Tailoring eg Orbital Occupancy of Fe in Ni-Doped Na4.3Fe3(PO4)2P2O7 Cathode for High-Performance Sodium-Ion Batteries

Abstract Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) is regarded as a prospective cathode for sodium-ion batteries (SIBs) because of its high structural stability and cost-effectiveness. However, its practical application is hindered by intrinsically low electronic conductivity. Herein, an unconventional electron transfer mechanism from Ni 2+ to Fe 3+ ions is unveiled in Ni-doped Na 4.3 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP-Ni) cathode, which facilitates electronic coupling within the Fe−O−Ni coordination unit and thereby effectively boosts electron transport. Moreover, the redox kinetics and reversibility of NFPP materials are predominantly governed by the degree of Fe−O covalency. The intermediate e g occupancy of Fe 2+ , modulated by the presence of Ni 2+ , optimizes the overlap between Fe d and O p orbitals. The adjustment of Ni dopant strikes a balance between accelerating Na + diffusion kinetics and mitigating lattice strain during cycling. As a result, the NFPP-Ni electrode displays impressive rate capacity (121.0 mAh g −1 at 0.1C / 80.9 mAh g −1 at 10C) and stable cyclability (89.1% capacity retention after 1000 cycles). More importantly, the relationship between Fe e g orbital occupancy and Fe−O covalency in NFPP as modulated by various transition metal cations (Ni 2+ , Mn 2+ , Zn 2+ , Co 2+ and Cu 2+ ) with different electron configurations are systematically elucidated, thereby providing insights for the commercial development of sodium-ion batteries (SIBs). Tuning the e g orbital occupancy of Fe in Na 4.3 Fe 3 (PO 4 ) 2 P 2 O 7 cathode can effectively optimize the spatial overlap between Fe d and O p orbitals with excellent rate capability for sodium-ion batteries. The e g could be a significant descriptor for Fe−O covalency that describes a volcano curve as a function of e g .