Download - DEVELOPMENT AND CHARACTERIZATION OF LiMn2O BASED …prr.hec.gov.pk/jspui/bitstream/123456789/2625/1/2695S.pdf · 2.3.2.6 CV 45 2.3.2.7 EIS 46 2.3.2.8 Charge/ discharge measurements

DEVELOPMENT AND CHARACTERIZATION OF

LiMn2O4 BASED CATHODE MATERIALS FOR

LITHIUM-ION BATTERIES

By

AZHAR IQBAL

INSTITUTE OF CHEMICAL SCIENCES

UNIVERSITY OF PESHAWAR

PAKISTAN

2013

DEVELOPMENT AND CHARACTERIZATION OF

LiMn2O4 BASED CATHODE MATERIALS FOR

LITHIUM-ION BATTERIES

By

AZHAR IQBAL

A Dissertation Submitted to the University of Peshawar in Partial

Fulfillment of the Requirements for the Degree of

Doctor of Philosophy in Chemistry



PAKISTAN

2013



It is recommended that the dissertation prepared by Mr. Azhar Iqbal entitled “Development

and Characterization of LiMn2O4 Based Cathode Materials for Lithium-Ion Batteries” be

accepted as fulfilling this part of the requirements for the degree of

“DOCTOR OF PHILOSOPHY IN CHEMISTRY”

SUPERVISOR Co-SUPERVISOR

Prof. Dr. Yousaf Iqbal Dr. Safeer Ahmed

Institute of Chemical Sciences Department of Chemistry

University of Peshawar Quaid-i-Azam University

Pakistan Islamabad, Pakistan

EXAMINER DIRECTOR

Prof. Dr. Imdad ullah

Institute of Chemical Sciences,

University of Peshawar

Pakistan

Dedicated

To

My Parents

Table of Contents

S. No Title Page No.

Acknowledgement i

Abstract ii

List of Tables iv

List of Figures v-xii

1 Chapter 1: Introduction 1-29

1.1 General Overview 1

1.2 Lithium-ion battery 3

1.3 Working principle and components of lithium-ion battery 6

1.4 What is a spinel LiMn2O4? 9

1.5 Jahn-Teller distortion effect in spinel LiMn2O4 12

1.6 Literature Review 14

1.7 Objectives of the present work 28

References 30-37

2 Chapter 2: Experimental 38-46

2.1 Chemicals used 38

2.2 Instrumentation 39

2.3 Procedure 40

2.3.1 Synthesis of samples 40

2.3.1.1 Sol-gel method 40

2.3.1.2 Procedure 40

2.3.1.3 Optimization of pH and synthesis temperature 42

2.3.1.4 Coin cells fabrication 44

2.3.2 Analysis protocol 45

2.3.2.1 TGA/ DTA 45

2.3.2.2 XRD 45

2.3.2.3 FTIR 45

2.3.2.4 SEM, TEM and EDX analysis 45

2.3.2.5 ICP-OES 45

2.3.2.6 CV 45

2.3.2.7 EIS 46

2.3.2.8 Charge/ discharge measurements 46

References

46

3 Chapter 3: Results and Discussion 47-123

3.1 Structural and morphological properties 47

3.1.1 Thermal analysis 48

3.1.2 X-ray diffraction studies 51

3.1.3 Chemical composition analysis 60

3.1.4 FTIR spectroscopic studies 62

3.1.5 SEM, TEM and EDX analysis 67

3.2 Electrochemical characterization 86

3.2.1 Cyclic voltammetric studies 86

3.2.2 Electrochemical impedance spectroscopy (EIS) 95

3.3 Charge/ discharge performance 100

3.3.1 Galvanostatic charge/ discharge studies 100

3.3.2 Cycling performance 101

3.3.3 Rate capability 117

Conclusions 124

References 127-130

List of Publications 131

i

ACKNOWLEDGEMENTS

All praise for Almighty Allah, who enabled me to complete this research work. I would

like to express my sincerest gratitude to my supervisor, Prof. Dr. Yousaf Iqbal for his advice

and encouragement throughout this research work. I feel great pleasure in expressing my

deepest appreciation to my co-supervisor Dr. Safeer Ahmed, who guided me in every step of this

research project. Prof. Dr. Imdad ullah, Director, Institute of Chemical Sciences, University of

Peshawar deserves my gratitude making available the necessary research facilities. Higher

Education Commission (HEC) Pakistan is highly acknowledged for providing financial support

through Indigenous scholarship scheme. I have no words to offer thanks to the chairman

department of chemistry, Quaid-i-Azam University for allowing me to carry out part of my

research work at Electrochemistry Lab. I am highly thankful to Prof. Zhiyong Tang and Prof.

Yan Gao, NCNST, Beijing, China, for providing experimental facilities and conducive research

environment. I am also thankful to my friends, Shakiaz Ahmed, Abdul Muqsit, Misbah ullah,

Wajid ur Rehman, Muhammad Hanif, for their prayers and moral supports.

Finally, all this is the fruit of the prayers, encouragement and moral as well as financial

support of my respected parents and lovely brothers. I have no words to explain the sacrifices

and support of my elder brother, Anwar Khan. My deepest gratitude goes to my family for

their encouragement and moral support. Moral support and good wishes from all my colleagues

and friends are highly acknowledged.

Azhar Iqbal

ii

Abstract

Spinel LiMn2O4 is one of the most attractive positive electrode materials for Li-ion

rechargeable batteries. In the present study, six series of low content bi-metal doped LiMn2O4

with nominal compositions of LiNixCryMn2-x-yO4, LiLaxZnyMn2-x-yO4, LiCuxCryMn2-x-yO4,

LiCuxZnyMn2-x-yO4, LiNixCuyMn2-x-yO4 and LiNixZnyMn2-x-yO4 (where x = y = 0.01-0.05) were

prepared by the sol-gel method. Thermogravimetric analysis (TGA) and differential thermal

analysis (DTA) confirmed the formation of the pure as well as the doped spinel LiMn2O4

between 285 oC and 350

oC. However, well crystallized spinel phase verified from the X-ray

diffraction studies was obtained at 750 oC. XRD measurements further confirmed that all the

synthesized compounds crystallized as single phase products in the cubic spinel Fd3m space

group. The results showed that doping LiMn2O4 with such small amount of metals has not

affected the original spinel structure. Inductively coupled plasma optical emission spectrometry

(ICP-OES) findings agreed the used nominal compositions. Energy dispersive X-ray analysis

(EDX) also confirmed the purity of all the synthesized samples. SEM and TEM images showed

that unlike the pure LiMn2O4, all the doped samples exhibited uniform size with smooth faceted

polyhedral particles. The average particle size ranges from about 42 nm to 250 nm. High

resolution TEM images also demonstrated the highly crystalline nature of all the six doped

series.

Cyclic voltammetric studies indicated that all the synthesized samples showed two pairs

of well-defined anodic and cathodic peaks at around 4.0 V that corresponded to the redox couple

of Mn3+

/ Mn4+

. However, for the doped samples, the oxidation and reduction peaks were much

closer to each other. The peak current was increased and the peak width was narrowed,

indicating the reduced polarization of the bi-metal doped LiMn2O4, resulting from the faster

iii

insertion/ extraction of Li+ ions into the spinel matrix. Electrochemical impedance spectroscopy

(EIS) was employed to have an insight about the synergetic effect of the bi-metal doping on the

electrochemical performance of spinel LiMn2O4. The Nyquist plots showed that the charge

transfer resistance (Rct) decreases upon doping with Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-

Zn. The observed faster kinetics of Li+

ions is attributed to the enhanced conductivities of the

doped samples. Galvanostatic charge/ discharge measurements performed between 3.0 and 4.8 V

for all the samples showed two plateaus around 4.0 V and 4.1 V vs. Li/ Li+ that clearly

demonstrated that insertion/ extraction of Li+ ions takes place in two steps. The improved cycling

performance of all the doped samples over the investigated 100 charge/ discharge cycles

indicated that low content bi-metal doping has stabilized the spinel LiMn2O4 structure by

suppressing Jahn- Teller distortion.

Rate capability of the pure and doped samples was also evaluated. The cells for each

material were charged to 4.8 V at constant low current rate (0.1 C) and discharged to 3.0 V at 0.1

C, 0.3 C, 0.5 C, 1 C, 2 C, and 5 C, respectively. Compared to the pure LiMn2O4 which retained

only 41% of the initial discharge capacity when cycled at high current rate of 5 C, the capacity

retention at 5 C for the Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn doped samples (x = y =

0.01) was 82%, 78%, 81%, 67%, 62%, and 58%, respectively. Among the various synthesized

bi-metal doped series, samples with the lowest doping metal contents LiM0.01M'0.01Mn1.98O4

(where M and M' are the various doping metal cations used in this study) appeared to be the best

composition both in terms of the initial discharge capacity as well as the rate capability.

iv

List of Tables

Table 2.1 Chemicals used with their essential specifications. 38

Table 3.1 Lattice parameter and unit cell volume for the pure and 52

Ni-Cr doped samples.


La-Zn doped samples.


Cu-Cr doped samples.


Cu-Zn doped samples.


Ni-Cu doped samples.


Ni-Zn doped samples.

Table 3.7 Chemical compositions of the pure and Ni-Cr and 60

La-Zn doped series.

Table 3.8 Chemical compositions of Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn series. 61

Table 3.9 Electrochemical performance of the pure and Ni-Cr doped LiMn2O4. 113

Table 3.10 Electrochemical performance of the pure and La-Zn doped LiMn2O4. 114

Table 3.11 Electrochemical performance of the pure and Cu-Cr doped LiMn2O4. 114

Table 3.12 Electrochemical performance of the pure and Cu-Zn doped LiMn2O4. 115

Table 3.13 Electrochemical performance of the pure and Ni-Cu doped LiMn2O4. 115

Table 3.14 Electrochemical performance of the pure and Ni-Zn doped LiMn2O4. 116

v

List of Figures

Fig.1.1 Comparison of energy densities (per unit weight and per unit volume) 4

of various rechargeable batteries.

Fig.1.2 A schematic representation of the working principle of 6

Lithium-ion battery.

Fig.1.3 Part of the unit cell of spinel LiMn2O4 representing the local structure 10

of octahedrally coordinated manganese in an ideal spinel frame work.

Fig.1.4 (a) Splitting of the atomic orbitals for Mn cation at octahedral site 13

(b) splitting of the eg orbital.

Fig. 2.1 Flow chart for the synthesis of the pure and doped LiMn2O4 41

by sol-gel method.

Fig.2.2 XRD patterns of LiMn2O4 synthesized at different temperature: 42

(a) 600 oC (b) 700

oC (c) 750

oC (d) 800

oC, (* = Mn2O3).

Fig.2.3 XRD patterns of LiMn2O4 synthesized at different pH: (a) 6.0 43

(b) 6.5 (c) 7.0 (d) 7.5 (e) 8.0.

Fig. 3.1 TGA/ DTA analysis of the pure LiMn2O4 sample. 48

Fig. 3.2 TGA/ DTA of (a) LiNi0.03Cr0.03Mn1.94O4 (b) LiLa0.03Zn0.03Mn1.94O4 50

(c) LiCu0.03Cr0.03Mn1.94O4 (d) LiCu0.03Zn0.03Mn1.94O4

(e) LiNi0.03Cu0.03Mn1.94O4 (f) LiNi0.03Zn0.03Mn1.94O4.

Fig.3.3 XRD patterns of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 51

(c) LiNi0.02Cr0.02Mn1.96O4 (d) LiNi0.03Cr0.03Mn1.94O4

(e) LiNi0.04Cr0.04Mn1.92O4 (f) LiNi0.05Cr0.05Mn1.90O4.

Fig.3.4 XRD patterns of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 55

(c) LiLa0.02Zn0.02Mn1.96O4 (d) LiLa0.03Zn0.03Mn1.94O4

(e) LiLa0.04Zn0.04Mn1.92O4 (f) LiLa0.05Zn0.05Mn1.90O4.

vi

Fig.3.5 XRD patterns of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 56

(c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4

(e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4.

Fig.3.6 XRD patterns of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 57

(c) LiCu0.02Zn0.02Mn1.96O4 (d) LiCu0.03Zn0.03Mn1.94O4

(e) LiCu0.04Zn0.04Mn1.92O4 (f) LiCu0.05Zn0.05Mn1.90O4.

Fig.3.7 XRD patterns of (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 58

(c) LiNi0.02Cu0.02Mn1.96O4 (d) LiNi0.03Cu0.03Mn1.94O4

(e) LiNi0.04Cu0.04Mn1.92O4 (f) LiNi0.05Cu0.05Mn1.90O4.

Fig.3.8 XRD patterns of (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 59

(c) LiNi0.02Zn0.02Mn1.96O4 (d) LiNi0.03Zn0.03Mn1.94O4

(e) LiNi0.04Zn0.04Mn1.92O4 (f) LiNi0.05Zn0.05Mn1.90O4.

Fig.3.9 FTIR spectra of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 63

(c) LiNi0.02Cr0.02Mn1.96 O4 (d) LiNi0.03Cr0.03Mn1.94O4


Fig.3.10 FTIR spectra of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 64



Fig.3.11 FTIR spectra of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 65

(c) LiCu0.02Cr0.02Mn1.96O4 (d) Li Cu0.03Cr0.03Mn1.94O4

(e) LiCu0.04Cr0.04Mn1.92O4 (f) Li Cu0.05Cr0.05Mn1.90O4.

Fig.3.12 FTIR spectra of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 65



vii

Fig.3.13 FTIR spectra of (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 66



Fig.3.14 FTIR spectra of (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 66



Fig. 3.15 SEM images of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 69



Fig. 3.16 SEM images of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 70



Fig. 3.17 SEM images of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 71



Fig. 3.18 SEM images of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 72



Fig. 3.19 SEM images of (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 73



Fig. 3.20 SEM images of (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 74



viii

Fig. 3.21 TEM images of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 75



Fig. 3.22 TEM images of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 76



Fig. 3.23 TEM images of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 77



Fig. 3.24 TEM images of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 78



Fig. 3.25 TEM images of (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 79



Fig. 3.26 TEM images of (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 80



Fig. 3.27 EDX profiles of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 81



Fig. 3.28 EDX profiles of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 82



ix

Fig. 3.29 EDX profiles of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 83



Fig. 3.30 EDX profiles of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 84



Fig. 3.31 EDX profiles of(a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 85



Fig. 3.32 Cyclic voltammograms for (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 89


(e) LiNi0.04Cr0.04Mn1.92O4 (f) LiNi0.05Cr0.05Mn1.90O4,

at a scan rate of 0.1 mV s-1

.

Fig. 3.33 Cyclic voltammograms for (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 90


(e) LiLa0.04Zn0.04Mn1.92O4 (f) LiLa0.05Zn0.05Mn1.90O4,


.

Fig. 3.34 Cyclic voltammograms for (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 91

(c) Li Cu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4

(e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4,


.

x

Fig. 3.35 Cyclic voltammograms for (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 92


(e) LiCu0.04Zn0.04Mn1.92O4 (f) LiCu0.05Zn0.05Mn1.90O4,


.

Fig. 3.36 Cyclic voltammograms for (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 93


(e) LiNi0.04Cu0.04Mn1.92O4 (f) LiNi0.05Cu0.05Mn1.90O4,


.

Fig. 3.37 Cyclic voltammograms for (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 94


(e) LiNi0.04Zn0.04Mn1.92O4 (f) LiNi0.05Zn0.05Mn1.90O4,


.

Fig.3.38 Nyquist plots for (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 96



Fig.3.39 Nyquist plots for (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 97



Fig.3.40 Nyquist plots for (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 98


(e) LiCu0.04Cr0.04Mn1.92O4(f) LiCu0.05Cr0.05Mn1.90O4.

Fig.3.41 Nyquist plots for (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 98



xi

Fig.3.42 Nyquist plots for (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 99



Fig.3.43 Nyquist plots for (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 99



Fig. 3.44 (A) First Discharge curves (B) Capacity vs. cycle number plots for 103

(a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 (c) LiNi0.02Cr0.02Mn1.96O4

(d) LiNi0.03Cr0.03Mn1.94O4 (e) LiNi0.04Cr0.04Mn1.92O4

(f) LiNi0.05Cr0.05Mn1.90O4.


(a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 (c) LiLa0.02Zn0.02Mn1.96O4

(d) LiLa0.03Zn0.03Mn1.94O4 (e) LiLa0.04Zn0.04Mn1.92O4

(f) LiLa0.05Zn0.05Mn1.90O4.


(a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 -

(d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4

(f) LiCu0.05Cr0.05Mn1.90O4.


(a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 (c) LiCu0.02Zn0.02Mn1.96O4

(d) LiCu0.03Zn0.03Mn1.94O4 (e) LiCu0.04Zn0.04Mn1.92O4

(f) LiCu0.05Zn0.05Mn1.90O4.

xii


(a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 (c) LiNi0.02Cu0.02Mn1.96O4

(d) LiNi0.03Cu0.03Mn1.94O4 (e) LiNi0.04Cu0.04Mn1.92O4

(f) LiNi0.05Cu0.05Mn1.90O4.


(a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 (c) LiNi0.02Zn0.02Mn1.96O4

(d) LiNi0.03Zn0.03Mn1.94O4 (e) LiNi0.04Zn0.04Mn1.92O4

(f) LiNi0.05Zn0.05Mn1.90O4.

Fig.3.50 Rate capability at 0.1-5 C of the pure LiMn2O4 and 118

LiNi0.01Cr0.01Mn1.98O4.

Fig. 3.51 Rate capability at 0.1-5 C of the pure LiMn2O4 and 121

LiLa0.01Zn0.01Mn1.98O4.


LiCu0.01Cr0.01Mn1.98O4.


LiCu0.01Zn0.01Mn1.98O4.


LiNi0.01Cu0.01Mn1.98O4.


LiNi0.01Zn0.01Mn1.98O4.

1

CHAPTER 1: INTRODUCTION

1.1 General Overview

Energy storage is, and will continue to be one of the most important and central research

areas in modern world due to ever increasing demand of energy. The extensive increase in the

portable electronic devices such as laptop, cameras, mobile phones etc., is a major sector which

not only indicates the consumption of huge amount of electrical energy but also imparts the

essential of energy storage devices. The emerging applications of low or zero-emission gas

electric and hybrid electric vehicles (EV/ HEV) as well as the portable electronic devices have

boosted the demand for clean and sustainable energy [1, 2]. Renewable energies obtained from

different sources (solar or wind) may not have the real anticipated impact unless there exists an

efficient way to store and use the electricity produced by them; batteries are considered an

attractive option for this [3].

“A battery is an electrochemical device or a transducer that converts chemical energy

stored in its active materials directly into electrical energy through an electrochemical redox

reaction”[4]. There are four main components of a battery, i.e., cathode, anode, electrolyte and

separator. Batteries can be classified into two main categories namely;

1. Primary Battery: These batteries can be discharged only once and then discarded. In other

words the electrode reactions are irreversible so, they are non-rechargeable batteries. For

example, Zn/ MnO2 alkaline cell.

2. Secondary Battery: They are rechargeable i.e. if once discharged they can be recharged or

restored to their original condition by reversing the current flow through the

electrochemical cell. Thus secondary batteries can be charged and discharged for multiple

cycles [5]. Examples include reversible electrochemical reaction(s) occurring within the

2

device. The secondary or rechargeable batteries are the most important and successful

technologies that can efficiently produce electricity from stored active materials and also

capable of converting electrical energy into the chemical energy. The overall

performance of secondary batteries mainly depends upon the electrodes (anode and

cathode) active materials and on the technologies used for the integration of battery

components [6].

Following are the definitions of some of the important terms:

Capacity (Ah or mAh): The energy stored by a battery is known as its capacity. Shortly it

represents the amount of charge that a battery contains. In other words it is the time for

which a battery can provide a current at the discharge rate at nominal voltage.

Specific capacity (mAh/g): Specific capacity is the amount of charge per unit mass of a

battery. This informs the effectiveness of the battery material.

C-rate: A common way to specify charge and discharge current is the C-rate. 1 C means

complete charge or discharge of a battery relative to its capacity in one hour. Similarly 0.5 C

takes 2 hours and 0.1 C takes 10 hours.

Energy density: Total amount of useful energy (extractable for useful work) a battery can

hold often expressed as W h/kg or W h/L.

Power density (W/L or W/Kg): Power density describes the rate at which a battery can

deliver energy per unit weight or volume.

Cycleability or Cycle life: To measure the life time of a battery for which it can work is

called its cycleability.

