Tailoring K ⁺ Dosage in K ⁺ /Zn ²⁺ Mixed Electrolytes via Lattice Expansion Regulation of Zinc Hexacyanoferrate for High‐performance Zinc Ion Batteries

ABSTRACT Incorporating K ⁺ into Zn‐based electrolytes can enhance the performance of aqueous zinc ion batteries, while the lack of a clear rule for matching specific K ⁺ dosage with the cathode lattice regulation hinders its practical advancement. Herein, we propose a current‐dependent critical dissolution concentration (CDC) as a quantitative reference to optimize K ⁺ content, aiming to realize suppressed cyclic degradation of zinc hexacyanoferrate (ZnHCF) cathodes through precise lattice expansion control. The spontaneous dissolution of ZnHCF is theoretically analyzed via Van't Hoff isothermal equation , and a quantitative relationship between ZnHCF dissolution and K ⁺ concentration is obtained. Further, HRTEM and XRD confirm that the K ⁺ concentration below the current‐free CDC causes ZnHCF dissolution due to the current enrichment effect, indicating that K ⁺ concentration above CDC can trigger spontaneous ZnHCF dissolution, while K ⁺ concentration is lower than CDC, the ZnHCF layer distance is not sufficiently expanded. Based on this, the long‐term cycling performance of gradient K ⁺ concentration under gradient current density and in situ electrode stability tests quantifies the optimal K ⁺ ratio at each current density and successfully predicts the optimal K ⁺ concentration corresponding to a certain current density. Overall, this work advances general design guidelines for alkali metal cation additives in aqueous zinc ion batteries.