Interfacial structure and electrochemical behavior of a ternary ionic liquid-based electrolyte with water and lithium salt: A computational analysis

This study investigates the interfacial structure and behavior of a ternary electrolyte composed of the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate, water, and lithium tetrafluoroborate for different water contents and voltage drops across planar electrodes. Polarizable molecular dynamics simulations were performed using explicit graphene monolayers as electrodes, with the image charge method ensuring constant potential boundary conditions. Structural and electrochemical properties, including density profiles, orientational order, charge distribution, coordination, and differential capacitance, were analyzed to identify key factors influencing electrolyte performance. A complementary computational scheme enabled direct evaluation of charge density and atomic polarization responses at the electrodes. Results show that small water additions minimally affect ionic organization, but at higher concentrations and voltages, free water accumulates near the negative electrode, destabilizing the interface while promoting lithium migration. Although moderate water levels can enhance differential capacitance and potentially improve battery performance, concentrations above 10% reduce electrochemical stability. Finally, the analysis of the charge response at the electrodes reveals a loss of short-range correlation in the surface charge on the negatively charged electrode as both the applied voltage and water content increase. Furthermore, this loss of correlation could serve as a direct indicator of water presence in these kinds of devices.