3.46 77 5 4.55 3.46 76 6 4.4 3.71 84

156

Table 6.7: Cyclic voltametry peak analysis of LiNi0.90Mn0.10PO4


Cathodic peak (V)

Reversibility %

1 No peak No peak ---- 2 4.19 3.31 78 3 4.19 3.30 78 4 4..27 3.07 72 5 4.34 2.85 66

6.4. GALVANOSTATIC CYCLING TESTS

Figure 6.11 shows the galvanostatic charge-discharge profiles for

pure LiNiPO4 at low current rate (C/20) for 10 cycles. The observed voltage

profile of the LiNiPO4 electrode evolves around the 4.6 V vs Li+/Li in

charged state with a depressed trend in discharged state. Under such a low

rate condition, pure LiNiPO4 material demonstrates very low capacity. The

first delivered capacity, i.e. 1.1 mA.h g-1 is far from the theoretical value of

167 mA h/g even if at a low current rate of C/20. No plateaus above 4.6 V

are observed for LiNiPO4. Similar results are also observed by wolfenstine

et al who observed the capacity of about <5mA h g-1 [Wolfenstine et al

(2004)]. Surprisingly, LiNiPO4 does not charge above 4.6 V even low

current at C/20 rate. The observed capacity is found to be nearly zero when

it is charged up to 5.1V at 1C rate. So the low discharge rate is selected in

the present study. Moreover, graphite carbon foams coated LiNiPO4

composites shows the discharge capacity of about nearly 120 mA.h g-1

[Dimesso et al (2012), (2013)]. Minakshi et al also observes the discharge

capacity of LiNiPO4 using aqueous electrolyte and it is found to be 50 mA.

h g-1 [Minakshi et al (2011)]. It is reported that the doping of isovalent and

supervalent cations with olivine cathode will improve the capacity, Li-ion

diffusion and rate capability [Wolfenstine et al (2004), kandhasamy et al

157

(2012), Luo Shaohua et al (2010), Jiezi Hu et al (2009)]. The following

paragraph discusses the electrochemical performance of doped LiNiPO4.

Figure 6.11: Charge-discharge curves of pristine LiNiPO4 measured at C/20.

Figure 6.12 & 6.13 shows the charge and discharge curves of 0.05

mol% La doped and 0.03 mol% Nd doped LiNiPO4measured at C/20 in the

voltage range of 2.8 V - 4.6 V vs. Li+ up to 10 cycles. In all cycles, voltage

plateaus close to 4.6 V are observed while charging and 3.5 V during

discharging. This indicates that the Li de-insertion/insertion process in the

LiNiPO4 and this result agrees with the CV data. Rare earth doped LiNiPO4

cathode materials show a substantial increase of discharge capacity at C/20

rate, especially for the La doped sample, which shows the capacity of about

30 mA.h g-1 than pure LiNiPO4 at C/20 rate, support the results of

conductivity discussed in chapter 5 and EIS.

158

Figure 6.12: Charge-discharge curves of 0.05 mol% La doped LiNiPO4 measured at C/20 rate.

Figure 6.13: Charge-discharge curves of 0.03 mol% Nd doped LiNiPO4 measured at C/20.

159

Due to the stable valence structure of La3+, doping of La3+ causes to

change the band structure of the semiconducting LiNiPO4 more evidently,

grants more high-mobile carriers in crystal and improve the intrinsic

conductivity of LiNiPO4. Moreover, doping of Nd3+ leads to the discharge

capacity of only 6 mA.h g-1. The low performance of Nd3+ substituted phase

may be due to the lattice contraction, which inhibits lithium ion diffusion in

the lattice [Kishore et al (2005)].

Figure 6.14 & 6.15 shows the charge-discharge curves for

LiNi0.85Zn0.15PO4 and LiNi0.90Mn0.10PO4 measured at C/20 in the voltage

range of 2.8 V-4.6 V vs. Li up to 10 cycles.

Figure 6.14: Charge-discharge curves of LiNi0.85Zn0.15PO4 measured at C/20.

