Unlocking Fast Na ⁺ Transport in Sodium Iron Sulfate Via Coupled Electronic–Ionic Modulation

ABSTRACT Alluaudite‐type sodium iron sulfate is an attractive cathode for sodium‐ion batteries owing to its high Fe 2 + /Fe 3 + redox potential and robust polyanionic framework; however, its rate capability is intrinsically limited by sluggish Na + transport arising from electronic localization within the Fe–O network. Here, we demonstrate that fast Na + transport in sodium iron sulfate can be unlocked through a coupled electronic–ionic modulation enabled by Fe‐site isovalent Zn substitution, which regulates the electronic structure while preserving the crystallographic framework. Density functional theory calculations reveal that Zn incorporation redistributes Fe–O electronic states, induces site‐dependent Na–O coordination, and significantly lowers the Na + migration barrier. These insights are corroborated by systematic experimental characterizations. As a result of the accelerated Na + transport kinetics, the optimized Na 2 . 6 Fe 1 . 65 Zn 0 . 05 (SO 4 ) 3 cathode delivers an initial reversible capacity of ≈109 mAh g −1 , maintains 81.5 mAh g −1 at 30 C, and retains 72.9 mAh g −1 with 87.7% capacity retention after 10 000 cycles at 20 C. This work establishes coupled electronic–ionic modulation as an effective strategy for unlocking fast Na + transport in polyanionic cathodes, offering mechanistic insights for the rational design of high‐rate and long‐life sodium‐ion battery materials.