| With the rapid development of social economy, resources and energy are becoming in shortage for human society, and soon green energy has become an important strategic choice for the sustainable development. As a new type of energy storage device, lithium-ion battery increasingly attracts many scientists because of their excellent high energy density, long cycle life and high working voltage. Since the cathode materials are the key components of lithium-ion batteries, which determines the performance of the whole battery, the design and preparation of high performance cathode materials for Li-ion battery becomes a decisive role, which could lead to a new generation of lithium-ion batteries with both high energy density and high power density. In numerous cathode electrode materials, monoclinic Li3V2(PO4)3 has good cyclic performance and rate performance, it is also simple to be synthesized, but the low discharge specific capacity (133 mAh g-1,3-4.3 V/197 mAh g-1,3-4.8 V) limits its practical application. Meanwhile, lithium vanadium oxides (LiVO2、LiVO3、LiV2O5、 LiV3O8, etc.) have the advantages of high specific capacity and low cost, but its structure is not stable in the process of cycling, so the capacity attenuate faster, unable to meet the application requirements either. To obtain cathode materials with both high specific capacity and long cycle life, this thesis will first attempt to organically combine lithium vanadium oxides and Li3V2(PO4)3 (we mainly focus on LiV3O8+Li3V2(PO4)3), using the synergistic effect modification to complement each other, and obtain the composite cathode material of lithium vanadium oxides+Li3V2(PO4)3 for lithium ion battery, the main research contents and results are as follows:(1) 0.6Li3V2(PO4)3·0.4Li-V-O was prepared by sol-gel assisted solvothermal method. The study found that the composite was composed of Li3V2(PO4)3 and Li-V-O (LiV2O5, VO2) phases. The morphologies of the composites were consist of nanoplates and irregular particles, which were dispersed on the surface of the nanoplates. It also shows good electrochemical performance. In the voltage range of 2.0-4.3 V, the first discharge capacity was 149.2 mAh g-1, much higher than that of Li3V2(PO4)3 under the same test conditions, and the composites had a better rate performance and higherdiffusion coefficient of Li ions(10-9.5-10-7.5 cm2 s-1).(2) The composite cathode material 2Li3V2(PO4)3·LiV3O8 was prepared by sol-gel、hydrothermal and ethanol combustion combined method. The composite shows better electrochemical properties than the single phases. In the voltage range of 2.0-4.3 V, the composite materials obtain a first discharge capacity of 162.8 mAh g-1, after 100 cycles, the capacity retention rate was up to 92.6%. The research shows that the special microstructure (nanoplates and nanorods) can effectively promote the charge transfer and increase the Li+ diffusion path and the contact area of the electrolyte, thus improving the performance of the materials.(3) A series of xLiV3O8·yLi3V2(PO4)3/rGO (x:y= 2:1,3:1,1:1,1:2,1:3) are prepared by mechanical mixing of Li3V2(PO4)3/rGO and LiV3O8.Electrochemical test results show that the 2LVO·LVP/rGO has the best electrochemical performance because of a better Li+ diffusion path and stable structureunder the ratio of x:y=2:1. GITT test show that the diffusion coefficient of Li ions (DLi+) in the composite is in the range of 10-11.5-10-9.5 cm2 s-1,higher than those of LVP/rGO and LVO. In the voltage range of 2.0-4.3 V the composite materials obtain a first discharge capacity of 197.0 mAh g-1,also showing a good cyclic performance and rate property. |
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