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.