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

Abstract

Bi-doped compounds Li₃V_2−xBi_x(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 Bi³⁺ 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 Bi³⁺ doping is beneficial in improving the electrochemical properties of Li₃V₂(PO₄)₃. Among all the samples, Li₃V_1.97Bi_0.03(PO₄)₃/C shows the best cycle and rate performance. At 3.0–4.3 V, the initial discharge capacity of Li₃V_1.97Bi_0.03(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 Li₃V_1.97Bi_0.03(PO₄)₃/C is almost 100% after 100 cycles at 3.0–4.3 V. In addition, Li₃V_1.97Bi_0.03(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 Li₃V_1.97Bi_0.03(PO₄)₃/C than the primary Li₃V₂(PO₄)₃/C. The excellent electrochemical performance of Li₃V_1.97Bi_0.03(PO₄)₃/C can be attributed to its larger Li ion diffusion channels, higher electronic conductivity, higher structural stability and smaller particle size.