Transport Phenomena of Nonaqueous Electrolyte Solutions at High Concentrations: A Comparison between the Li- and Na-Systems

In order to understand the solution chemistry of electrolyte solutions at concentrations far from infinite dilution, the solution density, viscosity and ionic conductivity of Li- and Na-TFSI dissolved in GBL and PC were measured at 0.1 ⩽ C /mol · dm − 3 ⩽ 2.0 and 278 ⩽ T /K ⩽ 328, where TFSI = bis(trifluoromethanesulfonyl)imide, GBL = γ-butyrolactone and PC = propylene carbonate. The results are compared to those of previously determined perchlorate salts. The partial molar volume of the solute, derived from the density, confirmed that the Na-systems occupy more volume in the solution than the Li-systems. On the other hand, the viscosity and ionic conductivity suggested that the Na-systems are more fluid and conductive than the Li-systems. The relative viscosity vs. the molarity follows a modified empirical Jones-Dole equation. The molar conductivity linearly decreased with respect to the cube-root of the molarity, which was analyzed by the pseudolattice model. The Raman spectroscopy revealed that, while the solvation number is comparable at 1-2 for either the Li- or Na-systems, Li + is more tightly bound to the solvent molecules than Na + . The higher fluidity and conductivity of the Na-systems than those of the Li-systems result from the less occurrence of the solvent-shared ion pairs in the former than in the latter.