Ionic conductivity and diffusivity in polyethylene oxode/electrolyte solutions as models for polymer electrolytes

Abstract

Conductivities and NMR 7Li and 19F self-diffusion coefficients have been measured in the system LiCF 3SO 3·PEO n as a function of concentration ( n = 5−15) and temperature (350–430 K). The temperature dependence of both sets of quantities is described by the Vogel-Tamman-Fulcher representation: Λ/ D = A exp{− B/ ( T − T 0)}. T 0 increases linearly with mole fraction of salt, consistent with the Li + ion forming cross-links between ethylene oxide segments of neighbouring chains. Ion diffusivities are independent of the concentration of salt, being simply a function of reduced temperature. This is shown to be consistent with the ionic mobility being controlled by the segmental motion of the PEO chains. In contrast, the molar conductivity varies in a complex manner with the concentration of salt: at fixed reduced temperature, it at first increases, passes through a maximum, and then decreases as the concentration of salt is increased. The “down-turn” in conductivity at the higher salt concentrations is attributed to the concomitant increase in the absolute temperature and the effect this has on the drift velocities of the ions under the influence of the applied electric field, whilst the “down-turn” to lower salt concentrations is attributed to correlated motions of neighbouring cations and anions. The latter contribute to the diffusivity, but not to the conductivity. This behaviour is manifest in deviations from the Nernst-Einstein equation. At low values of reduced temperature/high concentrations of salt, the diffusivities and conductivities are consistent with the Nernst-Einstein equation showing that both the mass and the change transport processes are dominated by the migration of discrete Li + and CF 3SO 3 - ions. Deviations do however occur as the reduced temperature is increased and they increase with increasing concentration of polymer. It is argued that the correlated motions of cations and anions giving rise to these deviations need to be distinguished from the discrete ionic pairs which are evident in vibrational spectra and whose concentration increases with increase in concentration of salt. The latter are considered to pertain to the “static structure” of the fluid and not to participate as discrete entities in the ionic transport process.