Electron Spin‐State Regulation Unlocks Structural Robustness and Stable Sodium Storage in Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 Cathode

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.