In order to obtain atomistic insights into the initial stages of the formation of the solid electrolyte interphase (SEI) in Na ion or Na metal batteries, we employ surface chemistry experiments and DFT calculations to study the interactions and reactions between a Na surface and the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI), a candidate to be used as electrolyte in batteries. Oxygen-free Na thin films, which were grown on Ru(0001) and characterized by X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), can be understood as model of a Na-rich electrode. After deposition of submonolayer to multilayer BMP-TFSI films on the Na thin films at room temperature, XPS measurements revealed partial decomposition and the formation of a 'contact layer' at the Na surface, consisting of mainly TFSI-based decomposition products. By comparison to core level binding energies obtained from density functional theory calculations for energetically feasible reaction products, the constituents of the 'contact layer' were identified both as atomic fragments of TFSI (F, O, S) and as larger fragments of TFSI (NSO2CF3, NSO2CF3SO2), presumably remaining at the surface due to kinetic barriers. Increasing the temperature results in cumulative decomposition towards the stable atomic species at or within the Na surface. Reveal SEI formation: X-ray photoelectron spectroscopy and density functional theory calculations are employed to study the initial stages of the solid electrolyte interphase formation in Na (ion) batteries. Reactive interactions between the ionic liquid BMP-TFSI and a Na thin film model electrode lead to the formation of both larger TFSI-based fragments and atomic (F, O, S) adsorbates at the Na surface.image
