Effects of dynamic spatial disorder on ionic transport properties in polymer electrolytes based on poly(propylene glycol)(4000)

Abstract

The equivalent ionic dc conductivity (Λ) generally exhibits a dramatic concentration dependence in electrolytic systems based on the host polymer poly(propylene glycol) of molecular weight 4000 (PPG4000). In particular, Λ typically increases rapidly with increasing salt concentration passing through a temperature dependent maximum at high concentration. Prompted by recent reports on a microscopic phase separation occurring in these electrolytes, we here report vibrational spectroscopic, ionic conductivity, and restricted diffusion data for ion-conductors based on PPG4000 complexed with the lithium salts LiCF3SO3 and LiN(CF3SO2)2, in an attempt to resolve seemingly contradictory results concerning ionic transport phenomena in these complexes. We find that the differential salt diffusion coefficient Ds, describing bulk salt motion over long time scales, exhibits a qualitatively similar concentration dependence as Λ. This is contrary to recent F19 pfg-NMR diffusion results for the PPG4000-LiCF3SO3 system which show that the anionic diffusion coefficient decreases monotonically with increasing salt concentration and is inversely proportional to solution shear viscosity. As determined from analyses of characteristic vibrational modes of the [CF3SO3]− and [(CF3SO2)2N]− anions, respectively, the spectroscopic data show very small changes in the distribution of anionic species over the range of electrolyte compositions corresponding to a sharp enhancement of Λ. The results are interpreted in terms of slowly fluctuating salt-rich electrolyte microdomains in equilibrium with salt-depleted polymer regions.