Effects of Transiotion Metal Co-Doping on the Electrochemical Properties of Li3V2-2xFexMnx(PO4)3/Graphene Cathode Material for Lithium Secondary Battery

A series of Fe and Mn co-doped Li₃V_2–2xFe_xMn_x(PO₄)₃/graphene (x = 0, 0.005, 0.010, 0.015, 0.020, and 0.025) samples have been fabricated via a sol-gel assisted microwave synthesis method. The crystal structure and morphology of the synthesized Li₃V_2–2xFe_xMn_x(PO₄)₃/graphene was investigated using an X-ray diffraction (XRD) and a scanning electron microscopy (SEM) systematically. Electrochemical measurement results prove that the Li₃V_1.98Fe_0.01Mn_0.01(PO₄)₃/graphene sample presents the best electrochemical performance as the cathode material for lithium ion batteries among the as-prepared samples in the potential ranges of 3.0–4.8 V. The resulting Li₃V_1.98Fe_0.01Mn_0.01(PO₄)₃/graphene cathode has an excellent specific capacity of 184.4 mAh g⁻¹ at 0.1C rate, and even at 10C rate, it still delivers the specific capacity of 110.5 mAh g⁻¹. The substitution of V³⁺ with an appropriate amount of Fe²⁺ and Mn⁴⁺ increases the Li⁺ diffusion coefficient assuming from the electrochemical impedance spectroscopy results, which is mainly responsible for the excellent electrochemical performance.