Tailoring K+ Dosage in K+/Zn2+ Mixed Electrolytes via Lattice Expansion Regulation of Zinc Hexacyanoferrate for High-performance Zinc Ion Batteries.

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.