Unlocking Electronic Spatiotemporal Confinement to Activate FeMn-Based Phosphates for High Energy Density and Ultra-Wide Temperature Na Storage

FeMn-based polyanionic phosphates, represented by Na 4 FeMn(PO 4 ) 3, are promising cathode materials for low-cost and sustainable sodium-ion batteries due to their high theoretical voltage and capacity. Nevertheless, their performance is constrained by long-standing and unexplained anomalous electrochemical inertness. This study unveils the physical origin of this inertness and identifies an intrinsic electronic “spatiotemporal confinement effect”. Spatially, PO 4 tetrahedra separate the redox-active MO 6 (M = Fe/Mn) units into mutually isolated compartments; temporally, spin-pairing constraints further impede charge transport within the d-electron configuration. Based on this mechanism, we propose a heterometallic bridging strategy by introducing V and Ti with empty 3d-orbitals as electronic bridges to reconstruct electronic connectivity within the Fe–Mn network. The prepared Na 3.6 Fe 0.6 Mn 0.6 Ti 0.5 V 0.3 (PO 4 ) 3 successfully breaks the spatiotemporal confinement, circumvents spin-forbidden barriers, and activates multielectron redox reactions. Consequently, it achieves a leap from an inert state to high energy density of 470.1 Wh kg –1 . Meanwhile, it exhibits exceptional wide-temperature adaptability, retaining 76.4 mAh g –1 at −80 °C and operating stably across 130 °C range. This work elucidates the nature of electrochemical inertness in FeMn-based polyanionic compounds through the lens of spatiotemporal confinement and provides a universal strategy for the design of high energy density polyanionic cathode materials.