Ionic Conductivity (Including Self-Diffusion)

Abstract

Ionic conductivity has become relevant. The recent revival of interest in electric automobiles and solid-state batteries has led to production of high-conductivity solid electrolytes. These new high-conductivity materials, such as RbAg₄I₅,^1,2 have greatly expanded the range over which ionic transport phenomena in solids have been observed. In Fig. 1, we show the conductivity of RbAg₄I₅ in comparison with the conductivity of the more common alkali and silver halides. We find grouped together on the left-hand, or high-temperature, side of the figure the alkali halide crystals. These materials have been extensively studied.* They are excellent insulators at room temperature and only have significant electrical conductivity within a few hundred degrees of their melting temperatures. Furthermore, in this high-temperature region, the conductivity of the alkali halides is strongly temperature-dependent, the conductivity changes by about 3% per degree Celsius, and the activation energy for conduction is about 2 eV. To the right in Fig. 1, we move to lower temperatures and to materials less well characterized than the alkali halides. The cesium and ammonium halides have ionic conductivities that are about equal in magnitude to the alkali halides but are less strongly temperature-dependent, with activation energies of 1.2 eV. With a conductivity higher by three orders of magnitude, we find silver chloride, with an activation energy for conduction of less than 1 eV.