报告题目：Discovery and Fundamental Studies of Phase
Transformative Materials for Energy Application
报 告 人：香港城市大学 刘奇教授
Discovery and Fundamental Studies of Phase Transformative Materials for Energy Application
Because of their high energy density, lithium ion batteries (LIBs) have become a rapidly growing energy storage technology, particularly in mobile applications, such as portable electronics, hybrid electric cars, etc. The cathode materials are considered to be the performance-limiting factor in research designed to increase cell energy and power density. During the cathode materials exploration, the advanced synchrotron-based characterization techniques, such as high-resolution synchrotron X-ray diffraction (HRXRD), in situ high-energy synchrotron X-ray diffraction (HEXRD), and in situ X-ray absorption spectroscopy (XAS), provide novel and powerful tools for exploring the structure evolution of battery materials. In my presentation, firstly I will briefly introduce how synchrotron-based techniques could be utilized for phase identification, fundamental study of structure dynamics, reaction mechanism, and doping mechanism during the cathode material exploration. Then, the presentation will be centered on the fundamental studies of V2O5 and LiCoO2 as the cathode materials for Li-ion batteries. Typically, the in-depth investigation of phase transformation behavior in V2O5-based and LiCoO2 in Li-ion batteries has been studied using advanced in situ synchrotron techniques. Take the LiCoO2 for example, theoretical and experimental investigations have shown that, when LiCoO2 is delithiated, the material will experience a series of phase transitions. Initially, there will be an insulator-metal transition in the low voltage region, resulting in a two-phase region. As the material continues to deintercalate and when approximately half of Li+ are removed from LCO, the material will experience an order-disorder transition, which drives the phase transition from the hexagonal structure to the monoclinic structure. Further delithiating LCO tends to induce the O3-O6(H1-3)-O1 phase transition process. Consequently, the unexpected phase transition and low Li+ diffusion at high voltage >4.3V prevent the lithium cobalt oxide from meeting the high-energy requirement. Here we develop a novel atomic-level multiple-element method to dope the LCO crystal structure with multiple elements. The resulting doped LiCoO2 (D-LCO) can withstand the increase in cell potential and still allow efficient lithium ion transport at high voltage, which exhibits extraordinary electrochemical performance: a high capacity of 190 mAh/g, approaching 70% of theoretical specific capacity of LiCoO2; a long cyclability (96% capacity retention over 50 cycles with a cut-off voltage of 4.5 V vs Li/Li+); and significantly enhanced rate capability. Such performance is the result of the combined effects of multiple doping elements on structural stability and lithium ion diffusion, which is supported via various electrochemical studies and synchrotron-based characterization. Especially, during the high voltage range, the O3-O6(H1-3)-O1 and order/disorder phase transition has been greatly suppressed.
刘奇博士, 香港城市大学物理系助理教授, 于2014年12月毕业于普渡大学机械工程学院。2014.12~2018.3在美国阿贡国家实验室先进光子光源部（APS）从事做博士后研究。目前主要致力于原位同步辐射技术在电化学反应、微波化学合成反应、材料合成、能量储存和转化等过程中原位检测、反应机理和反应动力学研究方面的应用；已在物理、化学、材料和工程领域做出众多原创性和突破性的工作。除了已经再包括Nature, Nature Energy, Nature Communications, JACS, Nano Letters, EES, Nano Energy, Journal of Materials Chemistry, ACS Applied Materials Interface, Electrochimica Acta 等国际知名期刊上发表高水平论文外，还作为第一负责人，搭建过国际上首台原位同步辐射和化成技术的综合测试平台，被美国能源部专家认为“为微波反应的机理研究提供了一个划时代的研究手段”。其多项研究成果不仅得到了相关领域国际学术届的广泛认可和关注，也引起了工业界的广泛兴趣。