The effect of Mg2+ doping on Na3V2O2(PO4)2F/C cathode materials for enhanced sodium storage performance

Na3V2O2(PO4)2F, a significant member of the polyanionic sodium superionic conductor family, exhibits high theoretical specific capacitance and operating voltage. However, its application potential is hindered by its limited intrinsic conductivity. In this study, a series of Na3V2–xMgxO2(PO4)2F/C cathode materials (x = 0, 0.05, 0.1, and 0.15) were synthesized using sol–gel and high-temperature solid-state methods. The impact of Mg2+ doping on the structural and electrochemical properties of Na3V2O2(PO4)2F was examined. Na3V1.9Mg0.1O2(PO4)2F/C demonstrated remarkable electrochemical performance, with a first cycle discharge specific capacity of 114.2 mAh/g and a capacity retention of 96.2 % after 500 cycles at 5C. The Na+ diffusion coefficient was 3.4 × 10–13 cm2 s−1. The full cell assembled with hard carbon as the counter electrode has a first cycle discharge specific capacity of 111.16 mAh/g at 0.1C and a capacity retention rate of 91.89 % after 50 cycles. This superior performance is attributed to the Mg2+ doping, which enhances the structural stability and accelerates sodium-ion diffusion.