Cation-Dependent Solvation Behavior of Aqueous Triflate Electrolytes

We investigated trends in the cation-dependent solvation behavior and electrochemical stability of aqueous electrolytes with ten trifluoromethanesulfonate (OTf) salts (Li+, Na+, K+, NH4+, NMe3Et+, Mg2+, Zn2+, Ca2+, Ba2+, and Al3+). Our findings show that electrochemical stability is predicted well by the pH of the electrolyte with kinetic limitations associated with concentration against hydrogen evolution playing an insignificant role when the solvent-to-anion ratio exceeds the coordination number. Furthermore, the abundance of contact ion pairs is generally inversely related to the charge density of the cation, except when the cation and anion have similar solvation enthalpies (e.g., Na+ and OTf-). These trends in cation-dependent solvation, solubility, proton activity, and electrochemical stability provide helpful guidance regarding the design of electrolytes for beyond-lithium energy storage technologies.