Accelerating Electrochemical Kinetics in Na₄Fe₃(PO₄)₂P₂O₇ Cathodes Through Oxygen Vacancy Modulation for...

Na₄Fe₃(PO₄)₂P₂O₇ (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-NaFePO₄ and low-energy-density Na₂FeP₂O₇, 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 Na₄Fe_2.79(PO₄)₂P₂O₇ (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.