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DEVELOPMENT AND CHARACTERIZATION OF LiMn2O BASED...

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  • 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

    INSTITUTE OF CHEMICAL SCIENCES

    UNIVERSITY OF PESHAWAR

    PAKISTAN

    2013

  • INSTITUTE OF CHEMICAL SCIENCES

    UNIVERSITY OF PESHAWAR

    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.

    Table 3.2 Lattice parameter and unit cell volume for the pure and 55

    La-Zn doped samples.

    Table 3.3 Lattice parameter and unit cell volume for the pure and 56

    Cu-Cr doped samples.

    Table 3.4 Lattice parameter and unit cell volume for the pure and 57

    Cu-Zn doped samples.

    Table 3.5 Lattice parameter and unit cell volume for the pure and 58

    Ni-Cu doped samples.

    Table 3.6 Lattice parameter and unit cell volume for the pure and 59

    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

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

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

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

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

    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

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

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

  • vii

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  • viii

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  • ix

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

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

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

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

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

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

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

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

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

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

    (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

    .

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

    (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

    .

    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,

    at a scan rate of 0.1 mV s-1

    .

  • x

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

    (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

    .

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

    (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

    .

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

    (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

    .

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

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

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

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

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

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

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

    (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 98

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

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

  • xi

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

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

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

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

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

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

    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.

    Fig. 3.45 (A) First Discharge curves (B) Capacity vs. cycle number plots for 107

    (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.

    Fig. 3.46 (A) First Discharge curves (B) Capacity vs. cycle number plots for 108

    (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. 3.47 (A) First Discharge curves (B) Capacity vs. cycle number plots for 109

    (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

    Fig. 3.48 (A) First Discharge curves (B) Capacity vs. cycle number plots for 110

    (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.

    Fig. 3.49 (A) First Discharge curves (B) Capacity vs. cycle number plots for 111

    (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.

    Fig. 3.52 Rate capability at 0.1-5 C of the pure LiMn2O4 and 122

    LiCu0.01Cr0.01Mn1.98O4.

    Fig. 3.53 Rate capability at 0.1-5 C of the pure LiMn2O4 and 122

    LiCu0.01Zn0.01Mn1.98O4.

    Fig. 3.54 Rate capability at 0.1-5 C of the pure LiMn2O4 and 123

    LiNi0.01Cu0.01Mn1.98O4.

    Fig. 3.55 Rate capability at 0.1-5 C of the pure LiMn2O4 and 123

    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

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DEVELOPMENT AND CHARACTERIZATION OF LiMn 2 O 4 BASED CATHODE MATERIALS FOR LITHIUM-ION BATTERIES By AZHAR IQBAL INSTITUTE OF CHEMICAL SCIENCES UNIVERSITY OF PESHAWAR PAKISTAN 2013
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