Exploration Of Composite Polyanionic Lithium Ion Insertion/extraction Materials

Composite polyanionic lithium ion insertion/extraction material is a new type of cathode material for lithium ion batteries. It has attracted intense research for its high capacity and operating voltage. However, the exploration of such material is relatively short time, and knowledge of this type of material is quite preliminary, so it need to been more comprehensive and profound study. Therfore, this dissertation intends to improve the performance through screening synthesis method, optimizing synthesis method and compositing modification.Firstly, composite polyanionic lithium ion insertion/extraction material Li VPO4 F was prepared with carbothermal reduction method and sol-gel method. The materials were characterized by X-ray diffraction(XRD), galvanostatic charge-discharge cycle tests(C&D),et al. Cathode material Li VPO4 F prepared with carbothermal reduction method exhibits an initial specific discharge capacity of 107.0m Ah/g. The capacity retention rate is only 19%after 50 charge and discharge cycles. Cathode material Li VPO4 F prepared with sol-gel method exhibits an initial discharge capacity of 80 m Ah/g. The material retains a specific discharge capacity of 86.0m Ah/g after 50 cycles, which is higher than the initial discharge capacity. Then composite polyanionic lithium ion insertion/extraction material Li2 Co PO4F was prepared with sol-gel method and hydrothermal method. The materials were initial characterized by XRD, C&D, et al. Cathode material Li2 Co PO4F prepared with sol-gel method exhibits an initial specific discharge capacity of 92.0m Ah/g. The material retains a specific discharge capacity is only 8.0m Ah/g after 25 cycles, which is only 9.0% of its initial capacity. Cathode material Li2 Co PO4F prepared with hydrothermal method exhibits an initial specific discharge capacity of 99.0m Ah/g. The material retains a specific discharge capacity is as high as 98 m Ah/g after 60 cycles, and the capacity retention rate is up to 99% of its initial capacity.Li VPO4F/C, as cathode material of lithium-ion batteries was prepared with carbon thermal reduction assisted sol-gel method. XRD, scanning electron microscopy(SEM),galvanostatic charge-discharge cycles, cyclic voltammogram(CV) and electrochemical impedance spectroscopy(EIS) were employed to investigate the effects of sintering time andtemperature on the structure and corresponding electrochemical performance of as prepared materials. Controlling the sintering time as 4h, pure phase Li VPO4F/C material could be obtained when the temperature is settled at 450℃. The as-produced Li VPO4F/C exhibits a specific discharge capacity of 193.2, 175.6 and 173.7m Ah/g at 0.1, 0.5 and 1.0C rates,respectively. Li3V2(PO4)3 impurities are formed and increased with raising calcination temperature. When sintered at 650℃, Li3V2(PO4)3 is turn out to be the main phase. On the other hand, optimal duration time at high temperature could also inhibit the decomposition of Li VPO4 F and decrease the formation of Li3V2(PO4)3 impurities, so that good electrochemical performance is realized. The optimal duration time is 3h when the precursor is sintered at550℃.Li2Co PO4 F, as cathode material of lithium-ion batteries was prepared with hydrothermal method. XRD, SEM, galvanostatic charge-discharge cycles, CV and EIS were employed to investigate the effects of hydrothermal temperature and sintering temperature on the structure and corresponding electrochemical performance of as prepared materials. Controlling the hydrothermal time as 12 h, pure phase Li2 Co PO4F material could be obtained when the temperature is settled at 200℃. The as-produced Li2 Co PO4F exhibits a specific discharge capacity of 145.2, 130.3, 115.3, 94.1, 45.7 and 23.5m Ah/g at 0.1, 0.5, 1.0, 2.0, 5.0 and 10 C rates, respectively. The specific discharge capacity is maintained above 136.4m Ah/g after 25 different high rate discharge cycles and the capacity retention is reach up to about 94% of its initial capacity. Controlling the hydrothermal temperature as 200℃, pure phase Li2 Co PO4F material could be obtained when the sintering temperature is settled at 650℃. We can clearly see from the XRD patterns, when the sintering temperature was 600℃, there is a clear formation of the Li2 Co PO4F phase, but with more impurities and less intensity peaks than the corresponding material sintered at 650℃. In addition, it is obvious that increasing the sintering temperature leads to the decomposition of the Li2 Co PO4F phase.Li1+x V1-x Cox PO4F(x=0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0), as composite cathode material was prepared with hydrothermal method. XRD, SEM and galvanostatic charge-discharge cycles were employed to investigate the effects of proportion between V and Co on the structure and corresponding electrochemical performance of as prepared materials. The better electrochemical performance can be achieved when the compositing amount is x=0.1. Theas-produced Li1.1V0.9Co0.1PO4 F exhibits an initial specific discharge capacity of 165.8m Ah/g and specific discharge energy of 486.8Wh/kg, which is much higher than Li VPO4 F and Li2 Co PO4F. The specific discharge capacity is about 214.7m Ah/g after 25 different high rate discharge cycles, and can also reach up to 174.2m Ah/g after 50 cycles.Li VPO4F/C, as cathode material of lithium-ion batteries was prepared with carbon thermal reduction assisted sol-gel method. The graphene with excellent conductivity is chosen to be coated on the particle surface of Li VPO4F/C, and the coating modified material is characterized with C&D. The specific discharge capacity is increased and the cycle stability is improved effectively at 0.1C. The coating modified material can also reach to 207.0m Ah/g after 50 cycles, and the capacity retention is reach up to about 86% of its initial capacity.Li2 Co PO4F, as cathode material of lithium-ion batteries was prepared with hydrothermal method. Al2O3 is chosen to be coated on the particle surface of Li2 Co PO4F, and the coating modified material is characterized with XRD and C&D. The specific discharge capacity is increased and can reach to 114.5m Ah/g.