Water activity: the key to unlocking high-voltage aqueous electrolytes?

Aqueous electrolytes offer enhanced safety and environmental friendliness for next-generation energy storage systems, but their application is limited by a narrow electrochemical stability window. This study provides a comprehensive analysis of the relationship between water activity and the electrochemical stability window of aqueous electrolytes, critically examining current expansion strategies. Our investigation reveals that stability window expansion is primarily driven by kinetic factors rather than thermodynamic ones. We demonstrate that decreasing water activity predominantly affects the oxygen evolution reaction, with minimal impact on hydrogen evolution. This asymmetric effect is quantified through Tafel analysis, showing a significant decrease in exchange current density with reduced water activity. Notably, this study is the first to establish a direct correlation between water activity and the electrochemical stability window for aqueous electrolytes, providing fundamental insights into how water activity influences electrode reaction kinetics and overall system stability. We critically evaluate existing approaches to reducing water activity, including high-concentration electrolytes, water-in-salt systems, and hydrophobic ions. While these methods widen the electrochemical window, they lead to decreased ionic conductivity and increased viscosity. In "water-in-salt" electrolytes, conductivity drops to levels comparable to organic electrolytes, while viscosity increases exponentially. This work challenges the focus on maximizing stability windows at the expense of other crucial properties. We argue for a balanced approach in aqueous electrolyte design, considering factors such as ionic mobility, salt solubility, viscosity, and operational temperature range alongside electrochemical stability.