Ionic Association, Solvation and Gelation in Super-Concentrated Electrolytes*

In super-concentrated electrolytes, ion-ion association can become extensive and complex. When the extent of ion association reaches a certain threshold, infinite percolating ionic gel networks can be formed spontaneously. We developed a thermodynamic model of reversible ionic association and gelation in super-concentrated electrolytes accounting for the competition between ion solvation and ion association. Our model is able to predict the populations of ionic clusters of different sizes as a function of salt concentration, as well as capture the onset of ionic gelation. We benchmark our model against molecular dynamics simulations of aqueous LiTFSI electrolyte for a large range of salt concentrations. We find that the ionic association and gelation observed in the molecular simulations are captured nearly quantitatively by our theory. The extent of ionic association and gelation greatly affects both the fraction of “free" ions in solution that can carry ionic current, as well as the viscosity of the mixture. Thus, knowledge of what governs these phenomena is essential in understanding the transport properties of super concentrated electrolytes.