High performance of Bi³⁺ and Cl⁻ co-doped Li₃V₂(PO₄)₃ as cathode for lithium-ion batteries

Bismuth (III) (Bi³⁺) and chloride (Cl⁻) ion co-doped Li₃V_1·97Bi_0·03(PO₄)_3−x Cl _x /carbon (C) compounds (x = 0·03, 0·05, 0·07, 0·09) were prepared by way of a simple sol–gel method. The X-ray diffraction patterns proved that a small amount of bismuth (III) and chloride ion doping does not change the crystal structure of lithium vanadium phosphate (Li₃V₂(PO₄)₃). Charge/discharge results showed that bismuth (III) and chloride ion co-doped samples exhibited higher initial capacities than lithium vanadium phosphate/carbon. Li₃V_1·97Bi_0·03(PO₄)_2·95Cl_0·05/carbon demonstrated the highest capacity of 188 mAh/g at 0·1 C. The cycling performance at 1 C showed that the capacity retention of Li₃V_1·97Bi_0·03(PO₄)_2·95Cl_0·05/carbon reaches 81·31% after 300 cycles. Electrochemical impedance spectroscopy and cyclic voltammetry testing indicated that Li₃V_1·97Bi_0·03(PO₄)_2·95Cl_0·05 has the smallest charge-transfer resistance and the largest lithium-ion (Li⁺) diffusion coefficient. The improvement can be assigned to the enhanced kinetics of lithium ions, reduction in polarization and decreased charge-transfer resistance of the electrode.