Bond-Defect Synergy Enabled Ultrastable and High-Rate Sodium Iron Phosphate Cathode through Zn/F Co Substitution

Achieving both long-term stability and superior rate capability in Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathodes remains a major challenge for sodium-ion batteries. Herein, we demonstrate a synergistic bond-defect strategy that circumvents this trade-off. The complementary interaction between Zn and F establishes a synergistic bond-defect environment. The strong bonding of Zn²⁺ mitigates the charge localization associated with F^- doping. Furthermore, the strategy narrows the electronic bandgap to near-metallic values and enhances the degree of graphitization in carbon coatings, resulting in a marked improvement in electronic conductivity. The optimized Na_3.95Fe_2.95Zn_0.05(PO₄)₂P₂O_6.95F_0.05 (NFZPPF) exhibits outstanding cycling stability with 76.25% retention after 16,000 cycles at 20 C and remarkable rate performance, delivering 68.6 mAh g^-1 at 50 C. Coupled with a hard carbon anode, the full cell retains 88.8% capacity after 200 cycles at 2 C, underscoring its viability for practical sodium-ion storage.