New Interpretation of X-ray Photoelectron Spectroscopy of Imidazolium Ionic Liquid Electrolytes Based on Ionic Transport Analyses

We reported a new perspective on the correlation between the electronic structure of an ionic liquid (IL)-based electrolyte probed by X-ray photoelectron spectroscopy and the transport properties analyzed by impedance spectroscopy. We highlighted the core level chemical shifts of 1-hexyl-3-methylimidazolium (bis(trifluoromethanesulfonyl)imide) (C(1)C(6)ImTFSI), 1-hexyl-3-methyl-imidazolium bis(fluorosulfonyl)imide (C(1)C(6)ImFSI), and 1-hexyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide (C(1)C(1)C(6)ImTFSI) laden with LiTF-SI salt and vinylene carbonate (VC) or fluoroethylene carbonate (FEC) with regard to the transport properties of cations and anions. We pointed out based on detailed binding energy shift analyses a clear effect of the anion on the local organization of Li+ ions. The significant peak shift in the case of C(1)C(6)ImTFSI laden with LiTFSI corroborates the formation of [Li(TFSI)(2)](-) complexes. On the contrary, the lower amplitude of the binding energy shift of C(1)C(6)ImFSI for both anion- and cation-related peaks indicates that the electronic distribution around the cation and the anion is not affected when the LiTFSI salt is added, which plays a strong role in the ion dynamics (lower viscosity) of the electrolyte. The X-ray photoelectron spectroscopy (XPS) result supports the preponderant role of imidazolium ionic liquid based on FSI anion to form an electrolyte less prone to form ionic complexes. The methylation of the imidazolium cation contributes to the reduction of the interaction between the C(1)C(1)C(6)Im cation and TFSI anion, while additives VC and FEC contribute to the change of the alkyl configuration in C(1)C(6)Im cation, leading to the modification of the macroscopic properties of the ILs.