Lithium solvation and anion-dominated domain structure in water-in-salt electrolytes

Water-in-Salt (WiS) electrolytes are an emerging class of high concentration aqueous electrolytes with large electrochemical stability windows, making them attractive as green alternatives in next-generation electrochemical energy storage devices. Recent work has highlighted the existence of water-rich and anion-rich domains in WiS electrolytes, but the extent, morphology and importance of these domains are still disputed. Here, we present neutron total scattering measurements of the archetypal WiS, lithium bis(trifluoromethanesulfonyl)imide, and use empirical potential structure refinement to match the structure of a simulated system to the experimental data for two technologically relevant concentrations, revealing ion solvation, geometric isomerism and long-range structures in unprecedented detail. Our analysis of the modelled WiS electrolyte suggests that water domains are small and isolated and points to a system dominated by percolating, anion-rich domains that assemble through the association of hydrophobic regions, extending throughout the entire system. This structural insight places restrictions on feasible transport mechanisms in WiSs and, more generally, will aid in the understanding of the structure and behaviour of WiS electrolytes, with implications for the design and manufacture of WiS-containing devices.