Nb‐Doping‐Derived Fe Vacancy Boosting Reversible Reaction Kinetics of Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ Cathode for Enhanced Sodium Storage

ABSTRACT Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP), with an intrinsic wide bandgap, is plagued by restricted electronic transport, leading to a significant capacity degradation for sodium‐ion batteries (SIBs). Herein, a Nb‐doping‐induced Fe vacancy strategy is proposed to increase carrier concentration and accelerate electronic conduction. The incorporated Nb 5+ ions, with unique 4d° configuration, preferentially occupy the Fe1 sites and trigger the formation of charge‐compensating Fe vacancies at Fe3 sites. Each Fe vacancy functions as a double‐hole donor, generating unpaired electrons that increase the spin state of Fe and induce a double exchange interaction (Fe 2+ ‒O‒Fe 3+ ). This significantly promotes charge transfer, elevating the electronic conductivity from 1.9 to 19.8 µS cm ‒1 , thereby activating previously dormant storage sites. Furthermore, the Fe vacancies effectively eliminate the maricite NaFePO 4 phase, promoting a quasi‐solid‐solution reaction mechanism with a minimal volume change of only 1.58%. The optimized Na 4 Fe 2.825 Nb 0.07 (PO 4 ) 2 P 2 O 7 /C exhibits outstanding rate capability (82.5 mAh g ‒1 at 20 C) and excellent cycling stability (94.6% capacity retention after 1000 cycles at 10 C). This work provides fundamental insights into targeted vacancy engineering from the perspective of defect chemistry, offering a viable approach for designing high‐performance NFPP cathodes for SIBs.