Local Electron Spin‐State Engineering at Fe Sites for Highly Reversible Sodium‐Ion Batteries

Developing robust cathode materials for sodium‐ion batteries (SIBs) requires the simultaneous mitigation of sluggish reaction kinetics and inadequate structural stability during cycling. Herein, a distinctive carbon‐coated sodium iron sulfide composite (Na 6 FeS 4 /C) is developed via a rotary evaporation‐carbothermal reduction strategy, and spin‐state engineering is first proposed to induce electronic spin polarization at Fe sites. The short and nearly identical axial (2.356 Å) and equatorial (2.355 Å) bond lengths in Na 6 FeS 4 indicate a thermodynamically stable crystal structure. Furthermore, the d ‐ p orbital hybridization between Fe 3d and S 3p orbitals in the [FeS 4 ] 6− tetrahedron can accelerate electron delocalization and optimize charge distribution, thus improving charge transfer dynamics and capacity output. More importantly, S coordination induces the electron transition of Fe 2+ from the d z 2 orbital to the d xz orbital to form high‐spin (HS, e 3 t 2 3 ) Fe atoms, which could generate more unpaired electrons on the 3d orbitals and enhance the electrochemical activity of Fe 2+ sites. Based on these merits, the well‐designed Na 6 FeS 4 /C composite cathodes exhibit exceptional Na storage performance, delivering a high initial reversible capacity (419.8 mAh g −1 at 0.1 C), excellent rate performance (192.2 mAh g −1 at 2 C), and superior long‐term cycling stability (83.3% capacity retention rate over 400 cycles at 1 C).