Superionic Conducting Oxides

Abstract

Fast ionic conduction in the solid state has been observed since the time of Faraday himself (1). Recently the increasing demand for new power sources and energy storage and conversion systems has led to a consider­ able interest in the field of solid electrolytes. Fuel cells and batteries are important devices in this respect. Solid superionic conducting materials have been extensively investi­ gated both for fundamental and technological aspects. Recent review books and papers (2-16) provide a comprehensive survey in that field. The present paper is limited to oxide compounds; special attention is paid to the latest results with emphasis on the relations between basic properties and observed fast ion conduction. Superionic conductors (SIC) are defined as materials with unusually high ionic conduction (0' > 10 2 n 1 em 1). Such values are normally found in molten salts, but some materials exhibit similar conductivity far below their melting point and even at room temperature. Their con­ ductivity may be purely ionic; the possible electronic contribution is several orders of magnitude lower. This property is required for solid electrolytes used in batteries, because excessive electronic conductivity will result in discharge of the system. Other compounds show mixed conductivity with high values for both ionic and electronic conduction, and they have useful applications mainly as electrodes in fuel cells or .batteries .