Physicochemical and Electrochemical Properties of Ionic Liquids Containing Aprotic Heterocyclic Anions Doped With Lithium Salts

Ionic liquids (ILs) have been received significant attention in recent years due to their outstanding properties, such as negligibly small vapor pressure, high thermal stability, relatively high conductivity and wide electrochemical windows. [1-3] Especially in the electrochemical field, ILs have been intensively investigated as alternatives for common flammable and volatile organic solvents. [4] A series of novel ionic liquids based on N,N′dialkyimidazolium, alkylphosphonium and N,N′dialkylpyrrolidinium cations paired with two aprotic heterocyclic anions (AHAs) were synthesized in the present study. In addition, their corresponding lithium salts were prepared. Physicochemical and electrochemical properties of the neat ILs and their lithium salt mixtures at various concentrations were measured from 283.15K to 343.15K at atmosphere pressure. Properties investigated include density (ρ), viscosity (η), electrical conductivity (σ), melting point (Tm), and thermal decomposition temperature (Td). Generally, the densities of all the neat ILs and their mixtures with the corresponding lithium salts follow a linear relationship with temperature and decrease with increasing temperature. The densities of all the IL mixtures with lithium salts increased with increasing lithium salt concentrations. The temperature dependence of the viscosity and the electrochemical conductivity were investigated for all neat ILs and lithium salt doped ILs. The empirical Vogel-Fulcher-Tammann (VFT) equation was used to analyze the viscosity and the electrical conductivity experimental results, which gave excellent fits over the temperature range studied.