Shear and conductivity relaxations of lithium ion electrolytes in polyethyleneglycol dimethyl ethers

Abstract

The frequency-dependent shear viscosity and electric conductivity of the solution of lithium perchlorate in tetraglyme (TEG) and polyethyleneglycol dimethyl ether (P10, average molecular weight is 500) are measured and compared with each other. The shear viscosity of neat P10 shows a relaxation around 100MHz, and its relaxation amplitude increases with the concentration of the salt, which can be ascribed to the large-scale intramolecular mode of a solvent molecule. In addition, another relaxation appears at 10MHz after the addition of the salt. No relaxation is found in the shear viscosity of TEG solution below 200MHz. The conductivity of P10 solution does not show any dispersion around 10MHz, which indicates that the ionic mobility is decoupled from the relaxation process at 10MHz. Although a relaxation of conductivity is found around 100MHz in P10 solution, the relaxation frequency increases with concentration, while the relaxation frequency of the shear viscosity little depends on the concentration. These two relaxations are therefore assigned to different mechanisms, indicating that the ionic mobility is decoupled also from the large-scale intramolecular mode of a chain. The comparison between the conductivity spectra of P10 and TEG solution shows that the mobility of the local mode of solvent is the determining factor of the ionic mobility.