Solubility, ionic conductivity and viscosity of lithium salts in room temperature ionic liquids

Abstract

Lithium salts used in batteries have poor solubility in many common room temperature ionic liquids, such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI). Lithium salt solutions in ionic liquids that contain the thiocyanate anion, in contrast, can be prepared at concentrations as high as 4 M. The solubility of three common lithium salts (LiClO₄, LiSCN and LiTFSI) used in lithium batteries were measured for four room temperature ionic liquids including 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆), 1-butyl-3-methylimidazolium acetate (bmimACET), 1-butyl-3-methylimidazolium thiocyanate (bmimSCN) and EMITFSI. The solubility was determined using Fourier Transform Infrared Attenuated Total Reflectance spectroscopy (FTIR-ATR), which was an efficient method for determining electrolyte solubility in electrolyte solvents. The ionic conductivity of room temperature ionic liquids with thiocyanate containing anions with high concentrations of lithium salts was determined using AC impedance. There was an optimal lithium salt concentration in the ionic liquid in terms of maximum ionic conductivity for the solutions.