3

1.2 Lithium-ion battery

Lithium-ion batteries are highly popular and widely used energy storage systems due to their

properties such as high energy density (210 WhKg-1

, 650 Whl-1

), high voltage, long cycle life,

low self-discharge (8-12% per month), no problem of memory effect, low toxicity, high

reliability and environmental friendliness [7, 8]. In the past two decades, lithium-ion batteries

have dominated the portable electronic market and are now on their way to be a potential

candidate for transportation applications (i.e., EV/ HEV), grid energy storage and utilization of

renewable energies [9]. The reasonable abundance of lithium compounds on the earth crust

makes Li a low cost and readily available material. Additionally, lithium compounds produce Li+

ions (and free electrons both of which are light, having low mass density, smaller size and fast

kinetics. Sony commercialized the first rechargeable lithium-ion battery in 1990s [10].

In rechargeable energy storage systems, energy density and power density are the important

parameters. The highest gravimetric and volumetric energy densities of lithium-ion batteries

compare to other rechargeable batteries systems can be seen from Fig. 1.1.

4

Fig.1.1 Comparison of energy densities (per unit weight and per unit volume) of various

rechargeable batteries [11].

Lithium metal due to its lowest potential and large theoretical capacity (3860 mA h g-1

) has

been considered a promising anode material and was used in lithium primary batteries in early

1970s. In 1989, Moli Energy (Canada) commercialized the first Li metal secondary battery.

However, this system failed due to the dendritic growth of lithium metal. Also, the high

reactivity of Li metal with the organic electrolyte has resulted in severe safety problems with

repeated cycling [12]. To overcome the safety issues associated with lithium metal batteries, a

new concept of lithium-ion rechargeable batteries has emerged. In lithium-ion concept, a low

potential Li insertion anode (usually carbon based) material is used with a high potential lithium

insertion cathode [13]. Thus, the active lithium is always present in the form of an ion rather than

a metal. In this way, the problems associated with lithium metal were solved as the insertion

5

electrodes can repeatedly accept and release Li+ ions during charge/ discharge process. An

insertion or intercalation compound is a solid host that can incorporate guest species (ions, atoms

or molecules) into normally unoccupied interstitial sites without any major structural disruption

[14].

A major problem associated with lithium-ion energy storage system is the consumption of

lithium ion by the secondary reactions that cannot be regained, and the specific capacity of

lithium-ion battery totally depends upon the available amount of lithium for the reversible

electrochemical reaction occurring in the cell. The various processes that are considered

responsible for the capacity fading in lithium-ion cells include lithium deposition due to cell over

charge, degradation of the active material, electrolyte decomposition at high voltages, phase

changes in the insertion electrode materials (e.g., structural degradation due to Jahn-Teller

distortion transforms LiMn2O4 from cubic to tetragonal phase) and inactive film formation on the

electrode and current collector surfaces [15-17]. With the advancement of research and

technology, presently commercial lithium-ion cells have several merits as mentioned in last

paragraph. In addition to those some distinguishing features include [18].

Lithium-ion batteries have a high operating potential

They have lightweight and compact.

Charging potential is very fast; In 1 h, battery can be charged to about 80-90% of their

full capacity.

Good rate capability and high discharge rate up to 3C are attainable.

Good cycleability.

Li-ion energy storage system has excellent safety.

Low self-discharged: only 8-12% per month.

Lithium-ion batteries have long shelf-life and are low maintenance system.

6

1.3 Working principle and components of lithium-ion battery

Fig. 1.2 shows the basic elements and working principle of the rechargeable lithium-ion

battery. Generally, the system consists of an anode, an aqueous/ non-aqueous electrolyte and a

cathode [19]. The two electrodes are physically separated by a separator material. The electrolyte

used is an ionic conductor but electronic insulator.

Fig.1.2 A schematic representation of the working principle of Lithium-ion battery [20].

7

Both the electrodes (cathode and anode) are capable of reversible insertion of Li+ ions.

During discharge process lithium atoms leave their outer conduction electron to form

energetically preferred Li+ ions with complete inner shell of electrons. The Li

+ ion formed tend

to move towards the cathode material developing a concentration gradient of the ions in this half

of the cell. However, this process results in an increasingly large negative charge at the anode.

Thus, flow of Li+ ion would soon stops because the charge of these leftover conduction band

electrons on the anode exerts an increasingly attractive force on the migrating cloud of Li+ ions.

The supply of cast-off electrons is put to work by connecting a load (electricity consuming

device) externally between the anode and cathode as shown in Fig. 1.2. In this way, the electrons

also get to the cathode through external circuit where they neutralize some of the Li+ ions. Thus,

more Li atoms in the anode are now free to leave behind their conduction electrons and join the

flow towards cathode. During charging (by applying an external voltage), the process get

reversed. Now the cathode material gives up Li+ ions that deintercalates from the cathode and

intercalate or diffuse back in to the anode, thus “recharging” makes the lithium-ion battery ready

to power a load again [21].

Due to shuttling of Li ions between anode and cathode a term “rocking chair” battery is also

used [22]. Storing Li in such a cell provides a convenient and efficient way of “storing

electricity”. In commercial lithium-ion batteries, the negative electrode is graphitic carbon that

holds Li in its layers, where as lithium metal oxides are commonly employed as positive

electrode materials. The basic chemical reactions (redox reactions) of lithium-ion battery can be

described as [23]:

8

As far as the nature of materials is concerned, the anode is usually composed of insertion

type materials (e.g., carbon, Li4Ti5O12, TiO2, etc.), conversion type materials (such as cobalt

oxides, iron oxides, nickel oxides, etc.) and alloying type materials (such as Sn, Si, etc.). The

typical cathode materials include Li containing metallic oxides such as LiCoO2, LiNiO2, LiVO2

and LiMnO2 with layered structure or tunnel-structured materials (LiMn2O4) or olivine LiFePO4

[8, 24]. Being the key components which determine the overall performance of the batteries, the

electrode materials are selected with the following prerequisites: (1) high specific charge/ charge

density, i.e., a higher number of charge carriers per unit mass or per unit volume of the material

(2) a high operating cell voltage and (3) a high reversibility of the redox reactions at both the

electrodes [25]. For lithium-ion batteries, it is the cathode material that plays an important role in

the determination of energy density, safety and cycleability (cycle life). Therefore, cathode

materials present point of enthusiastic research at main international meetings on lithium-ion

batteries [26]. Currently, LiCoO2 is the most favored positive material for lithium-ion batteries.

However, over the past decade extensive research in LiMn2O4 as the cathode material have been

motivated due to the high cost and high toxicity of LiCoO2 (cobalt being toxic and has limited

9

availability in nature). Although the theoretical capacity of LiMn2O4 (148 mAhg-1

) is lower

compared with LiCoO2 (275 mAh/g) but the practical accessible capacities of both materials are

almost same, since not all the Li ions can be extracted from LiCoO2. Advantages such as lower

cost, non-toxic nature, thermal stability, high power density, high reduction potential (4 V) and

abundant Mn resources, make LiMn2O4 an attractive material for the cathode component of

lithium-ion batteries [27].

1.4 What is a spinel LiMn2O4?

Spinel oxides are compounds with general formula AB2O4, where oxygen atoms form a face-

centered cubic packing at 32e sites of Fd3m space group. The A cations occupy tetrahedral 8a

location while the B cations are located in the 16d octahedral sites. In the spinel structure, the

16c sites remain vacant. In LiMn2O4 spinel, the Li+

ions are situated in the tetrahedral sites while

Mn3+

/ Mn4+

ions are situated in the 16d octahedral sites, in a cubic closely packed array of O2-

ions, located at the 32e sites as shown in Fig. 1.3. The 8a and 16c sites in LiMn2O4 spinel form a

three dimensional pathway or channel for lithium diffusion. About 107° angle is formed by the

successive straight spokes 8a-16c-8a [28]. Spinel LiMn2O4 is a candidate of choice for the

cathode component of rechargeable lithium-ion batteries due to its high operating potential and

environmental advantages.

10

Fig.1.3 Part of the unit cell of spinel LiMn2O4 [29].

The insertion of lithium-ion into the spinel LiMn2O4 occurs at 2.96 V according to the following

two phase reaction

Li + Li[Mn2]O4 → Li2[Mn2]O4 (4)

In this reaction, the spinel LiMn2O4 phase is converted to tetragonal Li2[Mn2]O4. This phase

transition leads to 16% increase in the c/a ratio of the unit cell. This large change in the unit cell

parameters makes it difficult for the spinel host material to maintain its cubic structure on

electrochemical cycling. However, the removal of Li+ ions from the tetrahedral sites of LiMn2O4

does not result in the change of the cubic structure. This reaction occurs at about 4 V vs. Li/Li+.

Thus, LiMn2O4 electrodes demonstrate excellent cycling behaviour at high voltage range. For Li-

ion battery applications, the lithium ion extraction region is very interesting [30]. In the cubic

spinel phase, the removal/incorporation of lithium ions from/ into the tetrahedral 8a sites occurs

at 4 V potential region that is associated with the Mn3+

↔ Mn4+

redox reaction.

11

Despite these merits, spinel LiMn2O4 has severe problem of rapid capacity loss during

charge/ discharge cycling. The capacity deterioration of LiMn2O4 during charge/ discharge

cycling can be attributed to the following possible factors: (1) decomposition of the organic

based electrolyte at high potentials, (2) manganese dissolution into the electrolyte (as Mn2+

) due

to the disproportionation reaction that results in the degradation of LiMn2O4 electrode.

2Mn3+

(solid) → Mn4+

(solid) + Mn2+

(solution) (5)

The dissolution of Mn2+

will not only result in the loss of the active material (i.e.,LiMn2O4) but

will also affect the anode by plating on the anode, thus will deplete the lithium content on the

anode since reduction of Mn3+

will oxidize Li metal from the negative electrode (anode), (3)

Jahn-Teller distortion effect of LiMn2O4 electrodes at deeply discharge state that reduces the

crystal symmetry of LiMn2O4 from cubic to tetragonal and (4) crystallinity loss during charge/

discharge cycling [31, 32]. Several strategies have been made in recent years to overcome the

capacity fading of LiMn2O4 by doping with other metal cations.

12

1.5 Jahn-Teller distortion effect in spinel LiMn2O4

The Jahn-Teller distortion is considered to be due the fact that degenerate orbitals can

experience a lowering of their ground state energy by lifting this degeneracy through a lattice

distortion [33]. In spinel LiMn2O4, lithium insertion results a phase transformation due to Jahn-

Teller distortion of Mn3+

O6 octahedron. It is known that in high spin electronic configuration,

Mn3+

cations lead to distortion of the octahedral sites. The main reason for this phenomenon

(lattice distortion) is that the high spin Mn3+

cations contain four 3d unpaired electrons. As a

result of crystal field splitting, the 3d band splits into two degenerate energy levels which are

conventionally denoted as t2g and eg. The t2g energy level consists of three orbitals which are dxy,

dyz, dzx whereas the eg energy level possesses two orbitals dx2

- y2 and dz

2.

The eg orbitals have lobes directly pointing to the neighboring oxygen, whereas the three

t2g orbitals have nodal planes in these three directions and do not bond to the oxygen ions. The eg

orbitals are of higher energy and possess stronger antibonding character because the energy

levels of these orbitals are degenerate. If Hund‟s rule is obeyed, three of the t2g orbitals and one

of the eg orbitals in high spin configuration are half filled. The unequal occupancy of the eg or t2g

orbital can give rise to Jahn-Teller distortion. Compared to t2g orbital, the distortion resulting

from the uneven occupancy of the eg orbital is expected to be greater. From Fig. 1.4, it can be

seen that the distortion along z-axis will result in the splitting of the eg orbital (i.e., will result in

an elongated octahedron). Insertion of lithium into the LiMn2O4 results in Mn4+

→ Mn3+

reduction. The d4 configuration of Mn

3+ is considered to be Jahn-Teller active [28]. The energy

levels for the Mn cation in an octahedral site and splitting of the eg orbitals from cubic to

tetragonal structure is shown in Fig 1.4 [29, 34].

13

(b)

Fig. 1.4 (a) Splitting of the atomic orbitals for Mn cation at octahedral site (b) splitting of

the eg orbital

14

1.6 Literature Review

To understand the current place and issues related to the cathode materials of Li-ion batteries

and the possibilities of improvement (particularly in the spinel LiMn2O4), a comprehensive

literature review report is being presented. Based upon which the aims of current research work

are outlined at the end.

Naghash and Lee [35] have synthesized spinel LiMn2O4 using stearic acid as a chelating

agent in tetramethylammonium hydroxide to co-precipitate lithium from the aqueous solutions of

the corresponding salts used. Their material exhibited an discharge capacity of 119 mAhg-1

. The

columbic efficiency was reported to be 96%.

Ye et al. [36] have synthesized LiMn2O4 spinel by a solid state reaction synthesis method at

500 o

C. They used ball milling in association with solid-state synthesis. The synthesized sample

was single crystalline in nature with particle sizes of 20 to 30 nm. The interplanar spacing of

about 0.47 nm indicated the orientation of (111) atomic plane of the LiMn2O4. The improved

high discharge capability was attributed to the strong nanomaterials interaction with Li+ ions that

take place often at the surface compared to the bulk of the material. However, to a certain extent

the cycleability of LiMn2O4 with nanostructure speedily deteriorated.

Yi et al. [37] prepared a series of LiMn2O4 spinel using adipic acid as a chelating agent by

sol-gel method at varying temperatures. Uniform cubic morphology of LiMn2O4 powder was

obtained at 800 oC. Charge-discharge experiments have shown that the retained discharge

capacity for the samples calcined at 350 o

C, 700 o

C and 800 o

C for the first 50 cycles was 93.6,

86.1 and 85.2%, correspondingly; however, the specific capacity at the 50th

cycle was 82.2,

104.8 and 110.8 mAhg−1

, respectively.

15

Hwang et al. [38] have evaluated the influence of different synthetic parameters (such as

temperature, pH, starting materials, molar ratio of citric acid and metal cations and amount of

water) on purity of LiMn2O4 by sol-gel method. The results showed that at optimum pH 6.0 and

molar ratio of chelating agent to total metal ions 1.0, pure spine LiMn2O4 phase can be

synthesized from the nitrate precursors.

Thirunakaran and co-workers [39] have synthesized the pure and LiAlxMn2-xO4 (x = Al:

0.00–0.40) by fumaric acid assisted sol-gel synthesis. These samples were heated in a 250-850 oC

temperature range. XRD showed some additional peaks corresponding to α-Mn2O3 and LiMn2O3

for the samples calcined at low temperature (250 and 450 oC). Samples calcined at 800

oC

showed highly intense peaks representing high crystallinity of LiAlxMn2-xO4 samples. Among all

the Al doped samples, LiAl0.1Mn1.90O4 showed the best electrochemical performance by

delivering 139 mAh/g discharge capacity at 1st cycle with 97% columbic efficiency. Low Al

doping with x = 0.1, was found to be effective to stabilize LiMn2O4 structure.

Sun et al. [40] synthesized spinel LiMn2O4 and LiMn1.95Ni0.05O4 powders glycine assisted

sol-gel method. The lattice constant increased linearly from 8.1992 Å to 8.2260 Å with

increasing temperature from 250-850 o

C. Compared with the pure LiMn2O4, LiMn1.95Ni0.05O4

cathode materials exhibited improved electrochemical performance attributed to the higher

lithium-ions diffusion during cycling and stabilization of spinel structure by Ni doping.

A microwave method was used for the synthesis of spinel LiMn2O4 and Nd-doped spinel

(LiNd0.01Mn1.99O4) by Yang and co-workers [41]. XRD measurement showed that Nd3+

substituted LiMn2O4 has the same phase as that of pure LiMn2O4. The diffusion coefficient for

the LiNd0.01Mn1.99O4 was bigger than the pure spinel LiMn2O4. Cyclic voltammetry showed that

the redox pair at 4.22-3.91 V (for the pure spinel LiMn2O4) and 4.20-3.92 V (Nd-doped spinel)

16

appears to be stable and were expected to control the discharge specific capacity at high current

rate. The charge/ discharge studies showed that LiNd0.01Mn1.99O4 has better reversibility and

cycling performance than the pure LiMn2O4.

Sahan and co-workers [42] have synthesized CaCO3 coated LiMn2O4 using glycine-

nitrate method. Charge-discharge measurement showed that at 25 oC and 55

oC, the pristine

LiMn2O4 cathode has capacity fade of 25.5 and 52 %, respectively. However, CaCO3 coated

LiMn2O4 cathode material showed 7.4% and 29.5% capacity loss at 25 oC and 55

oC,

respectively. The improved electrochemical performances were attributed to the surface coating

by CaCO3 that resulted in the reduction of Mn2+

dissolution into the solvent (i.e., electrolyte).

Alcantara et al. [43] have studied the effects of composition and preparation temperature

on the structure of LiMgyNi0.5-yMn1.5O4(y =0, 0.25, 0.5) cathode materials by EPR, FTIR, X-ray,

and neutron diffraction. On increasing the annealing temperature from 450 to 750 oC, cation

ordering in a P4332 superstructure takes place for the samples y ˃ 0.25. However, when synthesis

temperature increases from 700-800 oC, a loss of octahedral cation ordering and partial reduction

of transition metals were found for LiNi0.5Mn1.5O4. When LiMgyNi0.5-yMn1.5O4 compounds were

used as cathode materials in test lithium anode cells, a decrease in discharge capacity (at 5 V

region) was found with decreasing Ni content. Nevertheless, cycling in the 3 V regions indicated

a net improvement on increasing Mg content. Lithium removal from LiNi0.5Mn1.5O4 (up to 70%)

results in a loss of intensity in the EPR signal due to the of Ni2+

oxidation to Ni4+

without

considerable alteration in Mn4+

cation local environment.

He et al. [44] prepared spinel LiMn2O4 positive material, and its doped samples

LiM0.05Mn1.95O4 (where, M = Al, Co, and Zn) by solution gel method. They used a mixture of

acetate and ethanol as chelating agent. Among the various doped samples LiCo0.05Mn1.95O4

17

demonstrated the best electrochemical beviour. A capacity decay of 3% was noted after 30

charge/ discharge cycles.

Julien et al. [45] used wet-chemistry techniques to synthesize LiMn2-yAlyO4 spinel

phases. The structural properties were found to resemble that of the undoped sample. The

electrochemical performances showed that 95% of the specific discharge capacity in the 1st cycle

is maintained at 50th

cycle, this was explained by the change of the Mn3+

/ Mn4+

cations ratio in

the doped samples.

Thirunakaran et al. [46] obtained nanoparticles of LiMn2O4, LiCrxMn2-xO4 (x = Cr: 0.00–

0.40) by pthalic acid assisted sol-gel method. TEM micrographs of the pure LiMn2O4 and

LiCrxMn2-xO4 spinel particles showed well defined morphology. Compared to the undoped

LiMn2O4, LiCr0.10Mn1.90O4 delivered 138 mAh/g discharge capacity at 1st cycle. Over the

investigated 10 cycles, these cells delivered 100 mAh/g capacity which corresponds to the

capacity fade of 3.8 mAh/g.cycle.

Hernan and co-workers [47] reported three series of nominal stoichiometry LiMxMn2-xO4

(M = Fe, Co, Ni; x ≈ 0.3) using carbonate precursors. The samples were calcined by heat

treatment in air at 400 o

C, 600 o

C and 900 o

C. All the synthesized three series have the ability to

extract Li over 4.5 V, which was accompanied by the dopant metal cation oxidation. The Fe

doped spinel synthesized at 900 o

C loses its whole capacity after the first few cycles. Partial

extrusion of iron as Fe2O3 and non uniformity in particle size and shape may account for its poor

electrochemical behaviour. The Co and Ni doped spinels obtained at 900 o

C lack these features.

The cells exhibited excellent capacity retention upon extended cycling and were capable of

delivering 100 and 120 Ah/ kg, respectively, at an average voltage of 4.5 V.

18

Ammundsen et al. [48] investigated chromium substituted spinels of composition

LiCrxMn2-xO4 (0.2 ˂ x ˂ 1.0). XANES and EXAFS measurements were used to probe the local

structure and electronic states of Cr and Mn in the synthesized compounds. The replacement of

d4 Mn

3+ ions by d

3 Cr

3+ in the octahedral site of the spinels eliminated the local disorder present

in the lattice around [Mn3+

O6] octahedral. The data obtained from the XAFS indicated that while

the [Mn3+

O6] octahedra in the tetragonal phases are elongated along one axis, [Cr3+

O6] octahedra

retained a regular symmetry with six equal Cr-O distances.

Oh et al. [49] used co-precipitation method to prepare cobalt substituted spinel of

composition Li[Ni0.5CoxMn1.5−x]O4. The synthesized compounds presented a spherical

morphology with a diameter of 3 μm. From the data a cubic spinel structure was deduced and the

incorporated Co3+

ion was positioned in the Mn4+

location in Li[Ni0.5CoxMn1.5−x]O4. The

presence of Cobalt ions in the spinel framework was reasoned in the improvement of cyclic

performance.

