Spatially Confined Single‐Phase Ternary FeP 1‐x Se 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‐x Se x embedded in Se‐doped carbon polyhedra (FeP 1‐x Se 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‐x Se 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 Na 3 V 2 (PO 4 ) 3 exhibit excellent rate capability and cycling stability, highlighting an effective strategy for advanced sodium‐storage materials.