Relationship between Structural Relaxation, Shear Viscosity, and Ionic Conduction of LiPF6/Propylene Carbonate Solutions.

The structure and dynamics of the solutions of LiPF6 in propylene carbonate over a concentration range of 0-3 mol/kg are studied with neutron spin echo spectroscopy, alternating-current (AC) conductometry, and shear impedance spectroscopy. The neutron diffraction shows a prepeak at ≈10 nm(-1) in addition to the main peak at ≈14 nm(-1) when the concentration of the salt is no less than 2 mol/kg. Compared with the frequency-dependent shear viscosity and AC conductivity, the relaxation of the shear stress agrees with that expected from the structural relaxation of the main peak. On the other hand, the relaxation of the conductivity is slower than the shear relaxation at all the concentrations, and the former approximately matches with the relaxation of the prepeak at the highest concentration, 3 mol/kg, which is several times slower than that of the main peak. The possible contribution of the prepeak structure to the ionic conduction is discussed.