Frequency-dependent shear and bulk viscosity of aqueous and non-aqueous lithium battery electrolytes.

Transverse ultrasound was used to examine the frequency-dependent shear viscosities of three lithium bis(trifluoromethanesulfonyl)imide-based electrolytes: aqueous, non-aqueous, and hybrid aqueous-organic solutions. Frequency-dependent bulk viscosities of two of the electrolytes were measured with acoustic spectroscopy at low frequency and Brillouin scattering at high frequency. Both shear and bulk viscosities exhibit non-Newtonian behavior with a strong frequency dependence. The magnitudes of the two are similar, indicating the absence of long-range and slow dynamics involving volumetric and enthalpic changes that bring about ultrasonic relaxation. Increasing the solvent molecular size increases the magnitude of the viscosities and stretches out their frequency dependence, showing that the viscosities of the three solutions are coupled to the molecular scale relaxation. The shear viscosity relaxations match well with the correlation functions derived from neutron spin-echo measurements at specific scattering vectors in the range where diffraction measurements show strong intermolecular correlations, confirming that structural relaxation is an important factor in the viscosity mechanism. The frequency dependence of the shear viscosities obtained from molecular dynamics simulations agreed well with the experimental values and provides deeper insight into the coupling between time-dependent viscosity and molecular motion.