Abstract
Water-in-salt electrolytes are a fascinating new class of highly concentrated aqueous solutions with wide electrochemical stability windows that make them viable as aqueous battery electrolytes. However, the high ion concentration of water-in-salt electrolytes means that these systems are poorly understood when compared to more dilute electrolyte solutions. Here, we present direct surface force measurements across thin films of a water-in-salt electrolyte at several concentrations. We find that the electrolyte adopts a layered structure at charged interfaces composed of a nanostructure of hydrated cation and non-aqueous anion-rich domains. These observations will aid in the interpretation of capacitance and double layer behaviour of water-in-salt electrolytes with consequences for their use in energy storage devices. Electrolytes are ubiquitous: they form the planet's oceans, they exist within all living cells, and they are critical to the function of many modern technologies. In the dilute regime, electrolytes are well understood by considering the established ideal Debye-Hückel