Density and Dynamic Viscosity of Aqueous Electrolytes with Increased Li-Salt Solubility Due to Addition of Ionic Liquids

The limited solubility of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in water limits the use of aqueous media in Li-ion batteries. Enhancement of LiTFSI solubility in water may be achieved by adding room-temperature ionic liquids (RTILs) to the mixture of Li-salt and water. We present the density (ρ) and dynamic viscosity (η) of ternary electrolytic systems composed of LiTFSI, water, and four different RTILs: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [EMIm.TFSI], EMIm trifluoromethanesulfonate [EMIm.OTF], n-butyl-trimethylammonium TFSI [N4111.TFSI], and 1-butyl-1-methylpyrrolidinium TFSI [Pyr14.TFSI], respectively, over a temperature range of 293.15–363.15 K at different LiTFSI concentrations. The sensitivity of ρ toward T-change is more-or-less constant over the entire composition range. The T-dependence of η is correlated by the Vogel–Fulcher–Tammann model. Linear trends of ρ and η with Li-salt molality were observed for all RTILs except Pyr14.TFSI, where a sigmoidal increase better described the data, highlighting the predominant role of the IL cation in determining composition dependence. The extent of increase in ρ and η is much more profound in TFSI–-containing ILs as compared to that in OTf–. We believe that Li+ is preferentially solvated by anions of RTIL instead of water molecules, resulting in enhanced solubility and greater ionic interactions. The role of RTIL in achieving favorable results is established nonetheless.