Transport mechanism of fluorosulfonylamide-based molten alkali metal salts—Intermediate temperature ionic liquids

Molecular dynamics (MD) simulations in this study elucidated the transport mechanism of a series of intermediate temperature ionic liquids: molten MFSA, MFTA, and MTFSA, where M = (Li, Na, K, Rb, and Cs), FSA = bis(fluorosulfonyl)amide, FTA = fluorosulfonyl(trifluoromethylsulfonyl)amide, and TFSA = bis(trifluoromethylsulfonyl)amide. The following two peculiarities had been experimentally observed: (i) the electrical conductivity, σ, of Li-systems is extremely lower than that of the other alkali metal counterparts and (ii) the Nernst-Einstein conductivity, σNE, derived from the self-diffusion coefficients, D+ and D-, of LiFSA and LiFTA is lower than the real conductivity, σ > σNE, which is usually the other way around. Hypothetical MD simulations made by increasing the size or decreasing the valence of Li+ revealed that the strong interaction between neighboring cation and anion caused by the high surface charge density on Li+ is responsible for both (i) and (ii). Theoretical consequences derived from the momentum conservation and the separation of σ into its components in terms of velocity correlation coefficients proved that, in addition to these features, the significant mass difference between a cation and anion for the Li-systems leads to (iii) the almost exclusive contribution of Li+ to σ and (iv) a positive contribution of the Li+-Li+ cross correlation, which is negative for other systems. Hypothetical simulations at high temperatures, at which the anions actually decompose, suggested that features (i), (ii), and (iv) stem from the "intermediate" temperature range at which these salts are fluid.