A combined first principles and experimental study on Al-doped Na3V2(PO4)2F3 cathode for rechargeable Na batteries

Aluminum-doped Na3V2-xAlx(PO4)2F3 cathode materials were successfully synthesized using a sol-gel method for sodium ion batteries. Rietveld refinement results for pristine Na3V2(PO4)2F3 and Na3V1.93Al0.07(PO4)2F3 confirmed that no phase change occurred after Al doping. Composition-optimized Na3V1.93Al0.07(PO4)2F3 delivered the highest discharge capacity (121.3 mAh g−1 at 0.1C) and retained 75% capacity after 400 cycles at 5C. Rate capability testing indicates that Na3V1.93Al0.07(PO4)2F3 exhibits rates as high as 57 mAh g−1 at 10C, which was much higher (150%) than that of pristine Na3V2(PO4)2F3 (38 mAh g−1). Electrochemical impedance spectroscopy results showed that the diffusion coefficients of sodium ions could be enhanced from 3.8 × 10−14 cm2/s to 4.14 × 10−14 cm2/s after Al doping. Theoretical calculations on Na3V2(PO4)2F3 and doped material were also compared by first principle calculations in terms of bang gaps and density of states. The findings indicate that optimal Al doping makes NVPF as a promising cathode material for sodium-ion batteries.