The discharge capacity is found to be 4 mA.h g-1 and 2.5 mA.h g-1

for Zn2+and Mn2+ doped LiNiPO4 respectively. Similar result is also

observed by wolfenstine et al who observe the capacity of about < 5 mA.h

g-1 for 20% cobalt doped LiNiPO4. The disappointing capacity around 4

160

mA.h g-1 and 2 mA.h g-1 of the transition metal doped LiNiPO4 mainly

results from low intrinsic electronic conductivity and sluggish kinetics of

Li-ion transport [Xiao et al (2010), Xiao et al (2011), Wang et al (2011)].

Figure 6.15: Charge-discharge curves of LiNi0.9Mn0.1PO4 measured at C/20.

The above result thus clearly reveals the effects of doping helps to

some extent on improved electrochemical performance. Obviously, the

capacity loss in LiNiPO4 is a more complicated nature for both pure and

doped LiNiPO4. However, it is reported that the graphite carbon foams

coated LiNiPO4 or carbon coated LiNiPO4 exhibits better cyclic stability,

electronic conductivity and clear voltage plateaus [Dimesso et al (2013),

(2012), Wolfenstine et al (2004)]. Therefore, more efforts on exploring

experimental conditions to synthesize the carbon coated LiNiPO4 particles

with further reduced size by this method (in argon atmosphere) are still

needed in future.

161

6.5. SUMMARY

Pristine, 0.05 mol% La doped, 0.03 mol% Nd, LiNi0.85Zn0.15PO4 and

LiNi0.9Mn0.1PO4 samples are subjected into electrochemical

characterizations such as electrochemical impedance, cyclic voltametry and

galvanostatic charge-discharge studies. Electrochemical impedance

spectroscopy reveals that the presence of sluggish kinetic behavior in the

pure and transition metal doped LiNiPO4. The charge transfer resistance

value for pure LiNiPO4 is 1302Ω. Further, this values ( for fresh cell) of

0.05 mol% La doped, 0.03 mol% Nd doped, LiNi0.85Zn0.15PO4 and

LiNi0.90Mn0.10PO4 are found to be 45Ω, 266Ω, 57Ω and 1212Ω respectively.

The charge transfer resistance value gets decreased upon doping of both

rare earth and transition metal atoms. It is also evident that the charge-

transfer resistance is low value (for fresh cell) for 0.05 mol% La doped

LiNiPO4 compared with other samples. After cycling the Rct values are

increased. Especially for pure sample, Rct is increased suddenly even after

4th cycle. The abrupt increase of charge transfer resistance of the LiNiPO4

is considered as the major reason for the capacity fading which is reflected

in charge-discharge studies. From the CV analysis, 0.05 mol% La doped

sample shows the redox couples in the range of and 4.3-4.5 V (anodic

potential) 3.4-3.8 V (cathodic potential). This indicates that the

insertion/extraction process of Li-ions through Ni2+/Ni3+ redox reactions

become more irreversible with cycling. Similar results are also obtained for

pure, Nd, Zn and Mn doped samples. These results are deviated from the

literature in which the redox potential of LiNiPO4 is 5.1 V vs Li. The first

delivered capacity, i.e. 1.1 mA.h g-1 is far from the theoretical value of 167

mA h/g even if at a low current rate of C/20 for pure LiNiPO4.Rare earth

doped LiNiPO4 cathode materials show a substantial increase of discharge

capacity at 0.05C rate, especially for the La doped sample, which shows the

162

capacity of over 30 mA.h g-1. The discharge capacity of 0.03 mol% Nd

doped LiNiPO4, Zn2+ and Mn2+ doped LiNiPO4 is found to be 6 mA.h g-1

and 4mA.h g-1 and 2.5 mA.h g-1 respectively. The disappointing discharge

capacity of pure, Nd3+, Zn2+ and Mn2+ doped LiNiPO4 corresponds to the

low intrinsic electronic conductivity and sluggish kinetics of Li-ion

transport.

