Local Coordination Environment Engineering of Na3 Sites in Na₄Mn_1.5Fe_1.5(PO₄)₂P₂O₇ Cathode

Mn-Fe-based mixed polyanionic compound Na₄Mn_1.5Fe_1.5(PO₄)₂P₂O₇ (NMFPP) holds great promise for the development of high-energy-density commercial sodium-ion batteries. However, its practical energy density was constrained by first-cycle irreversible Na⁺ deintercalation associated with intrinsic structural defects, and the underlying structural origin in such compositionally and coordinatively complex frameworks remains incompletely understood. Herein, we show that local coordination environment reconstruction governs irreversible capacity loss and the ensuing long-term degradation in polyanionic cathodes. Atomic-scale reconstruction of transition-metal (TM) coordination polyhedra, together with short-range ordering within nanoscale domains, is found to dictate the thermodynamic accessibility and kinetic reversibility of the Na3 site. By constructing a series of site-directed doping models in conjunction with experimental validation, we demonstrate that specific dopant elements can induce the transformation of unstable TMO₅ units into more robust TMO₆ configurations, thereby generating locally reconstructed coordination environments without altering the overall framework structure. Electronic-structure analyses further suggest that this reconstruction stabilizes the Na3-related redox process by lowering the Na⁺ (de)intercalation barrier and enabling more effective Mn participation in charge compensation. Consequently, the long-standing first-cycle irreversible capacity loss in Mn-Fe-based mixed polyanionic cathodes is effectively mitigated, leading to simultaneous improvements in energy density and cycling stability. Altogether, these findings provide vital guidelines for defect-sensitive Na site engineering toward high-energy-density sodium-ion batteries.