Polarization-Induced Breaching of the Liquid/Liquid Interface Formed with Water-in-Salt Electrolytes

Abstract

The solvation properties of water-in-salt electrolytes (WiSEs) have been extensively studied by spectroscopic and computational means and were shown to impart them with unique chemical and physical properties when compared to more classical superconcentrated aqueous solutions. More specifically, the formation of ionic aggregates in solutions containing a large concentration of TFSI anions was shown to alter the water and anion reactivity at electrochemical interfaces, often improving the performance of aqueous rechargeable batteries. However, insights into the role of the WiSE solvation structure on ion transfer at electrochemical interfaces are scarce. Herein, interfaces between two immiscible electrolytes (ITIESs) are used to study the energetics for ion transfer between aqueous LiCl and LiTFSI solutions and dichloroethane. Combining electrochemical measurements at microinterfaces with metadynamics molecular dynamics (MD) simulations, the effect of solvation properties on the energy for transferring Li⁺ and Cl^–/TFSI^– ions across the liquid/liquid interface was studied. While increasing the LiCl concentration increases the amount of ion pairs, it only marginally impacts the ion transfer energy. Instead, using large LiTFSI concentrations at which ionic aggregates are formed, ion transfer across the liquid/liquid interface shows a unique behavior that departs from that observed for polarizable or nonpolarizable interfaces. Ions do not freely cross the interface, with a transfer energy found to be ≈8–10 kcal/mol. However, upon polarization, ionic aggregates are found to breach the liquid/liquid interface, locally mixing both solutions. We believe that such a finding calls for reevaluating our current understanding of ion transfer across chemical interfaces in superconcentrated electrolytes, including liquid/liquid interfaces used in membrane-less electrochemical systems.