Dual‑Site V/F Engineering in Na₄Fe₃(PO₄)₂(P₂O₇) Cathodes: Concurrently Boosting Ionic Kinetics and Interfacial...

Sodium iron pyrophosphate (Na₄Fe₃(PO₄)₂(P₂O₇), 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 V³⁺ occupancy on the Fe crystallographic site serves to rigidify the lattice while simultaneously creating an electronic conduction pathway through its t₂g orbitals, mediating electron transfer during Fe²⁺ oxidation. Concurrently, aliovalent V³⁺ doping triggers substantial bandgap narrowing, decreasing it from 1.138 to 0.179 eV, drastically improving electrical conductivity. The optimized Na₄Fe_2.955V_0.03(PO_3.985F_0.03)₂P₂O₇@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.