Thirunakaran et al. [50] have used glutamic acid as a chelating agent and sol-gel process

to synthesize LiMn2O4 and multi-doped spinel LiMgxSnyAlzMn2−x−y−zO4 (x, y, z = Mg, Sn, Al)

powders. The undoped spinel LiMn2O4 samples heat treated at 850 oC delivered a specific

capacity of 122 mAhg-1

at the initial cycle. For all the synthesized samples,

LiMg0.01Sn0.06Al0.30Mn1.6O4 delivered 115 mAhg-1

discharge capacity in the 1st cycle and

exhibited stable cycleability with a capacity loss of 1 mAhg-1

cycle-1

over the investigated 10

cycles.

Yang and co-workers [51] have demonstrated first principle computation to explore the

electronic, structural, and electrochemical properties of LiM1/2Mn3/2O4 (M = various metal

cations used). The computation results suggested that compared to Ni doping, substitution with

19

Cobalt or Copper cations can reduce lithium diffusion barrier. Detailed investigational

measurements on LiNi0.25Cu0.25Mn1.50O4 were carried out and the proposed explanation of the

computational study was proven. Although, the specific discharge capacity declines with

increasing Copper content, optimum composition like LiCu0.25Ni0.25Mn1.5O4 exhibited high

reversible capacities at high rates.

Lee et al. [52] have reported the Li1.1Al0.05Mn1.85O4 spinel electrochemical performance

by AlF3 coating. AlF3-coated layer thickness was found to be approximately 10 nm. Unlike the

pure uncoated sample, the electrochemical performances were significantly improved by AlF3

coating. The surface coating reduced the Mn dissolution into the electrolyte. AlF3 surface coated

spinel delivered 96.2% retention of the discharge capacity, while the pristine sample showed

85.3% capacity retention after 90 charge/ discharge cycles.

Kim et al. [53] investigated LiMn1.88Ge0.1Li0.02O4 material. The prepared material

exhibited 63% specific capacity retention of the 1st

cycle capacity (124 mAh/g) over the

investigated 100 charge/ discharge at 1C while the pure spinel (undoped) displayed only 44%

capacity retention.

Singhal et al. [54] have synthesized Nd doped spinel LiMn1.99Nd0.01O4 by chemical

synthesis route. LiMn1.99Nd0.01O4 delivered 149 mAhg-1

discharge capacity, while 91% of the

capacity is retained over the investigated 25 charge/ discharge cycles. Their finding indicated

that substituting part of Mn in LiMn2O4 with low content of Nd, the loss in capacity can be

significantly compensated.

Ju et al. [55] used co-precipitation method to prepare nanorod shaped LiNi0.5Mn1.5O4

cathode materials. The synthesized nanorod-shaped LiNi0.5Mn1.5O4 active materials delivered

initial specific capacity of 126 mAhg−1

at 0.1 C. The synthesized compounds demonstrated

20

improved cycleability and good rate capability compared to the sample of irregular shaped

morphology LiNi0.5Mn1.5O4 cathode materials.

Feng et al. [56] prepared Li1.02MxMn1.95O4−yFy (M = Cobalt, Yttrium, and Gallium) by

the rheological phase reaction method. The results showed that the non-stoichiometric spinels

Li1.02MxMn1.95O4−yFy had improved cycleability. Among the different compositions investigated,

Li1.02Mn1.95Co0.02Y0.01Ga0.01O3.97F0.03 sample besides having high specific capacity, but also

demonstrated superior charge/ discharge cycling behaviour. The results indicated that multiple-

catiions doping is a useful way to enhance the electrochemical properties of the spinel lithium

manganese oxide.

Goktepe et al. [57] have investigated the electrochemical performance of LiMxMn2−xO4

(M=Li, Fe, Co; x=0,0.05, 0.1, 0.15) and LiFe0.05MyMn1.95−yO4 (M=Li, Al, Ni, Co; y=0.05, 0.1).

The prepared samples displayed a pure structure with no impurity. Their experimental finding

indicated that the doped samples were more tolerant to the insertion/ extraction of Li+ ions

compared to the pure LiMn2O4 cathode material.

Thirunakaran et al. [58] have synthesized pristine spinel LiMn2O4 and zinc- cerium dual

doped samples [LiZnxCeyMn2−x−yO4 (x = 0.01–0.10; y = 0.10–0.01)] by the sol–gel method.

They used p-amino benzoic acid as a chelating agent to get small micro-particles and superior

performance to be used in rechargeable Li-ion batteries. Galvanostatic electrochemical studies of

LiMn2O4 samples annealed at 850 oC delivered a discharge specific capacity of 122 mAhg

−1.

specific capacity obtained and cyclic stability of LiZn0.01Ce0.01Mn1.98O4 were superior (124

mAhg−1

), with less capacity fading (0.1 mAhg−1

cycle−1

) for the inspected 10 charge/ discharge

cycles.

21

Churikov et al. [59] used melt-impregnation and sol-gel methods to synthesize spinel

with general formula LiMn2−yMeyO4 (Me = Cr, Co, Ni). Double doping of spinel LiMn2O4 with

low content of cobalt and nickel stabilized the discharge capacity for the investigated charge/

discharge cycles.

Arumugam et al. [60] coated LiMn2O4 with 0.5 wt%, 1.0 wt% and 1.5 wt% CeO2 by a

polymeric process. Among all the synthesized samples, coating of 1.0 wt% of the CeO2 resulted

in structural integrity of LiMn2O4, high reversible discharge specific capacity.

Yanwen et al. [61] investigated the variation in LiFePO4, spinel LiMn2O4 and Li1＋xV3O8

by yttrium substitution. The lattice constant „a‟ of LiMn2O4 after modification was smaller and

(100) crystal plane of Li1+xV3O8 was expanded as a result of Yttrium substitution. The grain

boundary of LiFePO4 happened to be smaller after yttrium doping. Yttrium doping resulted in

improved initial discharge capacities and electrochemical performance.

Arumugam and co-workers [62] prepared nanoparticles of Zn doped LiMn2O4 by sol-gel

method. Succinic acid was used as a chelating agent. The electrochemical measurements

demonstrated that LiZn0.10Mn1.90O4 when evaluated as a positive electrode material for

rechargeable Li-ion battery showed very low capacity fading, excellent rate capability at high

current rates and better charge-discharge reversibility.

Capsoni and co-workers [63] investigated the gallium doped spinel Li1.02GaxMn1.98-xO4

with 0.00 ˂ x ˂ 0.20 to investigate the lowest amount of dopant to inhibit the Jahn- Teller effect

and to clarify the allocation of Ga3+

at octahedral and tetrahedral sites of LiMn2O4. The

electronic and magnetic features of the spinel LiMn2O4 structure are found to be highly

dependent on the substitution of cationic sublattice. Ga3+

ions substitute mostly in the octahedral

22

sites. 1% dopant amount significantly modifies the temperature of the conductivity drop

associated with the Jahn Teller distortion effect of spinel LiMn2O4.

Jian-gang and co-workers [64] prepared SrF2-coated LiMn2O4 with excellent

electrochemical performance. Sample coated with 2.0% (molar fraction) when being cycled at 55

oC, 79% capacity was retained after 20 cycles, whereas the sample coated with 2.0% SrF2

delivered discharge specific capacity of 108 mAhg-1

, and 97% of the capacity in the 1st cycle

was retained.

Park et al. [65] synthesized mesoporous LiMn2O4 using SBA-15 or KIT-6 templates

between 500 o

C and 700 o

C. A specific capacity of about 100 mAhg-1

at 0.1 C was achieved and

at 10 C current rates, 76% of the initial capacity was preserved by the sample calcined at 600 o

C

in which KIT-6 was used.

Huang et al. [66] prepared LiCo1.09Mn0.91O4 by sol-gel method that delivered initial

discharge specific capacity of 87.1 mAhg-1

with two voltage plateaus 5.1 and 4.9 V.

LiCo1.09Mn0.91O4 was assembled with Li4Ti5O12 to form full cell that delivered 84.2 mAhg-1

specific capacity, good rate capability with 4.70 KW Kg-1

power density at 1700 mAg-1

.

Raja et al. [67] have synthesized nanocrystalline of LiMn2O4 by combustion method.

Electrochemical measurement showed that the synthesized spinel LiMn2O4 delivered a discharge

capacity of 78 mAh g-1

.

Balaji et al. [68] used co-precipitation technique to synthesize Nd3+

doped spinel

LiNdxMn2−xO4. The capacity retention for LiNd0·05Mn1·95O4, LiNd0·10Mn1·90O4 and

LiNd0·15Mn1·85O4 were found to be 88·4%, 97·1% and 96·8%, respectively after 50 cycles.

J. M Lloris and co-workers [69] studied LiCuxMn2-xO4 and LiCu0.5-yAlyMn1.5O4 series by

chemical analysis, XRD, X-ray absorption and electrochemical measurements. Lithium excess

23

synthesis resulted in decline of capacity, while Al doping resulted in significant improvement in

capacity, the value reaches 100 mAh/g.

Jayaparkash et al. [70] have synthesized a series of LiMn2O3.8F0.2, LiCr0.2Mn1.8O3.8F0.2

and LiCr0.2V0.2Mn1.6O3.8F0.2 by citric acid assisted modified (CAM) sol–gel method to know the

effect of synthetic method used and the effect of dual dopants, i.e., anion and/or cation upon

spinel LiMn2O4. The improved charge/ discharge performances of LiMn2O3.8F0.2 (130 mAh/g)

and LiCr0.2Mn1.8O3.8F0.2 (142 mAh/g) cathode materials demonstrated a possible increase in the

reversible capacity behaviour of the spinel LiMn2O4 upon F-

doping at 32e site and the

simultaneous substitution of Cr3+

and F- at 16d and 32e sites, respectively.

Todorov et al. [71] have calculated the theoretical capacity and cation vacancy of doped

LiMn2-xMxO4 spinel cathode materials. The theoretical capacity was found to increase with

increasing the oxidation number of the doped cations in the 16d sites of the spinel LiMn2O4.

Analysis also showed that cation doped spinel LiMn2O4 compounds having low contents of

vacancy promote high capacity.

Rong-hua et al. [72] have studied the inclusion/ extraction processes of Li+ ion in spinel

LiMn2O4 and Cu doped spinel (LiCuxMn2−xO4) by electrochemical impedance (EIS), CV and

XRD. The insertion/ extraction process of Li+ ion from/ into the spinel oxides was consisted of

three steps: charge transfer of Li+ ion on the surface of the spinel oxides, diffusion and

occupation of lithium ion in the lattice of the spinel framework.

Okada et al. [73] used melt impregnation method for the synthesis of Co and Ni doped

spinel LiMn2O4 at 800 o

C. Although, LiM0.5Mn1.5O4 (M = Co, Ni) cathode material demonstrated

a small capacity in the (3 + 4) V region compared with the pure LiMn2O4, it was a very useful

cathode material in controlling capacity fading in the 3 V region caused by the Jahn–Teller

24

distortion effect. Charge/ discharge studied have shown that Ni and Co doping in LiMn2O4

promote the cycling performance by stabilizing the spinel structure.

Sigala et al. [74] investigated the reversible lithium de-intercalation of Cr doped spinel

LiCryMn2-yO4 (0 ˂ y ˂ 1) at 3.4-5.4 V vs. Li. Sample with y ˂ 0.5, exhibited excellent cycling

performance while for compounds with y ˃ 0.75 rapid capacity loss was found. Low Mn/ Cr

substitution rate was found beneficial for the specific capacity, energy and cycling performance

of LiMn2O4.

Stroukoff et al. [75] investigated the thermal stability characteristics of spinel LiMn2O4

and oxyflouride materials Li1.1Mn1.9-yMyO4-zFz. The results indicated that the peak onset

temperature increases and the reaction enthalpy decreases with decreasing lithium content.

Aitchison et al. [76] studied the partial substitution of LiMn2O4 with Cobalt using

EXAFS and XANES techniques. The dopant ion (Co) was randomly distributed in the spinel as

trivalent ions replacing Mn3+

ions. The substituent ion was found to effectively stabilize the

spinel structure by suppressing the Jahn-Teller distortion of the lattice. The 5 V plateau was

attributed to the oxidation of Co3+

to Co4+

.

Liu et al. [77] have successfully fabricated zirconium and cobalt co-doped LiMn2O4 by

sol-gel method using porous anodic aluminum oxide (AAO) as a template. They claimed the

material as promising candidate for large scale production.

Iqbal and Ahmad [78] have evaluated the electrical and dielectric properties of rare earth

metals substituted LiRxMn2−xO4 (R=La3+

, Ce3+

, Pr3+

and x = 0.00−0.20) nanoparticles.

Substitution by the rare-earth elements improved the structural stability of the material and

dielectric properties which are believed to be helpful for better battery performance.

25

In the present study, in order to stabilize the spinel LiMn2O4 for the repeated charge/

discharge cycling and to improve the electrochemical performance at higher rates, part of Mn3+

ion in LiMn2O4 was substituted with low contents of various metal cations such as Ni2+

, Cr3+

,

Cu2+

, Zn2+

and La3+

. Different series with nominal compositions LiMxM'yMn2-x-yO4 (where M

and M' are the various doping metal cations used in this study) were synthesized by sol-gel

method. Nb5+

and Ge4+

were also tried to co-dope in LiMn2O4, but due to insolubility of

Niobium salts in water, this series was not further synthesized. The following cations in various

combinations were doped in LiMn2O4.

Various bi-metal doped (i.e., La-Zn, Ni-Cr, Cu-Cr, Cu-Zn, Ni-Zn and Ni-Cu) series of

LiMn2O4 materials were synthesized and their electrochemical performances were evaluated for

lithium-ion batteries. The above mentioned combinations of various metal dopants were

expected to result in better electrochemical performances for the doped LiMn2O4. The reasons

being as follows; the electrochemical properties of LiMn2O4 strongly depend upon the synthetic

method. Solid state reaction methods have been usually used for the synthesis of LiMn2O4 that

require extensive grinding and mechanical mixing which strongly affect the quality of the final

product. As a result of solid state reaction large particles with irregular morphology,

inhomogeniety and poor stoichiometry were obtained [40, 78]. The sol-gel method was used in

this study because of the advantages such as uniform particle morphology, narrow particle size

distribution, lower calcination temperature and short processing time.

Multiple cation-doping in LiMn2O4 has been reported and it has been found that co-doping

has a synergistic effect on the enhancement of the electrochemical performance [57, 79, 80].

Rare earth elements may be considered as possible dopants for Mn3+

, because of their large

binding energy, so they reduce the Mn3+

content, enhance the average valence of Mn, improve

26

the stability of the cubic structure and reduce the possibility of Jahn-Teller distortion [81, 82].

Lanthanum substitution in the spinel structure has been found to be beneficial as La3+

in the

lattice will act as a pillar in facilitating lithium ion mobility by preventing the collapsing of the

spinel lattice. So La3+

has been chosen as one of the dopant.

Zn2+

having ionic radius of 0.74 Å has been found to improve the cycling stability of

LiMn2O4. Also, Zn2+

has preference for tetrahedral sites in LiMn2O4. Thus substitution in

tetrahedral sites may produce very efficient spinel through stabilization of the tetrahedral

structure [83, 84]. Keeping in view the above mentioned advantages, Zn was selected as one of

the dopant in association with La in the present study. Therefore, the combined synergistic effect

of trivalent La3+

and divalent Zn2+

in the LiMn2O4 spinel is anticipated to enhance the cycling

performance by stabilizing the LiMn2O4 frame work.

Similarly, an attempt has been made to co-dope Ni+2

along with Cr3+

, Cu2+

and Zn2+

in spinel

LiMn2O4 to improve the electrochemical performance. Ni+2

ions (ionic radius 0.69 Å) have

strong octahedral preference and can easily substitute Mn3+

ion. The doping of Ni into LiMn2O4

has been found to result in stronger Ni-O bond than the Mn-O bond and therefore will give rise

to a smaller lattice parameter [85]. Thus, a stronger metal-oxygen bonding may restrain the Jahn-

Teller effect. The ionic radius of Cr3+

is 0.615 Å, which is smaller than Mn3+

(0.645 Å), also the

large binding energy of CrO2 (1142 kJ/ mol) compared to MnO2 (946 kJ/ mol) may also result in

the stabilization of the octahedral sites [86]. Thus co-doping of Ni+2

and Cr3+

may result in an

efficient spinel having good structural stability and high rate performance. Similarly, Cu2+

doping is found to result in higher electronic conductivity and stable cycling performance of

LiMn2O4 [51]. The combined synergistic effect of the bi-metal doping of Ni-Cr, Ni-Cu, Ni-Zn,

Cu-Cr, Cu-Zn and La-Zn in LiMn2O4 may result in the stabilization of the spinel frame work by

27

suppressing the Jahn-Teller distortion effect (Mn3+

being Jahn-Teller active ion). Thus, an effert

is made in this study to enhance the electrochemical performance of LiMn2O4 to be used as a

positive electrode material in lithium-ion batteries.

28

1.7 Objectives of the present work

Concluding the so far work in the area of spinel LiMn2O4 cathode material, following

objectives were set for the current research.

To synthesize the pure and bi-metal doped spinel LiMn2O4 by simple sol-gel method.

To optimize the synthesis conditions (pH, temperature) with the aim for large scale utility

of the method and materials.

To tune the low content dopants composition in the spinel matrix for Mn to reduce the

capacity fading of spinel LiMn2O4.

To achieve these objectives, part of Mn3+

ion in LiMn2O4 was substituted with low content of

various metal cations such as Ni2+

, Cr3+

, Cu2+

, Zn2+

and La3+

to synthesize different series of the

nominal compositions of LiMxM'yMn2-x-yO4 (where M and M' are the various doping metal

cations used in this study) by the sol-gel method. To stabilize the spinel structure and improve

the electrochemical performance of LiMn2O4, the following six (06) series of bi-metal cations

doped LiMn2O4 were synthesized;

(a) LiNixCryMn2-x-yO4

(b) LiLaxZnyMn2-x-yO4

(c) LiCuxCryMn2-x-yO4

(d) LiCuxZnyMn2-x-yO4

(e) LiNixCuyMn2-x-yO4

(f) LiNixZnyMn2-x-yO4, (where x = y = 0.01-0.05).

29

The synthesized samples were characterized by TGA/ DTA, XRD, EDX, SEM, TEM, for

morphology and structure. The chemical compositions and confirmation of the pure and doped

samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES)

and Fourier transformed infrared (FTIR) spectroscopy measurements. The electrochemical

investigations were made through cyclic voltammetry (CV) and electrochemical impedance

spectroscopy (EIS) techniques. Finally the charge discharge studies were carried out under

different set of conditions to evaluate the actual performance of the synthesized materials as

candidates for the cathode component of Li ion batteries.

It is hoped that the outcome of the present study will help in better understanding of the

structure, composition and function relationship of the synthesized cathode material. In particular

it will considerate the effect of „low content bi-metal doping‟ in the spinel LiMn2O4 matrix.

30

References

1. Choi, N. S.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y. K.; Amine, K.; Yushin, G.;

Nazar, L. F.; Cho, J.; Bruce, P. G. Challenges facing lithium batteries and electrical

double-layer capacitors. Angew. Chem. 2012, 51, 9994–10024.

2. Magasinki, A.; Dixon, P.; Hertzberg, B.; Kvit, A.; Ayala, J.; Yushin. G. High-

performance lithium-ion anodes using a hierarchical bottom-up approach. Nat. Mater.

2010, 9(4), 353–358.

3. Manthiram, A. Materials challenges and opportunities of lithium ion batteries. J. Phys.

Chem. 2011, 2, 176–184.

4. Shukla, A. K.; Kumar, T. P. Materials for next-generation lithium batteries. Current

Science. 2008, 94(3), 314–331.

5. Winter, M.; Brodd, R. J. What are batteries, fuel cells, and super capacitors? Chem.

Rev. 2004, 104, 4245–4269.

6. Cheng, F.; Liang, J.; Tao, Z.; Jun Chen. Functional materials for rechargeable batteries.

Adv. Mater. 2011, 23, 1695–1715.

7. Tarascon, J. M. Key challenges in future Li-battery research. Phil. Trans. R. Soc. A.

2010, 368, 3227–3241.

8. Liu, C.; Li, F.; Ma, L. P.; Cheng, H. M. Advanced materials for energy storage. Adv.

Mater. 2010, 22, E28–E62.

9. Li, H.; Zhou, H. Enhancing the performances of Li-ion batteries by carbon-coating:

present and future. Chem. Commun. 2012, 48, 1201–1217.

10. Belharouak. I. Lithium ion batteries – new developments. Publisher, In Tech: Croatia,

2012.

11. Tarascon, J. M.; Armand, M. Issues and challenges facing rechargeable lithium

batteries. Nature. 2001, 414, 359–367.

