Spectroscopic characterization of the interaction of lithium with thin films of the ionic liquid 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide

Abstract

In this experimental investigation the interaction of lithium with 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([OMIm]Tf₂N) is shown. For this purpose thin films of lithium and [OMIm]Tf₂N were successively vapor deposited on a copper substrate and analyzed by X-ray Photoelectron Spectroscopy (XPS) as well as by Ultraviolet Photoelectron Spectroscopy (UPS). When [OMIm]Tf₂N is evaporated on top of a thin lithium film a chemical shift analysis of XPS spectra shows a variety of reaction products like LiF, Li₂O and Li_xCH_y which reveals the instability of the IL against lithium. Time resolved XPS spectra were discussed to distinguish cation reactions from beam damage effects. In a second step lithium is deposited on a [OMIm]Tf₂N layer. The XPS spectra are in agreement with the results of the previous step, but show some differences concerning the [OMIm] cation. In a third step [OMIm]Tf₂N has been deposited on a passivated lithium layer. XPS results show nearly unaffected [Tf₂N]⁻ anions and partially decomposed [OMIm]⁺ cations. Interestingly the cation reactions show similarities when compared to the interaction of [C₄C₁Pyrr]Tf₂N (1-butyl-1-methylpyrrolidinium bis[trifluoromethylsulfonyl]amide) and lithium.