Complementary Operando Investigations of Graphite Cycled in Ionic Liquids By XPS and Raman Spectroscopy

Stable electrochemical cycling of graphite in ionic liquid electrolytes has recently been achieved in  presence of bis(fluorosulfonyl)imide ([FSI]^-) [1]. To get a better understanding of how FSI enables stable cycling, Raman spectroscopy (Raman) [2] and x-ray photoelectron spectroscopy (XPS) [3] were employed as two complementary operando techniques to monitor changes in the bulk structure and the chemical surface composition of graphite particle and HOPG model electrodes, respectively. These electrodes were cycled in 1-ethyl-3-methylimidazolium / bis(trifluoromethanesulfonyl)imide ([EMIM][TFSI] ionic liquid electrolyte containing either 1 M LiTFSI or 1 M LiFSI salt. The Raman experiments show that [EMIM]⁺ strongly competes with Li⁺ for intercalation sites in graphite and that Li⁺ intercalation only takes place at a detectable level in presence of [FSI]^- (Figure 1.a). XPS at the IL/electrode interface suggests that both [TFSI]^- and [EMIM]⁺ undergo reductive decomposition reactions resulting in the formation of a solid surface layer (Figure 1.b). A model describing the mechanism behind the cycling improvement in presence of FSI^- will be presented based on these results.

[1] M. Ishikawa, et al, Journal of Power Sources 162, 658-662 (2006)

[2] P. Lanz, P. Novák, J. Electrochem. Soc. 161, 10, A1555-A1563 (2014)

[3] D. Weingarth, A. Foelske-Schmitz, A. Wokaun, R. Kötz, Electrochem. Commun. 11, 619-622 (2011)