Long-Range Interactions in Salt-in-Ionic Liquids

Ionic liquids (ILs) are a promising class of electrolytes owing to a unique combination of properties, such as extremely low vapour pressures, non-flammability and being universal solvents. Doping ILs with alkali metal salts creates an electrolyte that is of interest for batteries, among others. These salt-in-ionic liquids (SiILs) are a class of super-concentrated, strongly correlated and asymmetric electrolytes. The transference number of the alkali metal cations has been found to be negative, owing to the small but highly negatively charged aggregates which form between alkali metal ions and the anions. Here, we investigate Na-based SiILs with a surface forces apparatus and by atomic force microscopy. We find evidence of confinement induced structural changes, giving rise to unprecedented long-range (non-exponentially decaying) interactions. This observation is supported by the soft structure revealed by the force curves, and supplemented by theory and simulations. The long-ranged interactions in SiILs are reminiscent of polymer-like interactions, suggesting analogous high aspect ratio aggregates at the mica interfaces, rather than a purely electrostatic origin. Remarkably, our aggregation framework and conclusions can also explain the negative transference number, often observed in these systems by the battery community.