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 Sad, Fad and TiNx. 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 Ti(3+) 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, Ti(3+) 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 Ti(3+) 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 Ti(3+) species, which gets completed at higher temperatures.
Highlights
Employing XPS and STM measurements for chemical and structural characterization, we have investigated the interaction of [BMP][TFSA] with a well-defined TiO2(110) surface, and the influence of Li thereon
The results of the present study in combination with previous results lead us to the following conclusions: (1) In the temperature range between 80 K and 380 K, [BMP][TFSA] adsorbs as intact ion pairs on the substrate, with both anion and cation in direct contact with the surface
At 80 K, the [BMP][TFSA] ion pairs are adsorbed in lines along the TiO2(110) (1 Â 1) [001] direction and uniformly cover the surface
Summary
Despite extensive literature on the physics and chemistry of TiO2 surfaces,[17,18] very little is known on the interaction of Li with these surfaces. 160 K the Li2s signal in the MIES spectra was found to vanish, pointing to Li insertion into the substrate.[20] In contrast, employing STM, Tatsumi et al reported the observation of small protrusions on a TiO2(110) (1 Â 1) surface upon Li adsorption at room temperature, which they attributed to adsorbed Li.[21] In an electrochemical environment, Li intercalation was found to take place in TiO2 substrates (rutile as well as anatase) at room temperature.[22,23,24] For rutile this intercalation was limited by slow diffusion of the Li into the bulk crystal.[25,26] Based on these findings, lithium intercalation could be a competing process in addition to the expected reaction with the adsorbed IL layer. Former studies of [OMIM][TFSA] in contact with bulk lithium[27] and of co-deposited Li and a [BMP][TFSA] adlayer on Cu(111)[28] revealed a decomposition reaction of the respective IL adlayer on the Li substrate or induced by coadsorbed Li, and the formation of various decomposition products such as LiF, Fad, Li2S, Sad, SOx,ad, CF3,ad, Li2O and LixCHy
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
