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, 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 postdeposition of Li and subsequent annealing has been investigated using a combined experimental and theoretical approach. For comparison, also the reaction between a bulk-like multilayer BMP-TFSI film and postdeposited Li is investigated. Employing X-Ray photoelectron spectroscopy and density functional theory-based calculations, it is found that postdeposition 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, and 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 >500–650 K, depending on the anion. ... mehrConsequences of these results for the molecular-scale understanding of the initial stages of SEI formation in an electrochemical environment are discussed.