Thermodynamic effects of solid electrolyte interphase formation from solvation and ionic association in water-in-salt electrolytes.

Water-in-salt electrolytes (WiSEs) are a promising class of next-generation electrolytes. Unlike classical dilute electrolytes or more conventional battery electrolytes, WiSEs are characterized by their super-concentrated salt content and low water content, which give rise to an expanded electrochemical stability window (ESW). The expansion of the ESW is due in part to the formation of an inorganic solid electrolyte interphase (SEI) that passivates the anode; this principle also proves important in commercial Li-ion batteries, where graphite and Li-metal anodes operate at potentials outside the ESW of conventional carbonate electrolytes, as well as extending beyond Li-ion technologies. Solvation structure and ionic association are key descriptors for understanding the expansion of the ESW. In particular, because the reactions that lead to SEI formation, or to cathode electrolyte interphase (CEI) formation, occur at the electrode-electrolyte interface, the distribution of reactants and their solvation environments is critical. In the absence of reactions, this interfacial distribution is referred to as the electrical double layer (EDL). Here, we further develop and analyze a recently proposed thermodynamic theory of hydration and ionic association in the EDL of WiSEs. We parameterize the theory using bulk molecular dynamics simulations and benchmark it against EDL simulations, finding good qualitative agreement. Using this thermodynamic framework, we rationalize an expansion in the ESW through changes in bulk electrolyte activity, via the Nernst equation, which directly alters electrolyte stability, and through thermodynamic effects on reaction kinetics, via the concentration of reactant species in the Helmholtz layer. Overall, this formalism directly links solvation and ionic aggregation to changes in reactivity, providing key insights into the thermodynamic factors that influence SEI formation in WiSEs. The framework can also be applied to other liquid electrolytes, including conventional carbonate electrolytes, solvent-in-salt systems, and Na-ion electrolytes, to understand how solvation and ionic association influence ESW expansion and SEI formation.