Experimental and theoretical studies of physicochemical properties of Gemini imidazolium ionic liquids: hydroxyl group in spacer chain and alkyl chain length of cation

Ionic liquids (ILs) represent a suitable category of electrolytes in new energy batteries and supercapacitors. Although mono-cation ionic liquids have been extensively studied, the electrochemical properties of Gemini ionic liquids (GILs) have not been thoroughly explored. In this study, GILs containing functional hydroxyl group in spacer chain and different alkyl chain in imidazolium cation were synthesized. The physicochemical properties, including melting point, density, thermogravimetric (TGA) analysis, electrical conductivity (EC), molar electrical conductivity, and electrochemical stability window (ESW) of GILs, were evaluated. The introduction of hydroxyl group on spacer chain enhanced the melting point and ESW, while decreased the density, EC, and the thermal stability of bis(trifluoromethyl sulfonyl)imide (TFSI)-based GILs. ESW of TFSI-based GILs increased as the length of alkyl chain changed from methyl to butyl. TFSI-based GILs showed wider ESW than those of Gemini imidazolium bromide salts. Density functional theory (DFT) was used to study the relationship between the change of structure (hydroxyl group in spacer chain, alkyl chain length, and anions) and ESW values of GILs. These results will shed light on potential application of TFSI-based GILs in batteries as lithium-ion or supercapacitors as electrolytes.