Synthesis and electrochemical properties of Zn-doped, carbon coated lithium vanadium phosphate cathode materials for lithium-ion batteries

Zn-doped Li3V2(PO4)3/C (Li3V2−xZnx(PO4)3/C, x = 0, 0.02, 0.04 and 0.06) cathode materials are synthesized by an improved sol-gel method of which pH value is controlled at 4. They are characterized by X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, linear sweep voltammetry, galvanostatic charge/discharge test, cyclic voltammetry, electrochemical impedance spectroscopy and potential step chronoamperometry. Li3V1.96Zn0.04(PO4)3/C has the highest electrical conductivity among the four samples. Although the initial discharge capacity for the doped samples at low current rate, such as 0.2C, presents no obvious enhancement compared with that for the undoped one, the cyclability and the rate performance are improved significantly. Zn-doped samples exhibit higher initial discharge capacity than the undoped one as increasing current rates. Among the three Zn-doped samples, Li3V1.96Zn0.04(PO4)3/C shows the highest initial discharge capacity of 105.5 mAh g−1 at 5C. Capacity retention for Li3V1.96Zn0.04(PO4)3/C remains 83.6% at 0.2C after 50 cycles, higher than 62.8% for Li3V2(PO4)3/C. It is believed that Zn substitution is beneficial to the rate performance and cyclic performance due to the lower charge transfer resistance and higher diffusion coefficient of lithium ions resulted from relatively higher intrinsic conductivity and smaller particle size.