Electron Spin‐State Regulation Unlocks Structural Robustness and Stable Sodium Storage in Na ₄ Fe ₃ (PO ₄

ABSTRACT

The low‐cost, structurally robust Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ (NFPP) polyanionic cathode for sodium‐ion batteries is constrained by electronic insulation and sluggish Na ⁺ transport. Herein, a spin‐state‐engineered cathode, Na ₄ Fe _2.94 Mo _0.03 Co _0.03 (PO ₄ ) ₂ P ₂ O ₇ (MC‐NFPP), is developed to modulate the local electronic structure of Fe, thereby unlocking additional unpaired electrons to simultaneously enhance intrinsic charge transport and lattice stability. The high‐valence Mo ⁶⁺ weakens the local ligand field via the inductive effect, while Co ²⁺ promotes spin reconfiguration through magnetic exchange interactions. This design induces a profound electronic reconfiguration, driving the transition of Fe from an intermediate‐spin (IS) to a high‐spin (HS) state (with the HS‐Fe ²⁺ fraction rising from 65.0% to 79.2%), which concurrently narrows the bandgap and lowers the Na ⁺ migration barrier. Meanwhile, the increased fraction of flexible HS‐state FeO ₆ octahedra activates a spin‐regulated structural breathing effect, enabling rapid relaxation of distorted [P ₂ O ₇ ] units and markedly reducing unit cell volume fluctuation from 5.78% to 3.89%, thus ensuring robust structural resilience. Consequently, MC‐NFPP achieves a high reversible capacity of 89.2 mAh g ⁻¹ at 30 C and retains 96.8% capacity after 1200 cycles in full cells. This spin‐state regulation effectively overcomes the intrinsic kinetic and structural bottlenecks of polyanionic cathodes, providing a viable pathway for their practical deployment.