In Situ Al₂O₃ Coating Stabilized the Interface of Na₂FePO₄F and Improved Its Sodium Ion Storage Performance

Na2FePO4F (NFPF) is an iron-based fluorophosphate that possesses a simple 2D sodium ion channel structure. It is regarded as a promising cathode material for sodium-ion batteries because of its low cost, abundant availability of resources, and nontoxic nature. Nevertheless, its application is significantly constrained by its limited intrinsic conductivity and poor cycling stability. In this study, we present an approach aimed at improving its chemical properties through an in situ Al2O3 coating. Al2O3 coating can inhibit particle agglomeration caused by the sol–gel synthesis process and enhance the ion transmission efficiency of the material. In addition, the uniform Al2O3 coating layer contributes to increasing stability of the crystal structure while simultaneously forming a stable interface layer on the electrode during cycling, which improves the material’s cycling stability. The optimized sample NFPF/C/Al-0.05 demonstrates a notable reversible capacity of 117 mA h g–1 at 0.1 C, a remarkable rate capability (69.6 mA h g–1 at 5 C), and commendable cycle performance (retention of 70.2% after 1000 cycles at 5 C). This study enhances the intrinsic conductivity and cycling stability of Na2FePO4F through an in situ Al2O3 coating strategy, offering a novel method for designing high-performance Na2FePO4F cathode materials.