TOWARDS HIGH-ENERGY AND HIGH-POWER DENSITY LITHIUM-RICH CATHODE MATERIALS FOR FUTURE LITHIUM-ION BATTERIES: EXPLORING AND UNDERSTANDING MECHANISMS AND ROLE OF TRANSFORMATION SONG BOHANG NATIONAL UNIVERSITY OF SINGAPORE 2013 TOWARDS HIGH-ENERGY AND HIGH-POWER DENSITY LITHIUM-RICH CATHODE MATERIALS FOR FUTURE LITHIUM-ION BATTERIES: EXPLORING AND UNDERSTANDING MECHANISMS AND ROLE OF TRANSFORMATION SONG BOHANG (B.Sc.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 Declaration DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously SONG BOHANG 09 August 2013 I Acknowledgements ACKNOWLEDGEMENTS First and foremost, I would like to express my deepest and the most sincere gratitude to my supervisors, Prof Lu Li and A/Prof Lai Man On, for their invaluable guidance and consistent support throughout my four years’ study at the National University of Singapore Their incisive mind and unique perspectives always enlightened me of new insights into research topics, and it is extremely pleasant to work with them I am also grateful to National University of Singapore for the financial support I would like to express my sincere thanks to my seniors, Dr Xiao Pengfei and Dr Wang Hailong for introducing me to this graduate program, teaching and inspiring me a lot in experiments and life I would also like to thank Dr Liu Hongwei and A/Prof Liu Zongwen for their professional skills on transmission electron mircoscopy characterizations In addition, I would like to thank Dr Ye Shukai, Dr Wang Shijie, Dr Zhang Zhen, Dr Xia Hui, Dr Yan Feng, Dr Zhu Jing, Dr Ding Yuanli, Mr Lin Chunfu, Mr Chen Yu, Mr Ding Bo, Mr Yan Binggong and Mr Helmy for their generous encouragement and valuable suggestions on lab work A special appreciation goes to my friend, Mr Mi Yu for the unstoppable support during my four years’ study I would like to acknowledge the following staff in Materials Laboratory: Mr Thomas Tan, Mr Ng Hongwei, Mr Khalim, Mr Juraimi and Dr Aye Thein for providing professional technical support Finally, I would like to express my utmost thanks to my parents and all other family members Without their understandings and endless love, I wouldn’t make it happen to accomplish this entire study II Summary SUMMARY Developing high-energy and high-power density cathode materials for next-generation lithium ion batteries (LIB) is important and urgent because of high demand of long lasting power sources, such as portable devices, power tools and electric vehicles (EVs) In addition to the cathode materials developed in the past two decades, a new family of Li-rich layered cathodes has received great interests due to their high theoretical and reversible capacities However, several drawbacks still gap them from real applications, for instance first irreversible capacity loss, poor rate capability and voltage decay during electrochemical cycling To conquer these critical issues, this study firstly focuses on exploring the mechanisms behind the voltage decay which is highly associated with inevitable phase transformation in local structure To solve this issue, a doping strategy taking advantages of cation ions is proposed to slow down the progress of this phase transformation Furthermore, inspired by a positive aspect on rate performance as a result of serious transformation, several surface modification strategies are proposed to enhance the rate capability of the Li-rich layered cathode, which involves a similar phase transformation during preparation but only occurs in the particle surface regions Systematic characterizations on crystal structure and solid state chemistry are performed to lead to comprehensive understandings on various evolutions of corresponding electrochemical behaviors III Table of Contents Table of Contents DECLARATION I ACKNOWLEDGEMENTS II SUMMARY III LIST OF FIGURES IX LIST OF TABLES XXII CHAPTER INTRODUCTION AND LITERATURE REVIEW 1.1 Basic Concepts of Rechargeable Li-ion Batteries 1.2 Literature Review 1.2.1 Overview of Electrode Materials 1.2.2 Cathode Materials Within Spinel Structure 1.2.3 