Research On Synthesis And Performance Of Lithium Vanadium Fluorophosphate Cathode For Li-ion Batteries

Currently, energy sources and environment problems have become more and more seriously, and green and environmental protection become the development theme of the21st century, so research of clean and renewable new energy will be one of the world’s first tasks in the future. As a new type of clean, secondary renewable energy, lithium ion battery is exclusive at notebook computer and mobile phone market. More over, it’s the most powerful competitor for electric vehicles (EV/PHEV/HEV) and energy storage areas. Lithium ion battery materials are the key technology of lithium ion battery, which has become a research focus in the field of new energy. The study object of this paper was lithium ion battery cathode material LiVPO4F. Lithium ion battery and its commonly used cathode materials were introduced at the beginning, and then the research progress of LiVPO4F cathode material was reviewed in detail. The aims of the present study were to focus on the preparation processes of LiVPOF synthesized by ball-milling assisted sol-gel method, and the modification of LiVPOF. The microstructure, surface morphology and electrochemical performance of LiVPOF were investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electron microscope (TEM) and electrochemical tests.(1) Lithium vanadium fluorophosphates, crystallizing with a triclinic structure was synthesized by a ball-milling assisted sol-gel method, the influences of heating temperature, heating time and concentrationon of H2O2on the physical performance and electrochemical behavior of LiVPOF were studied. The results showed that with the increased of sintering temperature, the peak intensity of XRD increased and the crystals become more perfect. The peak intensity changes slightly with the increases of calcined time at the same temperature, which apparently implies that the calcined time has little influence on the morphology of LiVPOF, but the size of crystals increased. With the increases of calcined temperature and time, the reversible capacity of LiVPOF increased at first, and then decreased. The sample synthesized at550℃for4h was pure phase, possessed good crystallinity and uniform particle size distribution. At0.1C rate, the LiVPO4F sample demonstrated charge and discharge capacity of about153.2mAh/g and125.7mAh/g at the first cycle respectively, the coulombic efficiency was82percent. The sample showed good electrochemical behavior with the concentration of H2O2was10percent (v/v).(2) LiVPO4F/C was synthesized at550℃for4h employing acetylene black as reducing agent, and acetylene black15wt.%,25wt.%,35wt.%,45wt.%,55wt.%and65wt.%excess respectively. Through the characterization of structure and morphology and electrochemical performance, the results indicated that with the increased of acetylene black, the peak intensity of XRD increased and the particles decreased gradually and tend to be spherical. The reversible capacity and electrochemical impedance were reduced after rising. The sample acetylene black45wt.%excess has small particles and uniform particle size distribution and presents good electrochemical performance. At high temperature, the sample possessed of stabile structural, and its discharge capacitity was higher than that of room temperature, and has good cycle life.(3) LiV1-xCuxPO4F/C was synthesized at550℃for4h by a ball-milling assisted sol-gel method employing copper acetate as copper source, and the x=0,0.02,0.04,0.06. The effects of Cu2+doped content on the crystal structure, microstructure, rate performance and EIS of LiVPO4F/C were studied. The results indicated that, the doping of Cu2+had no influence on the crystal structure of LiVPO4F. The material particles of LiVPOF/C increased with the increased doping amount of Cu2+. LiV1-xCuxPO4F/C showed the best discharge capacity when x=0.02. At2C rate, LiV0.98Cuo.o2P04F/C displayed discharge capacity of about82mAh/g at the first cycle, and76mAh/g after50cycles. The initial discharge capacity was80mAh/g at the rate of3C, and remained73mAh/g after50cycles. The capacity retention ratio of them both were above90percent.