163

CHAPTER 7

CONCLUSION

7.1. SUMMARY OF THE WORK DONE

The present research work reveals the investigation of rare earth and

transition metal doped LiNiPO4 for high voltage lithium ion battery

application. The synthesis conditions are optimized and the structural,

morphological, thermal, vibrational, impedance analysis and

electrochemical performance of the cathode materials are analyzed. The

important conclusion drawn from the present work is given as follows

The structural and morphological characterizations of olivine type

pure, rare earth doped and transition metal doped LiNiPO4 samples

are analyzed by X-ray diffraction, particle size distribution and SEM

analysis.

XRD analysis show that the doping of La3+ (up to 0.05 mol %) and

Nd3+ (up to0.03 mol %) is more favorable because it does not

collapse the lattice structure of LiNiPO4. Insertion of transition

metals also prevents the lattice structure but slightly affects the

lattice parameters within the concentration range studied.

Among all the doped samples La doped LiNiPO4get a maximum of

40% of nano sized particles among all undoped and doped samples

by polyol synthesis and nano particles correspond to shorter

diffusive distance favorable for Li-ions.

164

SEM analysis reveals that both undoped and doped samples exhibit

uniform morphology with sphere like particles. This kind of

morphology is more significant to achieve high capacity and good

cycleability.

Thermal properties of as prepared samples are characterized by

TGA. The weight loss of the precursor is observed up to 600°C then

there is no reduction in the weight. So, the calcination temperature is

fixed at 650°C irrespective of the samples and the result is

consistent with the XRD analysis which shows the formation of pure

LiNiPO4 occurs at 650°C.

FTIR analysis confirms that the dopant exactly occupy at Ni site of

the corresponding host lattice. Moreover, the dopants do not destruct

the structure of LiNiPO4 and this result is consistent with the XRD

results in which the lattice parameters are slightly affected upon

doping of rare earth atoms and do not destruct the lattice structure of

LiNiPO4.

Rare-earth ions and transition metal ions are occupying Ni site upon

doping with LiNiPO4which is also proved by Laser Raman analysis.

Emergence of new peak in the case of RE doped systems denotes the

removal of lithium ion from the lattice to maintain charge neutrality.

The sintering temperature of LiNiPO4 is optimized at 650°C; 5h with

the help of activation energy calculations. The calculated activation

energies are found as 1.02 eV, 0.54 eV and 0.97 eV for the pellet

sintered at 550°C, 650°C and 750°C respectively.

The electrical conductivity of LiNiPO4 is calculated as 4.36×10-6 at

300°C which is higher than the previously reported values.

Moreover, doping of both rare earth (La3+ and Nd3+) and transition

metal atoms (Zn2+& Mn2+) improve the electrical conductivity of

LiNiPO4. Among all dopants, 0.05 mol% La3+ doped LiNiPO4 shows

165

the maximum conductivity by the factor of two order, an increase

from <10-10 to 10-7 S/cm at ambient temperature

The enhancement of the conductivity of LiNiPO4 upon doping can

be explained as follows; substitution of rare earth or transition metal

produce structural defects (vacancies) in the lattice site which creates

the pathway wider that facilitates Li-ion mobility and the dopant can

reduce the interaction force between Li+ and Li+. This result is

consistent with the laser Raman analysis (as discussed in chapter 4),

in which the presence of new vibrational band at 966 cm-1 represents

the delithiation of lithium from LiNiPO4. Due to the formation of

these lithium ion vacancies, conductivity of LiNiPO4 is enhanced.

Moreover, the intensity of the vibrational peak positioned at 966 cm-

1 is lesser for Nd3+ doped sample compared with La3+ doped system.

It may be due to the presence of impurity peak (evidenced from the

XRD analysis) in the Nd3+ doped samples which may mask the

production of Li-ion vacancies hence the electrical conductivity of

Nd3+ doped sample is lesser than La3+ doped sample. Moreover, the

higher ionic radius of La3+ provides more path way to the mobility of

Li ions which support higher electrical conductivity. The higher unit

cell volume is observed for La3+ doped sample than Nd3+ doped

system. This increment in the lattice volume grants easier

intercalation/de-intercalation of Li-ions which is inferred from XRD

analysis.The magnitude of dielectric constant value is increased

upon doping of both rare earths and transition metals because of

increase of number of charge carrier concentration. The dielectric

loss spectra confirm the dc conduction mechanism is predominant

for all samples. Modulus formalism ensures that the non-Debye type

relaxation present in all prepared samples.

