Employing density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS), we identify products of the reaction of the ionic liquid N,N-butylmethylpyrrolidinum bis(trifluoromethylsulfonyl)imide (BMP-TFSI) with lithium in order to model the initial chemical processes contributing to the formation of the solid electrolyte interphase in batteries. Besides lithium oxide, sulfide, carbide and fluoride, we find lithium cyanide or cyanamide as possible, thermodynamically stable product in the Li-poor regime, whilst Li$_{3}$N is the stable product in the Li-rich regime. The thermodynamically controlled reaction products as well as larger fragments of TFSI persisting due to kinetic barriers could be identified by a comparison of experimentally and computationally determined core level binding energies.