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, <i>N</i>-propylammonium, <i>N</i>-methyl-1-propylpiperidinium, <i>N</i>-methyl-3-methylpyridinium, and <i>N</i>-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_<i>k</i>TFSI_<i>n</i>Ct_<i>m</i> (Ct represent one of five cations studied, <i>k</i>, <i>n</i>, <i>m</i> = 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