Influence of Lithium Solutes on Double-Layer Structure of Ionic Liquids.

The ionic liquid-electrode interface has attracted much recent interest owing to its importance for development of energy storage devices; however, the important step of adding electro-active ions is not yet well understood at the molecular level. Using direct force measurements across confined electrolyte films, we study the effect of added lithium-ion solute on the double-layer structure of an ionic liquid electrolyte with molecular resolution. We find anionic clusters involving lithium can persist adjacent to the surfaces, and in many cases, this inhibits direct adsorption of lithium ions to the negative surface. Two apparently similar ionic liquid solvents show diverging properties, with one facilitating and the other preventing direct Li-ion adsorption onto the negative surface. The results have implications for the selection of ionic liquids as electrolytes in lithium-ion batteries.