Influence of pressure on ionic transport in amorphous electrolytes: Comparison between glasses and salt polymer complexes

Abstract

Accurate conductivity measurements as a function of hydrostatic pressure (1 – 5000 bars) and temperature (20 – 150 °C) have been performed on a cationic inorganic glass and a cationic conducting polymer.

In both cases, the conductivity decreases with increasing pressure and the variation of Inσ at constant temperature as a function of pressure gives straight lines with slopes which allow an “activation volume”, ΔV*, to be obtained by the relationship (∂lnσ/∂P)_T=− (ΔV*/RT). In the case of silver metaphosphate glass, studied below its glass transition temperature, the activation volume (5 cm³⋅mol⁻¹) is temperature independent and equal to the molar volume of the silver cation. Since the transport mechanism implies a free energy barrier, this volume is a real activation volume, corresponding to the difference in volume between a mole of the moving species in its activated transition state and its volume at normal equilibrium.

In the case of the sodium conductive polymer, studied above its glass transition temperature, the previous thermodynamic definition does not hold any more because the ionic transport follows a V.T.F. behaviour rather than an Arrhenius law. Consequently, ΔV* is an “apparent activation volume” without a simple physical meaning. Experimental values are higher (20 to 30 cm³⋅mol⁻¹) and decrease with temperature. In this polymer, the mobility of the charge carriers is interpreted in terms of free volume mechanism. From the variations of the apparent activation volume with temperature, the critical free volume V_f* for an elementary displacement is estimated. For the Na⁺ conductive ionomer V_f* is estimated to be equal to 13 cm³⋅mol⁻¹. This large value would indicate the participation of macromolecular chain segments in the ionic transport.