Oxygen-Vacancy Engineering of Na₄Fe₃(PO₄)₂P₂O₇ Enables Fast and Wide-Temperature Sodium Storage

The polyanionic cathode Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) is regarded as a promising cathode for sodium-ion batteries owing to its low cost, intrinsic safety, and robust framework stability. However, the strongly localized electronic structure and sluggish Na⁺ transport kinetics impose coupled limitations on its rate capability and stability. Herein, we demonstrate a defect-engineering strategy to activate coupled electronic-ionic transport through the rational introduction of oxygen vacancies into NFPP. Combined experimental investigations and density functional theory calculations reveal that oxygen vacancies act as dual-functional kinetic regulators by simultaneously reconstructing the local Fe-O electronic environment and facilitating Na⁺ migration. The defect-induced electronic redistribution narrows the bandgap and accelerates electron transport (over 7 times), while expanded Na⁺ diffusion pathways and reduced migration energy barriers enable rapid ion diffusion (over 6 times). Consequently, the oxygen vacancy-enriched NFPP cathode delivers exceptional cycling stability with 90.46% capacity retention after 7000 cycles at an ultra-high rate of 20 C. This work establishes oxygen-vacancy engineering as an effective strategy for coupled transport regulation in polyanionic cathodes and provides fundamental insights into defect-mediated kinetic enhancement for advanced sodium-ion batteries.