Effect of Mn-doping on performance of Li3V2(PO4)3/C cathode material for lithium ion batteries

Li3V2-2/3Mnx(PO4)3(O≤x≤0.12) powders were synthesized by sol-gel method. The effect of Mn2+-doping on the structure and electrochemical performances of Li3V2(PO4)3/C was characterized by XRD, SEM, XPS, galvanostatic charge/discharge and electrochemical impedance spectroscopy(EIS). The XRD study shows that a small amount of Mn2+-doped does not alter the structure of Li3V2(PO4)3/C materials, and all Mn2+-doped samples are of pure single phase with a monoclinic structure (space group P21/n). The XPS analysis indicates that valences state of V and Mn are +3 and +2 in Li3V1.94Mn0.09(PO4)3/C, respectively, and the citric acid in raw materials was decomposed into carbon during calcination, and residual carbon exists in Li3V1.94Mn0.09(PO4)/C. The results of electrochemical measurements show that Mn2+-doping can improve the cyclic stability and rate performance of these cathode materials. The Li3V1.94Mn0.09(PO4)3/C cathode material shows the best cyclic stability and rate performance. For example, at the discharge current density of 40 mA/g, after 100 cycles, the discharge capacity of Li3V1.94Mn0.09(PO4)3/C declines from initial 158.8 mA·h/g to 120.5 mA·h/g with a capacity retention of 75.9%; however, that of the Mn-undoed sample declines from 164.2 mA·h/g to 72.6 mA·h/g with a capacity retention of 44.2%). When the discharge current is increased up to 1C, the intial discharge capacity of Li3V1.94Mn0.09(PO4)3/C still reaches 146.4 mA·h/g, and the discharge capacity maintains at 107.5 mA·h/g after 100 cycles. The EIS measurement indicates that Mn2+-doping with a appropriate amount of Mn2+ decreases the charge transfer resistance, which is favorable for the insertion/extraction of Li+.