Revealing Local Structure and Dynamics in Li-salt Electrolytes using Dielectric Relaxation Spectroscopy

Understanding the interplay between local structure and transport is critical for establishing design rules in polymer electrolytes. In this work, a set of Li-salt in liquid electrolytes is studied as a model system to examine correlations between several electrolyte properties, including conductivity, dielectric relaxation time and strength, ionicity, and viscosity. These properties were evaluated by changing ion concentration, solvent type, and anion type. Dielectric relaxation spectroscopy was used, in combination with other experimental techniques, to identify the relaxation processes associated with the different ion-solvent complexes. The dielectric relaxation time of the solvent-ion complexes, is independent of ion content at low salt concentrations, when solvent separated ion pairs dominate. At high ion concentrations, molar conductivity depends more strongly on this relaxation time as solvent dielectric constant is reduced, which we suspect is due to variation in shielding between ion pairs and aggregates. In contrast, there is no clear change in the dependence of molar conductivity on dielectric relaxation time with changing anion. This study provides insight into different local structures and dynamic processes that contribute to conductivity. While Li-salts in liquid solvents are presented here as a model system, the findings discussed in this work could be applied to polymeric systems, laying a foundation for the design of new polymer electrolytes.