Nanostructured Li4ti5o12 As Anode Material for Lithium Ion Batteries

Nanostructured Li4Ti5O12 as Anode Material for Lithium Ion Batteries

A thesis submitted in fulfillment of the requirements for the degree of Master of Science

By Ru Wen

The University of New South Wales

Faculty of Science

School of Materials Science and Engineering

July 2012

ORIGINALITY STATEMENT

‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’

Signed ……………………………………………......

Date ……………12/12/2012………………………………

List of publications

1. R. Wen, J. Yue, Z.F. Ma, AB. Yu, XC. Jiang. Fabrication of Graphitized

Li4Ti5O12 Nanostructures with High Lithium Storage Capacity. Submission for publication.

2. R. Wen, J. Yue, ZF. Ma, AB. Yu, XC. Jiang. Li4Ti5O12/TiO2 nanocomposite as anode material for high rate lithium ion battery. Submission for publication. 3. R. Wen, J. Yue, ZF. Ma, WM. Chen, AB. Yu, XC. Jiang. Self-Assembled

Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery. Submission for publication.

Abstract

Lithium ion batteries (LIBs) have grabbed increasing attention because they are the dominant power sources for most of portable electronics and the promising power sources for electric vehicles. Spinel lithium titanium, Li4Ti5O12 (LTO), has attracted significant attention as an alternative anode material for LIBs due to its unique properties such as high safety, long life time, easy fabrication, low cost and environmentally benignity. This work mainly focuses on the synthesis of LTO, LTO coupled with titanium dioxide (TiO2), and carbon-doped LTO materials (C-LTO) with morphology, size and function control. Four chapters are included in this thesis. In Chapter 1, the current research activities and developments of spinel LTO in the past years are summarized. Chapter 2 represents the synthesis of hollow structured LTO microspheres, followed by doping with TiO2 in LTO described in Chapter 3 and by modification using graphitized carbon in LTO discussed in Chapter 4. To obtain such materials, hydrothermal/solvothermal synthesis approaches were developed at 140-180 oC, followed by subsequent high-temperature calcination (500 oC) process for the spinel LTO materials. A number of advanced characterization techniques including SEM, TEM, HRTEM, SAED, XRD and Raman spectra are used to characterize the morphology and composition of the as-prepared LTO. The electrochemical measurements in terms of rate capability, charge-discharge profile and cycling performance are employed to evaluate the battery performance of LTO as an anode material. The results show that self-assembled LTO hollow microsphere is a promising anode material for high-power lithium-ion batteries. Modification of LTO by graphitic carbon doping enhances the capacity to 178 mAh/g at 10 C and can sustain a capacity of

169 mAh/g even after 100 cycles. LTO/TiO2 composite anode material can meet the high power requirement for large scale application, such as electric vehicles and energy storage systems based on desirable advantages of electrode materials.

iii

Table of Contents

ORIGINALITY STATEMENT ...... i List of publications ...... ii Abstract………………………………………………………………………………....iii Acknowledgements ...... V Abbreviations ...... VI List of Figures ...... VII List of Tables ...... XI Introduction ...... 1 Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications ...... 4 1.1 Introduction ...... 4 1.2 Synthesis of methods ...... 5 1.2.1 Solid state method ...... 7 1.2.2 Sol-gel method ...... 11 1.2.3 Hydrothermal method ...... 12 1.2.4 Solvothermal method ...... 14 1.2.5 Spray pyrolysis ...... 14 1.2.6 Solution-combustion method ...... 15 1.2.7 Other methods ...... 16 1.3 Modification ...... 17

1.3.1 Atomic doping Li4Ti5O12 ...... 18 1.3.2 Surface modification ...... 29 1.3.3 Effects of parameters ...... 31 1.4 Properties ...... 32 1.4.1 Physical and chemical properties ...... 32 1.4.2 Electronic and atomic structure ...... 35 1.5 Applications ...... 43 1.5.1 Anode material ...... 43 1.5.2 Coating for cathode materials ...... 46 1.6 Conclusions and perspectives ...... 47 Chapter 2 Self-Assembled hierarchical Li4Ti5O12 hollow microspheres for high performance lithium ion battery ...... 48 2.1 Introduction ...... 48 2.2 Experimental section ...... 49 2.2.1 Synthesis ...... 49 2.2.2 Characterization ...... 49 2.2.3 Electrochemical measurement ...... 50 2.3 Results and discussion ...... 50 2.3.1 Composition and morphology ...... 50 2.3.2 Growth mechanism ...... 55 2.3.3 Electrochemical properties ...... 57 2.4 Conclusions ...... 59

Chapter 3 Li4Ti5O12/TiO2 nanocomposite as anode material for high rate lithium ion battery…………………………………………………………………………………..61 3.1 Introduction ...... 61 3.2 Experimental section ...... 63 3.2.1 Synthesis ...... 63 iv

3.2.2 Characterization ...... 63 3.2.3 Electrochemical measurement ...... 63 3.3 Results and discussion ...... 64

3.3.1 Spherical LTO/TiO2 composite ...... 64 3.3.2 Porous LTO/TiO2 nanosheets ...... 70 3.4 Conclusions ...... 77 Chapter 4 Graphitized Li4Ti5O12 nanosheets with highly enhanced lithium storage capacity for lithium ion battery ...... 79 4.1 Introduction ...... 79 4.2 Experimental section ...... 80 4.2.1 Synthesis and graphitization ...... 80 4.2.2 Characterization ...... 81 4.2.3 Electrochemical measurement ...... 81 4.3 Results and discussion ...... 81 4.3.1 Composition and morphology ...... 81 4.3.2 Formation and growth mechanism ...... 84 4.3.3 Electrochemical performance...... 86 4.4 Conclusions ...... 89 Chapter 5 Summary ...... 90 References…...... 95

Acknowledgements

This thesis would never have been written without the scholarships from UNSW that I was awarded in 2010, and I wish to thank UNSW for offering me the chance of postgraduate study. I would like to thank the staffs at the School of Materials Science and Engineering for providing support, friendship and facilities.

I owe special thanks to Dr. Xuchuan Jiang, my principal supervisor, for his keenly support and continuous help from the very beginning of this project to the end of submission. The co-supervisor, Professor Aibing Yu often gives me useful instruction upon the research progress during group meeting or seminar. The group office manager, Ruiping Zou, encouraged and instructed me when I was in difficult situation. Wuming Chen has been a constant supporter during my study and life in Australia. Olivia, who is also my roommate, offered me a lot of help in my life. Canny always tried her best to give me suggestions and advices when I was not on the right point of research study. Valentino helps me on the proofreading of manuscripts. Jeffrey helps me on the thesis revision and papers for publication. Jason Scott assisted on the BET measurements. Jeff does some help on experiments. I want to thank all of them for the help and support of my life and research during the MSc. study.

I wish to thank the staff of UNSW library for providing me the training on how to start literature research and retrieving research articles from other libraries, financial support of the Australia Research Council (ARC) grants for the research and the access to the UNSW node of the Australian Microscopy & Microanalysis Research Facility (AMMRF).

I wish to thank all my friends and colleagues in the struggle for a better world, especially Ms Wen Xu. My family, especially my mum and grandmother, have given me much support to complete my study.

Abbreviations

AB Acetylene black AOT Dioctyl sodium sulfosuccinate BET Brunauer-Emmett-Teller CFSE Crystal field stabilization energy CTAB Cetyltrimethylammonium bromide CV Cyclic voltammetry C-LTO Carbon doped LTO DEC Diethyl carbonate EC Ethylene carbonate EG Ethylene glycol EV Electric vehicles HEV Hybrid electric vehicles HRTEM Transmission electron microscope LIBs Lithium ion batteries

LTO Lithium titanate [Li4Ti5O12] MWNTs Multi-wall carbon nanotubes NMP N-methyl-2-pyrrolidone n-LTO Nanosheet LTO OSPE Octahedral site preference energy PVDF Polyvinylidene difluoride PVP Poly(vinylpyrrolidone) SAED Selected area electron diffraction SEM Scanning electron microscope SXRD Synchrotron X-ray diffraction

TBT Titanium butoxide [Ti-(OC4H9)4] Ti-EG Titanium glycolate XRD X-ray powder diffraction

List of Figures

Fig.1.1 (a) & (b) Rate capabilities of the LTO electrodes prepared using different starting materials and milling methods at different charge–discharge rates. (c) & (d) Normalized capacity as a function of the C-rate for LTO electrodes prepared by ball milling and high energy milling, respectively …………………………………………..8

Fig.1.2 Scheme of a speculative reaction mechanism of Li4Ti5O12 formation from the intermediate mixture…………………..…………………………………………….… 10

Fig.1.3 TEM images of as-prepared LTO at different magnifications...... …...13

Fig.1.4 XRD patterns of: (a) Li4Ti5O12, and (b) doped Ag- Li4Ti5O12………….....…..23

Fig.1.5 (a) XRD patterns of synthesized Li4Ti5O12гxBrx (x = 0, 0.05, 0.1, 0.2, 0.3), (b)

Magnified (111) peaks of doped Br in Li4Ti5O12 with different amounts…………..….27

Fig.1.6 Crystal structure of spinel LTO with Fd3m space group………...………….....36

Fig.1.7 (a) Li4Ti5O12 spinel structure. Blue tetrahedra represent lithium, and green octahedra represent disordered lithium and titanium; and (b) Li7Ti5O12, rock salt. Blue octahedra represent lithium, and green octahedra represent disordered lithium and titanium………………………………………………………………………………...37

Fig.1.8 Discharge and charge curves of a Li/Li4Ti5O12k-A cell……………...…...……40

Fig.1.9 Comparison between calculated and experimental XAS spectra of Li4Ti5O12 at the OK edge…………………………..………………………………….………….….41

Fig.1.10 (a) TEM image of LTO nanotubes, (b) SEM image of a Li4Ti5O12 single crystal,

(c) SEM image of rod shaped single crystals Li4Ti5O12, (d) SEM images of Li4Ti5O12 nanofibers, (e) SEM image of Li4Ti5O12 nanowires, (f) SEM image of flower like

Li4Ti5O12, (g) TEM images of sawtooth-like Li4Ti5O12, insert is the TEM image at a higher magnification, (h) SEM image of spherical Li4Ti5O12, and (i) SEM image of 3D macroporous Li4Ti5O12………………………………………………………...……….42

Fig.1.11 SEM pictures of Li4Ti5O12-coated LiCoO2 with different contents (a) 0, (b) 3, (c) 5, and (d) 10 wt%...... 43

VII

Fig.2.1 XRD pattern of nanosheet LTO hollow spheres…………………...…….…....50

Fig.2.2 (a) SEM of precursor, (b) SEM images of the LTO precursor in a higher magnification, (c) SEM image, (d) TEM image, inserted in (d) is HRTEM image of LTO hollow microspheres……………………………….…………..………..…….…52

Fig.2.3 SEM images of LTO precursor prepared (a) r = 0 (b) r = 1:1, where r is the molar ratio of EG:TBT…………………………………………….…………….…….52

Fig.2.4 SEM images of LTO precursor obtained at different concentrations of CTAB: (a) k = 0.01, (b) k = 1, (c) k = 2……………………………………………….….……..…53

Fig.2.5 SEM images of samples obtained by heating at 180 °C for different reaction time: (a) 10 min, (b) 30 min, (c) 40 min, and (d) 2 h……………………………….….54

Fig.2.6 Formation mechanistic illustration of hollow LTO microspheres assembled from nanosheets obtained by CTAB surfactant-assisted hydrothermal process…………….56

Fig.2.7 (a) Charge and discharge profiles of the first cycle at the rate ranging from 1C to 50C over a potential window of 1.0-3.0 V; (b) rate capability of nanosheet LTO hollow microsphere and LTO-Sigma at rates ranging from 1 to 50 C; (c) cyclability of nanosheet LTO hollow microsphere at 10 C…………………………….……..………57

Fig.3.1 XRD pattern of nanosphere LTO/TiO2 composites…………………..…...…...64

Fig.3.2 (a) SEM image, (b) SEM image at a higher magnification of precursor; (a) SEM image, (b) SEM image at a higher magnification of nanosphere LTO/TiO2 composite……………………………………………………………………………….65

Fig.3.3 (a) TEM image, (b) TEM image at a higher magnification, (c) & (d)HRTEM image of nanosphere LTO/TiO2 composites………………………………………..….66

Fig.3.4 XRD pattern of samples prepared at different times (0.5, 1.5, 6, 24 h) combined with calcination at 500 oC for 2 h……………….……………………..……………….66

Fig.3.5 SEM images of precursors obtained at different hydrothermal time: (a) 0.5 h (b)1.5 h (c) 6 h (d) 24 h…………………………..………………………………...…..67

VIII

Fig.3.6 Electrochemical performance of nanospherical LTO/TiO2 composite: (a) cyclic voltammogram (CV); (b) charge-discharge profile (c) cycling performance at 2000 mA/g…………………...………………………………….……………………..……..69

Fig.3.7 XRD pattern of LTO/TiO2 nanosheets………………………………………....71

Fig.3.8 SEM images of (a) precursor, (b) precursor at a higher magnification, (c)

LTO/TiO2, (d) LTO/TiO2 at a higher magnification; TEM images of (e) LTO-TiO2, and

(f) LTO-TiO2 at a higher magnification………………………...….…………………..72

Fig.3.9 N2 adsorption-desorption measurements of mesoporous LTO/TiO2 nanosheets.

(a) Pore size distribution curve, and (b) N2 adsorption–desorption isotherm plot……………………………………………………………………………………...73

Fig. 3.10 SEM images of products prepared under hydrothermal treatment of 180 oC for 3h at the different concentrations of CTAB...………………………………………….73

Fig. 3.11 TEM images of product prepared under hydrothermal treatment of 180oC for 3h with PVP surfactant (a) at a low magnification (b) at a high magnification………..74

Fig. 3.12 (a) SEM image, (b) TEM image of product prepared under hydrothermal treatment of 180 oC for 3 h with CTAB-AOT dual surfactants…………..…………….74

Fig.3.13 (a) Cyclic Voltammogram of porous LTO/TiO2 nanosheets; (b) initial

Galvanostatic charge-discharge profiles of porous LTO/TiO2 nanosheet at different rates;

(c) galvanostatic charge-discharge profile of porous LTO/TiO2 nanosheet at 1st,100th cycle at the density of 10 C; (d) cycling performance of porous nanocomposite and commercial LTO at the rate of 10 C……………………………….…….....…………..78

Fig.4.1 SEM images of (A) LTO precursor, (B) LTO precursor at a high magnification, (C) LTO (denoted as n-LTO) nanosheet, and (D) graphitized LTO (denoted as C-LTO) nanosheets…………………………………………………………………………..…..82

Fig.4.2 (A) TEM images, (B) HRTEM images of n-LTO, (C) TEM image, (D) HRTEM images of C-LTO……………………………………………………………………….83

Fig.4.3 (A) XRD pattern of precursor, n-LTO and C-LTO nanosheet, (B) Raman spectra of n-LTO and C-LTO nanosheets..…………………………………………….84

Fig.4.4 Energy dispersive spectroscopic (EDS) mapping of C-LTO for the region shown in SEM………………………………………………………………………………….85

Fig.4.5 A scheme illustrating the formation of C-LTO nanosheets..…………………..86

Fig.4.6 (A) charge-discharge profiles of the electrode made of C-LTO nanosheet at the 1st, 2nd and 100th cycle at a high rate of 10 C; (B) cyclic voltammograms of the as- prepared C-LTO before cycling and after cycling performance of 100 times at the rate of 10 C with a scan rate of 0.2 mV·s-1; (C) cycling performance of electrode made by the as-prepared n-LTO and C-LTO at 10 C, respectively…….…….………………….86

List of Tables

Table 1.1 Properties of Li4Ti5O12 synthesized by different methods in terms of particle size, shape and electrochemical performance…………………….….………….………6

Table 1.2 The properties of Li4Ti5O12 doped by transitional metal…...…………...…..19

Table 1.3 Comparison of discharge capacity and cycling stability of the Li4Ti5O12 and doped Cu- Li4Ti5O12 anode materials…………………………………………..…...….22

Table 1.4 Comparison of discharge capacity and cycling stability of the Li4Ti5O12 and doped Ag- Li4Ti5O12 anode materials...………....…………………………...……...….24

Table 1.5 Doping properties of Li4Ti5O12 by metal in the main group…….…...... 25

Table 1.6 The factors on the synthesis of Li4Ti5O12….……….………………….....…31

Table 1.7 Properties and solution to improve electrical conductivity of Li4Ti5O12…...33

Table 1.8 The comparison between Li4Ti5O12 and other materials for lithium ion batteries……………………………………………………………………….……..….33

Table 1.9 Crystallographic data for Li4Ti5O12 and Li7Ti5O12……………..….…..…....36

Introduction

With the limitation of non-renewable energy sources and problems of global warming from combustion of natural energy source (e.g., oil, gas), lithium ion batteries (LIBs) have attracted tremendous interests for portable electronics (e.g. mobile phones, laptops) and are promising power source or back-up power source for large scale application, such as electric vehicles (EV) or hybrid electric vehicles (HEV), based on the essential advantages of no memory effect, high cell voltage, no toxic products, high volumetric and gravimetric energy.[1]

Since its discovery in 1990s by Sony Corporation, LIBs have been widely studied to satisfy the increasing demands of portable electronics. Carbon was firstly used as the anode material in practical market due to low cost, high capability and long life. However, carbon cannot be the ultimate goal for anode material, since they operate at a potential (~0.1 V vs. Li/Li+) close to metallic lithium. At such low voltage, electrolytes are not stable and would undergo decomposition. Lithium is likely to be deposited on the surface of the electrode material (so called lithium dendrite) and form a surface passivation film, which causes initial loss of capacity, safety issue, critical performance of electrode, and failure of the cell.[2-5]

Spinel lithium titanium Li4Ti5O12 (LTO) attracts significant attention as an alternative anode material for LIBs, attributed to the advantages of high safety, long life, easy fabrication, low cost and environmentally benignity properties. LTO has a stable operating voltage of ~1.5 V vs. Li+/Li, above the decomposition potential of electrolyte, thus avoiding the safety issue present in carbon.[6] LTO is classified as zero-strain material since there is no structural or volume change during lithium insertion/extraction process.[7] To our best knowledge, it was studied as early as in 1984 by Edwards et al,[8] then proposed to be cathode in 1989 by Colbow et al.[9] In 2005, it was commercialized by Altairnano corporation to replace the traditional graphite anode material in LIBs.

This thesis focuses on the synthesis and characterization of LTO with morphology control for lithium ion batteries. Firstly, the past research activities and developments of

spinel LTO are summarized. Specifically, the synthetic routes of LTO controlled in various morphologies, including, nanoparticles, nanorod, nanofibers, and approaches to modify LTO for further improvement of performance are analyzed.

Then, a facile and easily scaled-up hydrothermal method is developed to prepare LTO hollow microspheres assembled from nanosheets, which deliver a high capacity of ~110 mAh/g for 100 cycles at 10 C when used as an anode material. A formation mechanism for the formation of hierarchical LTO hollow microspheres was analysed with time dependent experiments. The anode LTO half-cell demonstrates excellent capacity retention with less than 4% of capacity loss even after 100 cycles.

The dual phase LTO/TiO2 composite as an anode material is synthesized and characterized. Spherical LTO/TiO2 nanocomposite with the size of 30-500 nm is synthesized by solvothermal method with acetone as the solvent. The as-prepared spherical LTO/TiO2 composite exhibits a high capacity of 110.2 mAh/g even after 100 cycles, corresponding to 99.1% of the fifth discharge capacity. Porous LTO/TiO2 nanosheets are prepared by a hydrothermal method. Titanium butoxide (TBT) is used as o the Ti source for both LTO and TiO2. High temperature calcination at 500 C was used to crystallize the precursor. Electrochemical measurements show that porous LTO/TiO2 nanosheet demonstrates enhanced lithium storage properties with excellent capacity retention at a current rate as high as 10 C even after 100 charge/discharge cycles.

Finally, spinel LTO is modified by graphitic carbon for further improvement of the electrochemical performance. A modified hydrothermal method is employed to synthesize graphitized LTO nanosheets in which carbon is homogeneously dispersed into the LTO nanosheets. The graphitized LTO nanosheet electrode delivers a remarkable reversible capacity of 169 mAh/g, with 96.5% capacity retention of its theoretical capacity after 100 cycles at a high rate of 10 C.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

1.1 Introduction

Lithium ion batteries (LIBs) are one of the successful achievements of modern material electrochemistry with the advantages of no memory effect, high cell voltage, low toxicity, high volumetric and gravimetric energy.[10, 11] As one of the solutions to reduce the global warming problem and to meet the requirement of energy storage, LIBs have been widely utilized as the power source for mobile electronics, portable entertainment devices and computing/telecommunication devices. However, its low rate capability has limited their application in electric vehicles (EVs), hybrid electric vehicles (HEVs), photovoltaic solar cells, and portable power tools. Improvement of this technology is urgently required due to the growing increasing usage of non-renewable energy sources, such as oil and gas.[12-14]

In today’s commercial fields, carbonaceous anodes are widely applied into the lithium- ion battery, because carbon has a longer cycle life and higher specific capacity than some lithium metal composite oxide anode materials. However, the potential of carbon (approximately 100 mV vs. Li+/Li) is very close to that of metallic lithium, at which most common electrolytes are not stable and lithium would be deposited on the surface of the electrode particles, which would result in safety issue and failure of the whole cell.[2-4, 15-18]

Recently, spinel lithium titanium Li4Ti5O12 (LTO) have attracted more and more attention as a potential anode material for lithium ion batteries, attributes to its excellent structural stability (no structural or volume change during Li insertion and extraction) and no risk of lithium plating during the fast charge/discharge rate with 1.55 V vs Li+/Li [6, 7] reduction potential of Li4Ti5O12. In comparison to carbonaceous anode, spinel lithium titanium Li4Ti5O12 has better electrochemical performance and higher safety. It is cheaper and easier to be synthesized compared with alloy-based anode in lithium ion

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

[19] batteries. Researchers have indicated that Li4Ti5O12 would be a good alternative for graphite in high-safety LIBs.[4] To our best knowledge, it was studied as early as in 1984 by Edwards et al,[8] then proposed to be cathode in 1989 by Colbow and co- workers.[9] In 2005, it became firstly commercialized by Altairnano corporation, who replaced traditional graphite materials for anode material. The science and technology [20] of Li4Ti5O12 has been reported in previous review, and will be discussed in more details below.