12. Jeong, G.; Kim, Y. U.; Kim, H.; Kim, Y. J.; Sohn, H. J. Prospective materials and

applications for Li secondary batteries. Energy Environ. Sci. 2011, 4, 1986–2002.

13. Aurbach, D. Review of selected electrode–solution interactions which determine the

performance of Li and Li ion batteries. J. Power Sources. 2000, 89, 206–218.

31

14. Bruce, P. G. Solid-state chemistry of lithium power sources. Chem. Commun. 1997,

1817–1824.

15. Aurbach, D.; Weissman, I. On the possibility of LiH formation on Li surfaces in wet

electrolyte solutions. Electrochem. Commun. 1999, 1, 324–331.

16. Richard, M. N.; Dahn, J. R. Accelerating rate calorimetry study on the thermal stability

of lithium intercalated graphite in electrolyte. I. Experimental. J. Electrochem. Soc.

1999, 146(6), 2068–2077.

17. Sacken, U. V.; Nodwell, E.; Sundler, A.; Dahn, J. R.. Comparative thermal stability of

carbon intercalation anodes and lithium metal anodes for rechargeable lithium batteries.

Solid State Ionics. 1994, 69(3-4), 284–290.

18. Eriksson, T. LiMn2O4 as a Li-ion battery cathode. From bulk to electrolyte interface.

PhD Thesis, Acta Universitatis Upsaliensis, Uppsala. 2001.

19. Jiang, J.; Li, Y.; Liu, J.; Huang, X.; Yuan, C.; Lou, X. W (David). Recent advances in

metal oxide-based electrode architecture design for electrochemical energy storage.

Adv. Mater. 2012, 24, 5166–5180.

20. Wang, Y.; Huang, H. Y. S. Comparison of lithium-ion battery cathode materials and the

internal stress development. Proceedings of the ASME 2011 International Mechanical

Engineering Congress & Exposition (IMECE2011), Denver, Colorado, USA. Nov. 11–

17, 2011,

21. Bewlay, S. L. Synthesis of novel high energy density cathode materials for lithium

rechargeable batteries. PhD Thesis, University of Wollongong, 2006.

22. Liu, R. S.; C.H. Shen, C. H. Structural and electrochemical study of cobalt doped

LiMn2O4 spinels. Solid State Ionics. 2003, 157, 95–100.

23. Cheng, F.; Tao, Z.; Liang, J.; Chen, J. Template-directed materials for rechargeable

lithium-ion batteries. Chem. Mater. 2008, 20, 667–681.

24. Feng, C.; Li, H.; Zhang, C.; Guo, Z.; Wu, H.; Tang, J. Synthesis and electrochemical

properties of non-stoichiometric Li–Mn-spinel (Li1.02MxMn1.95O4−yFy) for lithium ion

battery application. Electrochim. Acta. 2012, 61, 87–93.

25. Wang, Y.; Cao, G. Developments in nanostructured cathode materials for high-

performance lithium-ion batteries. Adv. Mater. 2008, 20, 2251–2269.

32

26. Gong, Z.; Yang, Y. Recent advances in the research of poly anion-type cathode

materials for Li-ion batteries. Energy Environ. Sci. 2011, 4, 3223–3242.

27. Liu, H.; Tang, D. The effect of nanolayer AlF3 coating on LiMn2O4 cycle life in high

temperature for lithium secondary batteries. Russ. J. Electrochem. 2009, 45(7), 762–

764.

28. Wakihara, M.; Yamamoto, O. Lithium ion batteries, fundamentals and performance.

Eds. WILEY-VCH., Germany, and Kodansha Ltd., Tokyo, 1998, p. 27.

29. Berg, H.; Goransson, K.; Nolang, B.; Thomas, J. O. Electronic structure and stability of

the LixMn2O4 (0<x<2) system. J. Mater. Chem. 1999, 9, 2813–2820.

30. Thackeray, M. M. Spinel electrodes for lithium batteries. J. Am. Ceram. Soc. 1999, 82

(12), 3347–3354.

31. Gummow, R. J.; Kock, A. de.; Thackeray, M. M. Improved capacity retention in

rechargeable 4 V lithium/ lithium manganese oxide (spinel) cells. Solid State Ionics.

1994, 69, 59-67.

32. He, B. L.; Zhou, W. J.; Liang, Y. Y.; Bao, S. J.; Li, H. L. Synthesis and electrochemical

properties of chemically substituted LiMn2O4 prepared by a solution-based gel method.

J. Colloid Int. Sci. 2006, 300, 633–639.

33. Keller, H.; Holder, A.B.; Müller, K. A. Jahn-Teller physics and high-Tc

superconductivity. Mater. Today. 2008, 11, 38–46.

34. Yamada, A.; Tanaka, M.; Tanaka, K.; Sekai, K. Jahn–Teller instability in spinel Li–

Mn–O. J. Power Sources.1999, 81–82, 73–78.

35. Naghash, A. R.; Lee, J. Y. Preparation of spinel lithium manganese oxide by aqueous

co-precipitation. J. Power Sources. 2000, 85, 284–293.

36. Ye, S. H.; Lv, J. Y.; Gao, X. P.; Wu, F.; Song, D. Y. Synthesis and electrochemical

properties of LiMn2O4 spinel phase with nanostructure. Electrochim. Acta. 2004, 49,

1623–1628.

37. Yi, T. F.; Hao, C. L.; Yue, C. B.; Zhu, R. S.; Shu, J. A literature review and test:

Structure and physicochemical properties of spinel LiMn2O4 synthesized by different

temperatures for lithium ion battery. Synthetic Met. 2009, 159, 1255–1260.

33

38. Hwang, B. J.; Santhanam, R.; Liu, D. G. Effect of various synthetic parameters on

purity of LiMn2O4 spinel synthesized by a sol-gel method at low temperature. J.Power

Sources. 2001, 101, 86–89.

39. Thirunakaran, R.; Sivashanmugam, A.; Gopukumar, S.; Dunnill, C. W.; Gregory, D. H.

Electrochemical behaviour of nano-sized spinel LiMn2O4 and LiAlxMn2-xO4 (x= Al:

0.00–0.40) synthesized via fumaric acid-assisted sol–gel synthesis for use in lithium

rechargeable batteries. J Phys Chem Solids. 2008, 69, 2082–2090.

40. Sun, Y. K.; Jin, S. H. Synthesis and electrochemical characteristics of spinel phase

LiMn2O4-based cathode materials for lithium polymer batteries. J. Mater. Chem. 1998,

8(11), 2399–2404.

41. Yang, S. T.; Jia, J. H.; Ding, L.; Zhang, M. C. Studies of structure and cycleability of

LiMn2O4 and LiNd0.01Mn1.99O4 as cathode for Li-ion batteries. Electrochim. Acta.

2003, 48, 569–573.

42. Sahan, H.; Goktepe, H.; Patat, S. A novel method to improve the electrochemical

performance of LiMn2O4 cathode active material by CaCO3 surface coating. J. Mater.

Sci. Technol. 2011, 27(5), 415–420.

43. Alcantara, R.; M. Jaraba, M.; P. Lavela, P.; Tirado, J. L.; Zhecheva. E.; Stoyanova, R.

Changes in the local structure of LiMgyNi0.5-yMn1.5O4 electrode materials during

lithium extraction. Chem. Mater. 2004, 16, 1573–1579.


properties of chemically substituted LiMn2O4 prepared by a solution-based gel method.

J. Colloid Int. Sci. 2006, 300, 633–639.

45. Julien, C.; Ziolkiewicz, S.; Lemal, M.; Massot, M. Synthesis, structure and

electrochemistry of LiMn2-yAlyO4 prepared by a wet-chemistry method. J. Mater.

Chem. 2001, 11, 1837–1842.

46. Thirunakaran, R.; Sivashanmugam, A.; Gopukumar, S.; Dunnill, C. W.; Duncan H.

Gregory, D. H. Phthalic acid assisted nano-sized spinel LiMn2O4 and LiCrxMn2-xO4 (x

= 0.00–0.40) via sol–gel synthesis and its electrochemical behaviour for use in Li-ion-

batteries. Mater. Res. Bull. 2008, 43, 2119–2129.

47. Hernan, L.; Morales, J.; Sanchez, L.; Castellon, E. R.; Aranda, M. A. G. Synthesis,

characterization and comparative study of the electrochemical properties of doped

34

lithium manganese spinels as cathodes for high voltage lithium batteries. J. Mater.

Chem. 2002, 12, 734–741.

48. Ammundsen, B.; Jones, D. J.; Roziere, J. Effect of chromium substitution on the Local

structure and insertion chemistry of spinel lithium manganates: Investigation by X-ray

absorption fine structure spectroscopy. J. Phys. Chem. B. 1998, 102, 7939–7948.

49. Oh, S. W.; Myung, S. T.; Kang, H. B.; Sun, Y. K. Effects of Co doping on

Li[Ni0.5CoxMn1.5−x]O4 spinel materials for 5 V lithium secondary batteries via Co-

precipitation. J. Power Sources. 2009, 189, 752–756.

50. Thirunakaran, R.; Ravikumar, R.; Vanitha, S.; Gopukumar, S.; Sivashanmugam, A.

Glutamic acid-assisted sol–gel synthesis of multi-doped spinel lithium manganate as

cathode materials for lithium rechargeable batteries. Electrochim. Acta. 2011, 58, 348–

358.

51. Yang, M. C.; Xu, B.; Cheng, J. H.; Pan, C. J.; Hwang, B. J.; Meng. Y. S. Electronic,

structural, and electrochemical properties of LiNixCuyMn2-x-yO4 (0 < x < 0.5, 0 < y <

0.5) high-voltage spinel materials. Chem. Mater. 2011, 23, 2832–2841.

52. Lee, D. J.; Lee, K. S.; Myung, S. T.; Yashiro, H.; Sun, Y. K. Improvement of

electrochemical properties of Li1.1Al0.05Mn1.85O4 achieved by an AlF3 coating. J. Power

Sources. 2011, 196, 1353–1357.

53. Kim, S. W.; Kumar, V. G.; Seo, D. H.; Park, Y. U.; Kim, J.; Kim, H.; Kim, J.; Hong, J.;

Kang, K. Preparation and electrochemical characterization of doped spinel

LiMn1.88Ge0.1Li0.02O4 cathode material. Electron. Mater. Lett. 2011, 7(2), 105–108.

54. Singhal, R.; Das, S. R.; Tomar, M. S.; Ovideo, O.; Nieto, S.; Melgarejo, R. E.; Katiyar,

R. S. Synthesis and characterization of Nd doped LiMn2O4 cathode for Li-ion

rechargeable batteries. J. Power Sources. 2007, 164, 857–861.

55. Ju, S. H.; Kang, Y. C.; Sun, Y. K.; Kim, D. W. Synthesis and electrochemical

properties of nanorod-shaped LiMn1.5Ni0.5O4 cathode materials for lithium-ion

batteries. Mater. Chem. Phys. 2012, 132, 223–227.

56. Feng, C.; Li, H.; Zhang, C.; Guo, Z.; Wu, H.; Tang, J. Synthesis and electrochemical

properties of non-stoichiometric Li–Mn-spinel (Li1.02MxMn1.95O4−yFy) for lithium ion

battery application. Electrochim. Acta. 2012, 61, 87–93.

35

57. Göktepe, H.; H. Şahan, H.; Ş. Patat, Ş.; Ülgen, A. Enhanced cyclability of triple-metal-

doped LiMn2O4 spinel as the cathode material for rechargeable lithium batteries. Ionics.

2009, 15, 233–239.

58. Thirunakaran, R.; Sivashanmugam, A.; Gopukumar, S.; Rajalakshmi, R. Cerium and

zinc: Dual-doped LiMn2O4 spinels as cathode material for use in lithium rechargeable

batteries. J. Power Sources. 2009, 187, 565–574.

59. Churikov, A. V.; Kachibaya, E. I.; Sycheva, V. O.; Ivanishcheva, I. A.; Imnadze, R.

I.; Paikidze, T. V.; Ivanishchev, A. V. Electrochemical properties of LiMn2–yMeyO4

(Me = Cr, Co, Ni) spinels as cathodic materials for lithium-ion batteries. Russ. J.

Electrochem. 2009, 45(2), 175–182.

60. Arumugam, D.; Kalaignan, G. P. Synthesis and electrochemical characterization of

nano-CeO2-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium

batteries. Electrochim. Acta. 2010, 55, 8709–8716.

61. Yanwen, T.; Xiaoxue, K.; Liying, L.; Chaqing, X.; Tao, Q. Research on cathode

material of Li-ion battery by yttrium doping. J. Rare Earths. 2008, 26(2), 279–283.

62. Arumugam, D.; Kalaignan, G. P.; Vediappan, K.; Lee, C. W. Synthesis and

electrochemical characterizations of nano-scaled Zn doped LiMn2O4 cathode materials

for rechargeable lithium batteries. Electrochim. Acta. 2010, 55, 8439–8444.

63. Capsoni, D.; Bini, M.; Chiodelli, G.; Mustarelli, P.; Massarotti, V.; Azzoni, C. B.;

Mozzati, M. C.; Linati, L. Inhibition of Jahn-Teller cooperative distortion in LiMn2O4

spinel by Ga3+

Doping. J. Phys. Chem. B. 2002, 106, 7432–7438.

64. Jian-gang, L.; Xiang-ming, H.; Ru-song, Z. Electrochemical performance of SrF2-

coated LiMn204 cathode material for Li-ion batteries. Trans. Nonferrous Met. Soc.

China. 2007, 17, 1324–1327.

65. Park, Y.; Shin, W.; Lee, J. W. Synthesis of mesoporous Li–Mn spinel without post-

template treatment. Micropor. Mesopor. Mat. 2012, 153, 137–141.

66. Huang, X.; Lin, M.; Tong, Q.; Li, X.; Ruan, Y.; Yang, Y. Synthesis of LiCoMnO4 via a

sol–gel method and its application in high power LiCoMnO4/ Li4Ti5O12 lithium-ion

batteries. J. Power Sources, 2012, 202, 352–356.

36

67. Raja, M. W.; Mahanty, S.; Ghosh, P.; Basu, R. N.; Maiti, H. S. Alanine-assisted low-

temperature combustion synthesis of nanocrystalline LiMn2O4 for lithium-ion batteries.

Mater. Res. Bull. 2007, 42, 1499–1506.

68. Balaji, S.; Manichandran, T.; Mutharasu, D. A comprehensive study on influence of

Nd3+

substitution on properties of LiMn2O4. Bull. Mater. Sci. 2012, 35(3), 471–480.

69. Lloris, J. M.; Leon, B.; Vicente, C. P.; Tirado, J. L.; Womes, M.; Fourcade, J. O.; J. C.

Jumas, J. C. Composition and electrochemical properties of LiCuxMn2-xO4 and LiCu0.5-y

AlyMn1.5O4. J Solid State Electrochem. 2004, 8, 521–525.

70. Jayaprakash, N.; Kalaiselvi, N.; Doh, C. H.; Gangulibabu.; Bhuvaneswari, D. A new

class of sol–gel derived LiM1xM2yMn2-x-yO3.8F0.2 (M1= Cr, M2= V; x = y = 0.2)

cathodes for lithium batteries. J. Appl. Electrochem. 2010, 40, 2193–2202

71. Todorov, Y. M.; Hideshima, Y.; Noguchi, H.; Yoshio, M. Determination of theoretical

capacity of metal ion-doped LiMn2O4 as the positive electrode in Li-ion batteries. J.

Power Sources. 1999, 77, 198–201.

72. Rong-hua, Z.; Wei-shan, L.; Dong-sheng, L.; Qi-ming, H.; Ling-zhi, Z. Insertion/

removal kinetics of lithium ion in spinel LiCuxMn2−xO4. Trans. Nonferrous Met. Soc.

China. 2007, 17, 1312–1318.

73. Okada, M.; Lee, Y. S.; Yoshio. M. Cycle characterizations of LiMxMn2-xO4 (M = Co,

Ni) materials for lithium secondary battery at wide voltage region. J. Power Sources.

2000, 90, 196–200.

74. Sigala, C.; Guyomard, D.; Verbaere. A.; Piffard. Y.; Tournoux. M. Positive electrode

materials with high operating voltage for lithium batteries: LiCryMn2-yO4 ( 0 ˂ y ˂1).

Solid State Ionics. 1995, 81, 167–170.

75. Stroukoff, K. R.; Manthiram, A. Thermal stability of spinel Li1.1Mn1.9-yMyO4-zFz (M =

Ni, Al, and Li, 0 ˂ y ˂ 0.3, and 0 ˂ z ˂ 0.2) cathodes for lithium ion batteries. J. Mater.

Chem. 2011, 21, 10165–10170.

76. Aitchison, P.; Ammundsen, B.; Jones, D. J.; Burns, G.; Roziere, J. Cobalt substitution

in lithium manganate spinels: examination of local structure and lithium extraction by

XAFS. J. Mater. Chem.1999, 9, 3125–3130.

37

77. Liu, X.; Wang, J.; Zhang, J.; Yang, S. Fabrication and characterization of Zr and Co co-

doped LiMn2O4 nanowires using sol–gel–AAO template process. J Mater Sci: Mater

Electron. 2006, 17, 865–870.

78. Iqbal, M. J.; Ahmad, Z. Electrical and dielectric properties of lithium manganate

nanomaterials doped with rare-earth elements. J. Power Sources. 2008, 179, 763–769.

79. Bonino, F.; Panero, S.; Satolli, D.; Scrosati, B. Synthesis and characterization of

Li2MxMn4-xO8 (M = Co, Fe) as positive active materials for lithium-ion cells. J. Power

Sources. 2001, 97-98, 389–392.

80. Rojas, R. M.; Amarilla, J. M.; Pascual, L.; Rojo, J. M.; Kovacheva, D.; Petrov, K.

Combustion synthesis of nanocrystalline LiNiYCo1−2YMn1+YO4 spinels for 5V cathode

materials: Characterization and electrochemical properties. J. Power Sources. 2006,

160, 529–535.

81. Liu, H.; Song, L.; Zhang, K. Er-Doped LiMn2O4. Inorg. Mater. 2005, 41(6), 646–649.

82. Arumugam, D.; Kalaignan, G. P.; Manisankar, P. Development of structural stability

and the electrochemical performances of „La‟ substituted spinel LiMn2O4 cathode

materials for rechargeable lithium-ion batteries. Solid State Ionics. 2008, 179, 580–586

83. Kock, A. de.; Ferg, E.; Gummow, R. J. The effect of multivalent cation dopants on

lithium manganese spinel cathodes. J. Power Sources. 1998, 70, 247–252.

84. Pistoia, G.; Antonini, A.; Rosati, R.; Zane, D. Storage characteristics of cathodes for

Li-Ion batteries. Electrochim. Acta. 1996, 41, 2683–2689.

85. Raja, M. W.; Mahanty, S.; Basu. R. N. Influence of S and Ni co-doping on structure,

band gap and electrochemical properties of lithium manganese oxide synthesized by

soft chemical method. J. Power Sources. 2009, 192, 618–626.

86. Yi, T. F.; Shu, J.; Zhu, Y. R.; Zhu, R. S. Advanced electrochemical performance of

LiMn1.4Cr0.2Ni0.4O4 as 5 V cathode material by citric-acid-assisted method. J Phys

Chem Solids. 2009, 70, 153–158.

http://www.sciencedirect.com/science/article/pii/S0378775306000218

http://www.sciencedirect.com/science/article/pii/S0378775306000218

38

CHAPTER 2: EXPERIMENTAL

This chapter is specified for the experimental part of the work, which describes the

chemicals used, instrumentation employed and the procedures adopted for the synthesis and

analysis work.

2.1 Chemicals used

The detail of the chemicals used is given in Table 2.1.

Table 2.1 Chemicals used with their essential specifications.

S. No Chemical Names Chemical Formula Mol. mass

( g.mol-1

) Purity

(%) Supplier

1

2

3

4

5

6

7

8

9

10

Lithium acetate

Manganese(II)acetate

Citric acid

Zinc nitrate hexa

hydrate

Copper(II)acetate

Chromium(III)nitrate

nona hydrate

Nickel(II)nitrate

hexa hydrate

Lanthanum(II)nitrate

hexa hydrate

Germanium(IV) Oxide

Niobium(V) Oxide

C2H3LiO2

Mn(CH3CO2)2

C6H8O7

N2O6Zn.6H2O

Cu(OOCCH3)2

Cr(NO3)3.9H2O

Ni(NO3)2.6H2O

La(NO3)3.6H2O

GeO2

Nb2O5

65.99

173.03

192.12

297.47

181.63

400.15

290.7

433.01

104.59

265.81

99.95

98.0

99.5

98

99.999

99.99

99.999

99.95

99.99

99.0

Aldrich

Aldrich

Sigma Aldrich

Alfa Aesar

Alfa Aesar

Aldrich

Aldrich

Sinopharm

Alfa Aesar

Alfa Aesar

39

Besides the above mentioned chemicals, the following compounds were used for making

cathode composites to be used in the coin cells. N-Methylpyrrolidone C5H9NO (NMP),

Polyvinylidene fluoride (PVDF) binder, Hiblack-4020 powder, LiPF6 dissolved in ethylene

carbonate/ dimethyl carbonate (1:1 in volume) an electrolyte for Li-ion battery. All these

chemicals were supplied by MTI corp.

