Effect of ester chain length and anion on electrochemical properties of imidazolium dicyanamide ionic liquids: experimental and theoretical studies

It is found that ester introduction into imidazolium or pyridinium cations can significantly improve the biodegradability property of ionic liquids and also influence the viscosity and the toxicity of ionic liquids. Ionic liquids based on dicyanamide anion reveal good electrochemical properties. Four chloride and dicyanamide ionic liquids with ester groups were prepared, and the effect of anion and cation structure of ionic liquids on electrical conductivity/electrochemical stability window was discussed from experimental studies and density functional theory (DFT) simulations. The introduction of the ester group in imidazolium cation decreased the refractive index and electrical conductivity, while increasing the electrochemical stability window of ionic liquids. The refractive index, electrical conductivity, and electrochemical stability window values decreased with the increase of the alkyl chain length of the ester group from ethyl to butyl. The refractive index values of chloride imidazolium ionic liquids were higher than those of dicyanamide ionic liquids, while the electrical conductivity and electrochemical stability window values of dicyanamide ionic liquids were much higher than those of chloride ionic liquids. DFT simulations revealed that ester introduction in imidazolium cation decreased the average hydrogen bond length and increased the cation–anion interaction. The average hydrogen bond length of chloride and dicyanamide ionic liquids increased with the increase of the alkyl chain length of the ester group from ethyl to butyl, while the absolute interaction energy (Δ E ) of chlorine and dicyanamide ionic liquids decreased with the increase of the alkyl chain length of the ester group from ethyl to butyl. DFT simulations demonstrated that ionic liquid structure (ester chain length and anion) was closely related to the electrical conductivity and electrochemical stability window properties of ionic liquids. These results will shed light on the design and optimization of functional ionic liquids as electrolytes for renewable energy and energy storage applications.