This chapter reviews the research on spinel lithium titanium Li4Ti5O12. Various methods for preparation of spine Li4Ti5O12 are introduced firstly. Then, the strategies to improve the electrochemical performance of spinel Li4Ti5O12 are summarized. The following sections fully illustrate the crystal structures and properties of spine Li4Ti5O12. The application of Li4Ti5O12 as an anode and surface coating material for cathode material are finally discussed.

1.2 Synthesis of methods

Various approaches have been reported for synthesis of lithium titanate, including solid- state method,[4, 13, 21, 22-36] hydrothermal or solvothermal method,[37-44] spray pyrolysis,[45- 48] composite molten-salt method,[49] template method,[50] emulsion-gel process,[51] reflux method,[52] flux method,[53] mechanochemical synthesis,[54] electrospinning method,[55] solution combustion synthesis,[56] modified solution combustion synthesis,[11, 16-18, 57] sonochemical method,[58] single step metal organic precursor method,[59] template-free hydrothermal process,[60] spray drying process,[61] modified rheological phase method,[62] and electrospray pyrolysis method.[63]

Conventionally, solid-state method is used for the synthesis of spinel LTO because it is simple and easy. However, this method would result in inhomogeneity, irregular morphology, and broad particle size distribution. The high temperature calcination is not desirable to pursue low cost in terms of fabrication process. Sol-gel method provides an alternative way to solve the above-mentioned disadvantages. However, the expensive organic materials prevent its massive application in energy storage devices. Therefore, numerous amounts of novel methods have been developed to prepare

Li4Ti5O12 with excellent electrochemical performance. Synthesis of Li4Ti5O12 by

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications different methods shows distinct properties based on particle size and shape, and electrochemical performance, as summarized in Table 1.1.

Table 1.1 Properties of Li4Ti5O12 synthesized by different methods in terms of particle size and shape, electrochemical performance

Methods Particle Shape Electrochemical performance Ref size (nm) Solid-state 70 spherical x 202 mAh/g at 0.01-2.5 V. [64, method 65] Sol-gel method 98- 136 nanoparticle x excellent cycliability [66] x 272 mAh/g - first cycle Sol-gel method 76- 500 round x 170 mAh/g - first cycle [67] with different x 150 mAh/g - 30 cycles - complex agents 0.5 mA·cm-2. Refluxing 5-400 spherical x 200 mAh/g - 0.4 mA·cm-2 [52] method - 60th cycles Flux method ---- rod-shape ----- [53] Heat treatment 6-11 nanotubes x ∼156 mAh/g - 0.1 C. [41] and an alkali- hydrothermal reaction Template-free 20 spherical x 114 mAh/g - 30 C [60] hydrothermal x 125 mAh/g - 200 cycles - process 20 C Single step metal >50 3-D ordered x 167 mAh/g - 1st cycle - [59] organic precursor macroporous 0.1mA·cm-2. method x 149 mAh/g - 0.125 mA·cm-2 x 155 mAh/g - 0.63 mA·cm-2. Carbon sphere 100-500 hollow x 100 mAh/g - 10 C [50] template method spheres x 150 mAh/g - 2 C - 200 cycle Electrospinning 230-270 nanofibers x 140-192 mAh/g - 0.5 C - [55] method first five cycles with a capacity loss of 1.0% per cycle x 87-170 mAh/g - first 30 cycles with a capacity loss of 1.6% per cycle at 1.5 C Combustion 40–80 Spherical -- [57] process particles Spray pyrolysis 980 Spherical x 171-168 mAh/g - 50 [46] cycle by heating at 800 ºC x Decreased from 172 to 129 mAh/g for 1000 ºC heating. 6

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

x Li4Ti5O12 heated at 600, 800 ºC have good retention capacity value on cycling while treating at 1000 ºC has poor cycle properties.

Composite 500-3000 octahedral x The Li4Ti5O12 powder [49] molten salt with molar ratio of method LiCl/KCl = 1.5 achieves initial discharge capacity of 169 mAh/g x charge/discharge efficiency of 94% at 0.2 C x good rate performances from 0.2 to 5 C. Solvothermal -- 0-1D x 125 mAh/g - 500 cycles - [40] reaction nanostructure 1 C, 114 mAh/g at 20 C.

1.2.1 Solid state method

Solid-state synthesis is normally conducted by mixing solid materials and then the mixtures are annealed at high temperature. This method is mostly used in both industries and experimental labs because of its simplicity. It is easy to realize this method with a furnace which could stand high temperature calcination (i.e. 1000 oC).

The solid materials are ground in a mortar before calcination at a furnace. Li4Ti5O12 powders are synthesized by a solid-state reaction of lithium and titanium salts at 700- o [4, 13, 21, 22-36] 1000 C. Generally, TiO2 ( i.e. anatase or rutile) is used as titanium source and Li2CO3 or LiOH as a lithium salt. Sometimes, solvent (i.e. ethanol) is used as the dispersant to provide a better environment to homogeneously mix the starting materials.

For example, stoichiometric amounts of TiO2 and LiOH·H2O were first dispersed in n- hexane for assuring homogeneity. Then, after removing the solvent, they were heated at o [5] [68] 800 C for 24 h in oxygen. Kataoka and co-workers employed Li2CO3 and TiO2 as starting materials, which were heated in air at 973 K for 12 h and subsequently annealed at 1123 K for 24 h.

Electrochemical performance is influenced by different synthetic parameters, such as the starting material (e.g, titanium or lithium salt), calcination temperature (i.e. 700, 1000 oC), mechanochemical activations,[69] calcination time.[69-71] Hong and co- [72] workers studied the influence of starting materials (e.g, anatase or rutile TiO2), annealing temperatures (e.g, 700, 800, 900 oC), and mechanochemical activations (e.g,

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

ball milling or high energy-milling). Li2CO3 and TiO2 (either anatase or rutile) were mixed with de-ionized water after adding 2 wt.% of the ammonium salt of polycarboxylic acid as a dispersant. The mixture powders were exposed to high energy milling for 3 h at a rotor speed of 3000 rpm with 0.4 mm ZrO2 beads or ball-milled for o 24 h using 5 mm ZrO2 balls. The dried powder was finally calcined at 700, 800, 900 C for 3 h in air. The rate performance and capacity retention of LTO prepared under different conditions are shown in Fig.1.1.

Fig.1.1 (a) (b) Rate capabilities of the LTO electrodes prepared using different starting materials and milling methods at different charge–discharge rates. (c) (d) Normalized capacity as a function of the C-rate for LTO electrodes prepared by ball milling and high energy milling, respectively. The dried powders named AB, AH, RB and RH were in terms of TiO2 type and milling method, where A and R stand for anatase- and rutile- [72] TiO2, and B and H for ball-milled and high energy-milled, respectively.

Rutile TiO2 is more desirable in acquiring high purity LTO than anatase due to the anatase to rutile phase transformation, which is found to be more rigid in the solid-state reaction than the intact rutile phase. Wet milling of Li2CO3 and TiO2 could produce 8

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications heterocoagulated mixtures with high dispersion stability and uniform distribution. Mechanochemical activation by high energy milling of the starting materials is more effective in decreasing the reaction temperature and particle size as well as increasing the LTO content of the final powder than those prepared by conventional ball milling.

The specific capacity of LTO prepared from anatase TiO2 depends significantly on the milling method and heat treatment temperature, whereas those from rutile TiO2 showed a uniform capacity of 160.6-165.1 mAh/g, regardless of the change in the main parameters owing to their high LTO content. Overall, LTO powder with a specific capacity of 165 mAh/g could be synthesized by optimizing the milling method and starting materials.[72]

Li2TiO3 is always coexisted as impurity along with Li4Ti5O12 obtaining by solid-state [22, 56, 70, 73, 74] method. It is noticeable that both Li2TiO3 and Li4Ti5O12 phases are layer structured, far from perfect, with their interplanar distances, Li2TiO3 (002) and

Li4Ti5O12 (111), being very close to each other, i.e. 4.80 and 4.84 Ǻ , respectively. These two substances most probably interlock with each other, at the stage of their coexistence. On the other hand, coexisting titania is also destabilized because of oxygen deficiency and partial substitution by nitrogen (under N2 atmosphere). This could enable short-range diffusion of Ti ions from the discontinued layers to enrich Ti concentration between layers of Li2TiO3 (002) toward Li4Ti5O12 (111), as illustrated schematically in

Fig.1.2. If indirectly, i.e. insertion of Ti to Li2TiO3 toward Li4Ti5O12, only in the case of coexistence of both layered phases mutually interlocked. The significance of the mechanical activation of the intermediates is, then, self-evident, as the layer structure of the intermediates and the final products are similar to each other as in the present case [74] of Li4Ti5O12 synthesis.

The conventional solid-state method would easily cause the aggregation of particles at the expense of the loss of material nanostructure. The desired properties of particles are difficult to realize as the expected morphology would be destroyed through particles aggregation. Modified synthesis of LTO based on solid-state method has been developed.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Fig.1.2 Scheme of a speculative reaction mechanism of Li4Ti5O12 formation from the [74] intermediate mixture, Li2TiO3 + TiO2.

Li et al.[65] demonstrated microwave reaction system to be used to prepare LTO. The starting materials of Li2CO3 and anatase TiO2 were thoroughly mixed using an agate mortar according to stoichiometric ratio. Since Li2CO3 and TiO2 could only absorb a small amount of microwaves at a relatively low temperature, double crucible system was applied to achieve the high temperature needed in the solid-state reaction while charcoal was used as a heating medium between two porcelain crucibles. The microwave reaction system was placed in the centre of a rotating plate of a modified microwave oven with irradiating at 500-700 W for 10-15 min.

A molten-salt method was a modified solid-state method by introducing salts such as LiCl, NaCl or KCl into the conventional solid-state method system.[75, 76] For halide fluxes, high quality and small crystals can be grown at halide fluxes as intermedia. The molar ratio of the starting materials LiOH·H2O:TiO2: LiCl-KCl is fixed to 4:5:10. The mixed powders were calcined at 800 oC for 8 h in air, and the resulting powders were thoroughly washed with distilled water and n-butanol to remove the residual molten salt. As for a single molten salt, e.g, employ LiCl or KCl, the melting point is around 612 oC or 789 oC, respectively. Whereas a composite LiCl-KCl can dramatically decrease the

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications melting point, thus a liquid reaction environment can be easily gained, which can accelerate the diffusion of the raw materials, and promote the crystal formation.[75]

1.2.2 Sol-gel method

The sol-gel process is a wet-chemical technique widely used in the fields of materials science and ceramic engineering. A colloidal solution (sol) acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are metalalkoxides, metal salts (such as chlorides and acetates), and organic metal compounds which undergo various forms of hydrolysis and polycondensation reactions.[77-81] Sol-gel method can avoid the problem caused by solid-state.[67] It is an environmentally friendly process as it uses an aqueous solution. This process can easily control the phase structure, composition homogeneity, crystallite size, monodispersity and microstructure.[82] It also has the advantages of realizing more uniform distribution of ions in oxides,[77] low fabrication cost, relatively easy stoichiometry control, and high deposition rate,[83] low synthesis temperature, shorter heating time, and better crystallinity.[84]

It is a desirable method to obtain Li4Ti5O12 with good homogeneity, uniform morphology, and narrow size distribution.[66] But it is difficult to control synthesis condition in terms of a large quantity of solvents and organic materials such as citric acid, ethylene glycol, polyvinyl alcohol, which prevents its massive application in energy storage.[29, 49] Moreover, sol-gel method also requires a post high-temperature calcination at 800 ºC or higher temperature to prepare a pure spinel phase Li4Ti5O12, which results in an undesirable particle growth.[43]

To obtain Li4Ti5O12 with excellent electrochemical performance, many efforts have been done to optimize the sol-gel method. Hao et al.[85] reported a novel sol-gel method with triethanolamine (TEA) exhibited an initial discharge capacity of 168 mAh/g and a subsequent charge capacity of 151 mAh/g. They also reported[3] citric acid sol–gel method to synthesize Li4Ti5O12, when a molar ratio R of citric acid to total metal ions is

1/2, Li4Ti5O12 exhibited an initial discharge capacity of 167 mAh/g at 23.5 mA/g and a subsequent charge capacity of 151 mAh/g. Besides these, they further investigated[86] oxalic acid-assisted sol-gel method to synthesize Li4Ti5O12, which exhibits 171 mAh/g in the first cycle and 150 mAh/g after 35 cycles under an optimal synthesis condition at

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

800 oC for 20 h while oxalic acid to titanium ratio R = 1.0. Kim et al.[87] reported the synthesis of Li4Ti5O12 by a novel sol-gel method with high energy ball milling (HEBM) of precursor. HEBM Li4Ti5O12 exhibited the first capacity of 173 mAh/g and excellent cycleability.

Alias et al.[78] used lithium tert-butoxide and titanium isopropoxide as starting materials, which were mixed together in ethanol, the residue was harvested and finally sintered at different temperatures (700-1000 oC) and times (1-5 h). Hao et al.[79] employed triethanolamine (TEA) chelating agent, while Khomane and co-workers[80] employed hexadecyltrimethylammonium bromide (CTAB) cationic surfactant to control the crystal growth of LTO. In a typical process, CTAB was dissolved in 100 ml ethanol under magnetic stirring. Four grams of lithium acetate dihydrate was dissolved in above solution with continued magnetic stirring. Titanium (IV) isopropoxide was added to the above solution dropwise by keeping the mole ration Li: Ti = 1: 1.25. The temperature of the solution was raised to 90 oC and stirred continuously to form the gel. The gel was aged at 100 oC for 24 h in air and the precursor was decomposed at 400 oC for 4 h followed by calcination at 800 oC for 12 h in air. The average discharge capacity of the

1 prepared LTO taken over 20 cycles is ∼60 mAh g a constant current density of 21.37 mA g1 .

1.2.3 Hydrothermal method

Hydrothermal synthesis is used in autoclaves under controlled temperature and/or pressure with the reaction in water solution and followed by a calcination step at 400- 600 oC. It is widely used for the production of uniform small particles in the industry. Many efforts have been dedicated to hydrothermal method.[38, 41, 43, 60, 88, 89, 90] For example, Lai et al.[89] reported a hydrothermal method to prepare LTO, they employed

TiOSO4 and LiOH·H2O as starting material and water as solvent with certain amount of urea. After thoroughly mixing the starting material in water containing urea, the resultant solution was transferred into a Teflon-lined autoclave and treated at 180 oC for 24 h. The precipitate was harvested and then calcined at 500 oC for 2 h under Ar atmosphere.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Fig.1.3 TEM images of as-prepared LTO at different magnification (inset in (d): illustration of the nanosheet perpendicular to us and SAED pattern taken from a single nanosheet).[38]

Chen and co-workers[38] reported a hydrothermal method to obtain LTO by using titanium tetraisopropoxide and LiOH as the starting material. Hydrogen peroxide was used to control the morphology of LTO. Typically, 1ml of 30% hydrogen peroxide was dispersed in 20 ml of 0.4 M LiOH, followed by the addition of 2 mmol titanium tetraisopropoxide. The solution was transferred into a 30 ml Teflon-lined stainless autoclave and maintained at 130 oC for 12 h. The resulting white precipitate was recovered by centrifugation, washed with deionized water thoroughly, and then dried in an oven at 80 oC. Finally, the as-prepared sample was calcined in a muffle at 550 oC for 6 h in the air. By this method, saw-tooth like LTO was obtained, as shown in Fig.1.3.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

1.2.4 Solvothermal method

The difference between solvothermal and hydrothermal method is the solvent used in the preparation process. Solvothermal method uses nonaqueous solution, e.g, ethanol, while hydrothermal method employs water as solution. Since a variety of organic solvents with high boiling points can be selected by using solvothermal method, the temperature can be elevated much higher than that in hydrothermal method. Besides, the solvothermal method normally has better control than hydrothermal methods of the size and shape distributions and the crystallinity of the LTO nanoparticles. The solvothermal method has been an alternative method to prepare LTO with small and uniform particles.[37, 91, 92] For example, lithium hydroxide (LiOH), tetrabutyl titanate and ethanol were used as starting materials. The reaction system was formed by 0.1 mol tetrabutyl titanate dissolved in 100 ml ethanol, and then followed by the addition of 0.1 or 0.08 mol LiOH. The undissolved LiOH particles were suspended in the orange solution. The alcoholic suspension was solvothermally treated at 140 oC for 24 h in a 150 ml autoclave under autogeneous pressure. The pressure generated in the autoclave at 140 oC is ~747 kPa. A Teflon liner was used to seal the reactive system and avoid any reaction with the stainless steel autoclave. After solvothermal reaction the hermetic system was cool down in the air, and the achieved solution is light orange, indicating that the reaction processed not completely. The produced powders were washed and filtered with ethanol and then deionized water five times to eliminate the unreacted reagents, followed by drying at 80 oC for 24 h in air.[37]

1.2.5 Spray pyrolysis

Spray pyrolysis technique has been developed to directly produce ceramic powders from solutions.[93] Spray pyrolysis is ceramic powder process for particles through decomposing precursor molecules at high temperature. A typical spray pyrolysis system consists of a reservoir for precursor solution, droplet generator, reactor and collection unit. Precursor solutions are firstly atomized and carried by a gas into a reactor in which droplets are evaporated and decomposed into solid particles. Nucleation and growth of monomer precursor are also involved in the process of particle formation. Electrical heating or flame provides heat for evaporation of solvent and decomposition of precursor.[94] Ju and co-workers reported spray pyrolysis method for synthesizing LTO.[45-47, 95] The precursor solution was prepared by dissolving a stoichiometric 14

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications amount of lithium nitrate and titanium (IV) tetraisopropoxide in distilled water. The as- prepared powders obtained by spray pyrolysis at a preparation temperature of 800 oC were post-treated in the box furnace at the temperature range between 600 and 1000 oC for 12 h in the air.[45]

1.2.6 Solution-combustion method

Solution-combustion method is a facile and economical technique for the preparation of nanomaterial for a variety of applications, such as catalysts, fuel cells, biotechnology and lithium ion batteries.[96] It has the advantage of relatively simple equipment, formation of high-purity products with size and shape control, and stabilization of metastable phases. The initial reaction medium is aqueous or nonaqueous solution, fuels are used as the source of C and H to liberate heat through combustion and complexes with the metal ions formed could facilitate homogeneous mixing of cations in solution.[97] Prakash et al.[56] reported a solution-combustion synthesis of LTO nanopowders using titanyl nitrate [TiO(NO3)2] and LiNO3 as the oxidant precursors and glycine as the fuel. In a typical preparation, an aqueous redox mixture containing stoichiometric amount of titanyl nitrate (0.0362 mol), 2 g of LiNO3 (0.0289 mol), and 4.22 g of glycine (0.0562 mol) were taken in a 120 ml alumina crucible and placed into a muffle furnace preheated to 800 oC. The reaction completes in a few seconds.

Raja et al.[57] reported a novel aqueous combustion process using a common amino acid alanine as fuel to synthesize Li4Ti5O12, which shows a uniform morphology with an average particle size in the range 40-80 nm and optical band gap at 1.80 eV. Yuan et [18] al. reported a glycine-nitrate combustion process for preparation of spinel Li4Ti5O12. LTO calcined at 700 ºC shows the best and high electrochemical performance, which reached a capacity of 125 mAh/g at 10 C with fairly stable cycling performance even at

40 ºC. Surface reaction kinetics of Li4Ti5O12 was improved significantly with the increase of its electronic conductivity by this method. Moreover, Yuan and co- workers[16] reported cellulose-GN combustion process to synthesize pure-phase spinel

Li4Ti5O12 at reduced temperature using anatase TiO2 solid as raw material of titanium.

Compared with solid-state reaction, the cellulose-GN process produced the Li4Ti5O12 oxides with smaller particle size and higher specific capacity due to the lower synthesis temperature. A reversible capacity of 103 mAh/g at 20 C and fairly stable cycling performance even at 40 C was achieved. The oxide delivered a high discharge capacity 15

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications of ∼170 mAh/g at 1 C rate and 103 mAh/g at 20 C. It also had much better high-rate performance than the oxide prepared by the solid-state reaction applying similar raw materials. Besides, it is also reported[17] that the cellulose-assisted combustion synthesis of Li4Ti5O12, which delivers a capacity of ~175 mAh/g (theoretical value) at 1 C rate, and even reached ~100 mAh/g at a high discharge rate of 10 C with high cycling stability. It is a promising method for the synthesis of Li4Ti5O12 with high rate performance and high cycling stability. Prakash et al.[56] reported the synthesis of nanoparticles Li4Ti5O12 crystallizing in cubic spinel-phase by single step solution combustion method in less than one minute. Li4Ti5O12 particles synthesized are flaky and highly porous in nature with a surface area of 12 m2·g-1. TEM images indicate the primary particles to be agglomerated crystallites of varying size between 20 and 50 nm with a three-dimensional (3D) interconnected porous network. During the galvanostatic charge-discharge stage at varying rates, Li4Ti5O12 electrodes yield a capacity value close to the theoretical value of 175 mAh/g at 0.5 C. The electrodes also exhibit promising capacity retention with little capacity loss over 100 cycles at varying discharge rates together with attractive discharge-rate capabilities yielding capacity values of 140 mAh/g and 70 mAh/g at 10 and 100 C discharge rates, respectively. Li4Ti5O12 prepared by the combustion technique is superior in terms of both high rate-capability and capacity retention compared with solid state method, which can be attributed to the high crystallinity and nanosized particles of LTO.