Cathode Materials Within Olivine Structure 1.2.4 Cathode Materials Within Layered Structure 11 1.2.4.1 Conventional LiCoO2, LiNiO2 and LiMnO2 11 1.2.4.2 Other Derivatives 13 1.2.5 Cathode Materials Within Two-phase Integrated Structure 19 1.2.5.1 Origins of Designation and Basic Concepts 19 1.2.5.2 Critical Issues and Achieved Improvements 22 1.3 Present Work on Improving Li-rich Layered Cathodes 26 CHAPTER EXPERIMENTAL APPROACH 28 2.1 Synthesis Routes 28 IV Table of Contents 2.1.1 Hydroxide Based Co-precipitation Method 28 2.1.2 Spray-dryer Assisted Sol-gel Method 29 2.2 Material Characterizations 29 2.2.1 Elemental Analysis 29 2.2.2 X-ray Diffraction and Rietveld Refinement 30 2.2.3 Raman Spectroscopy 30 2.2.4 Electron Microscopy 30 2.2.5 X-ray Photoelectron Spectroscopy 31 2.2.6 TGA/DSC Characterization 31 2.3 Characterization of Electrochemical Properties 31 2.3.1 Preparation of Positive Electrode and Battery Assembly 31 2.3.2 Galvanostatic Charge/Discharge Cycling 32 2.3.3 Cyclic Voltammetry 33 2.3.4 Electrochemical Impedance Spectroscopy 33 2.3.5 dQ/dV Plots 33 CHAPTER Ru DOPING ON 3a SITE IN Li-RICH LAYERED CATHODES 35 3.1 Motivation of the Doping Strategy 35 3.2 Material Preparation 36 3.3 Crystallographic Characterizations 37 3.4 Electrochemical Properties 41 3.5 Discussions on Facile Phase Transformation upon Long-term Cycling 56 3.5.1 Analysis Based on Electrochemical Behaviors 56 V Table of Contents 3.5.2 XPS analysis 59 3.5.3 Analysis Based on TEM-EDS and ICP 62 3.5.4 Analysis based on HRTEM 64 3.5.5 Analysis Based on Ex-situ XRD 69 3.5.6 Influence of Spinel-like Phase on Electrochemical Performance 70 3.6 Summary 72 CHAPTER Cr DOPING ON 3a SITE IN Li-RICH LAYERED CATHODES .74 4.1 Motivation of the Doping Strategy 74 4.2 Material Preparation 75 4.3 Crystallographic Characterization 76 4.4 Electrochemical Properties 82 4.5 Suppression of Phase Transformation upon Long-term Cycling 86 4.5.1 XPS Analysis 86 4.5.2 Cycle Performance 90 4.5.3 Analysis Based on Discharge Curves and dQ/dV 93 4.5.4 Ex-situ XRD 97 4.6 Summary 98 CHAPTER GRAPHENE-INVOLVED SURFACE TREATMENT ON LI-RICH LAYERED CATHODES 100 5.1 Motivation of the Modification Strategy 100 5.2 Material Preparation 101 5.3 Crystallographic Characterization 102 5.4 Electrochemical Properties 112 VI Table of Contents 5.4.1 Galvanostatic Charge/Discharge Behaviors 112 5.4.2 Cyclic Voltammetry 115 5.5 Discussions on Enhanced Rate Capability Led by Phase Transformation 117 5.5.1 XPS Analysis 118 5.5.2 Cycling Performance and Enhanced Rate Capability 121 5.5.3 Analysis Based on Discharge Curves and dQ/dV 125 5.5.4 Analysis Based on EIS 128 5.6 Summary 130 CHAPTER CARBON BLACK-INVOLVED SURFACE TREATMENT ON LI-RICH LAYERED CATHODES 131 6.1 Motivation of the Modification Strategy 131 6.2 Material Preparation 132 6.3 Crystallographic Characterizations 132 6.4 Electrochemical Properties 139 6.4.1 Galvanostatic Charge/Discharge Behaviors 139 6.4.2 Cyclic Voltammetry 141 6.5 Discussion on Enhanced Rate Capability Led by Phase Transformation 144 6.5.1 XPS Analysis 144 6.5.2 Analysis Based on TEM-EDS 146 6.5.3 Cycling Performance and Enhanced Rate Capability 148 6.5.4 Analysis Based on Discharge Curves and dQ/dV 151 6.5.5 Analysis Based on EIS 153 VII Table of Contents 6.6 Summary 155 CHAPTER SURFACE COATING OF CARBON ON LI-RICH LAYERED CATHODES 157 7.1 Motivation of the Modification Strategy 157 7.2 Material Preparation 157 7.3 Crystallographic Characterizations 158 7.4 Electrochemical Properties 166 7.5 Discussion on Enhanced Cyclability and Rate Capability 169 7.5.1 XPS Analysis 169 7.5.2 Enhanced Cyclability and Rate Capability 172 7.5.3 Analysis Based on Discharge Curves and dQ/dV 173 7.6 Summary 176 CHAPTER CONCLUSIONS AND RECOMMENDATIONS 177 8.1 Conclusions 177 8.2 Limitations and Recommendations 181 REFERENCES 184 LIST OF PUBLICATIONS 197 VIII References REFERENCES Tarascon, J.M and M Armand, Issues and challenges facing rechargeable lithium batteries Nature, 2001 414(6861): p 359-367 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