Coupling and decoupling of molecular reorientation and charge transport in Li-salt-doped cycloalcohol ion conductors

Abstract

Using broadband impedance spectroscopy and conductometry, we study the phase behavior and the reorientational and translational dynamics of cycloalcohols doped with 1 to 20 mol% of Li salt. At low salt concentrations, a plastically crystalline state is stable. In this regime, the dielectric spectra of the cycloalcohols reveal two relaxation processes. One of them originates from the α-relaxation and indicates an orientational glass transition. The other, much slower process, is related to ionic hopping. By comparing reorientation times with electrical conductivities, for 1 mol% Li doping, previous studies suggested an only weak decoupling of rotational and translational motions. Rather than analyzing disparate quantities, the present work directly compares peak frequencies corresponding to reorientational and translational rates. This approach allows one to resolve the mild, yet significant divergence of the two rates, i.e., a dynamic decoupling, also for higher doping levels and for different Li salts. The electrolytes that form at Li salt concentrations of 5 mol% and more display a qualitatively different behavior. In this compositional regime, a liquid-to-glass transition arises and the ion hopping time that we probe using lithium nuclear magnetic resonance and proton-detected diffusometry is compatible with the reorientational time scales. This observation implies a close coupling of both underlying degrees of freedom.