Unlocking Electronic Spatiotemporal Confinement to Activate FeMn-Based Phosphates for High Energy Density and...

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 Na_3.6Fe_0.6Mn_0.6Ti_0.5V_0.3(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.