1.2.7 Other methods

Besides the above-mentioned methods, other methods are also developed, such as reflux method,[52] continuous flow supercritical synthetic method,[91] sonochemical method[98] and etc.

Yoshikawa et al.[99] demonstrated a spray-drying method to prepare lithium titanate. The pure Li4+xTi5-xO12гδ (x = 0.06-0.08) phase materials shows a higher discharge capacity of 164 mAh/g in the range of 1.2-3.0 V, and exhibits excellent cyclability and superior rate [50] performance compared with Li4Ti5O12 containing impurity phases. He et al. reported hollow spheres Li4Ti5O12 prepared by using carbon spheres as template, which exhibits excellent rate capability and capacity retention, and a specific capacity of 150 mAh/g even after 200 charge and discharge cycles at 2 C. Yin et al.[62] used rheological method

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

o to synthesize Li4Ti5O12, which heated at 110 C exhibits discharge capacities of 161.6, 156.5 and 112.3 mAh/g after 50 cycles at current rates 1, 2.5 and 10 C, respectively.

Li4Ti5O12 has an average particle size ~140 nm and shows excellent high rate capability and cycling stability. Doi et al.[63] first reported the preparation of uniform nanosized particles Li4Ti5O12 by electrospray deposition method. Spinel Li4Ti5O12 particles have a fairly uniform particle size of ~12 nm and a distinct crystal form. Kim et al.[52] reported reflux method to obtain Li4Ti5O12 (6-8 nm), which is prepared from ethylene glycol solution of titanium tetra isopropoxide (Ti(O-iPr)4) and Li2O2 by refluxing at 197 ºC for 12 h, the obtained particles were filtered and dried at 100 ºC for 12 h, as a post-reaction step, and the dried powder samples further heated. Heated at 500 ºC for 3 h, Li4Ti5O12 exhibits the initial capacity of 320 mAh/g at a current density of 0.05 mA·cm-2 and an excellent rate capability over 60 cycles. Bai et al.[49] reported a composite molten-salt method to synthesize Li4Ti5O12. The optimized Li4Ti5O12 with molar ratio of LiCl: KCl is 1.5, which has an initial discharge capacity of 169 mAh/g and an initial charge- discharge efficiency of 94% at 0.2 C. When the current rates are 0.2, 0.5, 1, 2 and 5 C, the discharge capacities of Li4Ti5O12 are 169, 153, 142, 137 and 130 mAh/g, respectively, which means Li4Ti5O12 achieves good rate performances from 0.2 to 5 C. [47] Ju et al. reported the synthesis of Li4Ti5O12 by spray pyrolysis method. Li4Ti5O12 obtained from spray solutions with formamide, N,N-dimethylformamide and 1,4- dioxane have high initial discharge capacities of 167, 166 and 163 mAh/g at a constant current density of 0.1 C. Obtained from the spray solution with formamide, Li4Ti5O12 exhibits 95% of the initial discharge capacity after 60 cycles. They reported that

Li4Ti5O12 synthesized by spray pyrolysis can achieve dense structure, fine size, pure crystal structure and high discharge capacity with appropriate drying control chemical additive materials. Flame spray pyrolysis (FSP), proposed by Ernst et al,[48] synthesized

Li4Ti5O12 with primary crystallite size of 7-30 nm and high temperature stability. FSP process optimization could be used to further remove impurities. Moreover, FSP- prepared Li4Ti5O12 nanoparticles show good sintering stability at elevated temperatures and scalability of the process.

1.3 Modification

Since the oxidation state of Ti in Li4Ti5O12 is the highest possible valence (+4) for Ti,

Li4Ti5O12 is a very poor electronic conductor, which leads to low specific capacity and

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications poor capabilities at high rate. Great interests have been attracted to increase the electrical conductivity of Li4Ti5O12 in the science and technology of lithium ion batteries. Many efforts have been done as described as follows:

[100] 1. Doping noble metal or oxide nanoparticles into the Li4Ti5O12. For example, 4+ 3+ 4+ replacing some of Ti with M or M metals to prepare Li4Ti5O12, such as 3+ 3+ 3+ 3+ 4+ 4+ Li4/3−y/3MyTi5/3−2y/3O4 (e.g.Fe , Ni , and Cr ), Li4MyTi5−yO12 (Al , Mn , V , and 3+ [101] [102] Fe ), and Li4AlxTi5−xO12−yFy, Cr-, Fe-, Ni- or Mg-doped, substituting Li or Ti by other metal cations, such as V5+, Mn4+,Ga3+, Co3+, Ta5+, Cu2+;[103]

2. Incorporation of a second phase with high electronic conductivity. For instance, [104-106] [107-109] employing Ag, carbon, and Cu or CuOX as the conducting second [110] [111] phase, and loading SnO2 on Li4Ti5O12;

3. Addition of conductive fillers,[112] such as carbon black;[25]

4. Surface coating with a more conductive material, e.g, carbon and silver,[11] and TiN thin film;[113]

5. Novel synthesis methods to reduce the particle size and obtain desired structure,[107] especially the nanoparticles,[112] including combustion synthesis method with different agents,[16-18] and rheological method;[62]

6. Nitridation to form oxynitride species on its surface.[56]

1.3.1 Atomic doping Li4Ti5O12

Doping has been used as one of the methods in the research field to enhance the poor electronic conductivity of Li4Ti5O12. It can be realized by doping various metal cations (V5+, Mn4+, Fe3+, Al3+, Ga3+, Co3+, Cr3+, Ni2+, Mg2+) into Li or Ti sites,[2] or other anions such as Br-,[2] F-,[114] into O sites. Theoretical cation doping effects on the electronic [102] conductivity of Li4Ti5O12 has been reported. According to first-principles local- density calculations, Cr or Mg doping improves the electronic conduction of Li4Ti5O12, while Ni or Fe does not have such effect. Wen et al.[105] reported that Ag, Cu, C and the oxide CuxO composite-based LTO material enhances its electronic conductivity significantly. Co-doping, such as Mg-Al,[7] Al-F,[114] Mg-V,[13] can also enhance its electronic conductivity. 18

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Transition metal doping

Transition metal doping attracts much interest to improve conductivity of electrode [115] materials, including cathode materials, such as LiCoO2, LiNiO2, LiMnO2 and anode materials. Low conductivity of Li4Ti5O12 leads to a low specific capacity, limiting the application in industrial market. To Dope Li4Ti5O12 with transition metal is evidently a good way to solve this problem. Most of transition metal in the fourth period have been studied. The properties of transition metal doping on the LTO is summarized in Table 1.2.

Table 1.2 The properties of Li4Ti5O12 doped by transitional metal Electrochemical Doped formula Function Advantages Ref. performance Transition metal in the fourth period No effects on lattice parameter, x Improved blue shift on x Li Ti V O reversible Raman bands of 4 4.9 0.1 12 183 mAh/g - capacity, V5+ Li-O and Ti-O 100 cycles - Li Ti V O vibration. The x good [116] 4 5-x x 12 0.5-2.0 V cycling (0≤x≤0.3) discharge capacity x 229 mAh/g - performan decreases as the 130 cycle ce, content of V x low cost. increases. Increases average particle size, a x 151 mAh/g - little higher 2 C x High rate Li Ti Cr O [6] 4 2.5 2.5 12 conduction, no cycleability x 146 mAh/g - (up to 5 C). effects on 5 C crystallite size Mnn+(n=2,3,4) Li 1.333- No change on Mn Ti x/3 x 1.667- spinel structure, O , 2x/3 4 lattice parameter Li 1.333- and conductivity, Mn Ti -- -- [33] x/6 x 1.667- decreases with O 5x/6 4, increasing the Mn Li Mn Ti 1.333 x 1.667- content. O x 4 (x= 0.05, 0.167, 0.25, 0.50) Fe3+ Decreases the x 106 mAh/g -- [117] Li4Ti4.75Fe0.25O12 capacity Decreases the Co3+ x Decreasing the lattice parameters Li Co Ti O electro- -- [7] 3.85 0.15 4.9 1 and electronic chemical 1.9 conductivity. performance, 19

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

x 171.9 mAh/g - 0.15mA·cm-2 x 155.2 mAh/g - 40 cycle Improve the electronic x high rate x 209.2 mAh/g - Cu conductivity, discharge 1 C - 1st cycle Li Ti O /Cu almost two orders capacity [112] 4 5 12 x 142.5 mAh/g - composite of magnitude x cycling 10 C-1st cycle higher than the stability pristine Li4Ti5O12. x 163.3 mAh/g - 1st cycle - x = 0.075 x 169.5 mAh/g- 1st cycle- Lattice parameter x = 0.15 gradually Cu2+ increases with x, x 177.0 mAh/g - x High rate 1st cycle - performan [110] Li4Ti5CuxO12 +x optimal composition is x = 0.3 ce x 187.2 mAh/g - Li4Ti5Cu0.15O12.15. 1st cycle - x = 0.6 x Little capacity loss after 50 cycles. Transition metal in other period Does not enter x Improved the spinel reversible Ag structure, x 114.4 mAh/g- capacity [105] Li Ti O /Ag 4 5 12 improve 25 cycles - [104] composite 10 C x good electrochemical cycling performance. stability Does not x Less capacity degradation obviously change the structural with the rate increase. Zr characteristics, Li Ti Zr O greatly affects the x 141 mAh/g- x Improved 4 5−x x 12 [118] (x=0, 0.05, 0.1, morphology and 3 C - x = 0.05 the rate 0.2) particle size, x 146 mAh/g - capability optimum 3 C - x = 0.1 composite is x 126 mAh/g- Li 4Ti4.9Zr0.1O12 3 C - x = 0.2 Improves x 169.1 mAh/g - electronic 1 C Nb x excellent conductivity and rate [103] Li Ti Nb O x 115.7 mAh/g - 4 4.95 0.05 12 lithium-ion 10 C capability diffusivity -100cycles La3+ Improves x 161.5 mAh/g- x Improves La/Ti mol ratio electronic 0.1 C - 1st electroche [101] of 1:100 conductivity cycle mical 20

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

x 135.4 mAh/g- performan 0.1 C - 50 ce cycles

4+ + 5+ Partial substitutional Ti and Li in the spinel with V as Li4Ti5-xVxO12 was proposed [116] by Yi et al. The discharge specific capacities of Li4Ti5-xVxO12 are 167, 171, 167, and 166 mAh/g cycling in the voltage range of 1.0-2.0 V, when x = 0, 0.1, 0.15 , respectively; 208, 240, and 234 mAh/g cycling in 0.5-2.0 V, when x = 0, 0.1, 0.15, respectively; 303, 348, 333, 323, 312, and 254 mAh/g cycling in 0.0-2.0 V, when x = 0,

0.1, 0.15, 0.2, 0.25, 0.3, respectively. The capacity is higher than pure Li4Ti5O12 spinel.

Those results indicate that Li4Ti4.9V0.1O12 has the highest initial discharge capacity and cycling performance in the range of 0.0-2.0 V or 0.5-2.0 V. They further reported[119]

Li4Ti4.95V0.05O12 delivering discharge capacity of 218.4 mAh/g after 50 cycles. Low electrode polarization and high lithium ion diffusivity in solid-state body of

Li4Ti4.95V0.05O12 implies that vanadium doping is beneficial to the reversible intercalation and de-intercalation of lithium ions.

[33] Mn-substituted Li4Ti5O12 was demonstrated by Capsoni et al. The Fd3m spinel structure of the lithium titanate is preserved and the lattice parameter decreases with the increasing of Mn content. Mn2+ ions exclusively occupy the tetrahedral site up to 10% 3+ Mn-substituted Li4Ti5O12. Mn ions distribute both on octahedral and tetrahedral sites, with a constant value on the tetrahedral one, independent of the total Mn amount; Mn4+ ions are not detected. The presence of Mn ions on the tetrahedral site, which extrudes Li content in the material, decreases the electronic conductivity of spinel Li4Ti5O12.

Different amount of Co can influence the crystallize structure of spinel. Jovic et al.[120] reported that in the composition of Li1.33xCo2−2xTi1+0.67xO4 (0 ≤ x ≤ 1), space group is

Fd3m at 0 ≤ x ≤ 0.40 or x = 1, the space group is P4332 at 0.50 ≤ x ≤ 0.875.

Copper doping has been demonstrated to improve the electrochemical performance of [110] 2+ spinel Li4Ti5O12. Wang et al. reported the Cu doping into Li4Ti5O12 as

Li4Ti5CuxO12 +x, which largely improves rate performance compared with Li4Ti5O12. The optimal nominal composition is Li4Ti5Cu0.15O12.15 with increasing the conductivity of

Li4Ti5O12 by many orders of magnitude. When x = 0, 0.075, 0.15, 0.30, and 0.6, the -7 -6 -6 -6 -6 conductivity of Li4Ti5CuxO12+x is 4.2×10 , 3.0×10 , 4.7 ×10 , 5.3 ×10 and 8.2 ×10 -1 S·cm , respectively, while the conductivity of Li4Ti5O12 at room temperature is less 21

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications than 10-13 S·cm-1.[14] Huang et al.[112] also reported that the Cu additive effectively improves the electronic conductivity and electrochemical performance of the pristine

Li4Ti5O12, especially at high rate (see Table 1.3).

Table 1.3 Comparison of discharge capacity and cycling stability of the Li4Ti5O12 and [112] doped Cu- Li4Ti5O12 anode materials Discharge capacity cycle (mAh/g) Rate a (C) Anode materials 1st 10th Doped Cu- Li Ti O 209.2 171 1 4 5 12 Li4Ti5O12 180.6 164.2 Doped Cu- Li Ti O 184.8 166.6 2 4 5 12 Li4Ti5O12 162.2 155 Doped Cu- Li Ti O 173.4 153.6 4 4 5 12 Li4Ti5O12 150.7 117.3 Doped Cu- Li Ti O 165.7 144.6 8 4 5 12 Li4Ti5O12 98.9 72.5 Doped Cu- Li Ti O 142.5 141.6 10 4 5 12 Li4Ti5O12 80.3 61.2 a: charge-discharge rate

Ag as a second phase could effectively improves the electronic conductivity and [104] electrochemical performance of Li4Ti5O12. Huang et al. reported that Ag does not [106] enter the spinel structure, as shown Fig.1.3. They further reported that Li4Ti5O12/Ag composite displayed the highest discharge capacity at the 5wt.% Ag content. High rate discharge capacity and cycling stability of the Li4Ti5O12 are particularly increased by Ag dopant, which is summarized in Table 1.4. The study reported by Wen and co- workers.[105] also showed similar results.

Zirconium and Niobium are also studied to recognize their effects on spinel Li4Ti5O12. [118] Li et al. reported the effects of Zr substitution in the form of Li4Ti5гxZrxO12 (x = 0,

0.05, 0.1 and 0.2). The dopant Zr partly enters the lattice structure of Li4Ti5O12, and the excessive part existed as the impurity of ZrO2. Zr-doping does not change the electrochemical reaction process of Li4Ti5O12, but greatly influences its morphology and particle size. The particle size of the Zr doped Li4Ti5O12 sample was less than 100 nm and has less agglomeration, which result in the improvement of the rate capability. However, a high amount of Zr-doping is adverse to the electrochemical performance probably due to the amount of ZrO2 impurity contained in the Li4Ti5O12. Therefore, there is an optimum amount for Zr-doping. Li4Ti4.9Zr0.1O12 exhibits the best rate capability and cycling stability. At the charge-discharge rate of 5, 10 and 20 C, its

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications discharge capacities were 143, 132 and 118 mAh/g, respectively. It retains as 141 mAh/g even after 100 cycles at 5 C. Tian et al.[103] reported Nb doping effects on

Li4Ti5O12. The Li4Ti4.95Nb0.05O12 presents a higher specific capacity and better cycling performance than Li4Ti5O12 prepared by the similar process. Its exhibits an excellent rate capability with a reversible capacity of 169.1 mAh/g at 1 C and 115.7 mAh/g at 10 C even after 100 cycles. A higher electronic conductivity and faster lithium-ion diffusivity in Li4Ti4.95Nb0.05O12 than pure Li4Ti5O12 indicates that niobium doped lithium titanate

(Li4Ti4.95Nb0.05O12) is promising as a high rate anode for the lithium-ion batteries.

Table 1.4 Comparison of discharge capacity and cycling stability of the Li4Ti5O12 and [104] doped Ag- Li4Ti5O12 anode materials Discharge capacity cycle Capacity Rate a (C) Anode materials (mAh/g) degradation(%)b 1st 10th Doped Ag- Li Ti O 192.9 174.7 0.17 0.2 4 5 12 Li4Ti5O12 198.1 177.6 10.7 Doped Ag- Li Ti O 171.9 168.5 0.47 0.5 4 5 12 Li4Ti5O12 169 163.2 0.85 Doped Ag- Li Ti O 196.9 197.8 -0.46 1 4 5 12 Li4Ti5O12 180.6 164.2 4.59 Doped Ag- Li Ti O 189.8 178.1 0.11 2 4 5 12 Li4Ti5O12 162.2 155 1.40 Doped Ag- Li Ti O 163.3 156.2 0.32 4 4 5 12 Li4Ti5O12 150.7 117.3 11.8 a: charge-discharge rate, b: compared to the second cycle.

Fig.1.4 XRD patterns of: (a) Li4Ti5O12 (b) doped Ag-Li4Ti5O12. It was shown that except the characteristic patterns of Ag metal, all the peaks were in accordance with the [104] patterns of Li4Ti5O12. 23

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Most of the transition metals have been studied and shown good function on electronic improving. However, Ni and Zn in the fourth period have not been studied. Co3+ 2+ decreases the electrochemical performance of Li4Ti5O12, while Cu improves the rate discharge capacity and cycling stability of the pristine Li4Ti5O12. Ni as the transition metal between them in the same period may improve the electrochemical performance as Cu does. Zn, on the other hand, is a transitional metal following Cu in the period table, may follow the same effects by Cu substitution. Thus it is likely that Zn can improve the electrochemical performance of Li4Ti5O12. Additionally, some experiments should be undertaken to verify those conjectures.

Doped by metal in the main group

Some metals in the main group doping have attracted considerable interest and studied on the electric conductivity improvement of spinel lithium titanate. The metals studied presently are in the second, third and fourth period, their function and properties are summarized in Table 1.5.

Table 1.5 Doping properties of Li4Ti5O12 by metal in the main group Metals Formation Electrochemical performance and properties Ref. x Appropriate Li (x = 0.1) doping amount [35] improves electronic conductivity and Li Li4+xTi5-xO12 lithium-ion conductivity, resulting in better rate performance compared to the undoped Li4Ti5O12.

x Li4-xMgxTi5O12 increases the conductivity [27, of Li4Ti5O12 by many orders of magnitude; 121] x When x = 1.0, the conductivity of -2 -1 Li3MgTi5O12 is increased to 10 S cm -13 -1 from 10 S cm for pristine Li4Ti5O12; Mg Li4-xMgxTi5O12 x It improves the rate property of Li4Ti5O12.

x 3 % Mg-doped Li4Ti5O12 has the best charge (145 mAh/g) and discharge capacities (149 mAh/g) at the current density of 0.1 C. x It enhances the electronic conductivity [122] while decreases the lithium ionic conductivity; x The cycling stability at high Al Li4-xAlxTi5O12 charge/discharge rate is increased significantly;

x An optimal composition is Li3.9Al0.1Ti5O12, which shows the best rate performance under this experimental condition.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

3+ x Ga increases the capacity of Li4Ti5O12 to [7] some extent; x At a constant current density of 0.15 mA·cm-2 between the cut-off voltages of 2.3 and 0.5 V, the first discharge capacity Ga Li Ga Ti O 3.82 0.15 4.9 11.94 of it is 199.8 mAh/g; x at a current density of 0.15mA·cm-2 between 0.5 and 2.3 V, the reversible capacities after 40 cycles is 165.5 mAh/g with capacity loss of 4.39%.

Li and Mg have been demonstrated to improve the electrochemical performance due to the increasing of conductivity. Mg has been proved to be a good substitutional element in the spinel lithium titanate, doping Mg enhances greatly the conductivity of lithium titanate. Since these two metals obey the diagonal relationship, it is understandable that they have similar function while doping. Al has been investigated to have an optimal composition as Li3.9Al0.1Ti5O12, which shows the best rate performance. Be, which has the similar properties according to the diagonal relationship, may also obtain an optimal composition resulting in the best electrochemical performance by doping into Li4Ti5O12. However, further investigations are needed to confirm the prediction.

Doped by non-metal in carbon group

Carbon is usually employed to modify electrode materials (e.g. LTO) to improve the electrochemical performance due to its low cost and good conductive property. There are mainly three categories of carbon modification on LTO anode material; namely, carbon coating on the surface, carbon doped into the nanostructure, and carbon existed as a secondary phase by forming a composite. Carbon modified LTO can improve the electrochemical performance in terms of rate capability, lithium storage capacity and capacity retention.[108, 123, 124] The reasons for such enhancement in the performance are: (a) the carbon acts as bridges for electron pathways that electrically interconnect the LTO particles, hence, effectively improves the electronic conductivity of the hybrid material. The different morphology (e.g. nanoparticles, nanotube) or category (e.g. graphene, amorphous carbon) of carbon would improve the electric conductivity. (b) The nanosized LTO particles in the hybrid material provide a large electrode/electrolyte contact surface, shorten lithium diffusion path and allow fast electron/ion diffusion. The combined two parts is expected to provide LTO electrode with fast kinetics and lead to enhanced electrochemical performance compared with the bulk LTO.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

[29] [125] Li et al. and Lin et al. reported Li4Ti5O12/C composite improves reversible capacity and rate capabilities. The pyrolytic carbon incorporated into the spinel

Li4Ti5O12 particle reduces grain size and improve electronic conductivity, which is contributed to the electrochemical performances of Li4Ti5O12. The initial specific capacity of the composite is 130.0 mAh/g at 8.60 mA·cm-2, and excellent cycle performance is still maintained with the current density increase. Liu et al.[126] reported

Li4Ti5O12/C composite synthesized at 800 ºC for 15 h under argon, containing 0.98 wt% of carbon. The composite exhibits better electrochemical properties compared with the pristine Li4Ti5O12 due to the enhanced electrical conductive network of the carbon coating on the particle surface. The composite can deliver a capacity of 173.9 mAh/g at [127] 0.1 C rate while that of pure Li4Ti5O12 is only 165.8 mAh/g. Huang et al. reported that Li4Ti5O12/carbon nano-tubes presents an excellent rate capability and capacity retention. Its discharge capacities are 145 and 135 mAh/g, at the rate of 5 C and 10 C, respectively. After 500 cycles at 5 C, the discharge capacity retains as 142 mAh/g . [30] Recently, Li et al. further reported the composite of Li4Ti5O12/carbon/carbon nanotubes exhibiting better electrochemical performance than a Li4Ti5O12/carbon [31] composite. In addition, they investigated Li4Ti5O12/graphitized carbon nanotubes composite, which shows high specific capacity and good rate capability. The initial discharge capacity of the composite is 163 mAh/g at 0.5 C. Even at 10.0 C, its initial discharge capacity can reach 143 mAh/g, and 132 mAh/g after 100 cycles, which is about 86% of its capacity after the same number of cycles at 1.0 C (154 mAh/g).

