Structure, Energies, and Vibrational Frequencies of Solvated Li⁺ in Ionic Liquids: Role of Cation Type

This study examines the structure of five ionic liquids all of them containing bis[(trifluoromethyl)sulfonyl]imide (TFSI) as the anion with five different cations: Dimethylammonium, N-propylammonium, N-methyl-1-propylpiperidinium, N-methyl-3-methylpyridinium, and N-methylpyrrolidinium. This study is based on quantum chemical calculations of structure, energetics, and vibrational spectroscopy associated with solutions of Li⁺ in the five ionic liquids examined. We have shown that the Li-TFSI ion-pair stabilization is 2.5-4 fold larger than those of the ion pairs of five cations with TFSI. A large number of different species containing Li_kTFSI_nCt_m (Ct represent one of five cations studied, k, n, m = 0-2) were examined in detail. The results suggest that Li-(TFSI)₂ is a highly stable species and may be responsible for the transport of Li ions in these ionic liquids. The vibrational analysis suggests that the high stability of the Li-TFSI ion pair is mainly due to Coulomb interaction between the Li ion and two oxygen atoms bound to the two sulfur atoms in the TFSI anion. This O-Li-O bond exhibits stretching and bending modes that may allow monitoring of these ion pairs.