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

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 Na₂.₆Fe₁.₆₅Zn₀.₀₅(SO₄)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.