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 C_xH_y and LiC_xH_yN_z species and interact with the Tf₂N^- anions, forming LiF, Li_xO, 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.