2.2. Instrumentation

Spinel LiMn2O4 and its doped analogues were characterized both structurally and

electrochemically. The products were characterized for the, crystal structure, surface

morphology, particle size and qualitative and quantitative composition analysis. The techniques

employed with respective equipment model are; X-ray diffraction (XRD, Panalytical X'Pert-Pro

MPD), scanning electron microscopy (SEM) by Hitachi S-4800, energy-dispersive X-ray

spectroscopy (EDAX) by Horiba EMAX 7593-H, thermo gravimetric analysis (TGA/DTA) by

PerkinElmer Diamond, transmission electron microscopy (TEM) by Tecnai G20 S-TWIN, and

Fourier transform infrared spectroscopy (FTIR) by PerkinElmer spectrometer PE2000. Chemical

composition of the pure and doped LiMn2O4 was determined by inductively coupled plasma

optical emission spectrometry (ICP-OES) by Perkin Elmer Optima 5300DV.

Electrochemical characterizations were carried out using cyclic voltammetry (CV),

electrochemical impedance spectroscopy (EIS) and galvanostatic charge discharge

measurements. CV and EIS measurements were performed on an electrochemical workstation

CHI 660C. The galvanostatic charge/discharge experiments were performed between 3.0 and 4.8

V using LAND battery testing system (CT2001A).

40

2.3 Procedure

2.3.1 Synthesis of samples

2.3.1.1 Sol-gel method: Synthesis of inorganic oxides by sol-gel method involves the gelation of

aqueous/ non-aqueous precursors with concentration corresponding to the stoichiometry of the

final products followed by slow evaporation and calcinations to yield the desired product. On

drying the precursor solution under the set of conditions, the formed gel is heat treated to dried

powder which is converted to ultra-fine particles form by grinding. Among the various synthetic

methods, sol-gel method can produce highly homogenous particles with narrow size distribution,

fine particle size and uniform composition that result in better electrochemical performance.

Keeping in view the above mentioned advantages, sol-gel method was used in the present study

for the synthesis of the pure and various bi-metal doped spinels LiMn2O4.

2.3.1.2 Procedure: Spinel LiMn2O4 powders were prepared by the sol-gel method using citric

acid as a chelating agent. Stoichiometric amount of lithium acetate (1.0 mol) and manganese

acetate (2.0 mol) salts with the cationic ratio of Li : Mn = 1:2 were separately dissolved in the

de-ionized water and then mixed well with an aqueous solution of citric acid. Citric acid to metal

ions molar ratio was 1. Ammonium hydroxide was slowly added to this solution with a constant

stirring to control the pH at 6.0. The resultant solution was evaporated at 80 oC while being

mechanically stirred with a magnetic stirrer until a thick gel precursor was obtained. The gel

precursor obtained was dried in oven at 120 o

C for 5 h to remove moisture and thereby obtained

the dry mass. From the powder obtained, small sample was taken for thermo gravimetric analysis

to understand the thermal behaviour in air. Finally, the powder obtained was first heated at 400

oC for 5 h and then at 750

oC for 10 h at a heating rate of 5

oC per min to obtain fine black

powder of LiMn2O4. The flow chart for the sol-gel synthesis is shown in Fig. 2.1.

41

Fig. 2.1 Flow chart for the synthesis of the pure and doped LiMn2O4 by the sol-gel method.

Similarly, all the doped series of nominal composition LiMxM'yMn2-x-yO4 (M and M' are

the various doping metal cations used in this study, where x = y = 0.01-0.05) were also

synthesized using the same method by adding the stoichiometric amount of the doping metals

aqueous solutions.

Uniform mixing

with stirring Mixed aqueous solution

Citric acid

(aqueous solution)

Li(CH3CO2) &

Mn(CH3CO2)2

(aqueous solutions)

Aquous solution

Aqueous Ammonia,

pH adjusted at 6.0

Homogeneous solution

Stirring and heating

at 80 o

C

Annealing at 400 oC for

5 h & 750 o

C for 10 h

Gel precursors

Spinel LiMn2O4 powder

Drying at 120 o

C for

5 h

42

2.3.1.3 Optimization of pH and synthesis temperature: Spinel LiMn2O4 was synthesized by the

sol-gel method. The synthesis conditions particularly annealing temperature and pH were

optimized to get pure phase of LiMn2O4. To optimize the synthesis temperature, LiMn2O4 was

prepared at various annealing temperatures (i.e. 600-800 oC) for 10 h. The corresponding XRD

patterns are shown in Fig. 2.2. It can be seen that sample annealed at 600 o

C shows a small

impurity peak at about 32o, which can be assigned to Mn2O3 a common impurity found during

the synthesis of LiMn2O4. With the increase in annealing temperature to 750 oC the main

diffraction peaks of spinel LiMn2O4 such as (111), (311) and (400) became sharper and well

developed, showing the higher crystallinity. Thus, 750 oC was selected as the optimized

annealing temperature for the synthesis of the pure and doped LiMn2O4.

Fig.2.2 XRD patterns of LiMn2O4 synthesized at different temperature: (a) 600 oC (b) 700

oC

(c) 750 oC (d) 800

oC, (* = Mn2O3).

43

Similarly, spinel LiMn2O4 was synthesized at different pH (6.0, 6.5, 7.0, 7.5, and 8.0)

values (Fig. 2.3). The LiMn2O4 was calcined at 750 o

C for 10 h. The citric acid to metal ions

molar ratio was 1. No impurities were observed at all the pH values. However, highly intense

and sharp diffraction peaks were obtained for the samples synthesized at pH 6.0 and 6.5. Hence,

pH 6.0-6.5 was selected as suitable range for the synthesis of phase pure spinel LiMn2O4. Thus

the pure and all the doped series of LiMn2O4 were synthesized by sol-gel method at pH 6.0 and

750 oC temperature for 10 h annealing time.

Fig.2.3 XRD patterns of LiMn2O4 synthesized at different pH: (a) 6.0 (b) 6.5 (c) 7.0 (d) 7.5

(e) 8.0.

44

2.3.1.4 Coin cells fabrication: The electrochemical properties of the synthesized pure and

various bi-metals doped LiMn2O4 were analyzed by making CR2032 coin-type cells with lithium

metal as the negative electrode. Electrode materials are usually in the form of powder. Therefore,

the electrode material is casted on a thin sheet of metal substrate that will act as a current

collector. Li+ ions are known to form alloy with aluminum metal sheet below 0.5 V vs. Li metal,

but Cu does not. Therefore, Cu substrate is used to test the materials below that potential. On the

other hand, for potential above 3 V vs. Li metal, Cu gets oxidized but not the Al metal. Thus Al

substrate is used to test material at potential above 3 V. Here, also Al metal was used as a metal

substrate to act as a current collector. Since, LiMn2O4 is a semiconductor so carbon was used as

an electronic conductive agent. “The active material, acetylene black and polyvinylidene fluoride

(a binder) were mixed by mortar and pestle in a weight ratio of 80:10:10. N-methyl-2-

pyrrolidone (NMP) was used as a solvent to make paste or slurry. The blended slurry was then

casted onto an Al foil and dried at 120 oC for 12 h in a vacuum oven. Then, circular cathode

discs were punched from the Al foil and weighed to determine the amount of active materials

before being loaded into coin-type cells. Coin type-cells were assembled in an argon-filled glove

box”[1]. The electrolyte used was composed of 1 M LiPF6 dissolved in ethylene carbonate/

dimethyl carbonate.

The test material was in the form of cathode disc, Li metal and polyethylene separator.

Both the Li metal and separator were also in the form of circular discs. These discs were placed

in coin cell can in the following order; cathode disk, separator and Li metal (that act as an

anode). The electrolyte was added before sealing the coin cells. A flat metal disc was placed on

the top of the electrode stack. A metal spring was also placed on the top of the flat metal disc to

45

maintain pressure on the cell stack. Finally, the coin cell lid was crimped into the can to

complete the fabrication of the coin cell.

2.3.2 Analysis protocol

2.3.2.1 TGA/DTA: The thermogravimetric analysis was carried out of the LiMn2O4 powder

material dried at 140 o

C to assess the pure single phase temperature. The samples were heated

from 25 oC to 900

oC in air, while concluding 750

oC as the suitable temperature.

2.3.2.2 XRD: The samples were run for XRD measurements over a range of 10 to 80o (2θ) at a

step size of 0.02o and at a scan rate of 1 s/ step.

2.3.2.3 FTIR: FTIR spectra were recorded using KBr pellets. All the synthesized samples were

scan from 400-4000 cm-1

.

2.3.2.4 SEM, TEM and EDX analysis: For TEM and EDX analysis, samples were dissolved in

de-ionized water and then putted a drop of the dissolved sample on Cu or Ni grid support and

then dried. For SEM, the aqueous solution of the sample was drop on silicon substrate and then

after drying was analyzed.

2.3.2.5 ICP-OES: Aqueous solutions of the synthesized samples were used for ICP-OES to

determine the exact stoichiometric compositions.

2.3.2.6 CV: All the voltammetric measurements were carried out using CR2032 coin-type cells

with the synthesized sample as cathode (working electrode) and the Li metal as anode (reference

electrode). The potential scan was run between 3.4-4.8 V at a scan rate of 0.1 mV s-1

. Each

measurement was carried out at least three times to ensure the reproducibility of the results. The

technique was employed to find out the redox potential of the material and to assess the facility

of electron transfer of the processes concerned.

46

2.3.2.7 EIS: Similarly, the EIS was also done on two electrode system as mention above. The

Nyquist plots were obtained for the frequency range of 1.0 Hz to 100000 Hz at the open circuit

potential. The results were used to find out the charge transfer resistance of the Li/Li+

intercalation de-intercalation process.

2.3.2.8 Charge/ discharge measurements: Galvanostatic charge/ discharge measurements were

carried out to determine the electrochemical performances of the synthesized cathode materials

for lithium-ion batteries. The electrochemical properties of the products were analyzed by

making CR2032 coin-type cells with lithium metal as the negative electrode. The active material,

acetylene black and polyvinylidene fluoride were mixed in a weight ratio of 80:10:10. N-methyl-

2-pyrrolidone was used as a solvent to make the slurry. The slurry was then casted onto an Al

foil current collector and dried at 120 oC for 12 h in a vacuum oven. Then, circular cathode discs

were punched from the Al foil. The punched cathodes were weighed to determine the amount of

active materials before being loaded into coin-type cells. The coin cells were assembled in an

argon-filled glove box. The electrolyte composed of 1 M LiPF6 dissolved in ethylene

carbonate/dimethyl carbonate (1:1 in volume). Galvanostatic charge/discharge experiments were

performed between 3.0 and 4.8 V using LAND battery testing system (CT2001A).

References

1. Xi, L. J.; Wang, H. E.; Lu, Z. G.; Yang, S. L.; Ma, R. G.; Deng, J. Q.; Chung, C. Y.

Facile synthesis of porous LiMn2O4 spheres as positive electrode for high-power lithium

ion batteries. J. Power Sources. 2012, 198, 251–257.

47

CHAPTER 3: RESULTS AND DISCUSSION

The chapter describes the results of the experiments conducted to synthesize and

characterize the samples of six bi-metal doped series of spinel LiMn2O4. The results are

discussed and interpreted as per requirement of the data and supported with established facts

wherever found necessary. The chapter is divided into three parts. First of all the structural and

morphological aspects are discussed. Second part gives an account on the voltammetric and

impedance measurements; indeed it gives an insight about the electrochemical conduction,

kinetics and mechanism responsible for Li ions. The third part is dedicated for the charge-

discharge studies which actually invoke the electrochemical performance and efficiency of the

synthesized samples. It is attempted to develop a logical link between the performance and

composition and attributes of the material while discussing the results. At the end the salient

features of the work are summarized.

3.1 Structural and morphological properties

LiMn2O4 doped with various multiple metal cations, Ni-Cr, La-Zn, Cu-Zn, Cu-Cr, Ni-Zn

and Ni-Cu, was synthesized by the sol-gel method as described in detail in the experimental

section. Citric acid was used as a chelating agent The structural and morphological

characterizations of all the synthesized samples were carried out using thermal analysis (TGA/

DTA), X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR), scanning

electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-

ray spectroscopy (EDX). The chemical compositions were analyzed by inductively coupled

plasma optical emission spectrometry (ICP-OES).

48

3.1.1 Thermal analysis

Thermal analysis (TGA/DTA) was performed to determine the exact temperature for the

phase formation reaction of the pure and bi-metal doped LiMn2O4. Fig. 3.1 shows thermograms

of the pure LiMn2O4 sample in the range from 30 o

C to 900 o

C. For the pure LiMn2O4, the TGA

curve clearly exhibits two weight loss regions. The initial low weight loss of about 5% up to 150

oC is attributed to the removal of water due to the hygroscopic nature of the material. A further

major weight loss region between 150 o

C and 300 oC, which is also accompanied by a sharp

exothermic peak at 285 oC in the DTA curve, is attributed to the decomposition of the acetate

precursors and chelating agent [1, 2]. After 300 o

C, the TG curve becomes flat with no

appreciable weight loss and also no sharp peak can be seen in the DTA curve. This indicates that

no phase transformation takes place and also that further heating only enhanced the crystallinity

of spinel LiMn2O4.

Fig. 3.1 TGA/ DTA analysis of the pure LiMn2O4 sample.

49

The representative thermo-grams (TGA/ DTA) for the bi-metal doped LiMn2O4 samples

with Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn are shown in Fig. 3.2. All the doped samples

follow the same thermal behaviour like the pure LiMn2O4. A low weight loss (~ 4%) at about

170 oC corresponds to the removal of water, while a major weight loss below 400

oC which is

also accompanied by a strong exothermic peak in the DTA curve is due to the pyrolysis of the

acetate and chelating agent [2]. After about 350 oC, no appreciable weight loss occurs and the

TGA curves become flat indicating the formation of the doped LiMn2O4. From the thermal

analysis, it can be concluded that the spinel LiMn2O4 starts to form at about 300 to 350 oC, but it

has also been reported [1] that at low temperature (250 and 450 oC) some other impurities such

as α-Mn2O3 and LiMn2O3 are also present. So, all the pure and doped samples were synthesized

at 750 oC to get phase pure and well crystallized spinel compounds.

50

Fig. 3.2 TGA/DTA of (a) LiNi0.03Cr0.03Mn1.94O4 (b) LiLa0.03Zn0.03Mn1.94O4

(c) LiCu0.03Cr0.03Mn1.94O4 (d) LiCu0.03Zn0.03Mn1.94O4 (e) LiNi0.03Cu0.03Mn1.94O4

(f) LiNi0.03Zn0.03Mn1.94O4.

51

3.1.2 X-ray diffraction studies

X-ray powder diffraction (XRD) patterns of the pure LiMn2O4 and Ni-Cr substituted

series, i.e., Li[NixCryMn2-x-y]O4 (x = y = 0.01 - 0.05) are shown in Fig.3.3. All the XRD patterns

showed well-defined peaks, indicating that the product has gained single-phase spinel structure

without any impurities detectable from XRD measurements. Compared with the pure LiMn2O4

sample, the XRD patterns of all the Ni-Cr dual doped samples have no extra reflections. This

indicates that Ni and Cr have entered the structure of LiMn2O4 rather than forming impurities.

All the patterns can be assigned to well-crystallized spinel LiMn2O4 (JCPDS No. 35-0782). The

Bragg peaks of the pure and doped LiMn2O4 were indexed to a cubic system with space group

Fd3m in which the lithium ions occupy the tetrahedral (8a) sites and the Mn3+

/ Mn4+

cations, as

well as the doped metal ions reside at the octahedral (16d) sites [3]

Fig.3.3 XRD patterns of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 (c) LiNi0.02Cr0.02Mn1.96O4

(d) LiNi0.03Cr0.03Mn1.94O4 (e) LiNi0.04Cr0.04Mn1.92O4 (f) LiNi0.05Cr0.05Mn1.90O4.

52

The lattice parameters of the pure and Ni-Cr doped LiMn2O4 samples were calculated

from the XRD data, and are shown in Table 3.1. A decrease in the lattice parameter „a‟ for Ni-Cr

doped samples was confirmed. The observed change in the lattice parameter may reflect a

change in the M (Mn, Ni, and Cr)-O bonding, which is stronger than Mn-O bonding and finally

results in the reduction of lattice parameter [4]. This decrease in lattice parameters proved the

successful doping of Ni2+

and Cr3+

in LiMn2O4. The ionic radius of Cr3+

is 0.615 Å, which is

smaller than Mn3+

(0.645 Å), so the substitution of chromium into the spinel framework reduces

the unit cell volume [5]. Also, the large binding energy of CrO2 (1142 kJ/ mol) compared to

MnO2 (946 kJ/ mol) may also results in the stabilization of the octahedral sites [6]. Furthermore,

the reduction in size is also due to the substitution of Ni2+

and Cr3+

ions that resulted in the

formation of more Mn4+

cation that has a small ionic radius than that of Mn3+

, which causes the

contraction of the spinel framework [7]. The structural change during charge/ discharge cycling

that result in capacity fading may be suppressed in this way.

Table 3.1 Lattice parameter and unit cell volume for the pure and Ni-Cr doped samples.

S. No

Sample

Lattice constant „a’

(Å)

Unit cell Volume

(Å3)

1

2

3

4

5

6

LiMn2O4

LiNi0.01Cr0.01Mn1.98O4





8.2478

8.1805

8.1785

8.1753

8.1733

8.1545

561.06

547.44

547.04

546.40

546.00

542.24

53

The powder XRD patterns of La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn doped series are

shown in Fig. 3.4-3.8. From the XRD patterns, it is clear that all the synthesized compounds

reported in this research work crystallize as single phase products in the cubic spinel Fd3m space

group. Doping LiMn2O4 with such small amount of metals did not affect the spinel structure and

even for the higher dopants concentration, a well-defined spinel structure was retained. However,

due to the difference in the nature of different cations, their binding energies, ionic radii and the

site preferences, a slight shift in the peak position and variable peak widths was noted for the

doped compounds. Furthermore, no impurity peaks can be seen for the pure and various multi-

metal doped spinels LiMn2O4. However, at higher dopants concentration for La-Zn doped

LiMn2O4 (Fig. 3.4), a small impurity peak at 32.84° appeared that can be assigned to Mn2O3 [8],

a common impurity during the synthesis of LiMn2O4.

The lattice constant „a‟ and unit cell volume (V) for the pure and La-Zn, Cu-Cr, Cu-Zn,

Ni-Cu and Ni-Zn doped spinel LiMn2O4 are shown in table 3.2-3.6. The lattice constant „a‟ was

calculated from the XRD data using equation 2.2 as described in the experimental section. The

lattice constant and the unit cell volume of the doped compounds were smaller than that of the

pure LiMn2O4 (8.2478 Å). For La-Zn doped series there was a slight decrease in the lattice

parameter. Although the ionic radius of La3+

(1.032 Å) is larger than that of Mn3+

(0.645 Å) [5],

the observed decrease in the value of „a‟ may be explained as; lanthanum being a rare earth

element has larger binding energy than manganese and therefore has higher octahedral site

preference that result in the decrease of the bond length and lattice parameter. [9]. Also, La3+

doping results in the formation of more Mn4+

ions that are smaller than Mn3+

, so the lattice

constant decreased. A. de Kock et al., [10] has also found a slight decrease in lattice constant

upon Zn2+

doping in LiMn2O4.

54

The value of „a‟ for Ni-Cu, Ni-Zn, Cu-Cr, and Cu-Zn doped LiMn2O4 also showed a shrinkage

of the unit cell. However, with increasing dopant concentration the lattice constant also

increased. This may be due to the difference in the effective ionic radii of the doped metals and

manganese cations, because at higher dopant contents, it becomes difficult for the spinel cubic

lattice to accommodate larger cations. Thus, the unit cell gets increased, resulting in the higher

lattice constant. The initial decrease in the lattice constant for Ni-Cu, Cu-Cr, and Cu-Zn may be

due to the fact that when the amount of Cu2+

ions was smaller, most of the Cu2+

ions may

occupied the tetrahedral sites in the spinel LiMn2O4, thus resulted in smaller lattice parameter

than the pure LiMn2O4. Because, according to Shannon et al., [5] the effective ionic radius of

tetrahedral coordinated Cu2+

(0.57 Å) is somewhat smaller than Li+ ion (0.59 Å). However, when

the amount of Cu increases, Cu2+

will also occupy the octahedral sites that were initially

occupied by the Mn3+

ions. The lattice constant „a‟ will finally increase [11] with the increase of

doped Cu amount as the effective ionic radius of octahedral coordinated Cu2+

(0.73 Å) is larger

than Mn3+

(0.645 Å) ion.

