Spatially Confined Single-Phase Ternary FeP_1-xSe_x in Se-Doped Carbon Polyhedra for Fast and Stable Sodium Storage

Transition metal phosphides (TMPs) are considered promising anode materials for sodium-ion batteries owing to their high theoretical capacity and suitable redox potential, yet their practical application is limited by sluggish reaction kinetics and severe volume variation during cycling. Herein, a spatially confined single-phase ternary FeP_1-xSe_x embedded in Se-doped carbon polyhedra (FeP_1-xSe_x@C) is rationally constructed through a synergistic anion-substitution and spatial-confinement strategy. Partial substitution of P by Se effectively modulates the electronic structure, induces lattice distortion, and accelerates Na⁺ diffusion and charge-transfer kinetics. Meanwhile, the Se-doped carbon framework provides enhanced conductivity, abundant active sites, and robust structural confinement to alleviate volume expansion and suppress particle agglomeration. As a result, the FeP_1-xSe_x@C electrode delivers a reversible capacity of 412.0 mAh g^-1 after 1000 cycles at 1 A g^-1 and maintains 344.8 mAh g^-1 at 5 A g^-1, together with a high initial Coulombic efficiency of 74.8%. Furthermore, full cells paired with Na₃V₂(PO₄)₃ exhibit excellent rate capability and cycling stability, highlighting an effective strategy for advanced sodium-storage materials.