Interaction of the ionic liquid [BMP][TFSA] with rutile TiO2(110) and coadsorbed lithium

Abstract

Aiming at a fundamental understanding of the processes at the electrode|ionic liquid interface in Li ion batteries, we investigated the interaction of the ionic liquid n-butyl-n-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMP][TFSA] and of Li with a reduced rutile TiO₂(110) (1 × 1) surface as well as the interaction between [BMP][TFSA] and Li on the TiO₂(110) surface under ultrahigh vacuum (UHV) conditions by X-ray photoelectron spectroscopy and scanning tunnelling microscopy. Between 80 K and 340 K [BMP][TFSA] adsorbs molecularly on the surface and at higher temperatures decomposition is observed, resulting in products such as S_ad, F_ad and TiN_x. The decomposition pattern is compared to proposals based on theory. Small amounts of Li intercalate even at 80 K into TiO₂(110), forming Li⁺ and Ti³⁺ species. The stoichiometry in the near surface region corresponds to Li₇Ti₅O₁₂. For higher coverages in the range of several monolayers part of the Li remains on the surface, forming a Li₂O cover layer. At 300 K, Ti³⁺ species become sufficiently mobile to diffuse into the bulk. Li post-deposition on a [BMP][TFSA] covered TiO₂(110) surface at 80 K results in two competing reactions, Li intercalation and reaction with the IL, resulting in the decomposition of the IL. Upon warming up, the Ti³⁺ formed at low T is consumed by reaction with the IL adlayer and intermediate decomposition products. Post-deposition of [BMP][TFSA] (300 K) on a surface pre-covered with a Li₂O/Li₇Ti₅O₁₂ layer results in the partial reaction of [BMP][TFSA] with the Li⁺ and Ti³⁺ species, which gets completed at higher temperatures.