A novel approach to characterize the relationship between dissociation and conductivity in ionic liquid and solvent mixtures

Ionic liquid and solvent mixtures, in the recent development of electrochemical energy storage devices, such as supercapacitors and batteries, have emerged as promising potential avenues due to their high ionic conductivity. Particularly, the fraction of dissociated ions is thought to be highly influential on ionic conductivity, yet the nature of the link between the two properties has not been well established. In this work, we develop a novel theoretical approach, based on Wertheim’s thermodynamic perturbation theory and Turq et al.’s mean-spherical-approximation theory to rigorously connect ion dissociation and conductivity. We have validated this approach with molecular dynamics simulations for a large range of ionic liquid chemistries and solvent mixtures. A key insight is that although one might expect the dielectric medium to be the determining factor in ion dissociation, we find that the solvent dipole moment drives dissociation. Furthermore, we show that the Nernst–Einstein conductivity is directly proportional to ion dissociation. This novel approach could be used to predict the ionic conductivity of ionic liquid and solvent mixtures, aiding in the computer-aided molecular design for specific applications, such as battery electrolytes.