Cr/Br Doping Induced Electron Delocalization Tuning D‐Band Center Upshift in Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 Cathode for Unlocking Electron/Ion Transport

ABSTRACT The electron localization around Fe, caused by strong covalent bonds with the polyanions, has been identified as the primary reason for poor electron‐transfer capability and slow ion diffusion kinetics of Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) cathodes, which limits its high‐rate performance in sodium‐ion batteries (SIBs). Herein, an electron delocalization strategy is proposed to overcome this challenge. Specifically, the 3d 3 (t 2g 3 e g 0 ) electronic configuration of Cr 3+ and the 4p orbital effects of Br − reduce the electron density around oxygen, driving orbital overlap between Fe 3d and O 2p and facilitating the delocalization of electrons in Fe 3d orbitals. This promotes the Fe d‐band center toward Fermi level, thereby enhancing the electron transport rate and decreasing the diffusion energy barrier of Na + ions. Additionally, stronger Cr─O bonds serve as structural backbones to strengthen structural stability, while gradient‐distributed Br − ions alleviate surface degradation. Therefore, the obtained NFPP‐Cr‐Br cathode exhibits surprising capacity (120.5 mAh g −1 at 0.1C) and long‐term cycle stability at high rate (86.2% after 2000 cycles at 10C). It also demonstrates good compatibility with different hard carbon anodes, including in pouch cells (80.6% after 500 cycles at 1C). This work provides new perspective to design efficient polyanionic cathode through electronic structure modulation strategy for SIBs.