Interactions between Lithium, an Ionic Liquid, and Si(111) Surfaces Studied by X-ray Photoelectron Spectroscopy

Investigations of the solid-electrolyte interphase formation on a silicon anode are of great interest for future lithium-ion batteries. We have studied the interactions of the ionic liquid 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([OMIm]Tf2N) and of lithium with Si(111) surfaces on a molecular level by X-ray photoelectron spectroscopy. The interaction of Li with [OMIm]Tf2N on Si(111) results in the decomposition of both the cation and the anion and the intercalation of lithium. Lithium atoms donate the electrons to the [OMIm](+) cation, forming Li+, and at the same time the alkyl group is detached from the cation. Excessive Li could decompose the imidazolium ring, resulting in CxHy and LiCxHyNz species and interact with the Tf2N- anions, forming LiF, LixO, F3C-O2S N-Li+, and F3C-O2S-Li+ species. The formation of a stable Si/IL interface and of Si/Li surface alloys was proved to be an effective strategy in stabilizing Li for next-generation Li-ion batteries.