HeterovalentCation and Anion Codoping Strategy: SynergisticRegulation of Crystal Structure and Electrochemical Kinetics in Na4Fe3(PO4)2P2O7/C Cathodes for Sodium-Ion Batteries

Abstract This paper reports a novel heterovalent cation Bi3+ and anion F– codoping strategy for synergistically regulating the crystal structure and electrochemical kinetics of Na4Fe3(PO4)2P2O7/C cathodes. The novel Bi3+ and F– codoping Na4Fe2.91Bi0.06(PO4)2P2O7F0.06/C cathode (NFPP-B6F6) is proposed. Experimental analyses and theoretical calculations reveal that the combination of heterovalent cations Bi3+ with a larger ionic radius and F– with higher electronegativity efficiently reduces the band gap and modulates the crystal structure by introducing lattice distortion and Fe defects, promoting the migration of Na+ and providing additional reactive sites, which accelerate the reaction kinetics, while also forming stronger chemical bonds to stabilize the structure. Benefiting from this synergy, NFPP-B6F6 exhibits a large capacity of 92.3 mAh g–1 at 10C and improved retention of 90.2% after 1000 cycles. In-situ electrochemical impedance spectroscopy indicates that NFPP-B6F6 shows optimized electrochemical kinetics and lower charge transfer resistance, confirming the effective improvement of Bi/F codoping in conductivity. This study provides a valuable doping strategy for the design of NFPP, facilitating further practical applications.