Monochromatic X-ray Photoelectron Spectroscopy Study of Three Different Ionic Liquids in Interaction with Lithium-Decorated Copper Surfaces

A comparative study of the chemical reactions of three different ionic liquids prepared by physical vapor deposition on a Li/Cu surface was performed. In this investigation, the ionic liquids 1-butyl-1-methylpyrrolidinium bis[fluorosulfonyl]imide ([Py1,4]FSI), 1-butyl-1-methylpyrrolidinium bis[trifluoromethylsulfonyl]imide ([Py1,4]TFSI), and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([OMIm]TFSI) were characterized by monochromatic X-ray photoelectron spectroscopy after adsorption on Li/Cu. All spectra prove the formation of LiF and Li2O, which verifies the partial decomposition of the anions. This is found to be much less intense for [Py1,4]TFSI than for [Py1,4]FSI, indicating a more rapid decomposition of the FSI anion in the presence of lithium. In addition, changes of the cation structure, as well as changes of the anion/cation ratio, were observed in the spectra. In summary, cations were most likely displaced from the Li/Cu surface and were partially decomposed, especially in case of [OMIm]TFSI, due to the interaction with lithium. The results give an overview on the possible reaction products at the Li surface when using ionic liquids as electrolytes or additives for lithium batteries. Additionally, it gives an insight into the specific role of the cations and anions for the formation of the solid electrolyte interphase layer in battery systems.