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

Abstract

A series of novel halogen-free ionic liquids (ILs) based on N, N’-dialkylimidazolium, tetra-alkylphosphonium, and di-alkylpyrrolidinium cations paired with two aprotic heterocyclic anions (AHAs) were synthesized and tested as potential electrolytes. In addition, the corresponding lithium salts were prepared to investigate the impacts of the Li salts on the physical properties of the IL-Li mixtures. Physicochemical and electrochemical properties (density (ρ), viscosity (η), electrical conductivity (σ) and electrochemical window (EW) of the neat ILs and their mixtures with the lithium salts at various concentrations were measured at temperatures between 283.15 K and 343.15 K and at atmospheric pressure. The results indicate that ILs with both planar anions and cations exhibit lower viscosities, and better conductivities than those with more spherical geometries, while maintaining competitive EWs. The Walden plot behavior of the ILs provides qualitative insight into the suitability of these ILs as electrolytes. Based on systematic studies shown in this paper, it can be concluded that these new ILs are promising halogen-free electrolytes that deserve further investigation