Applying Sn as anode, high capacity of 990 mAh/g can be achieved, however, Sn anode is unstable and can expands the volume as high as 360% than its original volume during Li insertion and extraction.[128] One method to utilize the high capacity of Sn is to use it [11] as a doping element. Cai et al. reported that Li4Ti5O12/Sn composite displays much higher discharge capacity than pure Li4Ti5O12. Li4Ti5O12/Sn composite displays the first discharge capacity of 442 mAh/g at current density of 100 mAh/g, and a capacity of 224 mAh/g, which is higher than 195 mAh/g for the pure Li4Ti5O12, after 50 cycles at 100 mA/g current density.

Si has a capacity of 1000 mAh/g, which is much higher than that of Li4Ti5O12. Its high capacity may be attributed to improve the capacity of LTO if it is doped into the spinel. Further work on the Si doping is certainly worth performing.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Doped by non-metal as halogen

Many interests has been placed on the metal, especially transition metal, substitute for

Li or Ti in Li4Ti5O12. Recently, some studies on the O sites substituted by the halogen elements have been reported. Qi et al.[2] reported partial Br substitution for O site in

Li4Ti5O12 as the composition Li4Ti5O12гxBrx (0 İ x İ 0.3) by solid-state method. Br- doping does not change the XRD phase composition of Li4Ti5O12 (Fig.1.5), however, the crystallinity of the Li4Ti5O12 decreases with increasing Br-doping level. Br-doping increases the specific capacity significantly, as well as the rate-capability of Li4Ti5O12. The substitution of Brг into O2- sites can increase the amount of Ti3+/Ti4+ mixing as charge compensation and thus not only decrease the charge transfer resistance of

Li4Ti5O12 г xBrx, but also speed up the diffusion of lithium ion. With x = 0.2, the

Li4Ti5O12гxBrx presents the highest discharge capacity, and shows better reversibility and higher cyclic stability compared with pristine Li4Ti5O12, especially at high current rates. When the discharge rate was 0.5 C, the Li4Ti5O12гxBrx (x = 0.2) presents excellent discharge capacity of 172 mAh/g, while that of the pristine Li4Ti5O12 was 123 mAh/g. And maintains ~150 mAh/g after 50 cycles at the current density of 1.0 C.

Fig.1.5 (a) XRD patterns of synthesized Li4Ti5O12гxBrx (x = 0, 0.05, 0.1, 0.2, 0.3); (b) [2] magnified (111) peaks of doped Br in Li4Ti5O12 with different amounts.

[114] - Huang et al. reported F doping in Li3.683Ti5F0.22O11.732. It shows that F substitution reduces the particle size, and presents slightly decreased reversible capacity and cycling stability. When the cut-off voltage is 0.8 V, it exhibits a reversible capacity of 74.7

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications mAh/g after 20 cycles with capacity loss of 17.1% compared to its corresponding second cycle; when the discharge cut-off voltage is 0.5 V, it appears a platform at 0.68 V in addition to the main platform at 1.5 V, and the first discharge capacity is 172.1 mAh/g.

Although F reduces the particle size and slightly decreases reversible capacity and cycling stability of Li4Ti5O12, Br has been shown to improve reversibility and cyclic stability significantly, especially at high current rates. Cl, which is the element between

F and Br, may improve the electrochemical performance of Li4Ti5O12. I, belong to the same main group, may also modify the properties of Li4Ti5O12 by doping appropriately. All these are worth further investigation.

Doped by metal oxide

Up to date, some studies on metal oxide doping Li4Ti5O12 into have been reported.

Li4Ti5O12/CuxO composite anode material for lithium-ion batteries, proposed by Huang et al.[129], exhibits much better high rate discharge capacity and cycleability than pristine

Li4Ti5O12. The discharge capacity of the composite remains 137.6 mAh/g after 100 cycles at 10 C, and its capacity loss is only 5.56%.

Theoretically the first discharge capacity of SnO2 reaches 1491mAh/g, far more than that of Li4Ti5O12. Although only the second reaction is reversible, the reversible [11] discharge capacity of SnO2 is 781 mAh/g, which is still twice more than Li4Ti5O12. + However, due to its high corporation capacity of Li , SnO2 has been of great interests recently. Wang et al.[111] reported an initial discharge capacity of 476 mAh/g and specific capacity of 236 mAh/g after 16 cycles in Li4Ti5O12/SnO2 composites. SnO2 acts as a bridge between the spinel particles to reduce the interparticle resistance and as a [11] good material for the Li inserstion. Cai et al. reported that Li4Ti5O12/tin phase composites exhibits the first discharge capacity of 442 mAh/g at current density of 100 mA/g in 0.5-3.0 V. The capacity is maintained as 224 mAh/g after 50 cycles at 100 mA/g current density, which is higher than 195 mAh/g for the pure Li4Ti5O12 after the [130] same charge/discharge cycles. Hao et al. reported Li4Ti5O12/SnO2 (5%) has the best cycling behaviour, which delivers a discharge capacity of 189 mAh/g after 42 cycles at a current rate of 0.5 mA cm-2. The material shows a significantly improved cycle-life performance compared with SnO2.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

TiO2 is another promising dopant on improving the capacity of Li4Ti5O12. Rahman et [15] al. reported a nanocrystalline Li4Ti5O12-TiO2 duplex phase synthesized by a simple basic molten salt process (BMSP) using a eutectic mixture of LiNO3-LiOH-Li2O2 at 400-500 oC. Heat-treating at 400 oC for 3 h yielded the best electrochemical performance in terms of charge-discharge capacity and rate capability. It shows initial discharge capacities of 193 mAh/g at 0.2 C, 168 mAh/g at 0.5 C, 146 mAh/g at 1 C, 135 mAh/g at 2 C, and 117 mAh/g at 5 C. After 100 cycles, the discharge capacity is 138 mAh/g at 1 C with capacity retention of 95%. Its superior electrochemical performance can be mainly attributed to the duplex crystallite structure, composed of fine (<10 nm) and coarse (>20 nm) nanoparticles, where lithium ions can be stored within the grain boundary interfaces between the spinel Li4Ti5O12 and anatase TiO2.

Co-doping on Li4Ti5O12

There are also some investigations on several elements combining together as dopant. [114] 3+ − Huang et al. reported the co-doping effects of Al and F on Li4Ti5O12. At a constant current density of 0.15 mA cm-2 between the cut off voltages of 2.5 and 0.5 V, 3+ − the Al and F substitutions improves the first total discharge capacity of Li4Ti5O12. However, Al3+ substitution greatly increases the reversible capacity and cycling stability − of Li4Ti5O12 while F substitution decreases its reversible capacity and cycling stability 3+ − slightly. The electrochemical performance of the Al -F co-substituted Li4Ti5O12 is better than the F- substituted one but worse than the Al3+-substituted one. Shenouda et [13] al. reported the effect of Mg-V co doping on Li4Ti5O12 in the form of Li4-xMgxTi5- xVxO12 (0 ≤ x ≤ 1). The addition of Mg and V ions improves the conductivity of the −4 -1 material as observed in Li3.25Mg0.75Ti4.25V0.25O12 (2.452×10 Scm ). At a voltage + plateau located at about 1.55 V (vs. Li ), Li3.25Mg0.75Ti4.25V0.25O12 cell exhibits an initial specific discharge capacity of 198 mAh/g.

To our best knowledge, not too much work has been reported on the co-doping, and other work such as Ag-Cu, Al-Br doping in this field still requires development.

1.3.2 Surface modification

Various kinds of methods have been applied to modify the surface of Li4Ti5O12, such as depositing carbon materials, polymers and conductive agents. Wang et al.[109] reported that carbon-coated Li4Ti5O12 displays larger diffusion coefficient of lithium ions, higher 29

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications rate capability and excellent reversibility. Its reversible capacity is approximately 160 [108] mAh/g at the rate of 0.1 C. Yuan et al. reported carbon-coated Li4Ti5O12 synthesized by a cellulose-assisted combustion technique with sucrose as organic carbon source. The composite anode materials improve lithium insertion/extraction capacity and electrode kinetics, especially at high rates and low temperature. Its [107] conductivity increases ~3 orders of pure Li4Ti5O12. Ju Bin et al. reported carbon- coated Li4Ti5O12 obtained by calcining a mixture of Li4Ti5O12 and sucrose. The carbon coating does not break the structure of Li4Ti5O12 and improves the electrochemical [131] [132] performance of Li4Ti5O12. Zhu et al. and Jung et al. reported that carbon-coated microscale secondary LTO (~1-2, 10-20 μm) particles composed of nanoscale (<200 nm) microscale spherical LTO anodes exhibit ultra-high rate capability similar to that of nanomaterials. The dense LTO particles have a unique morphology comprising primary nanoscale particles and uniform carbon coating on each primary particle.

Matsui and co-workers[74] reported amino additives for modifying the properties of

Li4Ti5O12. They suggested that the primary effect of amino additives is to suppress agglomeration of the Li4Ti5O12 particles, but not to reduce its crystallite size. Ionic liquids EMIm-dca is demonstrated by Zhao et al. to be a new carbon precursors for decorating electrode material. A very thin and uniform N-doped carbon coating layer was formed simply by mixing the LTO and the ionic liquid EMIm-dca, and the resulted material exhibited superior rate capability and excellent cycling performance.

[133] Wang et al. reported the spinel Li4Ti5O12 nano-particles with double conductive surface modification of Ti (III) and carbon, which delivers high power performance. The carbonization of polyaniline (PANI) effectively restricts the particle-size growth of

Li4Ti5O12 and reduces the surface Ti (IV) into Ti (III). This can improve the surface conductivity and shorten the Li-ion diffusion path. Furthermore, both the Ti (III) surface modification and the tailored particles (50-70 nm) have the potential to increase the solid solution (single-phase insertion/extraction) during the electrochemical process.

TiN, which is electrically conductive and chemically inert to harmful reactions with the [113, 134] solvent/electrolyte, is a candidate material for coating on Li4Ti5O12. Snyder et al. reported that the substitution of the unmodified Li4Ti5O12 electrode for the Li4Ti5O12 anode coated with TiN thin film via atomic layer deposition improves the cell

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications performance. The specific charge capacity of the modified electrode maintains the value of ~170 mAh/g over many charge/discharge cycles and at different charge rates.

The effect of conductive additives and surface modification with NF3 and ClF3 on the charge/discharge behaviour of Li4Ti5O12 using vapor grown carbon fiber (VGCF) and acetylene black (AB), was proposed by Utsunomiya et al.[135] VGCF and VGCF-AB mixtures increases charge capacities of pristine Li4Ti5O12 and those fluorinated with

NF3 by improving the electric contact between Li4Ti5O12 particles and nickel current collector. Surface fluorination by NF3 increases the first charge capacity of Li4Ti5O12 at high current densities of 300 and 600 mA/g. Since NF3 is a mild fluorinating agent for o Li4Ti5O12 between 100 and 200 C while ClF3 highly fluorinated Li4Ti5O12 surface, NF3 is a better fluorinating agent for Li4Ti5O12 than ClF3.

1.3.3 Effects of parameters

The particle size, morphology and crystallinity influence the electrochemical performance of Li4Ti5O12. Smaller particles have shorter diffusion distances for intercalated Li-ions, resulting in a higher charge rate for intercalated anodes.[136] When particle agglomarization behaviour is similar, smaller crystallites provide better high- rate performance.[137] Optimal particle size, morphology and crystallinity, which would lead to excellent rate capability and high capacity, can be realized by controlling conditions on the synthesis steps. The factors such as oxygen atmosphere and heat treatment are summarized in table 1.6.

Table 1.6 The factors and its function on the synthesis of Li4Ti5O12 Factors Function Ref. Plays an important role in the formation of LTO with high [13] O 2 specific capacity. Proper heat treatment could smoothen the particle surface [34] of Li Ti O particles and increase the rate-capability of the Heating 4 5 12 electrode. time Overtime heat treatment would cause particle aggregation and hence result in a poor electrode kinetic process. Heating Appropriate heating temperature can effectively improve [138] temperature the capacity of Li4Ti5O12 Additives Improve the electronic conductivity [112]

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

1.4 Properties

1.4.1 Physical and chemical properties

Spinel Li4Ti5O12 has the advantages of excellent lithium ion insertion and extraction reversibility, negligible volume or structural change during charge/discharge processes, which classifies Li4Ti5O12 as zero-strain material. It provides a flat potential plateau at + 1.55 V vs. Li /Li, at which Li4Ti5O12 exhibits reversible peak. Solid electrolyte interface (SEI) film is often formed at the potential below 0.8 V versus Li+/Li, the potential of

Li4Ti5O12 is much above the reduction potential of most organic electrolytes, which consumes active lithium and results in a decrease of specific capacity. Thus the formation of SEI film can be avoided on the particle surface and it is safe by adopting [4] Li4Ti5O12 as anode in the electrochemical cell. Yao et al. have demonstrated that

Li4Ti5O12 has better thermal stability performance than natural graphite, especially below 230 oC.

One aspect should be pointed out, namely that Li4Ti5O12 has a low electrical conductivity, resulting in low specific capacity and poor capabilities at high rate.

Theoretically, the specific capacity of Li4Ti5O12 is 175 mAh/g, and the practical specific capacity is about 150-160 mAh/g, which is much less than other anode materials such as graphite (350 mAh/g). The excellent cycleability, thermal stability, cheap, safe and easy preparation makes Li4Ti5O12 an ideal material as anode for lithium ion batteries.

Due to the existence of available free octahedral sites in its lattice, Li4Ti5O12 is a good lithium-ion conductor. However, As the oxidation state of Ti in Li4Ti5O12 is the highest [25] possible valence for Ti (+4), Li4Ti5O12 is a very poor electronic conductor. More details about the properties and solution to improve electrical conductivity of Li4Ti5O12 are summarized in Table 1.7.

Applied as anode, spinel Li4Ti5O12 can be coupled with high voltage cathodes such as

LiCoO2, LiNiO2, or LiMn2O4, to fabricated lithium ion battery. When compared with its counterparts as anode, spinel lithium titanium Li4Ti5O12 is cheaper and easier to prepare than alloy-based anode and has better electrochemical properties and higher safety than [19] carbonaceous anode in lithium ion batteries. The comparison between Li4Ti5O12 and

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications other materials, including its counter electrode and counterparts, are summarized in Table 1.8.

Table 1.7 Properties and solution to improve electrical conductivity of Li4Ti5O12 x cheap, safe, and easy to prepare,[101] x good lithium-ion mobility, environmental friendliness,[38] x excellent lithium ion mobility than graphite,[39] x excellent high rate capabilities,[41] x negligible structure variation, lattice parameter is confined between 0.1% and 0.3%, free of solid electrolyte interphase (SEI) film,[24] Advantages x good Li-ion insertion and extraction reversibility, good structural stability,[100] x stable high-temperature performance even at 60 oC and high rate discharge performance even at 10 C,[139] x zero-strain insertion that provides little volume change during charge-discharge, little electrolyte decomposition (little solid electrolyte interface and little gas evolution), inexpensive raw [140] material. x low electronic conductivity,[110] x a poor high rate performance,[105] x perfect surface coatings and desired mixtures are often very [14] Disadvantages difficult to achieve, x suffered from a relatively lower reversible capacity (150–160 mAh/g) compared to other anode material such as graphite (∼300 [130] mAh/g). x a cubic close packed oxygen array, Li-ion diffusion coefficients are generally about 10-6 cm2·s -1,[141] x a theoretical specific capacity of 175 mAh/g, a practical specific capacity of 150-160 mAh/g,[85] x an energy density of 270 Whkg-1 in a Li metal cell,[134] Other properties x electronic conductivity is below 10-13 Scm-1 at room temperature[14, 102] (5.8×10−8 S·cm-1 at 140 oC[134]), x high columbic efficiency (>95% at 1 C), thermodynamically flat + [140] discharge profile at 1.55 V vs. Li/Li , x unique insertion–extraction mechanism that involves a two-phase [7] process between two compounds having the same symmetry.

Table 1.8 The comparison between Li4Ti5O12 and other materials for lithium ion batteries.

Counter electrode of Li4Ti5O12 materials Practical Safety Cost Comment Ref. Capacity (mAh/g) [10, LiCoO2 160, 140 Fair High Small-size [128] 142] 33

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

LiNiO2 220 Poor Fair impossible [128] LiMn2O4 110, Good Low HEV,EV [128] 125[143] T LiFePO4 170 , Good low --- [10] 134[143] LiFeO4 160 Good Low HEV, EV [128] Carbon anode Materials Practical Comment Ref. Capacity (mAh/g) Spherical 320-330 x Easy coating [128] graphitized mesocarbon microbeads(MCMB) Graphatized carbon 320-330 x Stopped to produce [128] fiber(MCF) Pitch base graphite 350, x Largest share in the market [128] 372T[10] Carbon-coated 360-365 x Less decomposition of [128] natural graphite electrolyte without additives Graphene 540 x Superior electronic [14] conductivity compared to graphitic carbon, x High surface area of over 2600 m2·g-1, x Chemical tolerance and a broad electrochemical window. Alloy anode [128] Li 1840, x Unsafe, short cycle life [144] 3600T[10], 3860T T[10] Si(Li4.4Si) 4200 x Maximum swelling: 400% [128] Sn 990 x Maximum swelling: 360% [128] SnB0.5Co0.5O3 600 x High irreversible capacity [128]

Li2.6Co0.4N 1200 x Unstable in the air [128] Si-C 1000 --- [10] LiC6 372 x Highly reactive, safety issue. [4, 142]

InSb, Cu2Sb 250-300 x Poor cycleability [142] TiO2 polymorphs anode TiO2-B 305 x Excellent capacity retention on [10] cycling TiO2-AB(acetylene 320 x Severe capacity fading [145] black) Rutile TiO2 270 x Poorer electrochemical [145] properties than anatase TiO2 T Anatase TiO2 336 x Low cost, nontoxicity, low [15] volume expansion (3-4%) during lithium insertion, x Environmental friendliness.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Phosphides LixTiP4(x=7,9) 700-900 x Fairly good cyclability [145] TiP2+2P 1600 x Good cycleability when [145] controlled charge capacity at 600 mAh/g. Li-Ti-O anode Li4Ti5O12 150-160, x EV, zero strain, safety, stable [85, 139] T 175 high-temperature performance, x Easy to prepare. T Li2Ti3O7 240 x Stable, good reversibility in Li [37] insertion and extraction, x rapid capacity fading at high current density of 525 mA/g. LiTi2O4 133.6 x Good electrochemical [146] reversibility, cyclic stability, and high rate performance. x Higher electric conductivity than Li4Ti5O12. x However, synthesis of pure phase LiTi2O4 is much more difficult. Li1+xTi2-2xO4 300 x Capacity of 190-200 mAh/g [147] and retains over 60 cycles.

Li4+xTi5гxO12гδ --- x x = 0.06-0.08, discharge [99] capacity of ~164 mAh/g at 1.55 V vs. Li, between the cut- off voltage of 1.2-3.0 V. x Excellent cyclability and superior rate performance in comparison with the Li4Ti5O12 phase containing impurity phases. Li2MTi6O14(M=Sr, 140 x A stable capacity of ~140 [148] T Ba) 242 (Ba), mAh/g for at least 40 cycles. 262T(Sr) T: theoretical capacity value

1.4.2 Electronic and atomic structure

Li4Ti5O12 has a face-centered cubic spinel structure based on space group symmetry of

Fd3m. In the Li4Ti5O12 spinel structure, tetrahedral 8a sites are fully taken up by lithium and the octahedral 16d sites are randomly occupied by lithium and titanium with an atomic ratio of 1:5 in a cubic close-packed oxygen array. Li4Ti5O12 can be described as

[Li]8a[Ti5/3Li1/3]16d[O4]32e

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Li4Ti5O12 is one of the end members of Li3+xTi6–xO12 (0 ≤ x ≤ 1) with Fd3m (space group) spinel structure[149] and lattice constant is 8.368 Å,[117] 8.365 Å.[150] Lithium ions occupy tetrahedral 8a site, and 1/6 of octahedral 16d sites, while the rest of octahedral sites were occupied by tetra valence Ti ions. Oxygen ions locate at 32e site. The structure can be clearly illustrated in Fig.1.6. The structure of the Li4Ti5O12 can be [151] written as Li3(8a)[LiTi5]16dO12(32e), where octahedral 16c sites and tetrahedral 8b and 48f sites are empty, which is suitable for lithium insertion and extraction.[24, 120] The

[TiO6] is the main frame for lithium insertion/extraction. There is no structural changes when less than 4.5 lithium ions insert/extract in the frame during the charge/discharge process,[152] by which LTO is also called zero-strain material, indicating that it is an excellent material in terms of cycleability performance as anode material for lithium ion batteries.

