Al-Doping Enables Low-Cost Preparation of High-Performance Na ₂ FePO ₄ F/C Using Commercial FePO ₄

Na 2 FePO 4 F (NFPF) is a promising cathode material for sodium-ion batteries, yet it is still confronted with a cost–performance trade-off. In this study, a low-cost preparation method of high-performance NFPF cathode materials was proposed via an Al-doping strategy, using commercial FePO 4 without premodification. The effect of the Al content on the crystal structure, sodium-ion diffusion kinetics, and electrochemical performance of NFPF was investigated in detail. X-ray diffraction and Rietveld refinements confirmed the high-purity phase. Na 1.98 Fe 0.98 Al 0.02 PO 4 F/C (NFPF-Al0.02) showed an excellent rate capability (57.7 vs 44.5 mAh·g –1 at 5.0 C) and better cycling stability (93.6% vs 66.7% after 100 cycles at 0.5 C) than the undoped NFPF sample. NFPF-Al0.02 also exhibited an excellent capacity retention of 84.6% after 1000 cycles at 5.0 C. GITT, CV, and EIS results revealed an order-of-magnitude increase in the sodium-ion diffusion coefficient of Al-doped NFPF-Al0.02 compared to that of pristine NFPF. A cost–performance comparison between this work and previous studies was conducted. In summary, this innovative method, which can produce high-performance NFPF cathode materials at low cost, is anticipated to significantly promote the commercialization of NFPF-based sodium-ion batteries.