Unlocking Bimetallic Activity in KCoFe(CN)₆ through Solvation Structure Reconstruction for High-Voltage Aqueous...

Although KCoFe(CN)₆ possesses an open framework and bimetallic redox activity for aqueous zinc-ion batteries, the utilization of the Co²⁺/Co³⁺ couple above 1.8 V (vs. Zn²⁺/Zn) is hindered by the competing oxygen evolution reaction. Herein, we unlock this capacity by introducing 30 vol % N,N-dimethylformamide (DMF) into a 2 M ZnSO₄ electrolyte to modulate the solvation structure. Spectroscopic and molecular dynamics analyses reveal that DMF breaks the hydrogen-bond network of water and enters the primary solvation shell of Zn²⁺. This coordination, evidenced by a reduced H₂O coordination number (from 5.18 to 4.77) and a new DMF coordination (0.29), raises the water oxidation overpotential, thereby widening the electrochemical window to 2.0 V. The expanded window allows reversible operation of both Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺ couples. Consequently, the Zn/KCoFe(CN)₆ cell delivers a specific capacity of 130 mAh g^-1 at 0.1 A g^-1, representing a 78% improvement over the cell using a conventional aqueous electrolyte (72.9 mAh g^-1). This enhancement is ascribed to the activated bimetallic redox activity, as confirmed by ex-situ XRD and XPS analyses, which demonstrate reversible Zn²⁺ (de)intercalation. Our work provides a practical electrolyte design strategy to access high-voltage capacity in Prussian blue analogues for aqueous zinc-ion batteries.