The Electrical Double Layer of Water-in-Salt Electrolytes: the Distribution of Ionic Associations and Hydration

Water-in-Salt electrolyte (WiSEs) are an exciting class of super-concentrated electrolytes of interest in energy storage application because of their promising transport properties and extended electrochemical stability window (ESW). At the 21m concentration, the ESW is expanded because there is very little free water at the interface, as most of it is hydrating the alkali metal cations, and the transport of the active cations remain facile because of the anion-dominated percolating ionic network which is interpenetrated by cation-rich water domains. In the bulk, these properties have been understood through molecular dynamics (MD) simulations, where the ionic associations and hydration effects have been quantified. This has been further rationalised within a polymer-inspired statistical mechanical theory, developed by McEldrew et al. [1]. However, until recently, predictions of how the ionic associations and hydration in WiSEs change within the electrical double layer (EDL) has received little attention. Here we perform atomistic MD simulation of the EDL of WiSEs at various concentrations, and use a graph-based analysis method to understand how the ionic associations and hydration change as a function of distance from the electrode. Furthermore, we present a simple theory to rationalise and predict the changes of ionic associations and hydration effects in the EDL of WiSEs. This theory has essentially no free fitting parameters, as the parameters of the theory can be determined from MD simulations. The results for the obtained differential capacitance are compared against experimental measurements. Overall, this methodology allows the changes in hydration and ionic associations within the EDL to be understood, which is essential for rationalising the reactions which occur at electrified interfaces that form the solid electrolyte interphase.



[1] McEldrew et al. Ion Clusters and Networks in Water-in-Salt Electrolytes Journal of The Electrochemical Society 168, 050514 (2021)