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Abstract The electrochemical stability window of water is known to vary with the type and concentration of dissolved salts. However, the underlying influence of ions on the thermodynamic stability of aqueous solutions has not been fully understood. Here, we investigated the electrolytic behaviors of aqueous electrolytes as a function of different ions. Our findings indicate that ions with high ionic potentials, i.e., charge density, promote the formation of their respective hydration structures, enhancing electrolytic reactions via an inductive effect, particularly for small cations. Conversely, ions with lower ionic potentials increase the proportion of free water molecules—those not engaged in hydration shells or hydrogen‐bonding networks—leading to greater electrolytic stability. Furthermore, we observe that the chemical environment created by bulky ions with lower ionic potentials impedes electrolytic reactions by frustrating the solvation of protons and hydroxide ions, the products of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. We found that the solvation of protons plays a more substantial role than that of hydroxide, which explains a greater shift for OER than for HER, a puzzle that cannot be rationalized by the notion of varying O−H bond strengths of water. These insights will help the design of aqueous systems. 

Abstract It remains a challenge to design aqueous electrolytes to secure the complete reversibility of zinc metal anodes. The concentrated water‐in‐salt electrolytes, e.g., 30 m ZnCl₂, are promising candidates to address the challenges of the Zn metal anode. However, the pure 30 m ZnCl₂electrolyte fails to deliver a smooth surface morphology and a practically relevant Coulombic efficiency. Herein, it is reported that a small concentration of vanillin, 5 mg mL_water⁻¹, added to 30 m ZnCl₂transforms the reversibility of Zn metal anode by eliminating dendrites, lowering the Hammett acidity, and forming an effective solid electrolyte interphase. The presence of vanillin in the electrolyte enables the Zn metal anode to exhibit a high Coulombic efficiency of 99.34% at a low current density of 0.2 mA cm⁻², at which the impacts of the hydrogen evolution reaction are allowed to play out. Using this new electrolyte, a full cell Zn metal battery with an anode/cathode capacity (N/P) ratio of 2:1 demonstrates no capacity fading over 800 cycles.