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.
We demonstrate reversible and switchable actuation using AC electric fields to bring two surfaces separated by a thin film of ionic fluid in and out of adhesive contact. Using a Surface Force Balance we apply electric fields normal to a crossedcylinder contact and measure directly the adhesive force and surface separation with sub-molecular resolution. Taking advantage of the oscillatory structural force acting between the surfaces across the fluid, which we show to be unaffected by the AC field, we pick between the distinct (quantized) adhesive states through precise tuning of the field. This proof-of-concept indicates exquisite control of surface interactions using an external field.
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