Na₄Fe₂Mn(PO₄)₂(P₂O₇) has received widespread attention due to high energy density and less structural variations. However, its rate capability and cycling performance is far inferior to expectations. This study unveils underlying failure mechanisms for the performance degradation: "electrostrictive" coupled-disruption driven by charge changes causes the Na⁺ channels closure, while irregular cathode electrolyte interphase (CEI) growth hinders interface Na⁺ diffusion and causes transition metal dissolution. Therefore, halogen elements (F, Cl, Br) are introduced into the material through defect-engineering. The strong electronegativity of F and spatial effects of Cl/Br effectively regulate the coordination environment to suppress the coupled-disruption. Furthermore, the surface halogen elements spontaneously combine with Na⁺, ultimately forming uniform, surface organic-rich and interior inorganic-rich CEI layers. Based on this, the modified material shows high-rate performance (55.0 mAh g^-1 at 200 C) with ultra-long cycle stability (98% after 20000 cycles at 50 C) and exhibits excellent electrochemical performance in full-cell and all-solid-state battery applications.

sodium-ion batteries; cathode materials; P2O7 distortion; Solid electrolyte interface