Reactive Interaction of (Sub-)monolayers and Multilayers of the Ionic Liquid 1-Butyl-1-methylpyrrolidinium Bis(trifluoro-methylsulfonyl)imide with Coadsorbed Lithium on Cu(111)

Abstract

The ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoro-methylsulfonyl)imide [BMP][TFSA] is a promising candidate for improved next-generation rechargeable lithium–ion batteries. We here report results of a model study of the reactive interaction of (sub-)monolayers and multilayers of [BMP][TFSA] with lithium (Li) on Cu(111), employing scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIRS) under ultrahigh vacuum (UHV) conditions. Upon post-deposition of Li on [BMP][TFSA] multilayers at 80 K, we identified changes in the chemical state of the [TFSA] anion and the [BMP] cation as well as in the IR absorption bands related to the anion. These changes are most likely due to the decomposition of the IL adlayer into a variety of products like LiF, Li2S, and Li2O upon anion decomposition and LiN3, LiCxHyN, and LixCHy upon cation decomposition, where the latter includes cracking of the pyrrolidinium ring. Deposition of Li on [BMP][TFSA] (sub-)monolayer-covered surfaces led to similar decomposition patterns, and the same was also observed for the reverse deposition order. The addition of the corresponding amounts of Li to a [BMP][TFSA] adlayer resulted in distinct changes in the STM images, which must be due to the surface reaction. After annealing to 300 K, the core-level peaks of the cation lose most of their peak area. Upon further heating to 450 K, the anion is nearly completely decomposed, resulting in LiF and Li2S decomposition products that dominate the interface.