Introduction

Abstract

The development of the electrochemistry of solids was made possible because of the existence of solid electrolytes. These are solid ionic conductors, solid —generally crystalline — compounds in which the electric current is carried by charged atoms, i.e. by ions. The passage of current is thus accompanied by mass transport. The ionic conductivity in solid electrolytes is sometimes as high as that in concentrated liquid electrolytes or molten salts. These solid electrolytes are often called superionic conductors. Some of these solid electrolytes with very high ionic conductivity have become very important for scientific as well as technological applications. Concerning the scientific applications one must emphasize the use of solid electrolytes in galvanic cells. These are important for thermodynamic as well as kinetic investigations. The emf of galvanic cells gives information about Gibbs reaction energies, reaction enthalpies and entropies, and also about activities, chemical potentials and partial pressures. From the kinetic point of view it is important that the current through suitably built galvanic cells with solid electrolytes can be a measure of reaction rates. The combination of reaction rate measurements via electric current with determinations of thermodynamic quantities via the emf often permits the analysis of kinetic processes. These processes may be diffusion in solids and liquids, diffusion-controlled solid-state reactions and phase boundary reactions at solid/solid phase boundaries or at solid/gas phase boundaries, e.g. evaporation processes. The technical applications include batteries with solid electrolytes, high-temperature fuel cells, sensors for measuring partial pressures or activities, display units and recently the growing field of chemotronic components. Science and technology involving solid-state electrolytes are sometimes called solid-state ionics, in analogy to the field of solid-state electronics.