Understanding the origin of the extended stability window in water-in-salt electrolytes

Water-in-salt electrolytes (WISEs) offer extended voltage stability, enabling the use of high-voltage materials in aqueous energy storage systems. The solid-electrolyte interphase (SEI) formed in some WISEs plays a key role in enabling this enhanced stability by passivating the electrode surface. However, non-SEI-forming WISEs also exhibit similar stability window, despite the lack of an SEI, though the mechanism behind remains insufficiently understood. In this work, we investigate the origins of the hydrogen evolution reaction (HER) suppression across a range of electrolyte concentrations of the non-SEI-forming electrolytes: from dilute solutions to WISE, using electrochemical techniques, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations. We disentangle the contributions of water activity, local pH changes induced by HER in non-buffering neutral electrolytes, and kinetic and mass transport effects. We establish a correlation between HER activity and active surface coverage by water molecules: in WISEs sluggish HER kinetics is observed, as indicated by a sevenfold decrease in exchange current density. Additionally, we reveal that in WISE HER is significantly limited by the sluggish water transport in the double layer, further contributing to the extended experimental voltage stability window.