Phase‐Purity Stoichiometric Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ Cathode Material via Reversible Reaction Modulation for High‐Performance Sodium‐Ion Batteries

Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP), a prominent polyanionic material, has garnered significant attention as a promising cathode for practical sodium‐ion batteries. However, the electrochemical performance of stoichiometric NFPP is often hindered by the persistent formation of the phase‐impurity maricite NaFePO 4 (NFP), which is difficult to eliminate during synthesis. In this study, it is tackled this challenge by reversible reaction modulation (RRM, ) to suppress NFP impurity. Through precise regulation, it is successfully synthesized phase‐purity stoichiometric NFPP‐0.25 which exhibits a significantly reduced NFP content and a more complete crystal structure with expanded Na⁺ diffusion channels compared to pristine NFPP. Consequently, NFPP‐0.25 delivers exceptional electrochemical performance, including a high reversible discharge capacity of 108.2 mAh g −1 at 0.2 C, excellent rate capability (∼87 mAh g −1 at 50 C), and remarkable long‐term cycling stability, retaining 95.8% capacity after 10,000 cycles at 10 C and 81.7% after 10,000 cycles at 50 C. Furthermore, NFPP‐0.25 demonstrates practical viability in an 800 mAh pouch cell, while DFT calculations provide deeper insights into its superior ion transport properties. Therefore, NFPP prepared by RRM strategy could emerge as a highly competitive candidate for stationary sodium‐ion batteries.