Correlating Ionic\nConductivity to Structure and Dynamics\nof Li-Ion Battery Electrolyte Systems: Raman Spectroscopy, Dielectric\nRelaxation Measurements, and Streak Camera Solvation Data Analysis

A correlation between ionic conductivity\nand electrolyte solution\nstructure and dynamics was explored by performing electrolyte concentration-\nand temperature-dependent measurements of conductivity, viscosity,\nand dielectric relaxation (DR) in solutions of lithium bis(trifluoromethane)sulfonimide\n(LiTFSI) in triethylene glycol dimethyl ether (also known as triglyme,\nG3). In addition, an electrolyte concentration-dependent Raman spectroscopic\nstudy and ultrafast dynamic fluorescence Stokes shift measurements\nof solvation of a dissolved solute by employing a streak camera detection\ntechnique were carried out. Measured conductivities (σ) and\nthe average DR times (⟨τ<sub>DR</sub>⟩) were found\nto be partially decoupled from the solution viscosities (η)\nand obeyed the relation, σ or ⟨τ<sub>DR</sub>⟩<sup>–1</sup> ∝(η/<i>T</i>)<sup>−<i>p</i></sup>, with <i>p</i> = 0.6–0.75 for σ\nand <i>p</i> = 0.2–0.45 for ⟨τ<sub>DR</sub>⟩<sup>–1</sup>. Raman data indicated the formation\nof ion pairs and ionic aggregates in these solutions, while the measured\nglass transition temperature increased with LiTFSI concentration.\nConductivities (σ) showed a nonlinear concentration dependence\nbut increased linearly with the solution static dielectric constants\n(ε<sub>s</sub>). The latter may be explained by considering\nthe temperature effects on complex electrolyte species and the subsequent\nsolution dielectric behavior. Interestingly, an inverse power-law\ndependence of σ on the measured DR or solvation time scales,\nσ ∝ (τ<sub><i>x</i></sub>)<sup>−<i>m</i></sup>, with <i>m</i> = 1.2–1.9, was observed.\nThis dependence may be explained by considering that the same environmental\nfriction regulates both the ion diffusion and the medium polarization\nrelaxation. The control of a slow process (ion translation) by relatively\nfaster medium dynamics (dipolar rotation), although quite fascinating\nfor the present system, warrants further experimental scrutiny for\nother electrolyte systems relevant to battery applications.