Synthesis And Property Studies Of Li₃V₂(PO₄)₃/C Cathode Materials For Lithium-ion Batteries

Monoclinic Li₃V₂(PO₄)₃has received extensive attention as cathode material forlithium-ion batteries in recent years, because it possesses stable three-dimensional framework,good lithium ion mobility, high operate voltage, high reversible capacity and safety.The cathode materials Li₃V₂(PO₄)₃/C were synthesized by sol-gel method in this paper,the structures and morphologies of the materiasl were observed by XRD and SEM, and theelectrochemical performances were characterized by galvanostatic current charge-discharge,cyclic voltamogram (CV) and electrochemical impedance spectroscopy (EIS). The effects ofsynthesis conditions on electrochemical performance of the materials Li₃V₂(PO₄)₃/C wereinvestigated, and the material Li₃V₂(PO₄)₃/C was modified by ion doping and carbon coated.The materials Li₃V₂(PO₄)₃/C were synthesized by using sucrose as carbon source,CH₃COOLi·2H₂O, V₂O₅, NH₄H₂PO₄and oxalic acid were used as starting materials. Theeffects of different sinter temperatures on electrochemical performance of Li₃V₂(PO₄)₃/Cmaterials were mainly researched, and testing high rate capability at the best synthesisconditions. The results show that Li₃V₂(PO₄)₃/C materials sintered for8h at800oC havebetter crystallinity, uniform particle sizes, low electrochemical impedance and have the bestperformance. The initial discharge capacity reaches128.2mAh/g between the voltage rangeof3.0-4.3V at1C rate, after100cycles, the capacity retention is97.0%, and they displayexcellent cyclic stability. The initial discharge capacity decreases by only10%from131.7mAh/g at0.5C to118.4mAh/g at10C, they display good rate capability.The materials Li₃V₂(PO₄)₃/C were coated by the carbon fabric, they were observed puremonoclinic Li₃V₂(PO₄)₃by XRD, but they do not have good morphologies andelectrochemical performance.Mg was chosen as doping element to aynthesize Li3V(2-2x/3)Mgx(PO4)3/C(x=0.03,0.06,0.09,0.12). The results show that doping small amount of Mg2+ion does not affect themonoclinic structure of the materials Li₃V₂(PO₄)₃/C, and doping the proper amount of Mg2+ion can improve electrochemical performance. The materials of coating amount x=0.06exhibit excellent electrochemical properties, and their initial discharge capacity and capacityretention are both enhanced. Their initial discharge capacity reaches167.6mAh/g at1C andafter100cycles the capacity retention is81.5%.