166

Pristine, 0.05 mol% La doped, 0.03 mol% Nd, LiNi0.85Zn0.15PO4 and

LiNi0.9Mn0.1PO4 samples are subjected into electrochemical

characterizations such as electrochemical impedance spectroscopy,

cyclic voltametry and galvanostatic charge-discharge studies.

Swagelok cell is used to test the electrochemistry of the cathodes.

The charge transfer resistance value gets decreased upon doping of

both rare earth and transition metal atoms. The charge-transfer

resistance (Rct) is low (for fresh cell) for 0.05 mol% La doped

LiNiPO4 compared with other samples.

After cycling, Rct values get increased. Especially for pure sample, it

is increased suddenly from 1302Ω to 4358Ω even after 4th cycle. The

abrupt increase of charge transfer resistance of the LiNiPO4 is

considered as the main reason for capacity fading.

Cyclic voltametry studies confirm that 0.05 mol% La doped sample

shows the redox couple in the range of and 4.3 - 4.5 V (anodic

potential) 3.4 - 3.8 V (cathodic potential). This indicates that the

insertion/extraction process of Li-ions through Ni2+/Ni3+ redox

reactions become reversible with cycling. Similar results are also

observed for undoped, Nd3+, Zn2+ and Mn2+ doped samples. The

higher unit cell volume supports the easier intercalation/ de-

intercalation of Lithium ions in LiNiPO4.

The first delivered capacity, i.e. 1.1 mA.h g-1 is far from the

theoretical value of 167 mA h g-1 even if at a low current rate of

C/20 for undoped LiNiPO4. Rare earth doped LiNiPO4 cathode

materials show a substantial increase of discharge capacity at C/20

rate, especially for the La doped sample, which shows the capacity

of 30 mA.h g-1 than undoped LiNiPO4. The discharge capacity of

0.03 mol% Nd doped LiNiPO4, LiNi0.85Zn0.15PO4 and

LiNi0.90Mn0.10PO4 doped LiNiPO4 is found as 6 mA.h g-1, 4mA.h g-1

167

and 2.5 mA.h g-1 respectively. The disappointing discharge capacity

of pure, 0.03 mol% Nd3+, LiNi0.85Zn0.15PO4 and LiNi0.90Mn0.10PO4

doped LiNiPO4 corresponds to the low intrinsic electrical

conductivity and sluggish kinetics of Li-ion transport.

7.2. FUTURE PLAN

The above result clearly reveals the effects of doping helps some

extent to improve the electrochemical performance. Obviously, the capacity

loss in LiNiPO4 is a more complicated nature for both pure and doped

LiNiPO4. However, it is reported that the graphite carbon foams coated

LiNiPO4 or carbon coated LiNiPO4 exhibits better cyclic stability,

electronic conductivity and clear voltage plateaus. Therefore, more efforts

on exploring experimental conditions to synthesize the carbon coated

LiNiPO4 particles with further reduced particle size by polyol method (in

argon atmosphere) are still needed in future.

168

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LIST OF PUBLICATIONS

International journals:

1. Structural and conductivity studies on LiNiPO4 synthesized by the polyol method

Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, R.S.Daries Bella, Journal of Alloys and compounds, 548 (2013) 65-69.

2. Structural and electrical studies on Zn2+ doped LiCoPO4, S.Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, R.S.Daries Bella, Journal of Electrostatics, 72 (2014) 181-186.

3. Structural, Morphological, Vibrational and Electrical studies on Zn doped nanocrystalline LiNiPO4 S.Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, R.S.Daries Bella, Materials science forum, 781 (2014) 145-153.

4. Structural, Thermal and Electrical conduction studies on LiNiPO4:RE (RE= La, Nd) prepared by polyol method S.Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, R.S.Daries Bella, Journal of New materials for Electrochemical systems 17 (2014) 159-166.

