Ramifications of Water-in-Salt Interfacial Structure at Charged Electrodes for Electrolyte Electrochemical Stability

Development of safe aqueous batteries and supercapacitors critically relies on expanding the electrolyte electrochemical stability window. A novel mechanism responsible for widening the electrochemical stability window of water-in-salt electrolytes (WiSEs) compared to conventional salt-in-water electrolytes is suggested based on molecular dynamics (MD) simulations of the electrolyte-electrode interface. Water exclusion from the interfacial layer at the positive electrode provided additional kinetic protection that delayed the onset of the oxygen evolution reactions. The interfacial structure of a WiSE at negative electrodes near the potential of zero charge clarified why the recently discovered passivation layers formed in WiSEs are robust. The onset of water accumulation at potentials below 1.5 V vs Li/Li⁺ leads to formation of water-rich nanodomains at the negative electrode, limiting the robustness of the WiSE. Unexpectedly, the bis(trifluoromethanesulfonyl)imide anion adsorbed and trifluoromethanesulfonate desorbed with positive electrode polarization, demonstrating selective anion partitioning in the double layer.