Effect of ether introduction and ether chain length on electrochemical stability window feature of imidazolium ionic liquids with different anions: experimental and DFT studies

In this paper, functional ether-based ionic liquids with bromide and dicyanamide anions were prepared and characterized. The introduction of ether group into imidazolium cation decreased the relative viscosity of ionic liquids, while increased density and electrical conductivity values of ionic liquids. The refractive index, density, thermal stability, electrical conductivity, and ESW values of ionic liquids decreased with the increase of alkoxyalkyl chain length from methoxyethyl to methoxypropyl, while the relative viscosity of ionic liquid increased from the methoxyethyl to the methoxypropyl group. The refractive index, relative viscosity, density, and thermal stability of bromide ionic liquids were higher than those of dicyanamide ionic liquids, while the electrical conductivity and ESW values of dicyanamide ionic liquids were higher than those of bromide ionic liquids. DFT simulation results demonstrated the introduction of ether group into imidazolium cation increased anion-cation interactions of ionic liquids, resulting as lower HOMO–LUMO (E_gap), lower interaction energy (ΔE), and shorter average hydrogen bond length values. The anion-cation interactions among ionic liquids decreased when ether group changed from methoxyethyl to methoxypropyl. The cation–anion interactions of dicyanamide ionic liquids were much stronger than those of bromide ionic liquids. These results will shed light on the design and preparation of functional ionic liquid electrolytes for the renewable energy storage devices.