Investigation of the Electrode/Ionic Liquid Interphase: Chemical Reactions of an Ionic Liquid and a Lithium Salt with Lithiated Graphite Probed by X-ray Photoelectron Spectroscopy

The solid electrolyte interphase (SEI) plays an important role in the performance of a Li-ion battery. A detailed insight into the chemical reactions between the electrolyte and lithium at the electrode/electrolyte interphase is thus of great interest. In the present paper, two different sequences of stepwise nanometer-thick depositions of the ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([EMIm]FSI) and of lithium bis(fluorosulfonyl)imide (LiFSI) on Li-intercalated highly oriented pyrolytic graphite were employed to examine the decomposition products of [EMIm]⁺ cations and FSI^- anions and their dependence on layer thicknesses and annealing temperature by X-ray photoelectron spectroscopy. The results indicate that the interaction of an IL adlayer with lithiated graphite leads to a stable passivation layer, which inhibits the deintercalation of Li. The passivation layer is composed of the decomposed products from both the [EMIm]⁺ cation and the FSI^- anion, such as methyl-, ethyl-imidazole, LiF, Li₂O, LiNSO₂, and LiSO₂F. Our work can provide valuable information about designing a suitable electrolyte to get a stable SEI layer for battery applications.