Li+ cation environment, transport, and mechanical properties of the LiTFSI doped N-methyl-N-alkylpyrrolidinium +TFSI- ionic liquids

Molecular dynamics ( MD) simulations have been performed on N-methyl-N-propylpyrrolidinium bis( trifluoromethanesulfonyl) imide (mppy(+)TFSI(-)) and N,N-dimethyl-pyrrolidinium bis( trifluoromethanesulfonyl)imide (mmpy(+)TFSI(+)) ionic liquids (ILs) doped with 0.25 mol fraction LiTFSI salt at 303-500 K. 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. MD simulations reveal that the Li+ environment is similar in mppy(+)TFSI(-) and mmpy(+)TFSI(+) ILs doped with LiTFSI. 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. Significant lithium aggregation by sharing up to three TFSI- anions bridging two lithiums was observed, 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 coordination shell with a smaller (similar to 30%) contribution also due to Li+ cations diffusing together with their first coordination shell. In both ILs, ion self-diffusion coefficients followed the order Li+ < TFSI- < mmpy(+) or mppy(+) with all ion diffusion in mmpy(+) TFSI- being systematically slower than that in mppy(+) TFSI-. Conductivity due to the Li+ cation in LiTFSI doped mppy(+) TFSI-IL was found to be greater than that for a model poly( ethylene oxide)(PEO)/LiTFSI polymer electrolyte but significantly lower than that for an ethylene carbonate/LiTFSI liquid electrolyte. Finally, the time-dependent shear modulus for the LiTFSI doped ILs was found to be similar to that for a model poly(ethylene oxide)(PEO)/LiTFSI polymer electrolyte on the subnanosecond time scale.