Fluorine Defect Modulation in Na ₂ FePO ₄ F _{1– <i>x</i>} as the Cathode Material for Sodium-Ion Batteries

Na2FePO4F emerges as a promising cathode material for sodium-ion batteries owing to its low cost, environmental benignity, and reasonably high theoretical capacity. Nevertheless, its practical application is hampered by limited electronic conductivity and ion diffusion rates. This investigation explores low-cost Na2FePO4F1–x (0 ≤ x ≤ 0.1) compounds with fluorine defects as potential cathodes for sodium-ion batteries. Specifically, the Na2FePO4F1–x (x = 0.04, denoted as NFPF-0.04) sample demonstrates a narrow band gap of 0.509 eV and a higher Na+ diffusion coefficient of 2.34 × 10–10 cm2 s–1 and also exhibits a volumetric change of only 1.76% during the first charge/discharge cycle. The NFPF-0.04 sample delivers a discharge capacity of 115.3 mAh g–1 at 0.1C, and the capacity retained 75.1% at 10C after 500 cycles. These improved electrochemical performances are attributed to changes in Na–O(F) and Fe–F1/F2 bond lengths induced by a small amount of fluorine defects. The insights into the enhancement of ion diffusion kinetics and mitigation of volume change are expected to accelerate the optimization of Na2FePO4F1–x electrodes for sodium-ion batteries.