5. Influence of metals on the structural, vibrational and electrical properties of Lithium Nickel Phosphate, S.Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, R.S.Daries Bella,Ionics, Doi: 10.1007/s11581-014-1192-2.

6. Conductivity studies on PMMA-Methanesulfonic acid based proton conducting polymer electrolytes, C.Ambika,G.Hirankumar, S.Karthickprabhu, R.S.Daries Bella, International journal of ChemTech Research, Vol.6, No.3, pp.1690-1692.

197

International conferences:

1. Structural and Conductivity Studies on lanthanum doped LiNiPO4 prepared by Polyol method, S.Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, R.S.Daries Bella, Paper published in proceedings of the 13th Asian Conference on Solid State Ionics to be held at Tohoku University, Sendai, Japan during

2. Effect of ZnO on the conductivity of LiCoPO4S.Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, R.S.Daries Bella, presented Oral presentation on 6th Asian Conference on Elecrochemical Power Sources to be held at Hotel Green park, Chennai during Jan 5-8, 2012.

3. Effect of La doping on the structural and conductivity of LiNiPO4S.Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, R.S.Daries Bella, presented Poster presentation on ICRAM 2012 to be held at VIT, Vellore during Feb 20-22, 2012.

National Conferences:

1. Influence of ZrO2 on the conductivity of LiCoPO4S.Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, R.S.Daries Bella, presented Oral presentation on NCAFMA 2011 to be held on Kalasalingam University, Krishnankovil during Dec 16-17, 2011.

2. Impedance Studies on Al3+ doped LiCoPO4S.Karthickprabhu, G.Hirankumar, A.Maheswaran, C.Sanjeeviraja, Paper published in proceedings of the III National conference on Advanced Materials to be held at PSN College of Engineering and Technology, Tirunelveli during Jan 23-25, 2013. ISBN: 978-93-82062-86-8.

3. Proton conducting polymer complexes (PVP-NH2SO3NH4) : AC Impedance Spectroscopy study R.S.Daries Bella, S.Asathbahadur, S.Karthickprabhu, A.Maheswaran, G.Hirankumar Paper published in proceedings of

198

the III National conference on Advanced Materials to be held at PSN College of Engineering and Technology, Tirunelveli during Jan 23-25, 2013. ISBN: 978-93-82062-86-8.

4. Vibrational and electrical conductivity studies on Zn2+ and Al3+ doped LiCoPO4, S. Karthickprabhu, G. Hirankumar, A. Maheswaren. Paper published in proceedings of the National seminar on Technologically Important Crysatlline and Amorphous Solids to be held at Kalasalingam University, Krishnankovil during 28th Feb-1st March, 2014.ISBN: 978-81-921319—4-8.

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CURRICULUM VITAE

PERSONAL DETAILS

KARTHICKPRABHU S 1/76, North Street, Athipatti, Periayur (Tk), Madurai (Dt). Mobile: 07708284384 [email protected]

Father’s Name: Sivabalan S Mother’s Name: Manickam S DoB: 12.02.1988 Nationality : Indian Blood Group: B+ve

RESEARCH EXPERIENCE

Department of Physics, Smart Materials Laboratory, Thiagarajar College of Engineering: India July 2010-Oct 2010 (Project Fellow) Department of Physics, Energy Materials Research Laboratory, Kalasalingam University: India, Oct’10 – Mar’13 (JRF and SRF)

EDUCATIONAL BACKGROUND

Doctor of Phoiloshopy: (Physics) Thesis submitted at Kalasalingam University, Krishnankovil. Master of Science (M.Sc) : (Physics with Gold medal) from 2008-2010 with 81% at Ayya Nadar Janaki Ammal College, Sivakasi. Bachelor of Science (B.Sc): (Physics with Gold medal) from 2005-2008 with 87% at Ayya Nadar Janaki Ammal College, Sivakasi. HSC with 88%, Rm.P.S.Ramiah Nadar Higher Secondary School, Athipatti, Tamilnadu. SSLC with 94%, Rm.P.S.Ramiah Nadar Higher Secondary School, Athipatti, Tamilnadu.

RESEARCH PUBLICATION

International Journals 6

International Conference 3

National Conference 4

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