A comparative study of ionic conductivity, translational diffusion, molecular motion, and physicochemical properties in lithium bis(trifluoromethanesulfonyl)imide-doped 1-methyl-3-pentyl- and 1,2-dimethyl-3-pentyl-substituted imidazolium-based ionic liquids

The ionic conductivity, translational diffusion, molecular motion, and physicochemical properties of ionic liquids (ILs) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-doped ILs with two different imidazolium cations (1-methyl-3-pentylimidazolium [MPI]⁺ and 1,2-dimethyl-3-pentyl-imidazolium [DMPI]⁺) are characterized. Self-diffusion coefficients D for the anion and the cation are measured by pulsed field gradient spin echo NMR (PGSE-NMR), the measured ion diffusion coefficients, viscosities, and ionic conductivity follow the Vogel-Tammann-Fulcher (VTF) equation for the temperature dependencies, and the best-fit parameters are determined. LiTFSI-doped [MPI][TFSI] shows a NOE signal between Li⁺ and MPI⁺ and a broad Li peak was observed in HOESY spectra, whereas there is no NOE signal in LiTFSI-doped [DMPI][TFSI], demonstrating that one additional methyl group at the C-2 position in the DMPI+ may prevent Li⁺ to be closed to DMPI⁺ via steric hindrance.