From Inert to Active: Breaking Mott-localization Enables High Na-Storage Performance in Na₄MnFe(PO₄)₃-based Cathode

As a sustainable cathode material for sodium-ion batteries, Na₄MnFe(PO₄)₃ (NMFP) is prized for high theoretical operating voltage and cost-effectiveness. However, its practical electrochemical activity is notoriously poor, contradicting theoretical predictions. Here, we reveal that this inactivity stems primarily from Mott localization, driven by strong electron correlations within the high-spin 3d⁵ electronic configuration (t_2g ³e_g ²) of Mn²⁺ and Fe³⁺. This symmetric, half-filled state leads to pronounced charge localization, severely suppressing the intrinsic redox activity. To address this limitation, we devised a symmetry-breaking reconstruction strategy which reorganizes the spin ordering to promote electron delocalization and activates multiple redox couples (Mn⁴⁺/Mn³⁺, Mn³⁺/Mn²⁺, and Fe³⁺/Fe²⁺). More critically, induce a novel "Na2 dp Na1" migration path for Na⁺, with a remarkably lower energy barrier than those of conventional paths (0.39 vs. 0.98 eV). Consequently, the engineered Na₄Mn_0.5Fe_0.5Cr_0.5Ti_0.5(PO₄)₃ delivers 138.84 mAh g^-1 at 0.1C, which represents a 12.74-fold breakthrough over the pristine NMFP (10.9 mAh g^-1). Our findings elucidate symmetry-breaking as a critical route for activating Mott-localized states in polyanionic frameworks and establish a new paradigm for designing redox-active and sustainable cathode materials.