Fig.1.6 Crystal structure of spinel LTO with Fd3m space group. Black : Li+. octahedron with 16d and 16c site share common edge, 16d and 16c site share common faces with tetrahedron 48f (highlighted with bold lines). Lithium can transport through 8a site to16c site and vice versa.[153]

Upon the insertion of lithium ions, three lithium atoms at 8a sites transfer to 16c sites, and the inserted three lithium ions move to 16c sites via 8a sites. During extraction, lithium atoms are extracted out from 16c sites via 8a sites, and the other lithium atoms [24] move back to 8a sites from 16c sites. Assured by the stable Li4Ti5O12 framework, the lattice parameters practically keep unchanged during the process. This is most likely caused by the Coulomb repulsion between nearest Li ions occupying the 8a-16c 36

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

[149] positions separated by only 1.81 Å. Finally, it transforms into Li7Ti5O12 4+ 3+ [24] ([Li2]16c[Ti5/3Li1/3]16d[O4]32e), and three Ti ions are reduced to Ti ions. The electrochemical reaction can be described as:

+ − [Li]8a[Li1/3Ti5/3]16d[O4]32e +Li +e ⇔ [Li2]16c[Li1/3Ti5/3]16d[O4]32e

The structure changed equation can be described as following:

(Li)[LiTi] [O] → (Li)[LiTi] [O]

→ (Li)[LiTi] [O] → [Li](Li)[LiTi] [O]

Through lithium intercalation, spinel structure of Li4Ti5O12 shifts into a rock salt [54] structure of Li7Ti5O12, as shown in Fig. 1.7. Their crystallographic date is summarized in table 1.9. The edge sharing [Li1/6Ti5/6]O6 octahedral form a three- dimensional network that connects the 8a sites via the 16c sites reflecting the most likely diffusion pathway for the Li-ions in spinel compounds.[154] Since the framework of Li4Ti5O12 is very robust, the lattice parameters remain almost unchanged during lithium insertion and extraction.[102] When more than 3 lithium per unit intercalates into

Li4Ti5O12, the additional 2 lithium began to occupy the empty tetrahedral 8a site with descending voltage to 0.01 V.[153, 155]

Fig.1.7 (a) Li4Ti5O12 spinel structure type. Blue tetrahedra represent lithium, and green octahedra represent disordered lithium and titanium. (b) Li7Ti5O12, rock salt. Blue octahedra represent lithium, and green octahedra represent disordered lithium and titanium.[59]

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

[156] Table 1.9 Crystallographic data for Li4Ti5O12 and Li7Ti5O12 Materials Atom Wyckoff Cell parameter occupancy Li4Ti5O12( Fd3m) site x y z Li1 8a 0.125 0.125 0.125 1.0 Li2 16d 0.5 0.5 0.5 0.1667 Ti 16d 0.5 0.5 0.5 0.8333 O 32e 0.2625(1) 0.2625(1) 0.2625(1) 1.0 Li7Ti5O12( Fd3m) Li1 8a 0 0 0 1.0 Li2 16d 0.5 0.5 0.5 0.1667 Ti 16d 0.5 0.5 0.5 0.8333 O 32e 0.2576(3) 0.2576(3) 0.2576(33) 1.0

Although many investigations have been done on Li4Ti5O12, there are still some important aspects of this compound are not clear in the literature so far. For instance, previous literature report that the theoretical capacity of Li4Ti5O12 is 175 mAh/g. [24, 151] However, some researchers state that Li4Ti5O12 can deliver a theoretical capacity of 293-296 mAh/g according to the reduction of all Ti4+ in the compound at the discharge voltage extended to approximately 0 V.

[64] [151] Hsieh et al. and Ge et al. reported the mechanism of the Li4Ti5O12 exhibited excellent electrochemical performance at different discharge voltage range, especially extended to 0 V. The Li-ion insertion sites can be three crystallographic sites, i.e., the

(8a), (16c), and (48f) sites, indicating that Li-ions can intercalate into Li4Ti5O12 at low potential (<1.0 V vs. Li/Li+). This low-potential intercalation process would lead to improvement of the energy density of spinel Li4Ti5O12 electrodes, relating to the numbers of octahedral (16c) and tetrahedral (8a) sites and their occupancies. Between 2.5 and 0.01 V vs. Li/Li+. Generally, additional Li-ions can be inserted into the lattice and located at octahedral sites in the intercalation process. Meanwhile, lithium ions initially located at tetrahedral sites also transport to octahedral sites. The intercalation/de-intercalation process can thus be expressed as:

4+ − + 3+ 4+ Li3(8a)[LiTi5 ](16d)O12(32e) +3e +3Li ↔ Li6(16c)[LiTi3 Ti2 ](16d)O12(32e)

The above reversible reaction takes place in the voltage range of 2.5-1.0 V vs. Li/Li+, and 3 mol Li-ions are capable of inserting into the octahedral (16c) sites of Li4Ti5O12 lattices, forming a rocksalt structure. In the voltage of 1.0-0.01 V vs. Li/Li+, other 2 mol of Li-ions could be intercalated into Li7Ti5O12 as there are still available interstitial sites in Li7Ti5O12. The insertion/de-insertion at low potential can be illustrated as:

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

3+ 4+ − + 3+ Li6(16c)[LiTi3 Ti2 ](16d)O12(32e) +2e +2Li ↔ Li2(8a)Li6(16c)[LiTi5 ](16d)O12(32e)

Here, vacant tetrahedral (8c) sites are available to accommodate lithium ions under 1.0 V vs. Li/Li+, thus enhancing a reversible capacity at a low potential. Accordingly, the second stage of charging plateau contributes to a significant portion of total capacity of

Li4Ti5O12 anodes. Due to this mechanism, Li4Ti5O12 can display a reversible capacity of 215.1 mAh/g within 2.5-0.01 V vs. Li/Li+ at 0.2 C, which is much higher than the conventional theoretical capacity of 175 mAh/g.

Several studies on the electrochemical performance of the Li4Ti5O12 electrode at the discharge voltage extended to approximately 0 V have been reported. Yao et al. [25] reported that the Li4Ti5O12 had excellent cycleability and a capacity of about 200 mAh/g in the voltage range of 0-3.0 V. During the discharge process, a 0.75V plateau was also observed in addition to 1.5 V plateau. Ge et al.[36] reported that reversible capacity and high rate performance of Li4Ti5O12 were improved when the discharge voltage extended from 0.6 to 0.01 V, while the cycling stability was not affected. Shu[24] demonstrated the formation of solid electrolyte interface (SEI) below 1 V and that the lithium titanate had reversible capacity below 1.55 V with a certain reversible intercalation processes.

Li4Ti5O12 delivers a charge capacity of 220 mAh/g in 0.0-4.0 V and a reversible specific capacity of 150 mAh/g at a current density of 7.8 mA·cm−2 in the cut-off 0.0-2.0 V. Improvement of the rate performance by discharging to 0 V is also proved in the all solid-state Li-In/Li2S-P2S5 solid electrolyte/Li4Ti5O12 cells in the voltage range between 0 and 3.0 V (versus Li) at room temperature by Kitaura et al.[157] The cells showed the larger reversible capacity of 225 mAh/g at 0.064 mA·cm-2 in the wide voltage range than the theoretical capacity of 175 mAh/g. The all-solid-state cells retained 90% of the first reversible of about 120 mAh/g capacity after 500 cycles at the density of 1.3 mA·cm−2.

The additional lithium (more than 3 lithium ions per unit) contributes higher reversible capacity for spinel Li4Ti5O12, which finally increase theoretical capacity of Li4Ti5O12 from 175 mAh/g of Li7Ti5O12 (reported by most of the present papers) to 293 mAh/g of [151] Li9Ti5O12, as demonstrated by Ge et. al. As the lithium ion insert into Li4Ti5O12, the electron of lithium moves into empty Ti d orbital, changing the valence of Ti from Ti4+ toTi3+. The electron in Ti d orbital finally generates energy splitting (d-shell splitting) in crystal field. As for Ti3+, the crystal field stabilization energy (CFSE) in an octahedral 39

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications site is 23.1 kcal/mol, while the CFSE in a tetrahedral site is 15.4 kcal/mol. According to this, the octahedral site preference energy(OSPE) of Ti3+ is 7.7 kcal/mol, which means more of 7.7 kcal/mol energy is reduced for Ti3+ by occupying the octahedral site rather than staying tetrahedral site.[158] Therefore, Ti3+ ions are still remained in the octahedral site rather than move to tetrahedral site.

The existence of available free octahedral sites in the lattice of Li4Ti5O12 makes it a good lithium-ion conductor. However, since the oxidation state of Ti in Li4Ti5O12 is +4, the highest possible valence for Ti, Li4Ti5O12 is a very poor electronic conductor. On the contrary, Li7Ti5O12 is a good electronic conductor attributes to the average oxidation state of Ti is +3.4, which means the coexistence of Ti3+ (60%) and Ti4+ (40%) in the lattice, and a very poor lithium-ion conductor due to the full occupancy of 16c octahedral site by Li+.[25]

[3] Fig.1.8 Discharge and charge curves of a Li/Li4Ti5O12k-A cell

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Fig.1.9 Comparison between calculated and experimental XAS spectra of Li4Ti5O12 at the O K edge. The observed differences in the amplitudes of the peaks are mainly due to the energy dependent broadening caused by the finite lifetime of the excited states which is not considered in the present calculation.[117]

Hao et al.[3] reported that it is a two phase reaction based on the Ti4+/Ti3+ redox couple during Li-ion intercalation and de-intercalation process, which can be concluded by the first three discharge and charge cycles for a Li/Li4Ti5O12-A cell in a liquid electrolyte with a constant current of 23.5 mA/g at room temperature (Fig.1.8). Accordingly, the overall insertion capacity of Li4Ti5O12 is limited by the number of free octahedral sites accommodating lithium ions. When the spinel host structure is in accordance with + [Li]8a[Li1/3Ti5/3]16d[O4]32e, it can accommodate maximum Li content without significant changes of lattice constants. In terms of this, the theoretical specific capacity of

Li4Ti5O12 is 175 mAh/g.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

[41] Fig.1.10 (a) TEM image of LTO nanotubes, (b) SEM image of a Li4Ti5O12 single [28] [53] crystal, (c) SEM image of rod shaped single crystals Li4Ti5O12, (d) SEM images of [55] [141] Li4Ti5O12 nanofibers, (e) SEM image of Li4Ti5O12 nanowires, (f) SEM image of [43] flower like Li4Ti5O12, (g) TEM images of sawtooth-like Li4Ti5O12, insert is the TEM [38] [19] image at a higher magnification, (h) SEM image of spherical Li4Ti5O12, (i) SEM [59] image of three-dimensionally ordered macroporous Li4Ti5O12.

During the phase transition by three titanium atoms from Ti (IV) to Ti (III), the change in the lithium site symmetry is very important for the insertion mechanisms and could be followed by XAS at the O K edge as predicted from LAPW calculations. Kubiak and [117] co-workers reported the calculated and experimental spectra of Li4Ti5O12 in the energy range between 525 and 545 eV, which showed the same main peaks labeled A-E in Fig. 1.9. The empty states which contribute to the peak C are mainly due to the hybridization between Li (8a) p-type orbitals and O 2p orbitals. There is no significant contribution of the Li atoms in octahedral sites. Thus, the peak C is expected to be modified by changes in Li site symmetry due to Li insertion.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Considerable efforts on different morphologies of Li4Ti5O12 have been done, such as [41, 44] [28, 159] Li4Ti5O12 nanotubes, single crystal Li4Ti5O12, single crystal rod shaped [53] [55] [141] Li4Ti5O12, Li4Ti5O12 nanofibers, Li4Ti5O12 nanowires, flower-like spinel [43] [38] [19, 42, 45, 51, 52, 61, Li4Ti5O12, sawtooth-like morphology, spherical Li4Ti5O12 powders, 64, 65, 160] [59] three-dimensionally ordered macroporous Li4Ti5O12 , as shown in Fig. 1.10. Spherical morphologies improve the electrode properties owing to packing [42] [141] efficiencies. Kim et al. reported that one demensinal-Li4Ti5O12 nanowires are suitable for Li-ion batteries in the high rate applications. It shows the initial discharge capacity of 165 mAh/g at a rate of 0.1 C and high capacity retention of 93% even at 10 C for lithium intercalation.

1.5 Applications

Recently, the cubic spinel Li4Ti5O12 are applied in asymmetric hybrid electrochemical supercapacitors, electrochemical generators, all-solid-state lithium ion batteries and room temperature Li metal batteries.[161]

[162] Du Pasquier et al. reported the application of Li4Ti5O12 into a new class of nonaqueous hybrid devices. The system consists of a nanostructured Li4Ti5O12 Li- intercalation anode, an acetonitrile-LiBF4 electrolyte, and an activated carbon double- layer (a theoretical maximum energy density around 20 W·kg-1), maintaining a high power density (approximately 1500 W·kg-1) and with excellent cycle-life over 250,000 cycles. It is suggested that replacing the activated carbon with an electronically conducting polymer of high specific capacitance and high voltage, such as poly (fluorophenyl) thiophene of 270 F·g-1, can enhance the energy density of this system. Composite positive electrodes consisting of a mixture of activated carbon and lithium transition metal oxides such as LiMn2O4 or LiCoO2 could be further surface modified by LTO as a coating layer.

1.5.1 Anode material

To abate the energy crisis of fossil fuels and to decrease the global warming effect from mass production of carbon dioxide, it is vital to develop the next-generation electric vehicles (EVs) and hybrid electric vehicles (HEVs), with which high powder and energy density are necessary for energy storage devices.[100] The spinel lithium titanate

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

(Li4Ti5O12) is such a promising candidate for the anode electrode material in high power and energy Li-ion batteries.

Spinel Li4Ti5O12 can be used as anode coupling with high voltage cathodes such as

LiCoO2, LiNiO2, or LiMn2O4, to provide a cell with an operating voltage of approximately 2.5 V, which is twice that of a nickel-cadmium or nickel-metal hydride cell. The electrode improves significantly safety features over conventional high voltage

(3.5 V) LiyC6/Li1−yCoO2 cells and the cost is relatively low compared with other alloy anode materials.[13, 15, 17, 19, 26, 57, 61, 65, 87, 102, 163]

A model hybrid capacitor cell consisting of a negative nc-Li4Ti5O12/CNF composite electrode and a positive activated carbon electrode shows high energy density of 40 Wh·L-1 and high power density of 7.5 kW·L-1 comparable to conventional EDLCs.[140]

LTO anode material is intensively studied mostly because it is one of the limitation in the best performance of whole-cell for LIBs. For example, Xiang et al.[164] reported 5 V spinel LiNi0.5Mn1.5O4 cathode coupled with the Li4Ti5O12 anode and fabricate the lithium-ion battery. It is recommended that LiNi0.5Mn1.5O4/Li4Ti5O12 cell whose capacity is limited by Li4Ti5O12 anode should be used to extend the application of the state-of-the-art lithium-ion batteries.

The properties of electrolyte play an ignorable role in the performance of LTO anode [157] material. Kitauta et al. reported that Li–In/Li4Ti5O12 cells using Li2S–P2S5 solid electrolytes exhibits excellent cycleability. When cycled in the voltage range of 1.0-2.0 V (versus Li) at a current density of 0.064 mA·cm-2, its reversible capacity retains more than 100 mAh/g over 300 cycle. And it remains 90% of the first reversible capacity of ~120 mAh/g after 500 cycle at 1.3 mA·cm-2.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

Fig.1.11. SEM pictures of Li4Ti5O12-coated LiCoO2 with different contents (a) 0 wt% (b)

3 wt% (c) 5 wt% (d) 10 wt%. The surface morphology of LiCoO2 is extremely smooth. From a comparison of this four powders surface morphology, it can be speculated that [139] the surface of the prepared LiCoO2 is covered with small Li4Ti5O12.

[162] -1 Du et al. reported a 40 Wh·kg Li-ion battery using a Li4Ti5O12 nanostructured anode and a composite activated carbon LiCoO2 cathode, which is built using plastic Li- ion processing based on PVDF-HFP binder and soft laminate packaging. The specific power of the device is similar to that of an electrochemical double-layer supercapacitor (4000 W·kg-1). They further reported[165] the highest ever measured rate capability and number of charge-discharge cycles of nano Li4Ti5O12/LiMn2O4 battery. The best devices can utilize the capacity of 190 mAh/g at 1 C, and still remain as 160 mAh/g at 80 C which is 49 Wh·kg-1 at 50 W kg-1and 20 Wh·kg-1 at 2000 W·kg-1 in terms of device power and energy, respectively.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

In the consideration of these factors (i.e. electrolyte, cathode, LTO) in LIBs, Reale and [23] co-workers reported the Li4Ti5O12/Py24TFSI-LiTFSI/LiFePO4 battery system. The

Li4Ti5O12/LiFePO4 electrode combination shows a good electrode behavior and appears very promising for the development of safe, reliable and long-lasting, IL-based lithium ion batteries.

[32] Peramunage et al. reported the Li(4+x)Ti5O12/solid polymer electrolyte/ LiMn2O4 cells. When x = 1.2, it exhibits a capacity fade rate of about 0.1% per cycle and an end utilization of 90 mAh/g at 1 C discharge rate. Li4Ti5O12/solid polymer electrolyte/LiMn2O4 cells shows full-depth extended cycling capability at a utilization of similar to 90 mAh/g for LiMn2O4 at 1 C rate and a capacity fade rate of about 0.08% per cycle. When fully packaged, the specific energy of Li4Ti5O12//PAN -1 electrolyte//LiMn2O4 cell is about 60 Wh·kg .

1.5.2 Coating for cathode materials

Some cathode materials (such as LiCoO2 and LiMn2O4) were modified by Li4Ti5O12. Yi [139] et al. reported enhanced cycling stability of microsized LiCoO2 cathode by

Li4Ti5O12 coating. The initial capacity of the Li4Ti5O12-coated LiCoO2 decreases as the contents of Li4Ti5O12 coating augments, but it shows enhanced cycling properties. The 3 wt.% Li4Ti5O12-coated LiCoO2 has the best electrochemical performance, showing capacity retention of 97.3% in the voltage of 2.5-4.3 V and 85.1% in 2.5-4.5 V after 40 cycles. Small Li4Ti5O12 particles are found to be highly dispersed on the coated LiCoO2 particles, as shown in Fig.1.11.

[166] Liu et al. reported that surface-coating of LiMn2O4 with Li4Ti5O12 is an effective way to improve the cycling properties at room and elevated temperatures. The 2 mol% and 5 mol% Li4Ti5O12-coated LiMn2O4 shows excellent capacity retention at room temperature and 55 ºC relative to pristine LiMn2O4. The enhancement of electrochemical performance is attributed to the suppression of electrolyte decomposition on the surface of LiMn2O4. Its coatings have been also shown to [167] improve the capacity retention during cycling of LiMn1.4Cr0.2Ni0.4O4.

Chapter 1 Literature review of nanostructured Li4Ti5O12: synthesis, modification, properties and applications

1.6 Conclusions and perspectives

Since nanoparticulate spinels can reduce diffusion lengths within particles, increase the number of active sites for surface reactions and decrease the local current density,[48] it is beneficial to synthesize nanosized Li4Ti5O12 for lithium ion batteries. To date, there are a plenty of methods as above mentioned to prepare Li4Ti5O12. However, these methods are time-consuming (at least 22 h). Further research on the methods of synthesis is needed to reduce reaction time and decrease the high temperature for industrial applications in lithium ion batteries. For instance, hybrid microwave method has more space for improvement in terms of this aspect.

The theoretical capacity study of spinel Li4Ti5O12, which is 293-296 mAh/g, opens new perspectives in research on practical discharge capacity of Li4Ti5O12, and its promising derivatives or possible insertion materials for anode in lithium-ions batteries field.

To date, many efforts have been done on the electrochemical performance of LTO by modification with transitional metal in fourth period (i.e. V, Cr, Mn, Fe, Co, and Cu). Ni,

Zn doping on Li4Ti5O12 might be an effective way to modify the properties of Li4Ti5O12. According to the diagonal relationship, Be, which has the similar properties as Al, may also have an optimal composition as Li4-xBexTi5O12 upon electrochemical performance. Other elements, such as Cl, I, belong to halogen may improve the electrochemical performance of Li4Ti5O12. Moreover, only Al-F and Mg-V co-doping have been done in co-substitution area, more other works in this field still requires development, such as Ag-Cu, Al-Br doping.

Some of the promising applications of Li4Ti5O12, such as anode or coating material to improve the stability of its parent are commonly researched. However, the use of

Li4Ti5O12 as a cathode of 1.5 V rechargeable lithium ion battery, proposed by Colbow and co-workers,[9] has rarely been investigated. Further work on this matter should be distributed if applied as cathode material in the market.

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery

Chapter 2 Self-Assembled hierarchical

Li4Ti5O12 hollow microspheres for high performance lithium ion battery

2.1 Introduction

With the rapid depletion of fossil fuels and environmental pollution caused by hazardous gas emission from their mass-consumption, there is increasing demand for lithium ion batteries to make efficient use of energy and to provide sustainable, clean energy sources.[168] The advocate of electric vehicles (EVs) and hybrid electric vehicles (HEVs) makes lithium ion batteries one of the most promising energy conversion and storage devices, besides their application in portable electronics, such as mobile phones, [169] laptops, watches and cameras. Li4Ti5O12 (LTO) is a potential anode material due to the advantages of easy fabrication, low cost, high thermal stability, intrinsically safe and excellent cycleability with zero strain during charge-discharge activities.[170, 171] However, LTO suffers from low electronic conductivity leading to slow reaction kinetics in terms of low practical capacity and poor rate performance.[172]

Currently, nanostructured electrode materials in lithium ion batteries are at the centre of research for enhancing power performance in lithium ion batteries by providing short lithium diffusion path and high surface area.[10, 173, 174] Unfortunately, nanomaterial tends to be unstable and can easily agglomerate into clusters at the expense of losing its original nanostructure. Instead of the expected high performance, electrode material in nanometer dimension performs poor rate capability, short cycle life and low specific capacity.

