Synthesis And Modification Studies Of Li₃V₂（PO）₄Cathode Materials For Li-ion Batteries

As polyanion cathode materials for lithiumion batteries, monoclinic lithium vanadium phosphate Li3V2(PO4)3is attractive for its high theoretical capacity (197mAh/g), stable structure, high voltage plateau, low cost, excellent cycle performance and preferable safety. However, its power performance is greatly limited by slow diffusion of lithiumions across the two-phase boundary and its low electronic conductivity. In order to overcome these defaults, the researches focus on the way of synthesizing small fine particles of Li3V2(PO4)3, carbon coating as PEG and ion doping with Na. The main points can be summarized as follows.The precusor of Li3V2(PO4)3was prepared via rheological phase reaction and the cathode material Li3V2(PO4)3/C was synthesized by carbothermal reduction reaction. The effects of PEG molecular weight, PEG combination and Na+doping on the structure, morphology and electrochemical properties of Li3V2(PO4)3/C had been studied. The phase structure, surface morphology, microstructure and electrochemical performances was characterized by XRD, SEM, constant current charge-discharge experiments, EIS, and CV, respectively.At the preparation using single molecular weight PEG as carbon source at700℃for15h, the results showed that with the increase of molceular weight of PEG, the particle size reduced, and the sample prepared with PEG10K (Mw=10000) revealed the best electrochemical properties and a discharge capacity of129.6mAh/g under0.1C was obtained with the capacity of117.8mAh/g over30cycles. At the rate of0.5C and1C, the discharge capacity arrived at123.8mAh/g and113.0mAh/g, respectively.In addition, we also researched the modification using both PEG with high molecula rweight and PEG with low molecular weight as carbon source. The results showed that using both PEG with high molecular weight and PEG with low molecular weight as carbon source was better than using only PEG as carbon source in controlling the morphology and size of the particles. And the sample using both PEG35K (Mw=35000) and PEG200as carbon source showed the best electrochemical performance with a fine particle size of0.5-1μm. Its initial discharge capacities under0.1C,0.5C, and1C rates were131.1mAh/g,124.3mAh/g, and124mAh/g, respectively. As well as high reversible corresponding discharge capacity of 123.6mAh/g,123.2mAh/g, and122.4mAh/g at0.1C,0.5C, and1C over30cycles, respectively.The effect of doping Na+in place of Li-site of Li3V2(PO4)3/C on the electrochemical properties was studied in detail. It showed that doping didn’t affect the structure of the material and indicated that the doping of Na+improved the electrochemical performance apparently and the Li2.95Nao.o5V2(P04)3/C sample exhibited the best performance with initial discharge capacity of130mAh/g,130.6mAh/g, and129.1mAh/g as it was discharged with0.1C,0.5C, and1C rates, respectively. As well as high reversible corresponding discharge capacity of128.1mAh/g,125.6mAh/g, and124.6mAh/g at0.1C,0.5C, and1C rates over30cycles, respectively.