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

Na₄Fe₃(PO₄)₂P₂O₇ (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 Ni²⁺ to Fe³⁺ ions is unveiled in Ni-doped Na_4.3Fe₃(PO₄)₂P₂O₇ (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 Fe²⁺, modulated by the presence of Ni²⁺, 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 (Ni²⁺, Mn²⁺, Zn²⁺, Co²⁺ and Cu²⁺) 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_4.3Fe₃(PO₄)₂P₂O₇ 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.