Li~+Transport in Lithium Sulfonylimide-Oligo(ethylene oxide) Ionic Liquids and Oligo(ethylene oxide) Doped with LiTFSI

摘要

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) (EO_(12)TFSI~-/Li~+) were determined and compared to those in a binary solution of the oligoethylene oxide with LiTFSI salt (EO__(12)/ 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 (EO_(12)/LiTFSI~-) was similar to that in EO_(12)TFSI~-/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 EO_(12) solvent accounted for 90% of Li~+transport in EO_(12)/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.