Improved Sodium Storage in Na ₂ FePO ₄ F via N-Doped Carbon Coating

Sodium-ion batteries (SIBs) are regarded as a promising alternative due to the natural abundance and cost-effectiveness of sodium. Na2FePO4F (NFPF) attracts significant attention as a cathode material for SIBs owing to its excellent structural stability and high operating voltage. However, a large band gap and low electronic conductivity hinder the application of NFPF. In this study, nitrogen-doped carbon-coated Na2FePO4F is synthesized via secondary heat treatment with polyacrylonitrile. The resulting uniform carbon layer effectively isolates the electrolyte from the active material, suppresses side reactions, and enhances both electronic and ionic conductivity. As a result, the modified material exhibits significantly improved electrochemical performance, with a specific capacity of 84.4 mAh·g–1 after 170 cycles at 1 C. The rate performance of the modified material is also excellent, delivering a capacity of 87.8 mAh·g–1 at 2 C.