Accelerating Electrochemical Kinetics in Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ Cathodes Through Oxygen Vacancy Modulation for Wide‐Temperature Ah‐Level Sodium‐Ion Batteries

Abstract Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) stands as a highly promising cathode material for sodium‐ion batteries, offering a favorable combination of operating voltage and theoretical capacity. Nevertheless, its commercial viability is significantly hindered by two primary factors: the prevalent formation of electrochemically inert impurity phases, such as maricite ‐NaFePO 4 and low‐energy‐density Na 2 FeP 2 O 7 , during conventional synthesis, coupled with its intrinsically poor electronic conductivity. Herein, we demonstrate a rational defect‐engineering strategy to enhance the electrochemical kinetics of NFPP cathodes through controlled oxygen vacancy formation. The optimized Na 4 Fe 2.79 (PO 4 ) 2 P 2 O 7 (NFPP‐2.79) exhibits superior phase purity, enhanced electronic conductivity, and facilitated Na + diffusion, as confirmed by multi‐scale characterization techniques. The NFPP‐2.79 cathode delivers a remarkable reversible capacity of 89.51 mAh g −1 at 10C and retains 96.42% capacity after 1500 cycles at 2C. Moreover, kilogram‐scale synthesis using cost‐effective raw precursors has been achieved via a sand‐milling‐assisted spray‐drying route. When assembled into Ah‐level pouch cells with hard carbon anodes, the NFPP‐2.79‐based cell demonstrates excellent rate capability (93.75% capacity retention from 0.5C to 4C), outstanding cyclability (75.63% retention after 3000 cycles), and superior safety under extreme conditions (overheating, crushing, nail penetration, overcharge, and external short‐circuit). This work highlights oxygen vacancy modulation as an effective pathway for advancing polyanionic cathodes toward practical applications.