ABSTRACT
The effect of water on the hydrophobic ionic liquid (IL) 1-n-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonylimide) and its Li(+) mixture was evaluated. The electrochemical stability, density, viscosity, and ionic conductivity were measured for both systems in different concentrations of water. The presence of Li(+) causes a large increase in the water absorption ability of the IL. The experimental results suggest a break of the interactions between Li(+) and Tf2N(-) anions in the strong aggregates formed in dried Li(+) mixtures, modifying the size and physicochemical nature of these aggregates. It is also observed that the size of the ions aggregates with formal charge increases at high temperature and decreases the mobility of the charge carrier, explaining the break in the Walden rules at high temperature. Raman spectroscopy and molecular dynamic simulations show the structural change of these systems. In neat ILs, the water molecules interact mainly among each other, while in the Li(+) mixtures, water interacts preferentially with the metallic cation, causing an important change in the aggregates present in this system.
ABSTRACT
The addition of lithium salts to ionic liquids causes an increase in viscosity and a decrease in ionic mobility that hinders their possible application as an alternative solvent in lithium ion batteries. Optically heterodyne-detected optical Kerr effect spectroscopy was used to study the change in dynamics, principally orientational relaxation, caused by the addition of lithium bis(trifluoromethylsulfonyl)imide to the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Over the time scales studied (1 ps-16 ns) for the pure ionic liquid, two temperature-independent power laws were observed: the intermediate power law (1 ps to approximately 1 ns), followed by the von Schweidler power law. The von Schweidler power law is followed by the final complete exponential relaxation, which is highly sensitive to temperature. The lithium salt concentration, however, was found to affect both power laws, and a discontinuity could be found in the trend observed for the intermediate power law when the concentration (mole fraction) of lithium salt is close to chi(LiTf(2)N) = 0.2. A mode coupling theory (MCT) schematic model was also used to fit the data for both the pure ionic liquid and the different salt concentration mixtures. It was found that dynamics in both types of liquids are described very well by MCT.