Long-Range Surface Forces in Salt-in-Ionic Liquids

Ionic liquids (ILs) are a promising class of electrolytes with a unique combination of properties, such as extremely low vapor pressures and nonflammability. Doping ILs with alkali metal salts creates an electrolyte that is of interest for battery technology. These salt-in-ionic liquids (SiILs) are a class of superconcentrated, strongly correlated, and asymmetric electrolytes. Notably, the transference numbers of the alkali metal cations have been found to be negative. Here, we investigate Na-based SiILs with a surface force apparatus, X-ray scattering, and atomic force microscopy. We find evidence of confinement-induced structural changes, giving rise to long-range interactions. Force curves also reveal an electrolyte structure consistent with our predictions from theory and simulations. The long-range steric interactions in SiILs reflect the high aspect ratio of compressible aggregates at the interfaces rather than the purely electrostatic origin predicted by the classical electrolyte theory. This conclusion is supported by the reported anomalous negative transference numbers, which can be explained within the same aggregation framework. The interfacial nanostructure should impact the formation of the solid electrolyte interphase in SiILs.