Surface Science and Electrochemical Model Studies on the Interaction of Graphite and Li-containing Ionic Liquid.

We have systematically investigated the process of solid electrolyte interphase (SEI) formation and its chemical composition on carbon electrodes in an ionic liquid-based, Li-containing electrolyte in a combined surface science and electrochemical model study using highly oriented pyrolytic graphite (HOPG) and binder-free graphite powder electrodes (Mage(c)) as model systems. The chemical decomposition process was explored by deposition of Li on a pre-deposited multilayer film of 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl) imide ([BMP][TFSI]) under ultrahigh vacuum conditions. Electrochemical SEI formation was induced by and monitored during potential cycling in [BMP][TFSI] + 0.1 M LiTFSI. The chemical composition of the resulting layers was characterized by X-ray photoelectron spectroscopy (XPS), both at the surface and also in deeper layers, closer to the electrode|SEI interface, after partial removal of the film by Ar + ion sputtering. Clear differences between chemical and electrochemical SEI formation, and also between SEI formation on HOPG and Mage(c) electrodes, are observed and discussed.