Development of high capacity Li-rich layered cathode materials for lithium ion batteries

Abstract

Since the launching of the first commercial lithium ion batteries (LIBs) in 1991, LIBs have been widely used to power portable electronic devices such as cell phones and laptop computers due to their high energy density. In the past decade, in response to the finite petroleum supply and the associated serious environmental concerns, low emission or even zero emission electric vehicles (EVs) have attracted increasing research and development interests and LIBs have been considered as one of the most potential power sources. However, the widely used layered LiCoO2 cathode material has very limited chance to meet the demand considering its low capacity, high cost and toxicity. Li2MnO3 based layered Li-rich cathode materials as promising cathode candidates of LIBs have attracted much recent attention mainly due to their superior high specific capacity, low cost and high working voltage. To date, although researchers have put much effort to this family of materials, they still face a few serious challenges to overcome in terms of the specific capacity, long-term cycling stability and rate performance. In addition, some fundamental issues are still under debates in the understanding of the crystal structures and the electrochemical reaction mechanisms. This thesis focuses on the development of new high capacity Li-rich cathode materials for LIBs.The first chapter starts with a general introduction of LIBs, followed by a brief summary of the LIBs component, working principle and three types of the well-developed cathode materials in Chapter 2. A detailed review of the recent development on Li2MnO3 based Li-rich cathode materials was also included in Chapter 2. The main objectives and the rationale behind this project are described in Chapter 3. All the experiment details including the material synthesis and characterization, coin cell fabrication and electrochemistry measurement are shown in Chapter 4.Chapter 5 presents a published work on a series of layered-spinel integrated Li-rich cathode materials with controllable capacity. The Co/Mn mole ratio is fixed while the Li/(Mn+Ni) ratio is varied to adjust the ratio of the layered/spinel phases. They exhibit steadily increased specific capacities upon cycling for several dozen of cycles due to the gradual activation of the initial Li-rich layered phase from the surface of the composite particles. Both experimental and computational results suggest that a small amount of Co dopants plays a critical role in the continuous activation process of these materials. In addition, the structural evaluation mechanism is also discussed. Based on this unique feature, controllable discharge capacities of these cathode materials can be achieved in a broad range from 30 to 240 mAh g-1 by deliberately activating the materials at a potential window of 2~4.8 V. Chapter 6 demonstrates a series of low-Co Li-rich cathode materials showing stepwise capacity increase over a few cycles from less than 50 mAh g-1 to around 250 mAh g-1. A systematic analysis on their compositions, crystal structures and the electrochemical performances reveals that the small change of Co content has negligible effect to the crystal structure and morphology, but plays an important role in adjusting the activation rate of the Li2MnO3 phase. In addition, optimized cycling potential window and current rate have been demonstrated to significantly ensure the effective Li2MnO3 activation and good long-term cycling stability.Chapter 7 describes a new class of Li-rich materials Li[Li1/3-2x/3Mn2/3-x/3Nix]O2 (0.09≤x≤0.2) with a small amount of Ni doping as cathode materials for LIBs. Anomalous gradual capacity growth up to tens of cycles due to the continuous activation of the Li2MnO3 phase is observed. Both experimental and computational results indicate that a small amount of Ni doping can promote the stepwise Li2MnO3 activation to obtain increased specific capacity and better cycling capability. On the contrary, excessive Ni will overly activate the Li2MnO3 and result into a large capacity loss in the first cycle. The Li1.25Mn0.625Ni0.125O2 material with an optimized content of Ni has shown a superior high capacity of ~280 mAh g-1 and good cycling stability at room temperature.Chapter 8 proposes a fundamental understanding on the Li2MnO3 activation process. Based on the platform of the low-Ni Li1.87Mn0.94Ni0.19O3 cathode material which exhibits exclusive stepwise capacity increase upon cycling as demonstrated in Chapter 6, the Li2MnO3 activation process was artificially retarded significantly and split into quite a few cycles. A combined study including the powerful in-situ XRD analysis and HAADF-STEM characterization revealed that the oxygen release sub-reaction is much faster than the TM-diffusion reaction. The latter is the key kinetic step to finalize the Li2MnO3 activation and results into the gradual capacity increase. Finally, conclusions and recommendations are presented in Chapter 9 summarising the key findings and achievements of the present work on Li-rich Mn-based cathode materials and also giving insights into the future research and development on this promising cathode material system.