Spectroscopic Identification of the Lithium Ion Transporting Species in LiTFSI-Doped Ionic Liquids

The solvation of the lithium ion in LiTFSI-doped ionic liquids based on alkyl-substituted imidazolium cations and bis(trifluoromethanesulfonyl)imide anions (TFSI-) was investigated by infrared and Raman spectroscopies. The spectral changes occurring for some TFSI- vibrations sensitive to the lithium coordination were analyzed with the help of DFT calculations. In addition, the vibrations of the lithium ion in its solvating cage were found to produce a broad IR absorption band centered at 374 cm(-1). For low to moderate LiTFSI mole fractions, 0.08 < x < 0.2, the [Li(TFSI)(2)](-) solvating cage was found to involve bidentate coordinations of Li+ with two oxygen atoms of one anion in the trans (C-2) conformation and two oxygen atoms of the other anion in the cis (C-1) conformation. At higher LiTFSI concentration, up to x = 0.5, the lithium ion-TFSI coordination number progressively becomes less than 2, indicating the possible formation of aggregates.