Physicochemical Properties of New Ionic Liquids and Plastic Crystal Based on Huckel Anions

There is increasing interest in ionic liquid electrolytes for battery applications due to their potential as safer alternatives to conventional liquid electrolytes. By carefully selecting cations and anions, the physicochemical properties of these salts can be optimised. However, investigation of new cations and anions is still required to achieve the desired electrolyte properties such as high conductivity, electrochemical stability and low corrosivity. Thus, here a new series of room temperature ionic liquids (RTILs) and an organic ionic plastic crystal (OIPC) were synthesized and characterized based on N-ethyl-N-methylpyrrolidinium ([C₂mpyr]⁺) and N-ethyl-N-methyloxazolidinium ([C₂moxa]⁺) cations, paired with 4,5-dicyano-2-(trifluoromethyl)imidazolide ([TDI]^-) and 4,5-dicyano-2-(pentafluoroethyl)imidazolide ([PDI]^-) anions, forming [C₂mpyr][TDI] (OIPC), [C₂mpyr][PDI] (IL), [C₂moxa][TDI] (IL), and [C₂moxa][PDI] (IL). Thermal properties were analysed using DSC and TGA. The plastic phase of the OIPC [C₂mpyr][TDI] is wide and spans most electrolyte application temperatures, extending from -40 to 56 °C. All salts exhibit thermal stability above 200 °C. The temperature dependence of the transport properties (viscosity, ionic conductivity, self-diffusion coefficient) were measured and fitted by the Vogel-Fulcher-Tamman (VFT) equation, and the ionicity of the ILs is reported. The electrochemical stability windows of the salts were analysed by CV, and the [C₂mpyr]⁺ cation-based ILs showed the widest window of ~5 V.