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 (C2) conformation and two oxygen atoms of the other anion in the cis (C1) 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.