Ion relaxation dynamics and nearly constant loss behavior in polymer electrolyte.

The broadband conductive spectroscopy covering the range 10(-2) to 10(6) Hz has been employed to examine both the ionic and the segmental motions in polymer salt complexes (PSCs), consisting of polyethylene oxide (PEO), LiClO4, or LiCF3SO3. The temperature dependence of dc conductivity has been analyzed using Vogel-Tamman-Fulcher (VTF) equation and the results suggest that the temperature dependence of the dc conductivity can be well described in terms of the free volume changes with temperature. The ac conductivity at low temperatures (below 223 K) remains unchanged with the temperature, exhibiting nearly linear frequency dependence. This observation, not previously reported in polymers, indicates clearly that nearly constant loss (NCL) phenomenon, usually observed in glassy and ceramic ion conductors, is also operative in polymer electrolytes. Furthermore, a crossover from a NCL to cooperative ion hopping at higher temperatures in the frequency dependence of the conductivity has been observed. The analysis of the temperature dependence of both dc and ac conductivities demonstrates that there exists a direct correlation between the ionic conductivity and the segmental relaxation processes.