Long-Range Surface\nForces in Salt-in-Ionic Liquids

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