Basic Properties of [C₃mim][NTf₂]/DEC/[Li][NTf₂] Systems

The hydrophobic ionic liquid (IL) 1-propyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Csrnim][NTf2]) was synthesized according to traditional methods. By adding different amounts of diethyl carbonate (DEC) solvent and lithium bis[(trifluoromethypsulfonyl]imide ([Li][NTf2]) salt to [Csrnim][NTf2] IL, eight solution systems were prepared. First, the thermodynamic properties of the eight solution systems were characterized by differential scanning calorimetry (DSC). The semi-stable temperature of the system gradually disappeared with increasing lithium salt content, but the melting point temperature was not apparent in the experiment. These results indicate that DEC and lithium salts can dissolve in ILs within the tested temperature range. The basic properties of the eight systems, including thermodynamic and dynamic properties, were systematically studied at different temperatures. The variation in the self-diffusion coefficient of lithium ion ([Li](+)) as a function of DEC concentration, density changes, viscosity, conductivity, and the viscosity/conductivity activation energy of the eight systems was calculated by the Vogel Fulcher Taman (VFT), Final Vogel Fulcher Taman (FVFT), and Arrhenius equations. The effect of temperature on the properties of the system was studied in detail. Within the temperature range measured herein, the deviation between the fitting equation and experimental value was small. Consequently, these equations were successfully used to calculate the properties of the system at various temperatures. All fitting parameters of the corresponding equations are provided herein. The viscosity for all systems decreased rapidly with increasing temperature, which increased the conductivity. Based on these experiments, the influence of DEC on the system microstructure was discussed in the context of the molecular dynamics simulation results. In particular, the interaction between [Li](+) and [NTf2](-)/DEC was examined. In all solution systems, [NTf2](-) coordinates to [Li](+) through only the O atom and not the N atom. Radial distribution function (RDF) analysis showed that the interaction between [Li](+) and [NTf2](-) weakened with increasing DEC concentration. DEC molecules were observed in the first solvation layer of [Li](+) coordinating to [Li](+) through the carbonyl O atom. Although the interaction between [Li](+) and DEC was weakened, competition between [NTf2](-) and DEC in the first solvation layer of [Li](+) was observed by the coordination number analysis of the O atom around [Li](+). Therefore, the introduction of DEC is beneficial for Li+ diffusion, which is consistent with the experimental results.