Micro-sized electrode materials are preferred for industrial application in lithium ion batteries due to their high stability. Combining the stability advantage of micro-sized electrode material with short diffusion path and high surface area advantage of nanosized electrode material, a secondary structure in micro-size consisting of primary nanosized material is proposed to solve the instability problem of nanostructured materials, through which high performance electrode materials in lithium ion batteries

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery for industrial large-scale application could be achieved. Such an idea has recently been embodied in a few reports.[175, 176] More specifically, besides the particular secondary- micro-primary-nano structure, carbon modification was further introduced to enhance the high rate performance of LTO. However, carbon surface modification suffers from complicated multi-steps and intact surface coating accompanied with low electrode’s kinetics.[177] Moreover, these methods for preparing micro-secondary-nano-primary structural LTO requires high temperature treatment (>800 oC). Therefore, an inexpensive, facile synthetic route to prepare micro-secondary-nano-primary structural LTO without carbon modification is still full of challenges.

In this chapter, we report a facile and easily scaled-up synthetic method to prepare LTO hollow microspheres assembled from nanosheets with a high capacity of ~110 mAh/g for 100 cycles at 10 C when used as anode material. A possible formation mechanism for the construction of hierarchical LTO hollow microspheres was described. The anode LTO half-cell demonstrates excellent capacity retention with less than 4% of capacity loss even after 100 cycles.

2.2 Experimental section

2.2.1 Synthesis

All chemicals were used as received without further purification. In a typical synthesis,

2 ml of titanium butoxide (TBT, Ti-(OC4H9)4, 97%, Aldrich) and 10 ml of ethylene glycol (EG) were mixed homogenously to obtain a mixture (denoted as TBT-EG). Cetyltrimethylammonium bromide (CTAB) was added into LiOH solution, with molar ratio of CTAB: Ti= 0.1 and Li:Ti= 4:5. Then 3 ml of TBT-EG solution was added and stirred for 1 h. The solution was transferred into a 50 ml Teflon-lined stainless autoclave, which was maintained at 180 oC for 3 h. Cooled to room temperature naturally, the resulting white precipitate was recovered by centrifugation, washed with ethanol, and dried in an oven at 60 oC. Finally, the as-prepared material was calcined in a muffle furnace at 500 oC for 2 h in the air.

2.2.2 Characterization

The phase and composition of the product was identified by X-ray diffraction (XRD, Philips X’pert Multipurpose X-ray Diffraction System (MPD) with Cu Kα radiation The 49

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery morphology of the product was analysed by the scanning electron microscopy (SEM, Fei Nova NanoSEM 230) and transmission electron microscopy (TEM, JEOL 1400, 100 kV). The crystalline structure was determined by high resolution TEM (HRTEM,

Philips CM200, 200 kV). The surface area was determined by N2 adsorption (Brunauer- Emmett-Teller (BET) method, Micromeritics Tristar).

2.2.3 Electrochemical measurement

The electrochemical cell was CR2032 coin-type cells. The electrode was made by mixing LTO powder with acetylene black (AB) and a polyvinylidene fluoride (PVDF) binder at a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) solution. The slurry was pasted onto pieces of Cu foil and dried at 100 oC in a vacuum oven. Lithium metal was used as the counter electrode and reference electrode. The electrolyte was 1

M LiPF6 in a 1:1 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The cells were charged and discharged at room temperature at the voltage range of 1.0- 3.0 V.

2.3 Results and discussion

2.3.1 Composition and morphology

Fig. 2.1 XRD pattern of nanosheet LTO hollow sphere

The structure and phase composition of LTO were analysed by XRD pattern. As shown in Fig.2.1, the characterization diffraction peaks at 2θ of 18.3, 31.7, 35.4, 43.1, 48.0, 50

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery

53.9, 57.1, 62.7o could be indexed to spinel LTO phase (JCPDS 49-0207; a=8.3588Å). o Diffraction peak at 2θ of 25.2 is ascribed to anatase TiO2 phase (JCPDS 83-2243), o whereas diffraction peaks at 2θ of 27.3 and 41.0 are allocated to rutile TiO2 phase (JCPDS 76-0319).

In the typical method, TBT and EG were firstly mixed to obtain homogeneous solution, in which titanium glycolates or mixed alkoxide/glycolate derivatives were formed. The detailed reaction equation is described below:[178]

Ti(OBu) +HOCHCH OH → Ti(OCHCH O)(OBu) +2HOBu (1)

Ti(OCHCH O)(OBu) +2 HOCHCH OH → Ti(OCHCH O) +4HOBu (2)

As EG (ethylene glycol) exceeds the stoichiometric ratio, all –OBu group is supposed to be displaced by –OCH2CH2O- group and titanium glycolate Ti(OCH2CH2O)2 (denoted as Ti-EG) was produced coexist with excess EG. After the formation of titanium glycolate, TBT-EG began to react with LiOH and formed the amorphous precursor under hydrothermal process. Finally, high-temperature calcination was employed to improve crystallization of the precursor obtained from hydrothermal process. After annealing at 500 oC for 2 h, the amorphous precursor became crystal LTO.

After the hydrothermal reaction at 180 oC for 3 h, the LTO precursor with hierarchical hollow structures was obtained. As proven by the SEM image in Fig. 2.2a, hollow microspheres with diameters of ~0.5-2 μm were produced. From the cracked parts of microspheres (indicated by white arrows), the hollow architectural feature can be clearly observed. Moreover, Fig. 2.2b shows that each microsphere is composed of numerous two-dimensional primary nanosheets with size of ~30 nm in thickness.

A subsequent annealing did not change the hierarchical appearance, and the overall hollow structural morphology is preserved, as shown in Fig.2c. The structural nature of microsphere is further investigated by TEM technique. As shown in Fig.2d, the dark and light contrast further indicates the hollow feature of LTO sphere. The lattice distance of the nanosheet is measured to be 0.482 nm (inserted image of Fig. 2.2d), corresponding to the interplanar spacing of plane (111) for spinel LTO.

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery

Fig. 2.2 SEM images of the LTO precursor under (a) low magnification, (b) high magnification; (c) SEM, (d) TEM images with inserted HRTEM image of LTO hollow microspheres.

Fig. 2.3 SEM images of LTO precursor prepared (a) r = 0, (b) r = 1, where r is the volume ratio of EG: TBT.

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery

In order to understand the role of EG, different volume ratios of EG:TBT (denoted as r) were investigated. At a low volume ratio (r=1) value, Fig.2.3a shows the packed nanoflakes and no hollow structures could be observed. At this concentration, it is likely that Ti(OCHCH O)(OBu) is formed according to equation 1 stoichiometrically. In the absence of EG, nanosheet with disorderly array is obtained (Fig. 2.3b), which is different from that of the precursor obtained at r=5 (Fig.2.2a).

Based on these results, we proposed that −(OCHCH O) − group plays a significant role in the formation of hierarchical hollow microspheres. The absence of

−(OCHCH O) − group in TBT will result in LTO precursor nanosheets with disordered array. Ti(OCHCH O)(OBu) containing one − (OCHCH O) − group will produce densely packed LTO precursor nanosheets. While two groups of

−(OCHCH O) − in titanium glycolate with encapsulated Ti ions functions as a bridge for combining these nanosheets together and finally forms the hierarchical hollow structure. Therefore, titanium glycolate is necessary to obtain the hollow structure as it acts as linking reagent, which is also reported by Cao et al,[179] Wang and co- workers.[180]

The effect of CTAB is studied by modifying its concentration in terms of molar ratio of

Fig. 2.4 SEM images of LTO precursor obtained at different concentrations of CTAB (a) k = 0.01 (b) k = 1 (c) k = 2, where k is the molar ratio of CTAB: TBT.

CTAB: TBT (denoted as k). In the typical CTAB surfactant-assisted system, the molar ratio of CTAB: TBT is 0.1 (k = 0.1). At a low concentration of CTAB, e.g., k = 0.01, spherical particles with size of ~ 30-50 nm were obtained (Fig.2.4a). Upon the introduction of high CTAB concentrations (e.g., k = 1, 2), hierarchical hollow feature

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery was lost and nanosheet precursor was observed (Fig. 2.4b and c), indicating that high concentration of CTAB surfactant counteracts the bridge-linking function of EG for the self-assembly of the nanosheets into hierarchical hollow sphere. Therefore, CTAB plays a significant role in the formation of hierarchical hollow structure, in which there is an optimal concentration (k = 0.1) for the synthesis of hierarchical hollow structure.

Fig. 2.5 SEM images of samples obtained at 180 oC for different reaction time: (a) 10 min (b) 30 min (c) 40 min (d) 2 h

Time dependent experiments were carried out to understand the formation mechanism of hollow LTO microspheres. SEM images in Fig.2.5a-d show the morphology evolution of LTO precursor prepared at 180 oC for different periods of time. At the beginning of heating treatment, no specific shape could be observed by SEM (not shown here). After a short reaction time of 10 min, small particles agglomerated to form spherical clusters. Besides some irregular particles, spherical secondary particles in 54

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery submicron sized dimension were observed (Fig. 2.5a). The defected sphere as indicated by the white arrow in Fig.2.5b shows that the agglomerated spheres became depressed after 30 min of reaction, indicating the dissolution of the primary particles. Upon increasing reaction time to 40 min, nanosheets started to form through further aggregation of small particles, which was accompanied by the disappearance of the original spherical clusters (Fig.2.5c). The formation of the nanosheets was completed in 2 h with no spherical clusters observed (Fig. 2.5d). The hierarchical hollow structure was finally formed by self-organization of the nanosheets (Fig. 2.2c).

The Brunauer-Emmett-Teller (BET) surface area for the synthesized LTO hollow microphere was 186.5 m2·g-1, which is much higher than its counterpart reported by Tang et al.[181] (hollow microsphere composed of nanosheet, 131.2 m2·g-1), reported by Chou et al. [182] (solid microsphere composed of nanosheet, 20.0 m2·g-1) and Yu et al.[183] (microsphere composed of nanoparticles 73 m2·g-1).

2.3.2 Growth mechanism

On the basis of the above experimental results, we propose a tentative mechanism to illustrate the formation process of LTO by CTAB-assisted method, as shown in Fig 2.6. The formation mechanism of LTO by the CTAB-assisted method can be explained as follows: initially, titanium glycolate (denoted as Ti-EG) is formed by reaction of TBT and EG. During the hydrothermal treatment, titanium glycolate (Ti-EG) combines with CTAB surfactant through hydrocarbon chains due to the attraction of similarly charged species.[184, 185] CTAB micelle-like species are then formed, in which titanium glycolate (Ti-EG) and Li+ are encapsulated in the core, and gradually aggregate to large spherical clusters.[186] Lamellar micelles can be formed by hydrophilic surfactant molecular in some conditions.[180] For example, lamellar structure has been reported to be a medium in the synthesis of tungsten nanowires under mild hydrothermal conditions.[187] In this system, we tried to adjust the surfactant concentration of CTAB. The continuing aggregation process transforms spherical clusters to highly ordered lamellar assemblies, followed by the dissolution of the primary nanoparticles, single crystal nanosheets are formed by Ostwald ripening process.[188] It is especially important to point out that the Ostwald ripening process is the key step for the formation of these nanosheets. While transforming from spherical particles to nanosheet particles, Li+ and titanium organic compound was wrapped into the sheet under the template function of CTAB surfactant. 55

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery

Part of Li+ would lose due to the repulsion between CTAB cation surfactant and Li+ + ions. Thus, the lack of Li ions resulted in as-prepared LTO with TiO2 as impurities (Fig.2.1).

Fig 2.6 Formation mechanistic illustration of hollow LTO microsphere assembled from nanosheet obtained by CTAB surfactant-assisted hydrothermal process.

Due to the driving force produced by the reduction in the surface free energy, the nanosheets self-assemble into three dimensional (3D) hierarchical hollow microspheres via the bridging linkage provided by EG.[189] With the increase in furnace temperature, the amorphous product becomes crystallized LTO while removing organic molecules by decomposition process.[185] Meanwhile, the morphology of the precursor is maintained during this calcination process and hierarchical hollow LTO microspheres are finally achieved. In general, hydrothermal treatment plays an important role in the formation of hollow structure, while the calcination step is necessary to obtain crystallized LTO.

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery

2.3.3 Electrochemical properties

Fig.2.7 (a) Charge and discharge profiles of the first cycle at the rate ranging from 1C to 50 C within a potential window of 1.0-3.0 V; (b) rate capability of nanosheet LTO hollow microsphere and LTO-Sigma at rates ranging from 1 to 50 C; (c) cyclability of nanosheet LTO hollow microsphere at a discharge density rate of 10 C. Where 1 C rate corresponds to the current density of 200 mA/g. 57

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery

The electrochemical performance of the LTO hollow spheres was analysed in terms of rate capability and cyclability. The LTO hollow microspheres were cycled at various charge/discharge rate increasingly stepwise from 1 C (200 mA/g) to 50 C within a potential window of 1.0-3.0 V and finally decreased back to 1 C. Typical charge and discharge profiles at the 1st cycle are shown in Fig.2.7a. The charge/discharge curves have a plateau at ~ 1.5 V, which is in good agreement with reported literature.[176] The coulombic efficiency of LTO electrode at density rate of 1, 2, 5, 10, 20, 30, 50 C is 94.7, 97.8, 97, 97.2, 99.1, 100, and 102.4%, respectively. The lowest Coulombic efficiency of LTO electrode at 1 C resulted from the irreversible capacity loss of the first cycle. At high rate (10-50 C), the cell displayed high Coulombic efficiency, indicating excellent rate capability. The electrode delivered a specific capacity of 94.8 mAh/g even at a current density of 50 C.

The rate capability performance at various rates in the range of 1 to 50 C is shown in Fig.2.7b. The discharge capacity of first cycle is 169.1 mAh/g at a current density of 1 C, and the capacity decreased with increasing current density. As the rate increases to 2, 5, 10, 20, 30, 50 C, the discharge capacity decreases to 142.8, 128.1, 117.9, 106.5, 102.5 and 99.7 mAh/g, respectively. After the high rate measurement, the current density is reduced back to 1 C, and a discharge capacity of 145.5 mAh/g can be recovered, indicating the good structural stability of the as-prepared electrode material. For comparison, the rate capability of LTO solid sphere purchased from Sigma, Aldrich (denoted as commercial LTO) was measured. The capacity of as-synthesised hierarchical hollow LTO sphere is much higher than commercial LTO at each density rate from 1 to 50 C, and the superior performance of hierarchical hollow LTO microsphere is much more obvious at high rate, for example, commercial LTO delivers only a reversible capacity of ~40 mAh/g at 30 C rate and ~12 mAh/g at 50 C, compared to ~102.5 and ~99.7 mAh/g of hollow LTO microspheres.

While the rate performance of hollow LTO microsphere is better than that of solid LTO microspheres composed of nanoflakes recently reported by Chou et al.( ~ 95 mAh/g at 20 C rate, ~70 mAh/g at 40 C rate),[182] solid microsphere made of nanoparticles (~85 mAh/g at 30 C, ~105 mAh/g at 20 C) [183], porous LTO microsphere (~92 mAh/g at 20 C rate),[90] and even better than Nb-doped LTO (~82.4 mAh/g at 40 C rate)[103] and nanosized LTO double surface modification of Ti(III)/carbon (~115 mAh/g at 7.5 C, 70

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery mAh/g at 15 C) (where 1 C corresponds to 200 mA/g);[190] the hollow LTO microsphere synthesized by Tang et al.[181] employed a multi-step solvothermal method which is time-consuming, and Zhang et al.[191] calcined at a much higher temperature (700 oC) combined with time-consuming steps.

The cycling performance was employed to verify the stability of the LTO hollow spheres for high rate lithium ion batteries at a current density of 10 C. Fig.2.7c shows discharge capacities as the function of the cycle numbers recorded with the current density of 10 C. No significant capacity loss is observed. As shown in Fig.2.7b, the first cycle of discharge capacity is 114.2 mAh/g and the reversible specific discharge capacity still maintains 110.4 mAh/g even after 100 cycles with less than 4% of capacity decay.

The significance of the above results clearly demonstrates that hierarchical hollow LTO microspheres exhibit good rate capability, excellent cycling performance and high specific capacity at high rate. The good electrochemical performance could be attributed to the combination of secondary microspherical structure and primary nanosheet. Two dimensional nanosheet provides short lithium diffusion path which lead to fast reaction kinetics at high rate through fast mass/charge transport. The microspherical structure, on the other hand, is beneficial in maintaining nanosheet structure. Moreover, hollow structure ensures good contact between nanosheet LTO and electrolyte.

2.4 Conclusions

In summary, hierarchical LTO hollow microspheres assembled from nanosheets have been synthesized by a hydrothermally CTAB-assisted method followed by subsequent high temperature calcination. A mechanism for the construction of LTO microspheres has been proposed: (1) formation of submicron-sized solid spheres from smaller particles, (2) smaller particles transform to highly ordered lamellar assemblies, (3) nanosheet precursor grows from these small particles through Ostwald ripening process, (4) macroscopic organization of nanosheets into precursor microspheres with internal space by reduction of free energy, and (5) crystalline LTO was finally prepared by calcination of precursor at high temperature for a short time. Hollow structure composed of nanosheets provides high surface area which improves the electrochemical performance in terms of rate capability and cyclability for high rate lithium-ion batteries. 59

Chapter 2 Self-Assembled Hierarchical Li4Ti5O12 Hollow Microspheres for High Performance Lithium Ion Battery

The resultant LTO hollow structures delivers a discharge capacity of ~110 mAh/g with no significant capacity loss after 100 cycles at a density rate of 10 C. A reversible capacity of ~95 mAh/g at 50 C is achieved while only ~12 mAh/g is maintained for commercial LTO solid powders. The superior electrochemical performance suggests the application of self-assembled LTO hollow microsphere as an anode material for high- power lithium-ion batteries.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Chapter 3 Li4Ti5O12/TiO2 nanocomposite as anode material for high rate lithium ion battery

3.1 Introduction

Lithium-ion batteries are the dominant power source for consumer portable electronics and considered to be promising for application in electric vehicles (EV) and hybrid electric vehicles (HEV). Carbon-based anode material occupies the commercial market since the first commercialization of lithium ion battery in 1990s.[192] The safety problem of lithium dendrite upon charge/discharge process greatly influences its usage for lithium ion batteries. An alternative with high Li intercalation voltage to carbon-based [193, 194] [195] anodes is required, and transition-metal oxide, such as TiO2, SnO2, Fe2O3, [196] [197] [198] [199] [200] [201] Co3O4, NiO, MnxOy, MoO3, CuO, V2O5 and Li4Ti5O12(LTO) have attracted great interests. Among all polymorphs of TiO2, anatase is the most electroactive Li-insertion host.[194, 202]

The design and synthesis of various morphologies and phases are important subjects in recent studies because they ultimately determine the device performance through influencing electrical, optical and other properties.[203] Proper control of morphology and phase is an effective strategy in nanoscience and nanotechnology to obtain desired physical, chemical and electrical properties. A variety of novel micro/nanostructures with 0D (nanoparticles[204]), 1D (nanorod,[205] nanowire,[206] nanotube,[207] nanofiber[208]), 2D (nanosheet, nanobridges), 3D (nanocubes,[209] nanoboxes,[210] and nanocages,[211] star-shaped,[212] urchin-like structure,[213] micro/nanosphere[179, 214]) have been obtained extensively. Recently, composite has attracted great interests in application as electronics,[215] water splitting,[216] photocatalyst,[217] and especially lithium ion battery,[218] due to their potential advantage from the synergistic function of each phase. Composite electrode would facilitate the development of lithium ion batteries for large- scale application. Many efforts has been made in the research of carbon/metal or oxide composite for lithium ion battery.[219] However, carbonaceous materials suffer from safety issues.[220] The lithium intercalation potential of carbon is as low lithium dendrite 61

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

voltage, at which most common electrolytes are not stable and flammable gas mixtures might be generated from decomposition of electrolyte on the surface of the electrode.[221, 222] + Anatase TiO2 and LTO have a high operating voltage of ~1.7 V and 1.5 V vs Li , respectively, which prevents lithium dendrite and electrolyte decomposition associated safety issues in lithium ion batteries. Many efforts have been conducted on either LTO or TiO2 to address these drawbacks. For example, synthesis of pure LTO or TiO2 by different methods, carbon coating on LTO or TiO2, doping or coating with metal ions/metal oxide. Most recently, surface modification with dual phases coating or doping are introduced, such as N-doped carbon coating on LTO,[222] nitridation anatase [223] by titanium oxynitride/titanium nitride (TiO2@TiOxNy/TiN) onto graphene sheets. However, surface modification in this way is complicated and expensive. Few reports has been focused on LTO-TiO2 nanocomposite with morphology controlling as anode for lithium ion batteries.[224]

Most recently, there are only a few reports on the preparation of LTO/TiO2 composite [224, 225] [226] has been made. Du et al. demonstrated that TiO2 component could enhance the conduction of composite LTO/TiO2 electrode by Synchrotron X-ray diffraction (SXRD) data. In this way, the electrochemical performance of the electrode is improved.

Considering the excellent cycleability of LTO and high capacity of TiO2, we propose the synthesis of LTO-TiO2 nanocomposite to employ their unique properties by hydrothermal method. Nanosheetlike nanocomposite with porous structure inside, instead of mesoporous TiO2 or LTO in which porous structure is the result of particles packing,[227] was firstly reported. To the best knowledge, we also firstly prepared the spherical LTO/TiO2 composite ranges from tens of nanometer to hundreds of nanometer.

