Effects of sodium and vanadium co-doping on the structure and electrochemical performance of LiFePO4/C cathode material for lithium-ion batteries

Sodium and vanadium co-doped olivine-type LiFePO4/C cathode materials are synthesized for the first time via a conventional solid-state reaction route. The effects of sodium and vanadium co-doping on the structure, morphology and electrochemical performance of LiFePO4/C are investigated in this study. X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images demonstrate the homogeneous existence of carbon on the surface of LiFePO4 particles with olivine-type structure and an average particle size of 500 nm. The electrochemical performance of the as-synthesized samples is studied by charge/discharge tests, and electrochemical impedance spectroscopy (EIS) measurements. Compared with the pristine LiFePO4/C material, the co-doped samples exhibit smaller particle size, higher discharge capacities, more stable cycle performance, better rate capability and smaller charge-transfer resistance. Especially, the Li0.97Na0.03Fe0.97V0.03PO4/C sample shows the best electrochemical performance with initial discharge capacity of 156.5 mA h g−1 at 0.1 C rate. Even at a high rate of 5 C, it delivers a high discharge capacity of 111.8 mA h g−1 with a capacity retention ratio of 90% after 30 cycles. It is also verified that the presence of a small amount of sodium and vanadium ions in the LiFePO4/C particles can reduce the charge-transfer resistance, resulting in an enhanced electrochemical performance.