Li+ transport in lithium sulfonylimide-oligo(ethylene oxide) ionic liquids and oligo(ethylene oxide) doped with LiTFSI

The Li+ environment and transport in an ionic liquid (IL) comprised of Li+ and an anion of bis- trifluoromethanesulfonyl) imide anion (TFSI-) tethered to oligoethylene oxide (EO) (EO12TFSI-/Li+) were determined and compared to those in a binary solution of the oligoethylene oxide with LiTFSI salt (EO12/ LiTFSI) by using molecular dynamics (MD) simulations and AC conductivity measurements. The latter revealed that the AC conductivity is 1 to 2 orders of magnitude less in the IL compared to the oligoether/salt binary electrolyte with greater differences being observed at lower temperatures. The conductivity of these electrolytes was accurately predicted by MD simulations, which were used in conjunction with a microscopic model to determine mechanisms of Li+ transport. It was discerned that structure-diffusion of the Li+ cation in the binary electrolyte (EO12/LiTFSI-) was similar to that in EO12TFSI-/Li+ IL at high temperature (> 363 K), thus, one can estimate conductivity of IL at this temperature range if one knows the structure-diffusion of Li+ in the binary electrolyte. However, the rate of structure-diffusion of Li+ in IL was found to slow more dramatically with decreasing temperature than in the binary electrolyte. Lithium motion together with EO12 solvent accounted for 90% of Li+ transport in EO12/LiTFSI-, while the Li+ motion together with the EO(12)TFSI(-)anion contributed approximately half to the total Li+ transport but did not contribute to the charge transport in IL.