Dual‑Site V/F Engineering in Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) Cathodes: Concurrently Boosting Ionic Kinetics and Interfacial Robustness

ABSTRACT Sodium iron pyrophosphate (Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), NFPP) is a compelling cathode candidate for sodium‐ion batteries (SIBs). Nevertheless, practical implementation is intrinsically constrained owing to deficient electrical conductivity and lethargic Na + transport kinetics. Herein, dual‑site V/F modification enhances electronic transport and interfacial robustness, offering a facile route to circumvent these limitations. Fluorine incorporation induces Fe─O bond elongation, effectively widening Na + migration channels to facilitate ionic transport while simultaneously enhancing both the mechanical robustness of the cathode‐electrolyte interphase (CEI) film and interfacial electronic conductivity. Furthermore, trace V 3+ occupancy on the Fe crystallographic site serves to rigidify the lattice while simultaneously creating an electronic conduction pathway through its t 2 g orbitals, mediating electron transfer during Fe 2+ oxidation. Concurrently, aliovalent V 3+ doping triggers substantial bandgap narrowing, decreasing it from 1.138 to 0.179 eV, drastically improving electrical conductivity. The optimized Na 4 Fe 2.955 V 0.03 (PO 3.985 F 0.03 ) 2 P 2 O 7 @C (NFV0.03PP‐F0.03@C) cathode exhibits exceptional electrochemical performance, retaining 93.9% capacity across 2000 cycles at 5 C (ultralow decay rate: 0.00356 mA h g −1 /cycle). Moreover, hard carbon||NFV0.03PP‐F0.03@C full cells retain 86.56% capacity after 400 cycles at 1 C. The present work clarifies how V/F dual‐site engineering optimises bulk structure and surface properties, thereby establishing reliable design principles for polyanion cathodes with fast‐charging durability.