55

Fig.3.4 XRD patterns of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 (c) LiLa0.02Zn0.02Mn1.96O4

(d) LiLa0.03Zn0.03Mn1.94O4 (e) LiLa0.04Zn0.04Mn1.92O4 (f) LiLa0.05Zn0.05Mn1.90O4.

Table 3.2 Lattice parameter and unit cell volume for the pure and La-Zn doped samples.

S. No

Sample

Lattice constant „a’

(Å)

Unit cell Volume

(Å3)

1

2

3

4

5

6

LiMn2O4

LiLa0.01Zn0.01Mn1.98O4





8.2478

8.2452

8.2463

8.2462

8.2497

8.2463

561.06

560.54

560.76

560.74

561.45

560.76

56

Fig.3.5 XRD patterns of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4

(d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4 (f) LiCu0.05Cr0.05Mn1.90O4.

Table 3.3 Lattice parameter and unit cell volume for the pure and Cu-Cr doped samples.

S. No

Sample

Lattice constant ‘a’

(Å)

Unit cell Volume

(Å3)

1

2

3

4

5

6

LiMn2O4

LiCu0.01Cr0.01Mn1.98O4





8.2478

8.2372

8.2384

8.2460

8.2467

8.2557

561.06

558.90

559.15

560.69

560.84

562.68

57

Fig.3.6 XRD patterns of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 (c) LiCu0.02Zn0.02Mn1.96O4

(d) LiCu0.03Zn0.03Mn1.94O4 (e) LiCu0.04Zn0.04Mn1.92O4 (f) LiCu0.05Zn0.05Mn1.90O4.

Table 3.4 Lattice parameter and unit cell volume for the pure and Cu-Zn doped samples.

S. No

Sample


(Å)

Unit cell Volume

(Å3)

1

2

3

4

5

6

LiMn2O4

LiCu0.01Zn0.01Mn1.98O4





8.2478

8.2351

8.2372

8.2381

8.2387

8.2433

561.06

558.47

558.91

559.08

559.21

560.15

58

Fig.3.7 XRD patterns of (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 (c) LiNi0.02Cu0.02Mn1.96O4

(d) LiNi0.03Cu0.03Mn1.94O4 (e) LiNi0.04Cu0.04Mn1.92O4 (f) LiNi0.05Cu0.05Mn1.90O4.

Table 3.5 Lattice parameter and unit cell volume for the pure and Ni-Cu doped samples.

S. No

Sample


(Å)

Unit cell Volume

(Å3)

1

2

3

4

5

6

LiMn2O4

LiNi0.01Cu0.01Mn1.98O4





8.2478

8.1658

8.1673

8.1681

8.1892

8.1897

561.06

544.49

544.79

544.95

549.19

549.29

59

Fig.3.8 XRD patterns of (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 (c) LiNi0.02Zn0.02Mn1.96O4

(d) LiNi0.03Zn0.03Mn1.94O4 (e) LiNi0.04Zn0.04Mn1.92O4 (f) LiNi0.05Zn0.05Mn1.90O4.

Table 3.6 Lattice parameter and unit cell volume for the pure and Ni-Zn doped samples.

S. No

Sample


(Å)

Unit cell Volume

(Å3)

1

2

3

4

5

6

LiMn2O4

LiNi0.01Zn0.01Mn1.98O4





8.2478

8.2052

8.1952

8.1835

8.1764

8.1342

561.06

552.42

550.40

548.05

546.62

538.20

60

3.1.3 Chemical composition analysis

The chemical compositions of the pure and doped samples were analyzed by inductively

coupled plasma optical emission spectrometry (ICP-OES). From Tables 3.7 and 3.8, it can be

seen that the stoichiometric molar ratio of all the synthesized samples is very close to the

nominal compositions. These data support the XRD patterns and the conclusion that dopant ions

fit well in the spinel structure while partially replacing Mn sites.

Table 3.7 Chemical Compositions of the pure LiMn2O4, Ni-Cr and La-Zn doped series

Series

Nominal composition

Experimental composition

Pure

Ni-Cr

La-Zn

LiMn2O4





LiNi0.05Cr0.05Mn1.9 O4






Li0.98Mn1.98O4

Li1.044Ni0.0098Cr0.011Mn1.976O4

Li1.042Ni0.0188Cr0.019Mn1.965O4

Li1.031Ni0.029Cr0.027Mn1.944O4

Li0.974Ni0.038Cr0.037Mn1.925O4

Li1.011Ni0.047Cr0.046Mn1.906O4

Li0.975La0.009Zn0.089Mn1.978O4

Li1.043La0.019Zn0.0175Mn1.956O4

Li1.021La0.028Zn0.026Mn1.935O4

Li0.982La0.039Zn0.036Mn1.917O4

Li0.954La0.048Zn0.047Mn1.879O4

61

Table 3.8 Chemical Compositions of Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn series

Series

Nominal composition

Experimental composition

Cu-Cr

Cu-Zn

Ni-Cu

Ni-Zn







LiCu0.02Zn0.02Mn1.96O4-










LiNi0.02Zn 0.02Mn1.96O4

LiNi0.03Zn 0.03Mn1.94O4

LiNi0.04Zn 0.04Mn1.92O4

LiNi0.05Zn 0.05Mn1.90O4

Li0.983Cu0.013Cr0.014Mn1.977O4

Li0.972Cu0.018Cr0.019Mn1.966O4-

Li1.051Cu0.027Cr0.028Mn1.936O4

Li1.012Cu0.038Cr0.037Mn1.925O4

Li1.054Cu0.049Cr0.048Mn1.893O4

Li1.034Cu0.013Zn0.011Mn1.985O4

Li0.983Cu0.023Zn0.019Mn1.964O4

Li0.962Cu0.028Zn0.027Mn1.938O4

Li1.053Cu0.044Zn0.046Mn1.919O4

Li1.013Cu0.056Zn0.049Mn1.894O4

Li1.023Ni0.015Cu0.013Mn1.978O4

Li1.011Ni0.025Cu0.023Mn1.958O4

Li1.004Ni0.033Cu0.035Mn1.945O4

Li0.986Ni0.044Cu0.046Mn1.916O4

Li1.042Ni0.049Cu0.054Mn1.897O4

Li1.043Ni0.014 Zn 0.015Mn1.976O4

Li1.012Ni0.022 Zn 0.024Mn1.956O4

Li1.031Ni0.035 Zn 0.033Mn1.935O4

Li1.001Ni0.043 Zn 0.045Mn1.925O4

Li1.002Ni0.053 Zn 0.051Mn1.865O4

62

3.1.4 FTIR spectroscopic studies

FTIR spectra of the pure and varying amount of Ni-Cr doped LiMn2O4 in the wave

number range of 400-4000 cm-1

are shown in Fig. 3.9. The bands observed in the range of 400-

600 cm-1

are characteristic of metal-oxygen vibration peaks, reflecting the local environment of

octahedral MO6 unit. These vibrational modes are susceptible to the point group symmetry of the

metal cations in the oxygen host lattice [12]. For the pure and Ni-Cr doped samples, two distinct

peaks in the region of 514-622 cm-1

were observed that can be assigned to the stretching vibration

of MO6 octahedral groups [13]. It has been suggested that the Li-O vibration of the spinel cannot

be recognized in this range. It has also been observed that in some cases the intensity and peak

widths of the alkali metal cations depend on the mass of the cations [14]. Compared to the pure

LiMn2O4, the IR bands for the Ni-Cr doped samples were shifted slightly towards higher wave

number. This clearly showed the octahedral site preferences of the doped cations (i.e., Ni2+

and

Cr3+

). It has been reported [15] that the increase in the frequency is caused by the decrease in the

site radius. Here, also the shift in the frequency towards higher wave number demonstrates the

fact that Ni2+

and Cr3+

doping resulted in the decrease of the octahedral site radius. This can be

attributed to the higher binding energies of Ni and Cr compared to the Mn-O. This observation

has been found to be in agreement with the XRD results shown in Table 3.1.

63

Fig.3.9 FTIR spectra of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 (c) LiNi0.02Cr0.02Mn1.96 O4


The FTIR spectra of La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn doped series are shown in

Figs. 3.10 to 3.14. For La-Zn doped samples, the characteristic bands for LiMn2O4 appeared at

507-617cm-1

. From Fig. 3.10, it can be seen that the peaks shift towards lower wave number (507

cm-1

) region upon La and Zn substitution. This shift may be related to the atomic masses of the

doped cations. The change in the IR peak positions also confirmed the successful incorporation

of La and Zn ions into the spinel matrix. The FTIR spectra of Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn

doped series were also similar to the pure LiMn2O4. The characteristic peaks observed in the

range of 515-626 cm-1

were assigned to the stretching vibration of MO6 octahedral groups. It

means that no structural change takes place for the low content doping in the spinel LiMn2O4.

However, there is a slight frequency shift towards higher wave number pointing to the successful

doping of the metal cations. Compared to the pure spinel LiMn2O4, the shift in the high wave

64

number band from 617 cm-1

to 626 cm-1

for the doped samples is also pointing towards the

increase of the average manganese valence [16] that will results in the suppression of Jahn-Teller

distortion and improved electrochemical performance as evidenced in later sections.

Fig.3.10 FTIR spectra of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 (c) LiLa0.02Zn0.02Mn1.96O4


65

Fig.3.11 FTIR spectra of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4


Fig.3.12 FTIR spectra of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 (c) LiCu0.02Zn0.02Mn1.96O4


66

Fig.3.13 FTIR spectra of (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 (c) LiNi0.02Cu0.02Mn1.96O4


Fig.3.14 FTIR spectra of (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 (c) LiNi0.02Zn0.02Mn1.96O4

(d) LiNi0.03Zn0.03Mn1.94O4 (e) LiNi0.04Zn0.04Mn1.92O4 (f) LiNi0.05Zn0.05Mn1.90O4

67

3.1.5 SEM, TEM and EDX analysis

It has already been accepted that, the morphology and particle size distribution are the

most important parameters that have a marked influence on the electrochemical performance of

lithium-ion batteries. To examine the surface morphology of the pure and doped LiMn2O4,

scanning electron microscopy (SEM) was used. SEM images of the pure and doped LiMn2O4 are

shown in Figs. 3.15 to 3.20. The existence of finer particle surface with well-defined morphology

is obvious from the SEM images. Compared with the pure LiMn2O4, low agglomeration of the

particles was found for the doped analogues. This also indicates that crystallites agglomeration to

form bigger particles has not occurred for the doped samples during sol-gel synthesis at 750 o

C.

Furthermore, it is also evident that all the doped samples exhibit uniform size with smooth

faceted polyhedral particles indicating their high crystallinity.

Figs. 3.21 to 3.26 show the TEM images for the synthesized samples. From the TEM

images it is evident that unlike pure LiMn2O4, the doped samples are composed of more uniform

and smaller particles. Furthermore, it can also be seen that the Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-

Cu and Ni-Zn doped series were consisted of particles that showed individual grain morphology

with well-separated grain boundaries. The average particle size ranged from about 42 nm to 250

nm. The particle size increased with increase in the dopant concentration which is consistent

with the XRD results. For La-Zn doped spinel LiMn2O4, TEM images showed that the samples

are composed of relatively smaller particles as compared to the other doped series. This may be

due to the large binding energy of the rare earth element (La), so it reduced the Mn3+

content,

increased the average valence of Mn (i.e., Mn4+

which has a smaller ionic radius than Mn3+

),

stabilized the spinel structure and decreased the possibility of co-operative Jahn-Teller distortion

68

effect. Thus is expected to result in the enhanced electrochemical performance [17]. For Ni-Cu,

Cu-Cr, and Cu-Zn doped samples the initial decrease in particle size may be due to the fact that

when the amount of Cu2+

ions was smaller, most of the Cu2+

ions may occupied the tetrahedral

sites in the spinel LiMn2O4, thus resulted in smaller lattice parameter than the pure LiMn2O4. As

mentioned earlier [5] the effective ionic radius of tetrahedral coordinated Cu2+

(0.57 Å) is

somewhat smaller than Li+ (0.59 Å). On the other hand, when the amount of Cu is increased,

Cu2+

will also occupy the octahedral sites that were initially occupied by the Mn3+

ions. So, the

particle size get increased as the effective ionic radius of octahedral coordinated Cu2+

(0.73 Å) is

larger than Mn3+

(0.645 Å) ion [11].

Inset of image (b) in Figs. 3.21 to 3.26 is the HRTEM image of the doped samples having

dopants amount x = 0.01, y = 0.01, which revealed a high crystallinity with no apparent

imperfections. The clear lattice fringes with an interplanar spacing of about 0.47 nm or 0.48 nm

point to the orientation of (111) atomic planes. The well-defined polyhedral particle morphology

of all the doped samples compared to the pure spinel LiMn2O4 is predictable to improved

electrochemical performance when used as cathode materials in lithium-ion batteries.

69

Fig. 3.15 SEM images of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 (c) LiNi0.02Cr0.02Mn1.96O4


70

Fig. 3.16 SEM images of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 (c) LiLa0.02Zn0.02Mn1.96O4


71

Fig. 3.17 SEM images of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4


72

Fig. 3.18 SEM images of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 (c) LiCu0.02Zn0.02Mn1.96O4


73

Fig. 3.19 SEM images of (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 (c) LiNi0.02Cu0.02Mn1.96O4


74

Fig. 3.20 SEM images of (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 (c) LiNi0.02Zn0.02Mn1.96O4


75

Fig. 3.21 TEM images of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 (c) LiNi0.02Cr0.02Mn1.96O4


76

Fig. 3.22 TEM images of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 (c) LiLa0.02Zn0.02Mn1.96O4


77

Fig. 3.23 TEM images of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4


78

Fig. 3.24 TEM images of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 (c) LiCu0.02Zn0.02Mn1.96O4


79

Fig. 3.25 TEM images of (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 (c) LiNi0.02Cu0.02Mn1.96O4


80

Fig. 3.26 TEM images of(a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 (c) LiNi0.02Zn0.02Mn1.96O4


81

Energy dispersive X-ray spectroscopy (EDX) was also used to analyze the composition

of the synthesized samples. Figs.3.27 to 3.31 present the EDX profiles of the pure and Ni-Cr, La-

Zn, Cu-Cr, Cu-Zn and Ni-Zn doped LiMn2O4. EDX measurements confirmed the co-existence of

all the doped elements in the synthesized samples. The reflections corresponding to the

constituent elements confirmed that the synthesized samples were pure. For Ni-Cr, La-Zn and

Ni-Zn doped spinel LiMn2O4, the Cu signal is originated from the copper grid support for EDX

analysis.

Fig. 3.27 EDX profiles of (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 (c) LiNi0.02Cr0.02Mn1.96O4


82

Fig. 3.28 EDX profiles of (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 (c) LiLa0.02Zn0.02Mn1.96O4


83

Fig. 3.29 EDX profiles of (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4


84

Fig. 3.30 EDX profiles of (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 (c) LiCu0.02Zn0.02Mn1.96O4


85

Fig. 3.31 EDX profiles of (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 (c) LiNi0.02Zn0.02Mn1.96O4


86

3.2 Electrochemical characterization

The electrochemical performances of all the synthesized cathode materials were

evaluated using CR2032 coin-type cells consisting of metallic lithium as the anode. The working

electrode (cathode) was composed of active material, carbon black and a binder (polyvinylidene

fluoride; PVDF) in a weight ratio of 80:10:10. The electrolyte used was 1M LiPF6 dissolved in

1:1 volume fraction of ethylene carbonate/ dimethyl carbonate. Coin cells were fabricated in an

argon filled glove box with moisture and oxygen concentrations below 1.0 ppm. Cyclic

voltammetry and electrochemical impedance spectroscopy (EIS) measurements were performed

on CHI660C electrochemical workstation.

3.2.1 Cyclic voltammetric studies

CV measurements were made to evaluate the effect of doping elements on the structural

stabilization of spinel LiMn2O4 cathode. Figs.3.32 to 3.37 show the cyclic voltammograms for

the pure and Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn doped LiMn2O4. CV curves were

recorded over the potential range of 3.4-4.8 V at a scan rate of 0.1 mV s-1

. All the synthesized

samples showed two pairs of well-defined anodic and cathodic peaks at around 4.0 V that

correspond to Mn3+

/ Mn4+

redox couple [18]. The anodic and cathodic peaks observed in the CV

curves represent the reversible redox reactions that correspond to Li ions removal and insertion

from/ into the spinel framework. Two pairs of peaks observed in the cyclic voltammograms of

the pure and various multi-metals (or bi-metal) doped LiMn2O4 indicated that insertion and

extraction of Li ions take place in two steps, and further confirmed that the intercalation and de-

87

intercalation of Li ions is a reversible process. The corresponding reactions for the insertion/

extraction of Li ions [19] can be written as

LiMn2O4→ Li0.5Mn2O4 + 0.5Li++0.5e (1)

(the reaction occurring at the 1st anodic peak)

Li0.5Mn2O4 → Mn2O4 + 0.5Li+

+ 0.5e (2)

(the reaction occurring at the 2nd

anodic peak)

Mn2O4 + 0.5Li+

+ 0.5e → Li0.5Mn2O4 (3)

(the reaction occurring at the 2nd

cathodic peak)

Li0.5Mn2O4 + 0.5Li+ + 0.5e → LiMn2O4 (4)

(the reaction occurring at the 1st

cathodic peak)

The presence of the characteristic peaks of LiMn2O4 at around 4.0 V clearly indicated that

even for the doped samples, Mn is the active material. In other words the specific capacity

obtained is due to the reversible oxidation/ reduction of Mn3+

/ Mn4+

redox couple and that the

doping cations do not take part in the redox process. However, for Ni-Cr and Ni-Cu doped

LiMn2O4, samples with relatively higher amount of doping elements (such as

LiNi0.04Cr0.04Mn1.92O4, LiNi0.05Cr0.05Mn1.90O4, LiNi0.04Cu0.04Mn1.92O4 and LiNi0.05Cu0.05Mn1.90O4)

showed two weak split peaks at around 4.7 V that correspond to Ni2+

/ Ni3+

and Ni3+

/ Ni4+

redox

couples [6]. As the amount of the doped Ni cations is smaller compared to the Mn ions that is

why weak peaks are obtained for these samples. Furthermore, the redox peaks for all the doped

compounds are sharp and showed well-defined splitting; indicating the structural stabilization of

various bi-metal doped spinel LiMn2O4 materials [3].

88

From the CV curves, it can also be seen that for all the doped samples, the oxidation and

reduction peaks are much closer (i.e., the difference between the anodic peak potential (Epa) and

the cathodic peak potential (Epc) is smaller), the peak current is increased and the peak width is

narrowed which means that the internal resistance for the doped LiMn2O4 cathodes is minimum

and the diffusion rate of Li+ ions is the fastest compared to the pure LiMn2O4. These results

substantiated that the polarization of LiMn2O4 has been reduced due to faster insertion/

extraction of Li+ ions into the spinel matrix. Thus, improved electrochemical performance of the

doped LiMn2O4 cathodes can be expected.

89

Fig. 3.32 Cyclic voltammograms for (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4

(c) LiNi0.02Cr0.02Mn1.96O4 (d) LiNi0.03Cr0.03Mn1.94O4 (e) LiNi0.04Cr0.04Mn1.92O4

(f) LiNi0.05Cr0.05Mn1.90O4, at a scan rate of 0.1 mV s-1

.

90

Fig. 3.33 Cyclic voltammograms for (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4

(c) LiLa0.02Zn0.02Mn1.96O4 (d) LiLa0.03Zn0.03Mn1.94O4 (e) LiLa0.04Zn0.04Mn1.92O4

(f) LiLa0.05Zn0.05Mn1.90O4, at a scan rate of 0.1 mV s-1

.

91

Fig. 3.34 Cyclic voltammograms for (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4

(c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4

(f) LiCu0.05Cr0.05Mn1.90O4, at a scan rate of 0.1 mV s-1

.

92

Fig. 3.35 Cyclic voltammograms for (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4

(c) LiCu0.02Zn0.02Mn1.96O4 (d) LiCu0.03Zn0.03Mn1.94O4 (e) LiCu0.04Zn0.04Mn1.92O4

(f) LiCu0.05Zn0.05Mn1.90O4, at a scan rate of 0.1 mV s-1

.

93

Fig. 3.36 Cyclic voltammograms for (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4

(c) LiNi0.02Cu0.02Mn1.96O4 (d) LiNi0.03Cu0.03Mn1.94O4 (e) LiNi0.04Cu0.04Mn1.92O4

(f) LiNi0.05Cu0.05Mn1.90O4, at a scan rate of 0.1 mV s-1

.