In this chapter, we successfully prepared LTO-TiO2 nanocomposite in terms of sphere and porous nanosheet utilizing a facile and simple one-step route. Titanium butoxide o (TBT) was used as Ti source for both LTO and TiO2. High temperature of 500 C was used to crystallize the precursor from hydrothermal synthesis method. Electrochemical measurements show that the porous LTO-TiO2 nanocomposite demonstrates enhanced lithium storage properties with excellent capacity retention at a current rate as high as 10 C even after 100 charge-discharge cycles.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

3.2 Experimental section

3.2.1 Synthesis

Synthesis of spherical LTO/TiO2 composite

All chemicals were used as received without further purification. 1 ml of TBT (Sigma, Aldrich, 97%) was mixed with 10 ml acetone for 10 min, then LiOH was added into the above mixture with stoichiometrically molar ratio Li:Ti of 2:5. Stirred for 1h, the solution was then transferred into a Teflon-lined autoclave and maintained at 180 oC for 3 h. Then cool to room temperature naturally, centrifuged and washed thoroughly with ethanol and water for several times. After dried in air at 60 oC overnight, the as- prepared powder was finally annealed in a muffle furnace at 500 oC for 2 h in the air.

Synthesis of porous LTO/TiO2 nanosheets

0.5ml of TBT (Sigma, Aldrich, 97%), LiOH, and CTAB was mixed in water, according to stoichiometric ratio Li:Ti of 4:5, CTAB: Ti ratio of 1:10. Stirred for 1h, the solution was then transferred into a Teflon-lined autoclave and heated to 180 oC for a period of 3h. Then cooled to room temperature, centrifuged, washed with ethanol for 3 times and dried in air at 60 oC overnight. Finally, the as-prepared sample was annealed in a muffle furnace at 500 oC for 2 h in the air.

3.2.2 Characterization

The X-ray diffraction (XRD) investigation was carried out on a Philips X’pert Multipurpose X-ray Diffraction System (MPD) with Cu Kα radiation. The morphology and crystal structure was examined by the scanning electron microscope (SEM, Fei Nova NanoSEM 230) and transmission electron microscope (TEM, Philips CM200,

200 kV). The specific surface area and the porosity were carried out by N2 adsorption- desorption Isotherm with Micromeritics Tristar (Brunauer-Emmett-Teller (BET) method).

3.2.3 Electrochemical measurement

The electrochemical measurements were carried out using CR2032 coin-type cells with pure lithium metal as the counter and reference electrodes at room temperature. The working electrode was composed of nanosheet LTO, a conductive agent (acetylene 63

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

black), and a polymer binder (polyvinylidene difluoride) in a weight ratio of 80:10:10. Cyclic voltammetry (CV) was at the voltage range of 1.0-3.0 V with 0.2 mV·s-1.

3.3 Results and discussion

3.3.1 Spherical LTO/TiO2 composite

3.3.1.1 Composition and morphology

Fig.3.1 XRD pattern of nanosphere LTO/TiO2 composite, A-TiO2: anatase TiO2, LTO:

Li4Ti5O12

Fig.3.1 shows the XRD pattern of submicro-sphere LTO/TiO2 nanocomposite. All diffractioin peaks were assigned to either LTO (JCPDS 04-006-5707) or anatase TiO2 (JCPDS 04-002-2751), as indexed in Fig.3.1. The peaks at 2θ of 25.5, 38.1, 54.2, 55.3, o 69.0, 70.5 were indexed to anatase TiO2, while all the other peaks were assigned to spinel LTO. The percentage of LTO in the composte was dertermined to be 48% while the rest 52% of the composite was attributed to TiO2.

Fig.3.2 shows scanning electron microscopy (SEM) images of the precursor and as- prepared submicro-sphere LTO/TiO2 composite. It is revealed that the samples consisted of dispersive submicrospheres with rough surfaces in diameter of 30-500 nm. It is noticeable that the shape and the size of nanosphere were preserved after high- temperature treatment at 500 oC.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig.3.2 (a) SEM image, (b) SEM image at a higher magnification of precursor; (a) SEM image, (b) SEM image at a higher magnification of nanosphere LTO/TiO2 composite.

Precursor:(a) SEM image (b) SEM image at a higher magnification. LTO-TiO2 mixture

(a) SEM image (b) SEM image at a higher magnification

The morphology and crystalline structure of nanosphere LTO/TiO2 composite was further examined by transmission electron microscope (TEM) and high resolution TEM (HRTEM). Submicrosphere composite was observed in Fig.3.3a. As clearly indicated at higher magnification of TEM image (Fig.3.3b), the submicro-sphere was composed of tiny particles, which in good agreement with the rough surface observed by SEM image (Fig.3.2d). Further investigation of these particles within submicro-spheres was revealed by HRTEM images in Fig.3.3c and d. The lattice fringe with a width of 0.50 nm corresponds to the (111) plane of spinel LTO, whereas the lattice distance of 0.36 nm was assigned to the (101) plane of anatase TiO2.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig.3.3 (a) TEM image, (b) TEM image at a higher magnification (c) (d) HRTEM image of nanosphere LTO/TiO2 composite.

3.3.1.2 Optimization of parameters

x xLTO iTiO x 2 x x x x x x 24h i x x 6h 1.5h

Intensity (a.u.) 0.5h

20 30 40 50 60 70 80 90 2T q

Fig.3.4 XRD pattern of samples prepared at different hydrothermal times (0.5, 1.5, 6, 24 h) combined with calcination at 500 oC for 2 h. 66

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig. 3.5 SEM images of precursors obtained at different hydrothermal time: (a) 0.5 h (b) 1.5 h (c) 6 h, and (d) 24 h.

In order to understand the role of the concentration of LiOH on the phase composition, we further conducted a different concentration of LiOH with the molar ratio of Li:Ti= 4:5 upon different hydrothermal heating time and analysed by SEM and XRD techniques. Fig.3.4 shows the XRD patterns of products obtained at different hydrothermal time and subsequent calcination at 500 oC for 2 h. Upon heating time of 0.5 h, all the peaks were assigned to spinel LTO (JCPDS 06-5707). As the reaction proceed to 1.5 h, besides the peaks assigned to spinel LTO, a peak located at 25.5 o was indexed anatase TiO2 (JCPDS 02-2751), indicating that anatase TiO2 was formed upon heating time of 1.5 h. After hydrothermal heating at 6 h, a peak at 25.5 o was still observed, but with higher intensity, indicating the higher crystallinity of anatase TiO2 than the product heated at a time of 1.5 h. Finally, as for the product obtained by hydrothermal treatment of 24 h, all the peaks were assigned to spinel LTO and no peaks

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

indexed to TiO2 were observed, indicating the disappearance of anatase TiO2 and high purity nature of LTO at 24 h heat treatment. These results indicate that the concentration of LiOH and hydrothermal treatment plays a significant role in obtaining the composition of LTO/TiO2 composite.

Fig.3.5 illustrates the SEM images of the precursors obtained at different hydrothermal time. At the time of 0.5 h, the precursor shows spheres in a diameter of about 1μm and nanoparticles less than 100 nm. Extended the heating time to 1.5 h, the spherical particles almost maintain the shape, but the nanoparticles reduce their size to less than 50 nm. Upon the introduction hydrothermal treatment of 6 h, the nanoparticles grow to larger spheres. There is slight deformation on the spherical particles. With long heating time of 24 h, the spheres become more perfect; probably due to the continuous mature of spherical particles. In general, the precursors obtained at different time (0.5, 1.5, 6, 24 h) had a spherical structure with diameters ranging from several nanometer to several micrometer, which involved in the maturity of spherical particles between submicrosized and nanosized particles. These results indicate that different hydrothermal time does affect the overall morphology of the precursors.

3.3.1.3 Electrochemical performance

The CV curve for the nanosphere LTO/TiO2 composite electrode at a scan rate of 0.02 mV·s-1 in the voltage range of 1.0-3.0 V are shown in Fig.3.6a. Two sets of peaks were detected. The peaks couple located at 1.46 and 1.84 V was assigned to the reduction/oxidation reaction of spinel LTO at the stage of lithiation/delithiation.[228] Another peaks at centered at 1.74 and 2.19 V corresponds to the lithium [202, 229] insertion/extraction of anatase TiO2. The result was in good agreement with the XRD pattern in Fig.3.1.

Fig.3.6b shows the voltage profile of nanosphere LTO/TiO2 composite during 1st, 5th, 100th cycle at the density of 2000 mA/g. At the first cycle, the discharge and charge capacity was 129.8, 132.3 mAh/g, respectively. However, the charge and discharge capacity were quickly reduced to 112.2, 111.2 mAh/g, respectively, during the 5th cycle. After that, the discharge/charge capacity was retained to be 110.2, 110.7 mAh/g, respectively at the100th cycle. The large capacity reduction from the first cycle to the 5th cycle was attributed to irreversible capacity loss during the first 5 cycles.[194, 230]

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig.3.6 Electrochemical performance of nanosphere LTO/TiO2 composite: (a) cyclic voltammogram (CV) at a scan rate of 0.02 mV·s-1 in the potential window of 1.0-3.0 V; (b) charge-discharge profile at 1st, 5th, 100th cycle at the high rate of 2000 mA/g; (c) cycling performance at the high rate of 2000 mA/g in the voltage range of 1.0-3.0 V.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

The potential differences between charge/discharge voltage of 1st, 5th, 100th cycle were 0.64, 0.27, 0.79 V, respectively, indicating that kinetics of the 5th cycle was the fastest while that of the 100th cycle was the slowest. These results could be attributed to the following: At the beginning, slow lithium transport occurring from surface defects and amorphous channels result in slow kinetics. Upon five cycles, kinetics of electrode material became faster accompanied with the formation of lithium channels for stabilization of the electrode. After that, the kinetics became slower again due to gradually damage of the electrode along with the increasing of cycles.[231]

The cycling performance of submicro-sphere LTO/TiO2 composite at density of 2000 mAg-1 is illustrated in Fig.3.6c. The first discharge was 129.8 mAh/g. After 5 cycles, the discharge capacity was decreased to 111.2 mAh/g. From the fifth cycle onwards, the discharge capacity was maintained up to 100 cycles with only slightly decreasing. A high capacity of 110.2 mAh/g was retained upon 100 cycles, corresponding to 99.1% of the fifth discharge capacity and average capacity loss of 0.0105 mAh/g per cycle from fifth cycle to 100th cycle, indicating the excellent cycleability of submicro-sphere

LTO/TiO2 composite at high current density (2000 mA/g). These results are comparable with carbon coated Li4Ti5O12/TiO2 nanocomposite reported by Rahman et. al. with 110 mAh/g at a current density of 2000 mA/g up to 100 cycles.[232] The high electrochemical performances can be attributed to the unique structure of sphere

LTO/TiO2 composite, in which the spherical structure can prevent the aggregation of nanoparticles and a long range diameter of 30-500 nm can provide an excellent ion diffusion and electronic conduction pathway.

3.3.2 Porous LTO/TiO2 nanosheets

3.3.2.1 XRD and morphology analysis

X-Ray diffraction (XRD) is investigated to determine the crystallographic phases of the porous LTO/TiO2 nanosheets. Characteristics peaks indexed to LTO (JCPDS 49-0207), rutile TiO2 (JCPDS 73-1764) and anatase TiO2 (JCPDS 76-0320) were detected, as marked separately in the Fig.3.7, indicating that three phases of LTO, rutile and anatase

TiO2 coexisted in the product instead of pure phase LTO or TiO2. Rutile TiO2 occupied

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery a small part compared with the other two phases. The products were then denoted as

LTO/TiO2.

Fig.3.7 XRD pattern of LTO/TiO2 nanosheet. A-TiO2 means anatase TiO2, R-TiO2 means rutile TiO2.

Fig.3.8 illustrates the morphology of precursor and nanosheet LTO-TiO2. Flower-like clusters were observed for precursor (Fig.3.8a). Better revealed by the high- magnification SEM image (Fig.3.8b), the precursor constituents exhibit nanosheet structure and were densely agglomerated to flower-like clusters. Fig.3.8c shows the o SEM image of LTO/TiO2, which was obtained by the calcination of precursor at 500 C for 2 h in the air. The morphology of flower-like was maintained. A closer observation (Fig.3.8d) reveals that the flower-like shape was composed of densely packed nanosheets with thickness of ~30 nm. The calcination step in this process does not change the morphology. TEM was used to further confirm the morphology of LTO obtained by annealing the precursor in the air. Nanosheet morphology of LTO-TiO2 observed in Fig.3.8e was consistent with that showed in SEM image (Fig.3.8d). As shown in Fig.3.8f, porous structure with diameter size of ~4-5 nm inside nanosheet was observed.

Brunauer-Emmett-Teller (BET) surface area measurement of the as-prepared porous 2 -1 LTO/TiO2 nanosheet yielded a value of 190 m ·g . The pore size distribution curve and

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

nitrogen adsorption-desorption isotherm curve of the as-prepared porous LTO/TiO2 nanosheet are shown in Fig.3.9.

Fig. 3.8 SEM images of (a) precursor (b) precursor at a higher magnification (c)

LTO/TiO2 (d) LTO/TiO2 at a higher magnification; TEM images of (e) LTO/TiO2 (f)

LTO-TiO2 at a higher magnification.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig.3.9 N2 adsorption-desorption measurements of mesoporous LTO/TiO2 nanosheet. (a)

Pore size distribution curve (b) N2 adsorption–desorption isotherm plot.

As presented in Fig.3.9a, the pore size of the as-prepared LTO/TiO2 nanosheet was ~8-

10 nm, indicating mesoporous nature of LTO/TiO2 nanosheet. The pore diameter is almost 2 times more than that observed by TEM image (4-5 nm, Fig.3.8f). This would attribute to the connecting of two pores. A type IV isotherm with a type H2 hysteresis loop at the relative pressure of 0.7-1.0 is observed in Fig.3.9b,[233] indicating the presence of mesopores and possible interconnecting pores. These interconnecting pores would be from the linking of two pores according to 8-10 nm pore size in Fig.3.9b and 4-5 nm of pore by TEM image (Fig.3.8f).

3.3.2.2 Optimization of parameters

Fig. 3.10 SEM images of products prepared under hydrothermal treatment of 180 oC for 3 h at the different concentration of CTAB (a) CTAB: Ti=0.00, (b) CTAB: Ti=0.11

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig. 3.11 TEM images of product prepared under hydrothermal treatment of 180 oC for 3 h with PVP surfactant (a) low magnification, and (b) high magnification.

Parameter, including the concentration of CTAB and other surfactant (e.g., PVP, AOT), are also studied. Fig.3.10 shows the SEM images of products obtained at different concentration of CTAB with hydrothermal treatment of 180 oC for 3 h. In the typical system, CTAB:Ti=0.1. As can be seen in Fig.3.10a and b, nanosheets agglomerated to each other with a slight change of the concentration of CTAB, indicating that there is an optimal concentration of CTAB in obtaining good ordered and perfect nanosheets.

Fig. 3.12 (a) SEM image, and (b) TEM image of product prepared under hydrothermal treatment of 180 oC for 3 h with CTAB-AOT dual surfactants.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig.3.11 shows the TEM images of products prepared with PVP surfactant rather than CTAB. Nanoparticles with the size of 50-100 nm and flagella-like shape with the width of 5-10nm and length of 100-200 nm are observed. When using both AOT and CTAB surfactant, spindle shaped nanoparticles are obtained, as clearly observed in Fig.3.12a and b. These results indicate that a specific surfactant is required to obtain the porous structural nanosheets.

3.3.2.3 Electrochemical performance

The electrochemical properties of mesoporous LTO/TiO2 nanosheet as anode material for high-power LIBs were investigated. Fig.3.13a shows cyclic voltammogram (CV) -1 scan of mesoporous LTO/TiO2 nanosheet nanosheet at a scanning rate of 0.2 mV·s in the voltage range of 1.0-3 V. Two couples of redox peaks were observed during the cathodic and anodic scans, respectively. The characteristic peaks at 1.52 and 1.75 V indicated the existence of spinel LTO,[103, 234] while the peaks at 1.70 and 2.20 V [235] indicated the presence of anatase TiO2. No voltage peaks for rutile TiO2 were detected due to the small amount of rutile TiO2 in the product. The cathodic peaks at 1.52 and 1.70 V, corresponding to the voltage plateau process of lithium insertion, while anodic peaks at 1.75 and 2.20 V correspond to the voltage plateau process of lithium extraction.[224, 236]

Fig.3.13b shows voltage profiles of porous LTO/TiO2 nanosheet at different specific currents density, ranging from 1 to 50 C. The discharge capacity is ~171, 142 mAh/g at a current density of 1, 2 C, respectively. The capacity decreased with the increasing current density. However, it can still deliver capacities of ~87, 74 mAh/g at a relatively high current density of 30 C and 50 C, respectively.

Fig.3.13c shows voltage profiles of porous LTO/TiO2 nanosheet electrode obtained after the 1st and the 100th charge-discharge cycle at a current rate of 10 C (Where 1 C corresponds to the current density of 200 mA/g) in the potential range from 1.0 to 3.0 V (vs. Li+/Li). During the first discharge-charge cycle, the discharge and charge voltage plateaus were located at ~1.1 and ~2.0 V, and calculated difference of discharge/charge voltage is ~0.9 V. After the 100th discharge-charge cycle, the electrode exhibited a discharge voltage of ~1.5 V and charge voltage of ~1.7 V, the differential discharge/charge voltage was ~0.2 V, which is significantly lower than that of the 1st

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

cycle (~0.9 V), indicating much faster kinetic of mesoporous LTO/TiO2 nanosheet after [237] the 100th cycle. More interestingly, the LTO/TiO2 electrode exhibited a discharge capacity of 103.7 mAh/g after the 100th cycle, while a discharge capacity of 95.1 mAh/g was obtained after the 1st cycle.

Generally, the discharge capacity decreases with the increasing of cycle number attributed to the capacity loss. However, in this situation, the relationship between discharge capacity and cycle number is different. To understand the mechanism of this phenomenon, the measurement of cycleability is necessary to reveal how the discharge capacity changed with cycle number.

Fig.3.13d shows the cycling performance of porous LTO/TiO2 nanosheet with a voltage window of 1.0-3.0 V at the rate of 10 C. The discharge capacity of the electrode firstly increases with the increasing of cycle number until the 44th cycle, at which a discharge capacity of 112.8 mAh/g was delivered. After that, the discharge capacity gradually decreases as cycle number increases up to the 100th cycle. Remarkably, as the previous paragraph pointed out, the discharge capacity of 103.7 mAh/g delivered by electrode after 100th cycle is higher than that of 95.1 mAh/g after 1st cycle. At first, the electrode is in a state that without good contact among the whole electrodes and the electronic path is not fully formed for the charge/electron transport.[238] With the discharge/charge process, the path for fast charge/discharge was gradually appears and the amorphous impurities disappear, thus the discharge capacity is gradually increased along with the cycle number. At the 44th cycle, the electronic path is fully developed with no amorphous impurities in the electrode, therefore, the discharge capacity achieves the highest value among the cycling in the system. After that, just like the literature reported,[239] the discharge capacity was decreased, which is ascribed to the irreversible capacity loss during the subsequent cycles. The discharge capacity of the 100th cycle is 97.2% of the 45th cycle from the point to decrease, and is 109% of the 1st cycle, indicating that porous LTO/TiO2 nanosheet exhibits 100% capacity retention from the first cycle up to 100 cycles.

In comparison, the cycling performance of commercial LTO (Sigma, Aldrich) was also measured at the same high rate of 10 C. At the beginning of cycling, capacity of commercial LTO was almost the same as porous LTO/TiO2. However, the capacity of commercial LTO gradually decreases with the increasing of cycling number. The 76

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

differential capacity between porous LTO/TiO2 and commercial LTO increased with the increasing of cycle number during the first 44 cycles and then maintained ~20 mAh/g until 100 cycles. Recently, Sun et al. reported anatase TiO2 with (001) exposed, which is considered to be favourable for lithium ion batteries, exhibited a capacity loss of ~ 50 mAh/g after 100 cycle at high rate of 10 C.[240] In our systems, only ~3 mAh/g capacity loss of the 45th cycle was observed after 100 cycle. These results indicated that porous

LTO/TiO2 exhibited good capacity retention at high rate of 10 C and excellent cycling performance.

The good electrochemical performance of porous LTO/TiO2 nanocomposite at high charge/discharege rate can be contributed to both the LTO/TiO2 nanocomposite and morphology of nanosheet with internal nanoporous strucure. LTO-TiO2 nanocomposite exhibits high capacity and excellent cycling performance at the rate of 10 C, which could be partly attributed to synergistic function of TiO2 and LTO. On the other hand, nanosize dimension provides short lithium diffusion path for both fast electron and ion transport. The porous structure inside nanosheet gives high accessibility of active material by allowing the electrolyte to penetrate the active electrode materials, and maintain structural stability by acting as a buffer to the local volume changes during [241] discharge/charge process.

3.4 Conclusions

Dual phase submicro-sphere LTO/TiO2 composite was successfully synthesized by a solvothermal method. The resulting LTO/TiO2 spheres exhibit a high capacity of 110.2 mAh/g at a high rate of 2000 mA/g after 100 cycles and excellent cycleability with average capacity loss of 0.0105 mAh/g per cycle from fifth cycle to 100th cycle.

Through material structure and composite design, LTO/TiO2 composite anode material can meet the high power requirement for large scale application, such as electric vehicles and energy storage systems.

Porous LTO/TiO2 nanosheets were prepared by a facile hydrothermal process and subsequent calcination at 500 °C. Porous LTO/TiO2 nanocomposites exhibited excellent capacity retention at high rate of 10 C than TiO2 and high reversible capacity during cycling performance at a high rate of 10 C.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig.3.13 (a) Cyclic Voltammogram of porous LTO/TiO2 nanosheet, (b) initial galvanostatic charge-discharge profiles of porous LTO/TiO2 nanosheet at different rates,

(c) galvanostatic charge-discharge profile of porous LTO/TiO2 nanosheet at 1st, 100th cycle at the density of 10 C, (d) cycling performance of porous nanocomposite and commercial LTO at the rate of 10 C. Where 1 C corresponds to the current density of 200 mA/g. All electrochemical measurements were carried out at room temperature in 2032 coin-type half-cells.

