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.
Dr. Qi Liu currently work as an assistant professor at department of physics in City university of Hong Kong. He obtained his Ph.D. degree in Mechanical Engineering from Purdue University in 2014. After that, Dr. Liu performed his postdoc training at Argonne National Lab. His research has been focusing on“Fundamental investigation of phase transformative materials for energy application”. Through conducting a series of cutting-edge research, he have equipped himself with expertise in a large variety of synchrotron X-ray physics, materials synthesis and characterization techniques. Particularly, he is a leading expert in utilizing multiple synchrotron X-ray techniques for fundamental studies of materials in energy application. In addition, he has considerable experience in fabricating and testing prototype batteries.To date, He has already published 50+ journal articles and given over 10+ international conferences.