94

Fig. 3.37 Cyclic voltammograms for (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4

(c) LiNi0.02Zn0.02Mn1.96O4 (d) LiNi0.03Zn0.03Mn1.94O4 (e) LiNi0.04Zn0.04Mn1.92O4

(f) LiNi0.05Zn0.05Mn1.90O4, at a scan rate of 0.1 mV s-1

.

95

3.2.2 Electrochemical impedance spectroscopy (EIS)

In an effort to better understand the synergetic effect of Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-

Cu and Ni-Zn dual doping on the electrochemical cycling performance of the LiMn2O4,

electrochemical impedance spectroscopy (EIS) measurement was performed. EIS is a powerful

technique to identify the kinetics of Li ions in oxide cathodes. Electrochemical impedance

measurements can very well explain and differentiate the contribution of lithium ions migration

through the electrode surface, charge transfer resistance through the electrode/ electrolyte

interface and the diffusion of Li ions through the bulk of the electrode materials [20]. The

impedance response for all the synthesized pure and various bi-metal doped cathode materials

exhibited a semicircle at high frequency region (lower Z' values) and a straight line at low

frequency region (higher Z' values). The high frequency semicircle can be ascribed to the charge

transfer resistance (Rct) and the inclined line at low frequency corresponds to the lithium

diffusion process within the active material or the so called Warburg diffusion. The numerical

value of the diameter of the semicircle on Z'real axis in the Nyquist plots is approximately equal

to the charge transfer resistance (Rct) between the cathode active material and the electrolyte

[21].

Fig.3.38 shows the Nyquist plots for the pure and Ni-Cr doped sample electrodes, it can

be seen that the semicircles for the doped electrodes are smaller compared to the pure LiMn2O4,

which indicated that Ni-Cr doped electrodes have experienced lowest charge transfer resistance

(Rct). Obviously, the pure LiMn2O4 has the highest charge transfer resistance (214 Ω) compared

to Ni-Cr doped samples (e.g. LiNi0.01Cr0.01Mn1.98O4 has Rct equal to 93 Ω) pointing to the higher

polarization of the electrode that will result in reduced lithium ion kinetics and poor cycling

performance. The higher impedance for the pure LiMn2O4 may be probably due to the formation

96

of a thick solid interface layer. This solid interface layer has a marked influence on the charge

transfer process as lithium ions have to pass through the electrode/ electrolyte interface. The

above result showed that for Ni-Cr doped electrodes, the solid electrolyte interface (SEI) layer

formation is reduced and the Li+ ions transfer rate through the spinel framework is enhanced [3].

Furthermore, the lower values of charge transfer resistance (Rct) for the doped samples also

indicated that Li+ ions can transfer more quickly through the electrode/ electrolyte interface due

to better electronic conductivity. This reveals that Ni and Cr dual doping improve the electronic

conductivity of the electrode as the polarization of the electrode is greatly reduced which may

result in better cycling performance.

Fig.3.38 Nyquist plots for (a) LiMn2O4 (b) LiNi0.01Cr0.01Mn1.98O4 (c) LiNi0.02Cr0.02Mn1.96O4


97

The Nyquist plots for the other doped series like La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn

are shown in Figs.3.39 to 3.43. All the doped samples exhibited similar impedance response like

Ni-Cr doped electrodes. Unlike pure LiMn2O4, the lower Rct values for the various multi-metals

doped electrodes clearly indicates anenhancement in the conductivity and consequent increase in

high rate cycling performance. Small impedance is favourable for the efficient extraction/

insertion of Li ions from/ into the spinel LiMn2O4 during charge/ discharge process. Similar

decrease in electrochemical impedance was also reported for LiMn2O4 doped with Cu, Al and Ti

as well as for olivine LiFePO4 [21, 22]. The data obtained from EIS and CV measurements

indicated that better electrochemical performance could be achieved for the various multi-metals

doped spinel materials due to the structural stability and enhanced conductivity.

Fig.3.39 Nyquist plots for (a) LiMn2O4 (b) LiLa0.01Zn0.01Mn1.98O4 (c) LiLa0.02Zn0.02Mn1.96O4


98

Fig.3.40 Nyquist plots for (a) LiMn2O4 (b) LiCu0.01Cr0.01Mn1.98O4(c) LiCu0.02Cr0.02Mn1.96O4

(d) LiCu0.03Cr0.03Mn1.94O4 (e) LiCu0.04Cr0.04Mn1.92O4(f) LiCu0.05Cr0.05Mn1.90O4.

Fig.3.41 Nyquist plots for (a) LiMn2O4 (b) LiCu0.01Zn0.01Mn1.98O4 (c) LiCu0.02Zn0.02Mn1.96O4


99

Fig.3.42 Nyquist plots for (a) LiMn2O4 (b) LiNi0.01Cu0.01Mn1.98O4 (c) LiNi0.02Cu0.02Mn1.96O4


Fig.3.43 Nyquist plots for (a) LiMn2O4 (b) LiNi0.01Zn0.01Mn1.98O4 (c) LiNi0.02Zn0.02Mn1.96O4


100

3.3 Charge/ discharge performance

The synthesized samples (both pure and bi-metal doped LiMn2O4) were systematically

investigated for their actual use as cathode materials for lithium-ion batteries. The study includes

the determination of initial capacity of the material, its variation with time (cycle life test) in the

given potential range and C-rate capability.

3.3.1 Galvanostatic charge/ discharge studies

The first discharge profiles for the pure and various bi-metal doped LiMn2O4 are shown

in the panels (A) of Fig.3.44-3.49. The cells were charged and discharged at a current density of

43 mAg-1

(0.3 C), while the voltage was maintained between 3.0 and 4.8 V. In this potential

range, the discharge curves for all the synthesized samples correspond to the characteristic

voltage profiles of the spinel LiMn2O4 associated with the occupation of lithium at tetrahedral

sites in agreement with the reported literature [23-25]. The discharge profiles for the pure and

doped cathode materials clearly depicted two plateaus at around 4.0 V and 4.1 V vs. Li/ Li+. The

presence of these two plateaus is due to the two step oxidation/ reduction process which

corresponds to the two pairs of redox peaks in the CV curves discussed earlier in section 3.2.1.

It has been reported that the higher voltage plateau at about 4.1 V corresponds to two

phase transition of λ-MnO2/ Li0.5Mn2O4 vs. Li/ Li+, while the second plateau at lower potential

(about 4.0 V) indicates single phase transition between Li0.5Mn2O4/ LiMn2O4. By suppressing

the formation of these unstable phases, the electrochemical performance of LiMn2O4 based

cathode materials could be improved [26, 27]. Compared to the un-doped LiMn2O4, the

boundaries of the two plateaus in the discharge curves of Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-Cu

and Ni-Zn doped cathode materials were not highly sharp. This may point towards the fact that

101

the doping elements have stabilized the spinel structure by suppressing the Jahn-Teller effect

which is considered to be one of the main cause of capacity fading in LiMn2O4 [27]. The

enhanced structural stability for the various bi-metal doped LiMn2O4 is due to the stronger M-O

(M = various doping elements like La, Cr, Ni etc. used in this study) binding energies compared

to the Mn-O.

3.3.2 Cycling performance

The cycling performance of the pure and doped LiMn2O4 cathode materials is shown in

the panels (B) of Figs.3.44 to 3.49. It can be seen that the cycling performance or cycleability of

the various bi-metal doped samples is notably improved. Although the pure LiMn2O4 has the

highest initial discharge capacity of 122 mAhg-1

in the first cycle, it only has 88 mAhg-1

after

100 cycles at 0.3 C. Thus 72% of the initial discharge capacity was retained after 100 cycles. The

pure LiMn2O4 has experienced a capacity fade of 0.34 mAhg-1

cycle-1

. Factors such as the

structural degradation due to co-operative Jahn-Teller distortion effect and the dissolution of Mn

into the electrolyte during charge/ discharge cycling have been suggested to be the main causes

for capacity fade [11].

Spinel LiMn2O4 samples doped with varying amount of Ni/ Cr delivered initial discharge

capacities of 113, 107, 106, 119 and 84 mAhg-1

and they retained 87, 83, 81, 80 and 84%

discharge capacities respectively after 100 cycles (Fig. 3.44B). It can be seen that the discharge

capacity decreased (except LiNi0.04Cr0.04Mn1.92O4) with increasing amount of the Ni and Cr. This

indicated that even for the doped samples, Mn3+

was the active material and contributes to the

specific capacities during charge/ discharge process. The higher discharge capacity for

LiNi0.04Cr0.04Mn1.92O4 was due to the fact that here some of the Ni ions get oxidized/ reduced

102

during the electrochemical reaction and thus contributed to the total discharge capacity. This

result is in agreement with the cyclic voltammetric studies (Fig. 3.32e) which showed two weak

split peaks at around 4.7 V that corresponded to Ni2+

/ Ni3+

and Ni3+

/ Ni4+

redox couples [28].

Among all the Ni-Cr doped cathode materials, LiNi0.01Cr0.01Mn1.98O4 showed the best

electrochemical performance.

103

Fig. 3.44 (A) First Discharge curves (B) Capacity vs. cycle number plots for (a) LiMn2O4

(b) LiNi0.01Cr0.01Mn1.98O4(c) LiNi0.02Cr0.02Mn1.96O4 (d) LiNi0.03Cr0.03Mn1.94O4


104

Similar trends were observed for the other bi-metal doped LiMn2O4 synthesized by the

sol-gel method in this study. Lanthanum and zinc co-doped spinel LiMn2O4 delivered initial

discharge capacities of 118, 111, 108, 106 and 105 mAhg-1

and experienced a capacity fade of

0.18, 0.22, 0.187, 0.165 and 0.26 mAhg-1

cycle-1

for the La : Zn contents corresponding to

0.01:0.01, 0.02:0.02, 0.03:0.03, 0.04:0.04, 0.05:0.05, respectively (Fig. 3.45B). The large

binding energy of the rare earth doped element (La) and Zn has stabilized the spinel structure by

suppressing the Jahn-Teller effect [17]. Gummow et al. have also reported that Zn2+

addition

improve the cycling stability of LiMn2O4. Thus some strong synergistic effect of La-Zn co-

doping may stabilize the spinel structure and resulted in improved cycling performance

compared to the pure LiMn2O4.

For Cu-Cr doped samples, the initial discharge capacities for the increasing amount of Cu

and Cr were 112, 112, 97, 56 and 68 mAhg-1

, respectively as shown in Fig.3.46B. After 100

charge/ discharge cycles, the synthesized Cu-Cr doped LiMn2O4 cathode materials delivered

discharge capacities of 93, 82, 72, 47 and 60 mAhg-1

that corresponded to the capacity retention

of 83, 73, 74, 84 and 88% of the initial discharge capacities. It can be seen that compared to the

pure LiMn2O4 that has 72% capacity retention after 100 cycles, the cycling performance of Cu-

Cr doped samples is better. Although LiCu0.04Cr0.04Mn1.92O4 and LiCu0.05Cr0.05Mn1.90O4 showed

higher capacity retention but their initial discharge capacities were low. This may be due to the

relatively higher amount of the doping elements. The sample with the lowest Cu/ Cr contents i.e.,

LiCu0.01Cr0.01Mn1.98O4 showed the best electrochemical performance both in terms of the initial

discharge capacity as well as the cycling stability. The large binding energy of CrO2 (1142 kJ/

mol) compared to MnO2 (946 kJ/ mol) has resulted in the stabilization of the octahedral sites and

thus enhanced the cycling performance [6]. Furthermore, copper doping in the spinel oxide has

105

been reported to induce higher electronic conductivity and lower diffusion barrier for the

intercalation/ de-intercalation of Li+ ions that resulted in higher rate capability [11]. The lower

charge transfer resistance (Rct) for the Cu-Cr doped LiMn2O4 is evident from the EIS

measurement shown in Fig. 3.40.

Similarly, Cu-Zn doped cathode materials also stabilized the spinel structure as is clear

from the excellent cycling stability. Galvanostatic charge/ discharge studies (Fig. 3.47B)

indicated that Cu-Zn doping could effectively improve the cycleability of LiMn2O4. Here, also

the improved electrochemical performance (Table 3.12) for all the Cu-Zn doped LiMn2O4

samples compared with the pure LiMn2O4 is attributed to the well-defined polyhedral particle

morphology (Fig. 3.24) as well as the stabilization of the spinel structure as evident from the

XRD and CV results.

Fig. 3.48(B) and Fig. 3.49(B) represent the cycling performance of Ni-Cu and Ni-Zn

doped spinel LiMn2O4. Ni-Cu doped cathode materials delivered initial discharge capacities of

113, 108, 107, 118 and 104 mAhg-1

and exhibited capacity retention of 88, 86, 83, 77 and 80%

for the Ni : Cu contents corresponding to 0.01:0.01, 0.02:0.02, 0.03:0.03, 0.04:0.04, 0.05:0.05,

respectively. The decrease in the initial discharge capacity with increasing amount of Ni and Cr

indicated that even for the doped LiMn2O4, only Mn3+

contributed to the charge/ discharge

capacity during the electrochemical reaction. However, just like Ni-Cr doped sample,

LiNi0.04Cu0.04Mn1.92O4 also delivered higher initial discharge capacity compared to the other

doped analogues, this may be due to the Ni2+

/ Ni3+

and Ni3+

/ Ni4+

redox couples [28] as is shown

in the CV curves (Fig. 3.36) by the weak split peaks at around 4.7 V. For LiNi0.05Cu0.05Mn1.90O4,

after 2nd

cycle the initial discharge capacity abruptly decreases from 104 to 87 mAhg-1

, this may

be due to some small short circuit of the coin cell, as it can retain 95% of the initial discharge

106

capacity at the 3rd

cycle over the investigated 100 cycles. Among all the investigated Ni-Cu

doped samples, LiNi0.01Cu0.01Mn1.98O4 showed the best electrochemical performance in terms of

initial discharge capacity and excellent cycling stability.

Fig. 3.49(B) presents the cycling performance of the Ni-Zn doped LiMn2O4. Compared

with the pure LiMn2O4, Ni-Zn doped samples also showed stability of the spinel structure for the

investigated charge/ discharge cycling.

107


(b) LiLa0.01Zn0.01Mn1.98O4 (c) LiLa0.02Zn0.02Mn1.96O4 (d) LiLa0.03Zn0.03Mn1.94O4


108


(b) LiCu0.01Cr0.01Mn1.98O4 (c) LiCu0.02Cr0.02Mn1.96O4 (d) LiCu0.03Cr0.03Mn1.94O4


109


(b) LiCu0.01Zn0.01Mn1.98O4 (c) LiCu0.02Zn0.02Mn1.96O4 (d) LiCu0.03Zn0.03Mn1.94O4


110


(b) LiNi0.01Cu0.01Mn1.98O4 (c) LiNi0.02Cu0.02Mn1.96O4 (d) LiNi0.03Cu0.03Mn1.94O4


111


(b) LiNi0.01Zn0.01Mn1.98O4 (c) LiNi0.02Zn0.02Mn1.96O4 (d) LiNi0.03Zn0.03Mn1.94O4


112

Tables 3.9 to 3.14 summarized the electrochemical performances of the pure and Ni-Cr,

La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn doped LiMn2O4. It is evident that, although the various

bi-metal doped series have low initial discharge capacities than the pure LiMn2O4, but they

exhibited better cycling performance compared to the pure LiMn2O4. Furthermore, the decrease

in the initial discharge capacity with increasing concentration of the doping elements indicated

that even for the doped materials, Mn3+

effectively contributed to the specific capacity during the

electrochemical reaction. For all the doped series, sample with the lowest doping contents i.e.,

LiM0.01M'0.01Mn1.98O4 (where M and M' are the various doping metal cations used in this study)

performed slightly better than the higher doping concentrations. This means that for the

synthesized various bi-metal doped LiMn2O4, the optimum dopant level in terms of capacity and

cycling stability is approximately x = 0.01, y = 0.01. A close look on the experimental CV

profiles showed that the charge transfer redox peaks are sharper and with large currents for x =

y= 0.01 mol dopant content than for the higher amounts. This reveals that, this very amount of

the dopants kinetically facilitates the Li+ intercalation/ deintercalation process, while a bit higher

amounts starts decelerting the process. A second evidence comes from EIS data giving the

smallest Rct values for 0.01 mol content of dopants and this value starts increasing for higher

values of dopants. Though a very small deviation in Ni-Cu series occured in favour of 0.02 mol

content that could be ignored as experimental error. Nevertheless, the overall picture supports the

same conclusion as stated above.

The better cycling stability for the doped LiMn2O4 at the expense of specific capacity is

mainly attributed to the inhibition of the Jahn-Teller effect on deep discharge of the various bi-

metal doped LiMn2O4 cathode materials. Furthermore, unlike pure LiMn2O4 that upon complete

extraction of Li from the spinel matrix results in an unstable λ-MnO2, all the Li cannot be

113

extracted from the doped samples. The presence of this residual Li in the delithiated bi-metal

doped spinel LiMn2O4 is considered to play a significant role in enhancing the structural stability

to the repeated Li insertion/ extraction processes [29].

Table 3.9 Electrochemical Performance of the pure and Ni-Cr doped LiMn2O4

S. No Sample Specific capacity

(mAhg-1

) at 1st

cycle

Specific capacity

(mAhg-1

) after

100 cycles

Capacity

Retention

(%)

1

2

3

4

5

6

LiMn2O4






122

113

107

106

119

84

88

98

89

86

95

71

72

87

83

81

80

84

114

Table 3.10 Electrochemical Performance of the pure and La-Zn doped LiMn2O4

Table 3.11 Electrochemical Performance of the pure and Cu-Cr doped LiMn2O4


(mAhg-1

) at 1st

cycle

Specific capacity

(mAhg-1

) after

100 cycles

Capacity

Retention

(%)

1

2

3

4

5

6

LiMn2O4






122

118

111

108

106

105

88

99.3

85.4

89.3

89.5

79

72

84

77

82

84

75


(mAhg-1

) at 1st

cycle

Specific capacity

(mAhg-1

) after

100 cycles

Capacity

Retention

(%)

1

2

3

4

5

6

LiMn2O4






122

112

112

97

56

68

88

93

82

72

47

60

72

83

73

74

84

88

115

Table 3.12 Electrochemical Performance of the pure and Cu-Zn doped LiMn2O4

Table 3.13 Electrochemical Performance of the pure and Ni-Cu doped LiMn2O4


(mAhg-1

) at 1st

cycle

Specific capacity

(mAhg-1

) after

100 cycles

Capacity

Retention

(%)

1

2

3

4

5

6

LiMn2O4






122

113

108

107

118

104

88

99

93

89

91

83

72

88

86

83

77

80


(mAhg-1

) at 1st

cycle

Specific capacity

(mAhg-1

) after 100

cycles

Capacity

Retention

(%)

1

2

3

4

5

6

LiMn2O4






122

113

105

104

88

85

88

96

87

85

75

68

72

85

83

82

85

80

116

Table 3.14 Electrochemical Performance of the pure and Ni-Zn doped LiMn2O4


(mAhg-1

) at 1st

cycle

Specific capacity

(mAhg-1

) after

100 cycles

Capacity

Retention

(%)

1

2

3

4

5

6

LiMn2O4






122

114

82

89

82

57

88

91

69

72

65

40

72

80

84

81

79

70

117

3.3.3 Rate capability

Besides good cycling performance, the high rate capability is also of great importance

especially for better electrochemical performance as well as high power applications and was a

major aim of this study to be investigated for the synthesized samples. The pure LiMn2O4 and the

representative sample LiM0.01M'0.01Mn1.98O4 electrodes (where M and M' are the various doping

metal cations used in this study) from each of the doped series were cycled at various rates

within a cut-off voltage of 3.0-4.8 V. The cells for each material were charged to 4.8 V at

constant low current rate (0.1 C) and discharged to 3.0 V at 0.1 C, 0.3 C, 0.5 C, 1 C, 2 C and 5 C

values.

Fig.3.50 shows the rate capability of the undoped LiMn2O4 and LiNi0.01Cr0.01Mn1.98O4 at

various C-rates. As expected, LiNi0.01Cr0.01Mn1.98O4 electrode exhibited good cycling stability at

varying C-rates. The specific capacities were 114, 111, 104, 102, 99 and 94 mAhg-1

, when the

current rates were 0.1, 0.3, 0.5, 1, 2 and 5 C, respectively. At high current rate of 5 C, 82% of the

initial discharge capacity in the first cycle was retained. Remarkably, after deep cycling at higher

rates, a discharge capacity of 104 mAhg-1

was resumed upon reducing the current rate to 0.1 C

which was 91% of the specific capacity in the first cycle. For comparison, the cycling

performance of the pure LiMn2O4 under the same conditions was also investigated. A very low

capacity 51 mAhg-1

, which is about 41% of the initial discharge capacity can be delivered at 5 C.