The good cycling performance of the porous LTO/TiO2 nanocomposite at high charge/discharge rate can contribute to: (1) the combined LTO/TiO2 nanocomposite through which high capacity of TiO2 and excellent cycling performance of LTO are maintained, (2) nanosheet morphology provide short lithium diffusion length for electron/charge transport, and (3) the unique internal pore structure provide more active sites for lithium insertion/extraction, large surface area for good contract between electrolyte and active material and structural stability by acting as a buffer to the local volume changes during lithium insertion/extraction. The LTO/TiO2 hybrid would be promising in the design and synthesis of battery electrodes.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Chapter 4 Graphitized Li4Ti5O12 nanosheets with highly enhanced lithium storage capacity for lithium ion battery

4.1 Introduction

Lithium ion batteries have achieved significant success in the commercial market of portable electronics and are considered to be power sources for electric vehicles (EV) and hybrid electric vehicles (HEV), due to excellent energy/power density, low cost and eco-friendly properties.[242] Various battery materials have been explored in the past, [221, 243] [244] including anode materials (e.g., carbon ) and cathode materials (e.g., LiCoO2, [245] LiFePO4 ). Despite some success, the limited diffusion rate of lithium ions in the solid-state electrode materials leads to poor capacity, especially at high rate, thus limiting their access to the current large-scale commercial market for applications in EV and HEV.[173, 246]

Of the achieved titanium-based electrode materials so far, Li4Ti5O12 (LTO) has been an attractive anode candidate for high power lithium ion batteries because of zero strain insertion properties (no structural change in the process of lithium insertion/extraction), excellent lithium ion mobility, highly thermal stability, and safety properties compared with the currently commercialized carbon anode.[170, 247] However, LTO suffers from poor capacity at a high rate, mainly due to its low electronic conductivity, which hinders [172] practical application in lithium ion batteries for EV, HEV.

To overcome these limitations, considerable efforts have been dedicated, which include (1) tailoring the particle size to nanoscale dimension;[248] (2) coating carbon on LTO particle surface;[249] (3) doping with cations (Ag+, Mg2+, Co3+, Ga3+, or Ta5+)[7, 250] or anions (F−, Cl−).[251] Among all of these, nanosized LTO modified by electronically conductive carbon is an effective strategy to achieve high lithium storage capacity at high rate, which is attributed to the short Li diffusion lengths, fast electrochemical reaction kinetics and enhanced electronic conductivity. Typically, one step solid-state method at a high temperature of over 800 oC and a heating time of over 12 h is usually used to prepare carbon modified LTO.[123, 252, 253] For instance, Yang et al. prepared 79

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

LTO/C with starting materials of TiO2, Li2CO3 and Super-P-Li carbon black heated at 850 oC for 16 h.[252] Recently, a solution-based method and followed by calcination at a lower temperature has been introduced. Xiang et al. reported a facile sol-gel method to obtain LTO/graphene composite, which required a calcination temperature of 800 oC and a heating period of 12 h , the use of ethanol instead of water as solvent further increases the cost.[254] However, those methods are rather complicated with multiple steps or required high temperature (>700 oC) calcination for a long period of time (>4h).[255, 256] Furthermore, the expensive starting materials, such as multi-wall carbon nanotubes(MWNTs) and benzyl alcohol,[256] make these methods unsuitable for commercialisation. Therefore, a simple and low cost method in terms of inexpensive staring materials and low temperature calcination in short time to prepare carbon modified LTO is still a challenge. Currently, there is little reported LTO with a high capacity (approaching to its theoretical capacity of ~175 mAh/g) at a high rate (10 C) and excellent capacity retention upon cycling.

In this chapter, we report a facile and low-cost strategy to prepare graphitized LTO nanosheet via a hydrothermal method followed by a calcination process. The crystal structure and morphology size of nanoparticles will be characterized using the state-of- the-art techniques such as TEM, HRTEM, SEM, XRD, and Raman spectroscopy. The electrochemical performance will be measured based on CR2032 coin-type cells. The graphitized LTO nanosheet electrode delivers a remarkable reversible capacity of 169 mAh/g, which is nearly its theoretical value of 175 mAh/g, with 96.5% capacity retention of its theoretical capacity after 100 cycles at a rate of 10 C. The possible mechanism for the highly enhanced performance will also be discussed.

4.2 Experimental section

4.2.1 Synthesis and graphitization

All chemicals were used as received without further purification. 0.5 ml TBT (Sigma, Aldrich, 97%) and LiOH (molar ratio of Li: Ti=4:5) were mixed in 10 ml of water containing glucose. After stirring for 1 h, the solution was transferred into a Teflon- lined autoclave and heated at 140 oC for 68 h. The as-prepared white precipitates were collected by centrifuged and repeatedly washed with ethanol and water. After drying in

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

o o air at 60 C overnight, the white powder was calcined at 500 C for 2 h in the air or N2 atmosphere to obtain pristine LTO or graphitized LTO, respectively.

4.2.2 Characterization

The morphology of the product was examined by the scanning electron microscope (SEM, Fei Nova NanoSEM 230). Transmission electron microscope (TEM) and high resolution Transmission electron microscope (HRTEM) was carried out on Philips CM200 at 200 kV. The X-ray powder diffraction (XRD) patterns were obtained on a Philips X’pert Multipurpose X-ray Diffraction System (MPD) using Cu Kα radiation. The Raman spectroscopy was performed one Renishaw Invia Raman microscope with an excitation wavelength of 514 nm.

4.2.3 Electrochemical measurement

The electrochemical performance was measured based on CR2032 coin-type cells. Pure lithium metal was used as the counter and reference electrodes, the electrolyte was 1 M

LiPF6 in a 1:1 (volume ratio) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The working electrode was composed of as-prepared active material, a conductive agent (acetylene black), and a polymer binder (polyvinylidene difluoride) in a weight ratio of 80:10:10. The cyclic voltammetry (CV) was obtained within electrochemical window of 1.0-3.0 V at the scan of 0.2 mV·s-1.

4.3 Results and discussion

4.3.1 Composition and morphology

The morphology and crystal structure of the precursor and as-prepared nanosheets via our proposed hydrothermal combined calcination approach were examined by SEM, TEM and HRTEM. The LTO nanosheet (denoted as n-LTO) and graphitized LTO nanosheet (denoted as C-LTO) can be obtained by annealing the precursors at 500°C in the air and N2 atmosphere, respectively. As shown in Fig.4.1A, the flower-like structure was observed in LTO precursor. A high-magnification SEM image in Fig.4.1B revealed that the flower-like structure was composed of nanosheet with thickness of 25-50 nm. It was found that the nanosheet-like structure was maintained even after calcinations at a o high temperature of over 500 C either in air (Fig. 4.1C) or in N2 atmosphere (Fig.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

4.1D). Both n-LTO and C-LTO nanosheets have a thickness range of 20-40 nm, which is a little smaller than the size of precursor, probably caused by the shrinkage due to decomposition of organic molecules (e.g. carbon-based molecules).

Fig.4.1 SEM images of (A) LTO precursor, (B) LTO precursor at a high magnification, (C) LTO (denoted as n-LTO) nanosheet, and (D) graphitized LTO (denoted as C-LTO) nanosheets. n-LTO was obtained from calcination at 500 oC for 2 h in the air, while C- o LTO was obtained by calcination at 500 C for 2 h in N2 atmosphere.

Fig.4.2A shows TEM image of n-LTO nanosheets, which possessed a feature of smooth surface. It is remarkable that C-LTO nanosheet ends up with rough surface (Fig.4.2C), which is attributed to graphitic carbon modification. The interplanar spacing of 0.49 nm (Fig.4.2B, 2D) corresponds to the lattice spacing of (101) plane for spinel LTO crystals. A lattice distance of 0.34 nm in C-LTO (Fig.4.2D) is ascribed to graphitic carbon.[257, 258]

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

A coating layer or coating layers could not be observed, indicating that the carbon is dispersed well into LTO rather than coating as a surface layer.

Fig.4.2 (A) TEM images, (B) HRTEM images of n-LTO; (C) TEM image, (D) HRTEM images of C-LTO.

XRD pattern and Raman spectra were carried out to determine the phase composition of precursor and as-prepared nanostructured LTO. As shown in Fig.4.3A, no peaks of precursor assigned to spinel LTO were detected. Characteristic peaks as labelled in precursor were assigned to Li0.94Ti2O4 (JCPDS No. 88-0609), LiTi1.826O4 (JCPDS No.

70-2689), rutile TiO2 (JCPDS No. 76-0321), and anatase TiO2 (JCPDS No. 73-1764), o respectively. After calcination at 500 C in air or N2 atmosphere, n-LTO and C-LTO naonosheets show similar XRD patterns. Li0.94Ti2O4 and TiO2 in precursor were converted to spinel LTO (JCPDS No. 72-0426) after calcination. One peak related to 83

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

either Li0.94Ti2O4 or TiO2 (anatase or rutile) as impurity was observed. As shown by Raman spectra in Fig.4.3B, n-LTO and C-LTO nanosheets have characteristic peaks at ~232, 270, 353, 422, 672 and 750 cm-1, which is in good agreement with reported spinel LTO in the literature.[260] As for C-LTO nanosheets, a different peak centred at ~1590 cm-1 is assigned to G-band of highly ordered carbons, indicating the presence of graphitic carbon with sp2 carbon-type structure.[261] With regard to the D mode, however, there is no disorder induced band, located at ~1300 cm-1, observed in C-LTO nanosheets.[262, 263]. The intensity of the Raman shift peaks for C-LTO is little higher than those for n-LTO sample. This might be due to the higher electronic energy in C- LTO by carbon modification.

Fig.4.3 (A) XRD pattern of precursor, n-LTO and C-LTO nanosheet. A-TiO2: anatase

TiO2, R-TiO2: rutile TiO2. (B) Raman spectra of n-LTO and C-LTO nanosheet. The exciting laser wavelength is 514 nm.

Energy dispersive spectroscopic (EDS) mapping/imaging was used to understand the distribution of C in the C-LTO nanosheet. EDS mapping of Ti, O, and C composition in Fig.4.4 shows that C is well dispersed in the system along with Ti and O elements. Li could not be detected by the EDS because it is a light element.

4.3.2 Formation and growth mechanism

Fig.4.5 illustrates the formation process of C-LTO nanosheet. Upon hydrothermal treatment, nanosheet precursor was formed based on the function of hydroxyl group introduced as the source of LiOH. After calcination at 500 oC, the precursor was

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

converted to spinel LTO. In the N2 atmosphere, high temperature calcination converted carbon-based material to graphitic carbon, which was uniformly dispersed in the nanosheet C-LTO. Due to the decomposition of organic molecules, the thickness of as- prepared nanosheets is reduced after the annealing of precursor at 500 oC. The content of C in the system is calculated to be 3.6%, further calcination of C-LTO in the air confirms that carbon is 3.2%.

Fig.4.4 Energy dispersive spectroscopic (EDS) mapping of C-LTO for the region shown in SEM.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig.4.5 A scheme illustrating the formation of C-LTO nanosheet.

4.3.3 Electrochemical performance

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

Fig.4.6 (A) charge-discharge profiles of the electrode made of C-LTO nanosheet at the 1st, 2nd and 100th cycle at a high rate of 10 C; (B) cyclic voltammograms of the as- prepared C-LTO before cycling and after cycling performance of 100 times at the rate of 10 C with a scan rate of 0.2 mV·s-1; (C) cycling performance of electrode made by as-prepared n-LTO and C-LTO at the high rate of 10 C. Where 1 C rate corresponds to 200 mA/g. The potential window is 1.0-3.0 V. The electrode composition is active material (80 wt%), carbon (10 wt%) and binder (10 wt%).

The electrochemical performance was evaluated in CR2032 type coin cells with lithium foil as the counter electrode and reference electrode, 1M LiPF6 in 1:1 ethylene carbonate (EC) and diethyl carbonate (DEC) as the electrolyte and active materials (n- LTO, C-LTO) mixing with acetylene black, polyvinylidene fluoride (PVDF) at a weight ratio of 80:10:10 as work electrodes.

Charge and discharge profile of C-LTO nanosheet anode during the 1st, 2nd and 100th cycle at a high rate of 10 C was performed with electrochemical window of 1.0-3.0 V vs Li+/Li. Where a rate of 1 C corresponds to a current density of 200 mA/g. As shown in Fig.4.6A, the first discharge and charge capacities of C-LTO nanosheet were as high as ~188 and ~177 mAh/g, corresponding to a coulombic efficiency of 94.1%. The second discharge and charge capacities of C-LTO were ~178 and ~176 mAh/g with coulombic efficiency of 98.8%. The initial irreversible capacity loss would possibly be from incomplete conversion reaction and formation of solid electrolyte interphase (SEI) film along with graphitic carbon modification.[264] The higher capacity value than the theoretical capacity (~175 mAh/g) of spinel LTO indicates the existence of additional lithium storage sites in C-LTO nanosheet, which is attributed to the reduction/oxidation reaction of graphitic carbon. After 100 charge/discharge cycles at a high rate of 10C, the electrode shows a reversible discharge capacity of 169 mAh/g, corresponding to ~96.5% of its theoretical capacity.

Fig.4.6B shows cyclic voltammogram (CV) curves of as-prepared C-LTO nanosheet before cycling performance and after 100 cycles at the high rate of 10 C within the voltage window of 1.0-3.0 V at a scan of 0.2 mV s-1. After 100 cycles, the cathodic peak shifted to a higher potential value with an increase in peak intensity, while anodic peak

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

showed lower potential with increase in peak intensity, revealing the lithium insertion and extraction process with some irreversibility. The reduction/oxidation peaks for anatase and rutile TiO2 was not found because they just occupied a small part in the system. Fig.4.6C shows cycling performance of electrode made by as-prepared C-LTO nanosheets at the same rate of 10 C up to 100 cycles at room temperature. Within the last 98 cycles, the capacity loss is only 5.1% of the second cycle, indicating the excellent cycling stability. For comparison, the cycling performance of n-LTO at the rate of 10 C is also shown. Even at the first cycle, n-LTO nanosheets exhibited a discharge capacity of 105.5 mAh/g, corresponding to ~60.3% of theoretical capacity. Noticeably C-LTO showed more initial capacity loss than that of n-LTO. Carbon doping in the LTO might be the reason for this phenomena. At the initial cycle, the capacity of C-LTO would contribute to both the carbon and LTO, in which carbon is the main source of higher capacity loss. These results indicate that graphitized LTO nanosheet can greatly enhance reversible capacity with excellent capacity retention at a current rate as high as 10 C even after 100 cycles.

Of all modified methods for LTO, carbon modification has attracted tremendous interests. Most recently, Kang et al.[263] reported LTO/carbon nanocomposite obtained at 700oC exhibited a capacity of ~110 mAh/g at the high rate of 10 C after 100 cycles. Jung et al.[132] reported the cycling performance of carbon coated LTO prepared at a high temperature of 900 oC for 20 h, its delivers ~150 mAh/g after 100 cycle at a rate of 10C. Zhu et al.[265] reported carbon coated nano-sized LTO microsphere prepared at 800oC for 10h delivered a capacity of 126 mAh/g at 10 C with capacity retention of [222] o ~81%. Zhao et al. reported LTO modified by N-Doped Carbon at 600 C exhibited a capacity of ~140 mAh/g at 10 C rate. These either use high temperature (>500 oC) or long calcination time.

A reversible discharge capacity of 169 mAh/g after 100 cycles at the rate of 10 C for C- LTO nanosheet in our synthetic system ranks the highest among the reported carbon modified LTO. Besides, we annealed the precursor at a low temperature of 500 oC for 2 h, compared to 600-900 oC for a period time of over 10 h.[132, 265] In general, our synthetic method is facile, time and energy saving. Such outstanding electrochemical performance combined with the facile, low cost method, making C-LTO nanosheet a promising anode material for large scale high power application.

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

The C-LTO showed greatly enhanced electrochemical performance in terms of excellent cycling performance and high lithium storage capacity at high discharge/charge rates. The possible reasons for such high performance are: (1) sheet-like shape is in nanosized dimension, and fast reaction kinetics is possible by providing excellent ion diffusion and electronic conduction pathway. (2) Graphitic carbon is sp2 structural two dimensional sheet with excellent electronic conductivity, which would be a good electronically conductive additive for nanostructured electrodes.[266] Zhang et al. reported that the enhanced photocatalytic activity of TiO2 modified by graphite-like carbon is attributed to d-π conjugate between TiO2 and graphite-like carbon through π bonds of graphite [257] and d-orbital of TiO2. Wang et al. also reported the d–π interaction between graphite-like carbon and TiO2 nanotube for the improved photocatalytic performance.[267] Similarly, the d-π conjugate can be employed to explain the enhanced lithium storage capacity of C-LTO nanosheet at high rate in our system. The π bonds of graphitic carbon would interact with the d-orbitals of Ti 3d sites in LTO[268], through which the electrical conductivity of LTO is greatly improved. Moreover, the graphitic carbon can provide sufficient conductive networks for electrolyte penetrating the active material. Hence, the capacity is highly improved at a high charge/discharge rate. (3) The specific capacity of C-LTO is ~169 mAh/g, the volumetric capacity might be relatively low due to the void space originated from intersperse of each nanosheet. However, its volumetric capacity would be tremendously increased to a high value by densifying nanosheet with physical pressure.

4.4 Conclusions

We have developed a facile and efficient method to synthesize graphitized LTO by employing glucose as carbon source. These graphitized LTO nanosheets greatly enhance the reversible capacity with excellent capacity retention at a current rate as high at 10 C after 100 cycles. The capacity was ~188 mAh/g at the first cycle and was sustained as a reversible capacity of ~169 mAh/g, corresponding to 96.5% of its theoretical capacity (~175 mAh/g), over 100 cycles at the high rate of 10 C. This superior performance could be attributed to the short diffusion path of nanosheet for fast electron/ion transport, d-π conjugate between LTO and graphitic carbon for improvement of electrical conductivity, and high volumetric capacity realized by

Chapter 4 Graphitized LTO Nanosheets with Highly Enhanced Lithium Storage Capacity for Lithium Ion Battery

densifying nanosheets with physical pressure. The as-prepared C-LTO nanosheets are a promising anode material for large scale application in lithium ion batteries.

Chapter 5 Summary

In this thesis, specific morphologies of LTO, carbon-doped LTO and LTO/TiO2 composites with control have been investigated on the base of spinel LTO as anode material for lithium ion batteries. Hydrothermal/solvothermal synthesis approaches at 140-180 oC were employed, and followed by combination of high temperature calcination at 500 oC to synthesize the electrode materials. More details are given in the followings:

In Chapter 2, Hierarchical LTO hollow microspheres with a diameter of 0.5-2 μm assembled from nanosheets. This specific structure reduces the lithium diffusion path and can provide high surface area. The hollow LTO microspheres deliver a discharge capacity of ~ 110 mAh/g with no significant capacity loss after 100 cycles at a density rate of 10 C. A reversible capacity of ~95 mAh/g at the rate of 50 C was achieved while only ~12 mAh/g was maintained for commercial LTO solid sphere powder. The superior electrochemical performance suggests that self-assembled LTO hollow microspheres can be used as a promising anode material for high-power lithium-ion batteries.

In Chapter 3, Graphitized LTO is synthesized by employing glucose as a carbon source. These graphitized LTO nanosheets greatly enhance the reversible capacity with excellent capacity retention at a current rate as high as 10 C after 100 cycles. The capacity was ~188 mAh/g at the first cycle and was sustained as a reversible capacity of

~169 mAh/g, corresponding to 96.5% of its theoretical capacity (~175 mAh/g), over 100 cycles at the high rate of 10 C. This superior performance could be attributed to the short diffusion path of nanosheet for fast electron/ion transport, d-π conjugate between LTO and graphitic carbon for high electrical conductivity, and high volumetric capacity realized by densifying nanosheets with physical pressure. The as-prepared C-LTO nanosheet is a promising excellent anode material for large scale application in lithium ion batteries.

Chapter 5 Summary

In Chapter 4, Dual phase sub-microsphere LTO/TiO2 composites have been successfully synthesized by a solvothermal route by using acetone as a solvent. The resulting

LTO/TiO2 sphere exhibits high capacity of 110.2 mAh/g at 2000 mA/g after 100 cycles and excellent cycleability with average capacity loss of 0.0105 mAh/g per cycle from fifth cycle to 100th cycle. Porous LTO/TiO2 nanocomposites exhibit excellent capacity retention at a high rate of 10 C than TiO2 and high reversible capacity during cycling performance at 10 C. The good cycling performance of porous LTO/TiO2 nanocomposites at high charge/discharge rate can be contributed to: (a) the synergetic effect of LTO and TiO2 in the nanocomposite though which both high capacity of TiO2 and excellent cycling performance of LTO are maintained; (b) nanosheet morphology provide short lithium diffusion length for electron/charge transport; and (c) the unique internal pore structure provides more active sites for lithium insertion/extraction, large surface area for good contract between electrolyte and active material and structural stability by acting as a buffer to the local volume changes during lithium insertion/extraction. The LTO/TiO2 hybrid materials with excellent cycleability and high reversible capacity would be a promising anode material in the design and synthesis of battery electrodes.

The continuing breakthroughs in the synthesis and modifications of LTO nanomaterial will be distributed to new properties for high power applications with long cycle life. With this aspect, the modification of LTO in specific compositions (e.g. LTO/Ag, LTO/C) has been intensively reported to improve its electrochemical performance in LIBs. However, the mechanisms for lithium storage have rarely been investigated. Future research of theoretical tools and models, including simulation methods (e.g, first principle calculations, and molecular dynamics), to understand the mechanisms on lithium intercalation/deintercalation would establish systematic data on the relationship between electrochemical performance and various structural stability, composition, crystallinity and shape of LTO anode, thus speed up the development of LTO anode for large scale application in lithium ion batteries.

However, the current achievement good electrochemical of LTO are subject to lab-scale, and can hardly be produced or applied in large-scale for commercial purpose. It is still a long way for current lab research of high performance LTO to penetrate the current LIBs market for EVs. The consideration of LTO in terms of capacity, rate capability and

Chapter 5 Summary

cycle life, need to be further refined and investigated for high power devices (e.g. EVs, HEVs) upon industrialization for broad market. The optimization of parameters for the LTO synthetic systems is still required to meet the specific demands of EVs.

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