Physical and electrochemical properties of 1-(2-hydroxyethyl)-3-methyl imidazolium and N-(2-hydroxyethyl)-N-methyl morpholinium ionic liquids

Various ionic liquids (ILs) were prepared via metathesis reaction from two kinds of 1-(2-hydroxyethyl)-3-methyl imidazolium (HEMIm]⁺) and N-(2-hydroxyethyl)-N-methyl morphorinium (HEMMor]⁺) cations and three kinds of tetrafluoroborate (BF₄]⁻), bis(trifluoromethanesulfonyl)imide (TFSI]⁻) and hexafluorophosphate (PF₆]⁻) anions. All the HEMIm]⁺ derivatives were in a liquid state at room temperature. In particular, HEMIm]BF₄] and HEMIm]TFSI] showed no possible melting point from −150 °C to 200 °C by DSC analysis, and their high thermal stability until 380-400 °C was verified by TGA analysis. Also, their stable electrochemical property (electrochemical window of more than 6.0 V) and high ionic conductivity (0.002-0.004 S cm⁻¹) further confirm that the suggested ILs are potential electrolytes for use in electrochemical devices. Simultaneously, the HEMMor]⁺ derivatives have practical value in electrolyte applications because of their easy synthesis procedures, cheap morpholinium cation sources and possibilities of high Li⁺ mobility by oxygen group in the morpholinium cation. However, HEMMor]⁺ derivatives showing high viscosity usually had lower ionic conductivities than HEMIm]⁺ derivatives.