Evidently, LiNi0.01Cr0.01Mn1.98O4 has shown improved electrochemical performance than the

pure sample. With the co-doping of Ni and Cr, the lattice volume is decreased which stabilized

the spinel structure. As Ni2+

and Cr3+

ions were substituted for Mn3+

, so the content of Mn3+

in

the spinel was decreased. This resulted in the suppression of Jahn-Teller distortion which is

considered as one of the main reason for capacity fading in LiMn2O4 electrode. Also the strength

118

of the M-O bond (M = Ni, Cr) is greater than Mn-O bond [4, 18, 30] that also prevented the

degradation of the material, so the Ni-Cr co-doping has stabilized the spinel structure and finally

resulted in better electrochemical performance. Furthermore, the smaller nanoparticles with well-

defined polyhedral morphology of LiNi0.01Cr0.01Mn1.98O4 will shorten the diffusion length of Li+

and electron as evidenced from the EIS measurement (Fig.3.38). This will result in efficient Li+

insertion/ de-insertion processes that give rise to improved cycling stability. Based upon the

above results, LiNi0.01Cr0.01Mn1.98O4 may be considered a good candidate for the cathode

component of Li-ion batteries. The recently published work [31] about similar bi-cation doping

along with surface coating of CuO could be an interesting investigation in future with such low

content doping as presented in this study.

Fig.3.50 Rate capability at 0.1-5 C of the pure LiMn2O4 and LiNi0.01Cr0.01Mn1.98O4.

119

The rate capability tests for La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn doped LiMn2O4 are

shown in Figs.3.51 to 3.55. Compared with the pure spinel LiMn2O4, LiLa0.01Zn0.01Mn1.98O4

showed greatly improved cycling response to continuously varying current rates (Fig. 3.51). The

discharge capacities of the LiLa0.01Zn0.01Mn1.98O4 were 118, 112, 109, 107, 102, 92 mAhg-1

at 0.1

C, 0.3 C, 0.5 C, 1 C, 2 C and 5 C respectively. Capacity retention of 78% was recorded at the

high current rate of 5 C. After the high rate cycling, the current rate was reduced back to 0.1 C

and a discharge capacity of about 113 mAhg-1

was restored, which is 95% of the discharge

capacity in the first cycle. In contrast, the undoped LiMn2O4 delivered a discharge capacity of

122.3 mAhg-1

at 0.1 C. At 5 C current rate, the capacity reduced to 51 mAhg-1

, which is about

41% of the initial discharge capacity, much lower than the La-Zn doped sample. This severe

capacity loss was attributed to the Jahn-Teller distortion effect, structural instability and slow

dissolution of the materials. It has been known that the solvent (i.e., electrolyte) oxidation also

plays a very important role in the dissolution of spinel LiMn2O4 [32]. The improved rate

capability and electrochemical performance of LiLa0.01Zn0.01Mn1.98O4 originated from smaller

particle size, better chemical and structural stability with lower Jahn-Teller distortion which was

also consistent with the CV results shown in Fig.3.33. The above results showed that

LiLa0.01Zn0.01Mn1.98O4 may be a good candidate for the cathode component of lithium-ion

batteries.

Fig.3.52 shows the discharge capacities at varying currents for the pure LiMn2O4 and

LiCu0.01Cr0.01Mn1.98O4. It can be seen that Cu-Cr doped sample exhibited high capacity retention

than the pure spinel LiMn2O4. As a comparison, LiCu0.01Cr0.01Mn1.98O4 cathode material was

able to deliver initial discharge capacity of 114 mAhg-1

at 0.1 C. However, cycling at high

current rate (5 C) provided 93 mAhg-1

discharge capacity, which is about 81% of the initial

120

discharge capacity at 0.1 C. After deep cycling at high current rates, a discharge capacity of 106

mAhg-1

was recovered that is 93% of the initial discharge capacity. This electrochemical

performance was superior to that of the pure LiMn2O4 which showed only 41% capacity

retention at high current rate of 5 C. The improved rate performance for LiCu0.01Cr0.01Mn1.98O4

may be due to the higher electronic and ionic conductivities as is clear from the EIS results

(Fig.3.40). It has been reported that although Cu doped spinel provide lower discharge capacity,

it results in stable electrochemical cycling [11]. The higher octahedral stabilization energy of

Cr3+

may also contribute in the stability of the LiMn2O4 structure [33]. The higher conductivity,

small particle size and good structural stability resulted in the improved electrochemical

performance of Cu-Cr doped LiMn2O4.

Similarly, Cu-Zn doped spinel (LiCu0.01Zn0.01Mn1.98O4) also showed good cycling

performance at different rates as shown in Fig.3.53. The discharge capacities at 0.1 C, 0.3 C, 0.5

C, 1 C, 2 C and 5 C were 114, 112, 104, 103, 99 and 78 mAhg-1

respectively. At high current rate

of 5 C, it can retained 67% of the initial discharge capacity (114 mAhg-1

) at 0.1 C. Upon

reducing back the current rate to 0.1 C after the high rate measurement (5 C), about 95% of the

initial discharge capacity is restored which is far better than the pure LiMn2O4 that retained only

41% of the capacity when cycled at 5 C. This also showed the stabilization of the spinel

LiMn2O4 structure resulting from the Cu-Zn co-doping.

Just like Ni-Cr, La-Zn, Cu-Cr and Cu-Zn doped spinel LiMn2O4 cathode materials,

LiNi0.01Cu0.01Mn1.98O4 and LiNi0.01Zn0.01Mn1.98O4 also resulted in improved rate capability when

cycled at varying current rates. From Fig.3.54 and Fig.3.55, it can be seen that both

LiNi0.01Cu0.01Mn1.98O4 and LiNi0.01Zn0.01Mn1.98O4 showed stable cycling performance at higher

121

current rates compared to the pure LiMn2O4. This clearly verified that the spinel LiMn2O4

framework has been stabilized by the Ni-Cu and Ni-Zn co-doping.

Based upon the above results, it can be concluded that bi-metal doping of LiMn2O4 with

small amount of metal cations can effectively reduce the capacity fading by suppressing the

Jahn-Teller distortion effect, improve the rate performance at high current rates by reducing the

charge transfer resistance (Rct) as evident from the EIS measurement.

Fig. 3.51 Rate capability at 0.1-5 C of the pure LiMn2O4 and LiLa0.01Zn0.01Mn1.98O4.

122

Fig. 3.52 Rate capability at 0.1-5 C of the pure LiMn2O4 and LiCu0.01Cr0.01Mn1.98O4.

Fig. 3.53 Rate capability at 0.1-5 C of the pure LiMn2O4 and LiCu0.01Zn0.01Mn1.98O4.

123

Fig. 3.54 Rate capability at 0.1-5 C of the pure LiMn2O4 and LiNi0.01Cu0.01Mn1.98O4.

Fig. 3.55 Rate capability at 0.1-5 C of the pure LiMn2O4 and LiNi0.01Zn0.01Mn1.98O4.

124

Conclusions

Looking at the experimental findings of the current study while focusing on the

objectives set earlier in chapter 1, the study can be concluded as follows.

LiMn2O4 and its doped analogues (i.e., Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn

doped LiMn2O4) were successfully synthesized by the sol-gel method at 750 oC using

citric acid as a chelating and combusting agent. The dopant content was kept between

0.01 to 0.05 moles in each case. XRD measurement confirmed that all the synthesized

compounds reported in this research work crystallized as single phase products in the

cubic spinel Fd3m space group. Doping LiMn2O4 with such small amount of metals has

not affected the spinel structure and for all the dopant concentrations, the well-defined

spinel structure was retained. EDX analysis also showed the purity of the synthesized

samples.

ICP-OES analysis has shown that stoichiometric compositions of the pure as well as

doped LiMn2O4 are in good agreement with the nominal compositions. SEM and TEM

images showed that compared to the pure LiMn2O4, all the doped samples exhibited

uniform size with smooth faceted polyhedral particles indicating their higher crystallinity.

HRTEM images also confirmed the highly crystalline nature of all the six doped series.

Cyclic voltammetric measurements established that for all the doped samples, the

oxidation and reduction peaks are much closer to each other. Furthermore, on doping, the

peak current increased and the peak width is narrowed which imply that the internal

resistance for the doped LiMn2O4 cathodes is smaller and the diffusion rate of Li+ ions is

faster compared to the pure LiMn2O4. These results revealed that the polarization of

LiMn2O4 has been reduced due to faster insertion/ extraction of Li+ ions into the spinel

125

matrix. The direct consequence of reduced polarization during charge/ discharge cycling

is the improved electrochemical performance of the doped LiMn2O4 cathode materials.

The charge transfer resistance (Rct) was found to decrease upon doping spinel LiMn2O4

with Ni-Cr, La-Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn. This behaviour demonstrated the

enhanced conductivities of the doped samples that resulted in faster kinetics of Li ions.

Thus when evaluated as cathode materials for lithium-ion batteries, improved rate

performance was observed for the doped LiMn2O4.

The discharge profiles for the pure and doped cathode materials clearly depicted two

plateaus. The presence of two voltage plateaus at around 4.0 V and 4.1 V vs. Li/Li+

in the

discharge profiles verified the two step oxidation/ reduction process for the insertion/

extraction of lithium ions into the spinel matrix. This also indicated that the doped metals

were not involved in the oxidation/ reduction process. Thus only Mn3+

ions contributed to

the specific capacity during the electrochemical reaction. That is why, the various bi-

metal doped LiMn2O4 cathode materials have lower initial discharge capacity than that of

the pure LiMn2O4.

The cycling performance of the various bi-metal substituted samples was significantly

improved due to the stabilization of the spinel LiMn2O4 structure. The stable cycling

performance over the investigated 100 charge/ discharge cycles clearly depicted that

doping spinel LiMn2O4 with small amount of metal pairs (Ni-Cr, La-Zn, Cu-Cr, Cu-Zn,

Ni-Cu and Ni-Zn) is an effective strategy to suppress the Jahn-Teller distortion effect,

lower the charge transfer resistance and improve the electrochemical performance.

The results also imparted that the rate capability for the doped LiMn2O4 has been

enhanced. For example, at high current rate of 5 C, the capacity retention for Ni-Cr, La-

126

Zn, Cu-Cr, Cu-Zn, Ni-Cu and Ni-Zn doped samples (x = 0.01, y = 0.01) was 82%, 78%,

81%, 67%, 62% and 58%, respectively compared with the pure LiMn2O4 which retained

only 41% of the initial discharge capacity when cycled at high current rate of 5 C.

For all the synthesized doped series, sample with the lowest doping metals contents

LiM0.01M'0.01Mn1.98O4 (where M and M' are the various doping metal cations used in this

study) appeared to be the best composition both in terms of the initial discharge capacity

and the rate capability. Thus, it can be concluded that low content metal doping is an

effect way to improve the electrochemical performance of LiMn2O4 based cathode

materials for lithium-ion batteries.

In summary, the low content bi-metal doping into spinel LiMn2O4 matrix through

sol-gel process considerably improved the stability of the material. The low content of

metals did not disturb the crystal structure of LiMn2O4 but on the other hand remarkably

improved the cycling performance with decreased capacity fading both at low and high

C-rates. The concept of low content bi-metal doping is proposed for large scale cathode

material development. The study also point towards the future work ideas such as; (i) tri-

metal doping (ii) calculation of kinetic parameters and mechanistic model development,

and (iii) computer simulations for such cases followed by target synthesis and due

characterization.

127

References

1. Thirunakaran, R.; Sivashanmugam, A.; Gopukumar, S.; Dunnill, C. W.; Duncan H.

Gregory, D. H. Phthalic acid assisted nano-sized spinel LiMn2O4 and LiCrxMn2-xO4 (x =

0.00–0.40) via sol–gel synthesis and its electrochemical behaviour for use in Li-ion-

batteries. Mater. Res. Bull. 2008, 43, 2119–2129.

2. Arrebola, J. C.; Caballero, A.; Cruz, M.; Hernán, L.; Morales, J.; Castellón. E. R.

Crystallinity control of a nanostructured LiNi0.5Mn1.5O4 spinel via polymer-assisted

synthesis: A method for improving its rate capability and performance in 5 V lithium

batteries. Adv. Funct. Mater. 2006, 16, 1904–1912.


properties of chemically substituted LiMn2O4 prepared by a solution-based gel method. J.

Colloid Int. Sci. 2006, 300, 633–639.

4. Wei, Y. J.; Yan, L.Y.; Wang, C. Z.; Xu, X. G.; Wu, F.; Chen. G. Effects of Ni doping on

[MnO6] octahedron in LiMn2O4. J. Phys. Chem. B. 2004, 108, 18547–18551.

5. Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic

distances in halides and chalcogenides. Acta Cryst. 1976, A32, 751–767.

6. Yi, T. F.; Shu, J.; Zhu, Y. R.; Zhu, R. S. Advanced electrochemical performance of

LiMn1.4Cr0.2Ni0.4O4 as 5 V cathode material by citric-acid-assisted method. J Phys Chem

Solids. 2009, 70, 153–158.

7. Bellitto, C.; Bauer, E. M.; Righini, G.; Green, M. A.; Branford, W. R.; Antonini, A.;

Pasquali, M. The effect of doping LiMn2O4 spinel on its use as a cathode in Li-ion

batteries: neutron diffraction and electrochemical studies. J Phys Chem Solids. 2004, 65,

29–37.

8. Zhang, D.; Popov, B. N.; White, R. E. Electrochemical investigation of CrO2.65 doped

LiMn2O4 as a cathode material for lithium-ion batteries. J. Power Sources. 1998, 76, 81–

90.

9. Chen, M.; Li, S.; Yang, C. Structure and electrochemical properties of La, F dual-doped

LiLa0.01Mn1.99O3.99F0.01 cathode materials. J. Univ. Sci. Technol. Beijing. 2008, 15(4),

468–473.

128

10. Kock, A. de.; Ferg, E.; Gummow, R. J. The effect of multivalent cation dopants on

lithium manganese spinel cathodes. J. Power Sources. 1998, 70, 247–252.

11. Yang, M. C.; Xu, B.; Cheng, J. H.; Pan, C. J.; Hwang, B. J.; Meng. Y. S. Electronic,

structural, and electrochemical properties of LiNixCuyMn2-x-yO4 (0 < x < 0.5, 0 < y < 0.5)

high-voltage spinel materials. Chem. Mater. 2011, 23, 2832–2841.

12. Helan, M.; Berchmans, L. J. Synthesis of LiSm0.01Mn1.99O4 by molten salt technique. J

Rare Earth. 2010, 28(2), 255–259.

13. Raja, M. W.; Mahanty, S.; Basu. R. N. Influence of S and Ni co-doping on structure,

band gap and electrochemical properties of lithium manganese oxide synthesized by soft

chemical method. J. Power Sources. 2009, 192, 618–626.

14. Wu, C.; Wang, Z.; Wu, F.; Chen, L.; Huang, X. Spectroscopic studies on cation-doped

spinel LiMn2 O4 for lithium ion batteries. Solid State Ionics. 2001, 144, 277–285.

15. Balaji, S.; Manichandran, T.; Mutharasu, D. A comprehensive study on influence of Nd3+

substitution on properties of LiMn2O4. Bull. Mater. Sci. 2012, 35(3), 471–480.

16. Wang, L.; Li, H.; Huang, X.; Baudrin, E. A comparative study of Fd-3m and P4332

“LiNi0.5Mn1.5O4”. Solid State Ionics. 2011, 193, 32–38.

17. Liu, H.; Song, L.; Zhang, K. Er-doped LiMn2O4. Inorg. Mater. 2005, 41(6), 646–649.

18. Jayaprakash, N.; Kalaiselvi, N.; Gangulibabu.; Bhuvaneswari, D. Effect of mono- (Cr)

and bication (Cr, V) substitution on LiMn2O4 spinel cathodes. J Solid State Electrochem.

2011, 15, 1243–1251.

19. Eftekhari, A. On the fractal study of LiMn2O4 electrode surface. Electrochim. Acta. 2003,

48, 2831–2839.

20. Shaju, K. M.; Subba Rao, G. V.; Chowdari, B. V. R. Li ion kinetic studies on spinel

cathodes, Li (M1/6Mn11/6) O4 (M = Mn, Co, CoAl) by GITT and EIS. J. Mater. Chem.

2003, 13, 106–113.

21. Liu, H.; Cao, Q.; Fu, L. J.; Li, C.; Wu, Y. P.; Wu, H. Q. Doping effects of zinc on

LiFePO4 cathode material for lithium ion batteries. Electrochim. Acta. 2006, 8 1553–

1557.

129

22. Xiong, L.; Xu, Y.; Tao, T.; Goodenough, J. B. Synthesis and electrochemical

characterization of multi-cations doped spinel LiMn2O4 used for lithium ion batteries. J.

Power Sources. 2012, 199, 214–219.

23. Julien, C.; Ziolkiewicz, S.; Lemal, M.; Massot, M. Synthesis, structure and

electrochemistry of LiMn2-yAlyO4 prepared by a wet-chemistry method. J. Mater. Chem.

2001, 11, 1837–1842.

24. Xu, W.; Yuan, A.; Wang, Y. Electrochemical studies of LiCrxFexMn2−2xO4 in an aqueous

electrolyte. J Solid State Electrochem. DOI 10.1007/s10008-011-1347-2.

25. Ding, Y. L.; Xie, J.; Cao, G. S.; Zhu, T, J.; Yu, H. M.; Zhao, X. B. Enhanced elevated-

temperature performance of Al-doped single-crystalline LiMn2O4 nanotubes as cathodes

for lithium ion batteries. J. Phys. Chem. C. 2011, 115, 9821–9825.

26. Ding, Y. L.; Zhao, X. B.; Xie, J.; Cao, G. S.; Zhu, T. J.; Yu, H. M.; Sun, C. Y. Double-

shelled hollow microspheres of LiMn2O4 for high performance lithium ion batteries. J.

Mater. Chem. 2011, 21, 9475–9479.

27. Xiong, L.; Xu, Y.; Zhang, C.; Zhang, Z,; Li, J. Electrochemical properties of tetravalent

Ti-doped spinel LiMn2O4. J Solid State Electrochem. 2011, 15, 1263–1269.

28. Kovacheva, D.; Markovsky, B.; Salitra, G.; Talyosef, Y.; Gorova, M.; Levi, E.; Riboch,

M.; Kim, H. J.; Aurbach, D. Electrochemical behavior of electrodes comprising micro-

and nano-sized particles of LiNi0.5Mn1.5O4: A comparative study. Electrochim. Acta.

2005, 50, 5553–5560.

29. Gummow, R. J.; Kock, A.de.; Thackeray, M. M. Improved capacity retention in

rechargeable 4 V lithium/lithium-manganese oxide (spinel) cells. Solid State Ionics. 1994

69, 59–67.

30. Yi. T. F.; Li, C. Y.; Zhu, Y. R.; Shu, J.; Zhu, R. S. Comparison of structure and

electrochemical properties for 5 V LiNi0.5Mn1.5O4 and LiNi0.4Cr0.2Mn1.4O4 cathode

materials. J Solid State Electrochem. 2009, 13, 913–919.

31. Liua, T.; Zhaob, S. X.; Wang, K.; Nan, C. W. CuO-coated Li[Ni0.5Co0.2Mn0.3]O2 cathode

material with improved cycling performance at high rates. Electrochim. Acta. 2012, 85,

605– 611.

130

32. Hwang, B. J.; Santhanam, R.; Liu, D. G.; Tsai, Y. W. Effect of Al-substitution on the

stability of LiMn2O4 spinel, synthesized by citric acid sol–gel method. J. Power Sources.

2001, 102, 326–331.

33. Thirunakaran, R.; Babu, B. R.; Kalaiselvi, N.; Periasamy, P.; Kumar, T. P.; Renganathan,

N. G.; Raghavan, M.; Muniyandi, N. Electrochemical behaviour of LiMyMn2–yO4 (M =

Cu, Cr; 0 ≤ y ≤ 0.4). Bull. Mater. Sci. 2001, 24(1), 51–55.

131

List of Publications

1. Azhar Iqbal, Safeer Ahmed, Lin Chang, Yan Gao, Yousaf Iqbal, Zhiyong Tang,

“Enhanced Electrochemical Performance of La and Zn co-doped LiMn2O4 spinel as the

cathode material for lithium-ion batteries”, J. Nanopart. Res (2012) 14:1206, DOI

10.1007/s11051-012-1206-9.

2. Azhar Iqbal, Safeer Ahmed, Lin Chang, Yan Gao, Yousaf Iqbal, Abdul Muqsit khattak,

Zhiyong Tang, “Effective low content Ni and Cr co-doped LiMn2O4 cathode material for

lithium-ion batteries” (submitted)