Metal electrode potential diverges with ion additions

The electrode redox potential, which varies substantially with the electrolyte, governs both the direction and thermodynamic driving force of electrochemical reactions. However, a comprehensive understanding and effective control of redox potential shifts remain unestablished. Here we report the pronounced effect of coordination-shell ions on the redox potential of metal electrodes. Cooperative interactions between the metal ion and surrounding anions and cations with large differences in hardness/softness amplify the redox potential shift, as corroborated by liquid Madelung potential. Specifically, in the Zn/Zn2+ system, the addition of soft anions paired with hard cations destabilizes the Zn2+ solution structure and upshifts the redox potential, whereas hard anions with soft cations stabilize the structure and induce a downshift. Following this principle, a potential gap exceeding 0.6 V was achieved—a remarkably large but reasonable value for a divalent system. This ion hardness/softness-based strategy also demonstrates practical benefits in Zn plating/stripping tests, where electrolytes with upshifted redox potentials enable an average Coulombic efficiency exceeding 99.9%. The role of ion coordination environments in influencing metal-ion redox potentials has remained elusive for electrochemical systems. Now it has been shown that the hard/soft character of coordinating ions shapes the electrostatic potential of metal ions, enabling substantial shifts in redox potential and offering a handle for rational electrolyte design.