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

Although KCoFe(CN) 6 possesses an open framework and bimetallic redox activity for aqueous zinc-ion batteries, the utilization of the Co 2+ /Co 3+ couple above 1.8 V (vs. Zn 2+ /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 ZnSO 4 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 Zn 2+ . This coordination, evidenced by a reduced H 2 O 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 Fe 2+ /Fe 3+ and Co 2+ /Co 3+ 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 Zn 2+ (de)intercalation. Our work provides a practical electrolyte design strategy to access high-voltage capacity in Prussian blue analogues for aqueous zinc-ion batteries.