From Inert to Active: Breaking Mott‐localization Enables High Na‐Storage Performance in Na ₄ MnFe(PO ₄ ) ₃ ‐based Cathode

Abstract

ABSTRACT As a sustainable cathode material for sodium‐ion batteries, Na 4 MnFe(PO 4 ) 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 3d 5 electronic configuration (t 2g 3 e g 2 ) of Mn 2+ and Fe 3+ . 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 (Mn 4+ /Mn 3+ , Mn 3+ /Mn 2+ , and Fe 3+ /Fe 2+ ). 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 Na 4 Mn 0.5 Fe 0.5 Cr 0.5 Ti 0.5 (PO 4 ) 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.