快离子导体Li_3V_2(PO_4)_3包覆LiFePO_4的结构和性能

Structure and Electrochemical Performance of LiFePO_4 Cathode with Coating Super Iron Conductor Li_3V_2(PO_4)_3

通过机械活化将快离子导体Li3V2(PO4)3包覆在LiFePO4表面,制备了性能优异的复合正极材料9LiFePO4@Li3 V2(PO4)3.用XRD,SEM,HRTEM,EDS和电化学测试等手段研究了材料的物理化学性能.结果表明,包覆后的材料含有橄榄石结构的LiFePO4、单斜晶系的Li3V2(PO4)3和正交晶系的Li3PO4;LiFe-PO4颗粒表面包覆了一层Li3 V2(PO4)3,且部分V3+进入LiFePO4晶格内部,使其晶格参数减小,包覆后的LiFePO4的交换电流密度和锂离子扩散系数均提高了1个数量级.电化学测试结果表明,包覆后的LiFePO4的倍率性能及循环性能都得到显著改善,在1C和2C倍率下,包覆后的LiFePO4的首次放电比容量较包覆前分别提高了34.09%和78.97%,经150次循环后容量保持率分别提高了27.77%和65.54%;并且5C时容量为121.379 mA.h/g(包覆前LiFePO4在5C下几乎没有容量),循环350次后的容量保持率高达94.03%.

The composite 9LiFePO4@Li3 V2（PO4）3 was prepared by coating super ion conductor Li3 V2（PO4）3 on the surface of LiFePO4 particles through mechanical activation method.X-ray diffraction,scanning electron microscopy,high resolution transmission electron microscopy,energy disperse spectroscopy and electrochemical measurements were used to evaluate the properties of the cathodes.The coated LiFePO4 contains olivine LiFePO4,monoclinic Li3 V2（PO4）3 and orthorhombic Li3 PO4 phases.As LiFePO4 particles are encapsulated with a layer of Li3 V2（PO4）3 and some V3＋ ions are incorporated into the olivine structure,the LiFePO4 lattice parameters decrease.Subsequently,the exchange current density and lithium ion diffusion coefficients of coated LiFePO4 are separately improved one order of magnitude.Electrochemical measurements indicate that the rate capability and cycle performance of coated LiFePO4 are significantly enhanced.Compared with bare LiFePO4,the first discharge capacity of coated LiFePO4 is raised 34.09% and 78.97% at 1C and 2C rates,while the capacity retention increases 27.77% and 65.54% after 150 cycles,respectively.Furthermore,the 9LiFePO4@Li3 V2（PO4）3 exhibits initial discharge capacity of 121.379 mA·h/g at the rate of 5C and its capacity retention is 94.03% after 350 cycles,while the pure LiFePO4 almost shows no capacity at the same rate.