A Bi-doped Li3V2(PO4)3/C cathode material with an enhanced high-rate capacity and long cycle stability for lithium ion batteries.

Bi-doped compounds Li3V2-xBix(PO4)3/C (x = 0, 0.01, 0.03, 0.05, 0.07) are prepared by a sol-gel method. The effects of Bi doping on the physical and electrochemical properties of Li3V2(PO4)3 are investigated. X-ray diffraction (XRD) analysis indicates that Bi doping does not change the monoclinic structure of Li3V2(PO4)3. A detailed analysis of the XRD patterns suggests that Bi(3+) ions partly enter into the crystal structure of Li3V2(PO4)3 and enlarge the lattice volume of Li3V2(PO4)3. According to the results of cycle and rate performance measurements, moderate Bi(3+) doping is beneficial in improving the electrochemical properties of Li3V2(PO4)3. Among all the samples, Li3V1.97Bi0.03(PO4)3/C shows the best cycle and rate performance. At 3.0-4.3 V, the initial discharge capacity of Li3V1.97Bi0.03(PO4)3/C is as high as 130 mA h g(-1), close to the theoretical specific capacity of 133 mA h g(-1). The capacity retention of Li3V1.97Bi0.03(PO4)3/C is almost 100% after 100 cycles at 3.0-4.3 V. In addition, Li3V1.97Bi0.03(PO4)3/C exhibits excellent low-temperature and high-rate performance. Impedance spectroscopy (EIS) and cyclic voltammetry (CV) curves indicate lower charge transfer resistance and a larger Li ion diffusion rate of Li3V1.97Bi0.03(PO4)3/C than the primary Li3V2(PO4)3/C. The excellent electrochemical performance of Li3V1.97Bi0.03(PO4)3/C can be attributed to its larger Li ion diffusion channels, higher electronic conductivity, higher structural stability and smaller particle size.