Glass transition, viscosity, and conductivity correlations in solutions of lithium salts in PEGylated imidazolium ionic liquids

Ionic liquids (ILs) are attractive as non-volatile and non-flammable electrolytes for lithium ion batteries. In this study, ionic conductivities of the solutions of lithium salts in ILs derivatized with poly(ethylene glycol) (PEG) were correlated with the viscosities and glass transition temperatures of these solutions. The ILs studied include 1-(monomethoxy PEG)-3-methylimidazolium bistriflamide and 1-(ω-perfluoroalkyl PEG)-3-methylimidazolium bistriflamide, the properties of which were compared with 1-(monomethoxy PEG)-3-methylimidazolium iodide, 1-(ω-perfluoroalkyl PEG)-3-methylimidazolium iodide, and 1-hexyl-3-methylimidazolium iodide. Binary solutions of lithium salts, lithium bistriflamide or lithium iodide, in ILs with common anions, were characterized using electrochemical impedance spectroscopy, cone and plate viscometry, and differential scanning calorimetry. Although the dissolution of the lithium salt would increase the concentration of charge-transporting ions in the solution, a decrease in conductivity was observed. The decrease was related to an increase in the glass transition temperature and an increase in viscosity. It was found that for a particular IL and lithium salt combination, the variation of viscosity or ionic conductivity with temperature could be characterized by the Vogel–Fulcher–Tammann expression, exp{A+BT0/(T-T0)}, with fixed values of parameters A and B, that were independent of the concentration of the lithium salt in the solutions, and only temperature T0, as the concentration-dependent parameter, which showed correlation to the glass transition temperatures of the solutions.