(28h) Structure and Transport in Pyrrolidinium- and Imidazolium- Tfsi Ionic Liquids Doped with Litfsi

We present structure and dynamics of pure and doped with
0.25 mol LiTFSI N-methyl-N-propyl pyrrolidinium⁺TFSI^- (mppy⁺TFSI^-)
and ethyl-methyl-imidazolium⁺TFSI^- ionic liquids. The
liquid density, ion self-diffusion coefficients and conductivity predicted by
MD simulations were found to be in good agreement with experimental data, where
available. The Li⁺ cations were found to be coordinated on average
by slightly less than four oxygen atoms with each oxygen atom being contributed
by a different TFSI- anion in both imidazolium and pyrrolidinum-based ionic
liquids.  Significant lithium aggregation through up to three TFSI^-
anions bridging lithiums was observed (Li⁺?(O=S=O)_m?Li⁺),
m=1,2,3, particularly at lower temperatures where the lithium aggregates were
found to be stable for tens of nanoseconds. Polarization of TFSI^-
anions is largely responsible for the formation of such lithium aggregates.  Li⁺
transport was found to occur primarily by exchange of TFSI^- anions
in the first solvation shell with a smaller (~30%) contribution also due to Li⁺
cations diffusing together with their first solvation shell. Conductivity contribution
due to the Li⁺ cation in LiTFSI doped mppy⁺TFSI^-
ionic liquids was found to be greater than that for a model poly(ethylene
oxide)(PEO)/LiTFSI polymer electrolyte but significantly lower than that for
ethylene carbonate/LiTFSI liquid electrolyte.   Finally, the time-dependent
shear modulus for the ionic liquids is compared with that for polymers and
nonionic liquids