Study Of Properties And Mechanism Of Carbon Coated Li₃V₂（PO₄）₃ As A Cathode Material For Lithium Ion Batteries

Lithium vanadium phosphate（LVP） material has been considered as an extremely promising cathode material because of its high specific capacity, excellent rate performance at low temperature and good stability. However, the intrinsic low electronic conductivity of LVP has greatly limited the commercial use of this material. Carbon coating is one of the most effective methods to solve this problem. In the past decades, most research efforts focused on the morphology control and developing novel preparation method, while only few studies on the interaction between LVP and carbon have been reported. In order to figure out this issue, LVP-C composites using different carbon sources have been prepared via a simple solution-based method. The electrochemical performance, morphology, LVP-C interface structure and structural evolution upon cycling have been studied systematically.LVP/reduced graphene oxide（LVP/r GO） composites with different r GO contents have been prepared using grapheme oxide as carbon source. SEM images illustrate that adding r GO effectively restrain the growth of LVP particles, which leads to decreasing of LVP particle size upon increasing r GO content. But at high r GO content（16%）, the small particles tend to reunite to form large grains, presumably during calcination. In order to separate the contribution of each single component（r GO, binder and additional conductive additive） to the overall performance of an electrode, both normal and binder free cathodes were prepared for comparison. For normal cathodes, the one using LVP/r GO（8%） composite as active material shows the best electrochemical performance. For binder-free cathodes, the electrochemical performance keeps getting better upon the increasing of r GO content; and eventually, reaches an optimized point with LVP/r GO（16%）. The roles played by r GO, binder and additional conductive additive are analyzed and discussed base on the results of charge/discharge tests and EIS measurements.For comparison, LVP/graphene（LVP/G） composites with different graphene contents have been prepared using graphene as carbon source. Graphens shows less morphology/size control effect on LVP particles, according to SEM results. The morphology and particle sizes of this series of composites are similar with pure LVP. For normal cathodes, LVP/G（8%） composite shows the best electrochemical performance, with a discharge capacity of 127.0 m Ah·g during the first cycle and the capacity retention of 97.87% after 200 cycles at 1 C. While LVP/G（16%） composite owns the best rate performances. For binder-free cathodes, the two with composites with high grapheme contents（12% and 16%） show better electrochemical performance.In addition, three other carbon sources glucose, citric acid and super P have been chosen in the preparation of LVP-C composites, base on the different interactions between these carbon sources and vanadium cations in precursor; and pure Li3V2（PO₄）3 was also synthesized for comparison. Different carbon sources affect the surface morphologies in different ways. The results of charge/discharge test shows that LVP/G has the highest discharge capacity at 1C while the capacity of LVP/SP is only slightly higher than pure LVP. LVP/CA and LVP/r GO share the best rate performance. The diffusion coefficients of Li+ for these composites were assessed from cyclic voltammetry tests. It has been demonstrated that the diffusion coefficients of carbon coated LVP is about 2³ times higher than pure LVP.⁷Li&³¹P magic angle spinning nuclear magnetic resonance（MAS NMR） has been used to investigate the structural change of LVP/r GO during the first charging process; and the results indicate that V5+ has formed in Li V2（PO₄）3, the relaxation and change of material structure is different in contrast to the chemically delithiated samples reported in literature. 1D and 2D 7Li MAS NMR tests of LVP-C composites indicate that different carbon sources have profound effects on the local structure of the LVPcarbon interface. Also, the results of 13 C MAS NMR demonstrate that the local structure of carbon coating varies with different carbon sources.