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.