Correlating Ionic Conductivity to Structure and Dynamics of Li-Ion Battery Electrolyte Systems: Raman Spectroscopy, Dielectric Relaxation Measurements, and Streak Camera Solvation Data Analysis

Abstract

A correlation between ionic conductivity and electrolyte solution structure and dynamics was explored by performing electrolyte concentration- and temperature-dependent measurements of conductivity, viscosity, and dielectric relaxation (DR) in solutions of lithium bis(trifluoromethane)sulfonimide (LiTFSI) in triethylene glycol dimethyl ether (also known as triglyme, G3). In addition, an electrolyte concentration-dependent Raman spectroscopic study and ultrafast dynamic fluorescence Stokes shift measurements of solvation of a dissolved solute by employing a streak camera detection technique were carried out. Measured conductivities (sigma) and the average DR times (<tau DR >) were found to be partially decoupled from the solution viscosities (eta) and obeyed the relation, sigma or <tau DR >-1 proportional to(eta/T)-p , with p = 0.6-0.75 for sigma and p = 0.2-0.45 for <tau DR >-1. Raman data indicated the formation of ion pairs and ionic aggregates in these solutions, while the measured glass transition temperature increased with LiTFSI concentration. Conductivities (sigma) showed a nonlinear concentration dependence but increased linearly with the solution static dielectric constants (epsilon s). The latter may be explained by considering the temperature effects on complex electrolyte species and the subsequent solution dielectric behavior. Interestingly, an inverse power-law dependence of sigma on the measured DR or solvation time scales, sigma proportional to (tau x )-m , with m = 1.2-1.9, was observed. This dependence may be explained by considering that the same environmental friction regulates both the ion diffusion and the medium polarization relaxation. The control of a slow process (ion translation) by relatively faster medium dynamics (dipolar rotation), although quite fascinating for the present system, warrants further experimental scrutiny for other electrolyte systems relevant to battery applications.