Lithium solvation and diffusion in the 1‐butyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquid

The Raman spectra of (1 − x )(BMITFSI), x LiTFSI ionic liquids, where 1‐butyl‐3‐methylimidazolium cation (BMI + ) and bis(trifluoromethane‐sulfonyl)imide anion (TFSI − ) are analyzed for LiTFSI mole fractions x < 0.4. As expected from previous studies on similar TFSI‐based systems, most lithium ions are shown to be coordinated within [Li(TFSI) 2 ] − anionic clusters. The variation of the self‐diffusion coefficients of the 1 H, 19 F, and 7 Li nuclei, measured by pulsed‐gradient spin‐echo NMR (PGSE‐NMR) as a function of x , can be rationalized in terms of the weighted contribution of BMI + cations, TFSI − ‘free’ anions, and [Li(TFSI) 2 ] − anionic clusters. This implies a negative transference number for lithium.