Surface chemistry and electrochemistry of an ionic liquid and lithium on Li₄Ti₅O₁₂(111)-A model study of the...

Aiming at a detailed molecular understanding of the initial stage of the solid|electrolyte interphase (SEI) formation in Li-ion batteries, we have investigated the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) (solvent/electrolyte) and Li (Li⁺ ion shuttle) on well-defined Li-poor Li₄Ti₅O₁₂(111) and Li-rich Li_4.3Ti₅O₁₂(111) surfaces/electrodes in a combined surface science and electrochemical model study. X-ray photoelectron spectroscopy (XPS) measurements reveal that postdeposition of Li⁰ under ultrahigh vacuum (UHV) conditions on a Li-poor Li₄Ti₅O₁₂(111) surface precovered with a molecularly adsorbed [BMP][TFSI] adlayer leads to little IL decomposition at 80 and 300 K. We assume that most of the Li diffuses through the IL adlayer and rapidly inserts into the Li₄Ti₅O₁₂(111) bulk. More pronounced IL decomposition was obtained upon IL deposition on a Li-rich Li_4.3Ti₅O₁₂ phase at 80 K and subsequent heating to 300 K. Cyclic voltammograms (CVs) recorded on the Li₄Ti₅O₁₂(111) electrodes in Li-TFSI/[BMP][TFSI] indicate an almost reversible Li (de-)insertion, with a slight decay of the amount of (de-)inserted Li with increasing cycle number. XPS measurements performed on the electrode after potential cycling show low intensity signals of IL decomposition products, in addition to dominant signals from residual IL electrolyte, which are related to reaction of the adsorbed IL with Li inserted into/extracted from Li₄Ti₅O₁₂ during the CV. The results indicate a close similarity between IL decomposition products formed under UHV and under electrochemical conditions, underlining the validity of this experimental approach and the potential of such kind of model studies for obtaining detailed understanding of the SEI formation.