Correlation between ionic and molecular dynamics in the liquid state of polyethylene oxide/lithium perchlorate complexes

Broadband electrical spectroscopy was used to investigate the correlation between ionic conductivity and dielectric relaxation in the liquid state of polyethylene oxide (PEO)/lithium perchlorate (LiClO4) complexes over the broad ranges of salt concentration Li/O, molecular weight of PEO Mw, and temperature T. The introduction of LiClO4 induced a split of the dielectric relaxation into fast and slow modes. The fast mode decreased its relaxation strength with increasing Li/O while keeping the frequency constant, whereas the slow mode first increased its strength and then became constant while decreasing the frequency. On the basis of their Mw dependences, we concluded that the fast mode is associated with the segmental motion of free PEO and that the slow mode arises from the slowed segmental motion due to the cation coordination of ether oxygens. The molar conductivity first increased and then decreased with increasing Li/O in parallel with the slow-mode relaxation frequency showing a plateau near Li/O = 3%. The ionic diffusion coefficient was shown to be proportional to the slow-mode relaxation frequency, with the coefficient of proportionality being independent of Li/O, Mw, and T. The use of a random walk scheme yielded a microscopic feature in which ions hop for a distance of 0.3 nm at the rate of the slowed relaxation frequency. Discussions are extended to the conduction mechanisms in the partially crystallized states.