In Situ\nAl<sub>2</sub>O<sub>3</sub> Coating Stabilized\nthe Interface of Na<sub>2</sub>FePO<sub>4</sub>F and Improved Its\nSodium Ion Storage Performance

Na<sub>2</sub>FePO<sub>4</sub>F (NFPF) is an iron-based\nfluorophosphate\nthat possesses a simple 2D sodium ion channel structure. It is regarded\nas a promising cathode material for sodium-ion batteries because of\nits low cost, abundant availability of resources, and nontoxic nature.\nNevertheless, its application is significantly constrained by its\nlimited intrinsic conductivity and poor cycling stability. In this\nstudy, we present an approach aimed at improving its chemical properties\nthrough an in situ Al<sub>2</sub>O<sub>3</sub> coating. Al<sub>2</sub>O<sub>3</sub> coating can inhibit particle agglomeration caused by\nthe sol–gel synthesis process and enhance the ion transmission\nefficiency of the material. In addition, the uniform Al<sub>2</sub>O<sub>3</sub> coating layer contributes to increasing stability of\nthe crystal structure while simultaneously forming a stable interface\nlayer on the electrode during cycling, which improves the material’s\ncycling stability. The optimized sample NFPF/C/Al-0.05 demonstrates\na notable reversible capacity of 117 mA h g<sup>–1</sup> at\n0.1 C, a remarkable rate capability (69.6 mA h g<sup>–1</sup> at 5 C), and commendable cycle performance (retention of 70.2% after\n1000 cycles at 5 C). This study enhances the intrinsic conductivity\nand cycling stability of Na<sub>2</sub>FePO<sub>4</sub>F through an\nin situ Al<sub>2</sub>O<sub>3</sub> coating strategy, offering a novel\nmethod for designing high-performance Na<sub>2</sub>FePO<sub>4</sub>F cathode materials.