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 Na4FeMn(PO4)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, PO4 tetrahedra separate the redox-active MO6 (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 Na3.6Fe0.6Mn0.6Ti0.5V0.3(PO4)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.