The Relationship between Ionic Conductivity and Solvation Structures of Localized High-Concentration Fluorinated Electrolytes for Lithium-Ion Batteries

Localized high-concentration electrolytes (LHCEs) combinea diluentwith a high-concentration electrolyte, offering promising properties.The ions, solvent, and diluent interact to form complex heterogeneousliquid structures, where high salt concentration clusters are embeddedin the diluent. Optimizing LHCEs for desired electrolyte propertieslike high ionic conductivity, low viscosity, and effective solid electrolyteinterphase (SEI) formation ability within the vast chemical and compositionaldesign space requires deeper understanding and theoretical guidance.We investigated the structures and conductivities of LHCEs based ona fluorinated solvent with two different diluents at varying concentrations.2,2,3,3-Tetrafluoro-propyl trifluoro-acetate (TFPTFA) entersthe solvation cluster due to its stronger Li-ion interactions, whereas1,1,2,2-tetrafluoro-ethyl 2,2,2-trifluoro-ethyl ether (TFETFE)enters only at extremely high diluent concentrations. The ionic conductivityincreases with decreasing diluent concentrations, with a slope changeduring cluster percolation. Overall, TFETFE demonstrates higher effectivenessthan TFPTFA, forming higher local salt concentration clusters andresulting in higher ionic conductivity.