Experimental and DFT Studies on Electrochemical Performances of Ester-Containing Vinylimidazolium Ionic Liquids: Effect of the Ester Substituent and Anion

Four imidazolium ionic liquids (ILs) with vinyl and ester groups were synthesized and characterized by 1H NMR, 13C NMR, and element analysis. The refractive index, electrical conductivity (EC), and electrochemical stability window (ESW) of the ILs were determined. The effect of the alkyl chain length of the ester group and anions on electrochemical properties was analyzed by density functional theory (DFT) calculations. At the same temperatures, the EC of the ILs decreased with the increase of the alkyl chain length. The ester group and vinyl group of cations showed a negative effect on the EC. The ESW increased with the increase of the alkyl chain length. The introduction of the ester group and vinyl group decreased the cathodic limiting potential of ester-based vinylimidazolium cations. DFT simulations showed that the structure of the IL was closely related to the EC and ESW. The average hydrogen bond length increased with the increase of the alkyl chain length of the ester group. The absolute interaction energy of ILs decreased with the increase of the alkyl chain length. Introduction of the vinyl group and ester group into the imidazolium cation resulted in a stronger cation–anion interaction, strengthening the ion mobility and charge transfer, resulting in a lower EC. The information is useful for the design and optimization of ILs as electrolytes for energy storage applications.