Reaction between the Ionic Liquid BMP-TFSI and Li: Comparison between Monolayer Films and Thick Multilayer Films on Graphite (0001

Aiming at a molecular-scale understanding of the initial stages of the solid-electrolyte interphase (SEI) formation in Li ion batteries, we have investigated the chemical reaction of a monolayer of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) adsorbed on a graphite (0001) substrate during post-deposition of Li and subsequent annealing in a combined experimental and theoretical approach. For comparison, also the reaction between a bulk-like multilayer BMP-TFSI film and post-deposited Li was investigated. Employing x-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) based calculations, we find that already post-deposition of Li at room temperature leads to a significant modification of both monolayer film and bulk BMP-TFSI, including the formation of (adsorbed) molecular fragments, binary Li compounds and desorption of volatile, C- and F-containing species. The initial reaction with Li is highly exothermic and non- or little activated, products are identified by comparison of experimental XP spectra and calculated core level binding energies. Further reaction steps, leaving only binary Li compounds or comparable adsorbed species, are considerably activated and require annealing to >500 K – 650 K, depending on the anion. Consequences of these results for the molecular-scale understanding of the initial stages of SEI formation in an electrochemical environment are discussed.