Preparation And Modification Of Li₃V₂（PO₄）₃as Cathode Materials For Lithium-ion Batteries

Monoclinic Li3V2(PO4)3is considered as a potential cathode material due to its high operating voltage, high theoretical capacity and good safety performance. However, Li3V2(PO4)3has an intrinsic low electronic conductivity, which largely limits its large-scale application. Carbon coating, which is an effect way to overcome this restriction, is investigated in this work. Carbon coating can not only improve the conductivity of Li3V2(PO4)3but also improve the rate capability of Li3V2(PO4)3. The main contents are summarized as follows:Li3V2(PO4)3/C composites were synthesized by sol-gel method. The effects of carbon sources, calcination temperature and calcination time on the performance of materials were studied in detail. The results showed that the Li3V2(PO4)3/C sample which was synthesized with PVP as carbon source and sintered at750℃for8h had the best electrochemical performance. The initial discharge capacity of the sample at0.1C was126.4mAh g-1.Carbon coating was used to overcome the restriction of Li3V2(PO4)3. The effects of different PVP contents in the precusor on the structure, morphology and performance of the material were investigated. The results showed that the structure of materials was not changed, but the electrochemical performance was improved by carbon coating. The sample synthesized with9wt.%PVP was coated with a carbon layer in a thickness of about12nm. The sample had small and well-dispersed particle size, which presented excellent rate capability and good cyclic performance. The initial discharge capacity of the sample was127.2mAh g-1at1C. Even at discharge rate of20C, it still has81.8mAh g-1with10.7%capacity loss after120cycles.The electrode process kinetics of the Li3V2(PO4)3/C composites was studied by Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). CV analysis indicated that the lithium diffusion coefficient (DLi+) of Li3V2(PO4)3/C composites was between5.74×10-8cm2s-1and6.43×10-8cm2s-1. DLi+calculated by EIS is in the range of 3.65×10-12～1.89×10-10cm2S-1. The results showed that this electrode had good lithium insertion/de-insertion capability.