Beyond Conventional Sodium Superionic Conductor: Fe-Substituted Na ₃ V ₂ (PO ₄ ) ₂ F ₃ Cathodes with Accelerated Charge Transport via Polyol Reflux for Sodium-Ion Batteries

Na 3 V 2 (PO 4 ) 2 F 3 (NVPF), featuring a sodium superionic conductor structure, is a promising cathode for sodium-ion batteries (SIBs), because of its robust framework and high operating potential. However, its application is limited by low electronic conductivity and moderate cyclability. Herein, we synthesized Fe 2+ -substituted NVPF (Na 3 V 1.8 Fe 0.2 (PO 4 ) 2 F 3, NVFPF) via a simple polyol reflux method to overcome these limitations. The Fe 2+ substitution significantly enhanced the rate capability and structural stability. Consequently, NVFPF exhibited excellent rate performance (∼96 mAh g –1 at 30 C) and outstanding cycling stability (81% retention after 1000 cycles at 5 C). Improved electronic conductivity was predicted by density functional theory calculations and verified experimentally by four-point probe measurements. Furthermore, in-situ X-ray diffraction and X-ray absorption near-edge structure analyses elucidated the underlying reaction mechanisms responsible for the enhanced sodium storage kinetics. This study addresses conductivity challenges in high-voltage SIB cathodes, presenting a viable pathway for the development of high-performance materials for practical energy storage applications.