Diffusion of Lithium Cation in Low-Melting Lithium Molten Salts

The self-diffusion coefficients of the lithium cation (D(Li+)) and counteranion (D(anion)) in the molten lithium amide, such as lithium bis(fluorosulfonyl)amide (Li[FSA]) and lithium fluorosulfonyl(trifluoromethylsulfonyl)amide (Li[FTA]), were measured by a pulsed-gradient spin-echo nuclear magnetic resonance method at 150 degrees C. In the relationships between viscosity and the resulting self-diffusion coefficient (the D(Li+) is 1.4 X 10-11 m(2).s(-1) and the D(anion) is 5.5 X 10(-12) m(2).s(-1) for the Li[FSA] and the D(Li+) is 1.7 X 10(-12) m(2).s(-1) and the D(anion) is 6.6 X 10(-13) m(2).s(-1) for the Li[FTA]), the Li[FTA] and Li[FSA] have a stronger intercalation between ions than the high-temperature lithium molten salts. However, their ionic conductivities are higher than those estimated by D(Li+) and D(anion); specifically, that of the Li[FTA] is three times higher. Therefore, the Li[FTA] would have special conductive behavior, which leads to the superionic nature of the Walden plot and the high rate performance against much higher viscosity in lithium secondary battery. Such unique conduction and battery behaviors using the Li[FTA] might be due to the robust and asymmetric structure of the FTA(-).