The interdisciplinary area of science and engineering dealing with solid electrolytes and mixed conductors is frequently known as solid-state ionics. It concerns materials that show rapid ionic motion with or without electronic conductivity, from basic science through application. Interest in solid-state ionic materials has continued for the past few decades due to several important, promising applications, such as fuel cells, batteries, sensors, and electrochemical pumps. The principle behind these applications is simply either the Nernst law (as exemplified by Equation 1 in the article by Singhal in this issue) or Faraday's laws of electrochemistry (which connect current flow to mass flow), as applied to a cell consisting of an electrolyte and two electrodes. However, the technological issues are complex, and demands on materials can be very diverse, as illustrated by the five articles in this issue. These articles are based, in part, on the invited talks presented at a symposium in April 1999 on the same subject at Lehigh University to commemorate G.C. Farrington's inauguration as its president.

The high oxygen-ion conductivity over wide ranges of temperature and oxygen pressure in stabilized cubic zirconia has led to its use as a solid-oxide electrolyte in a variety of electrochemical applications. Zirconia-based oxygen sensors are widely used for combustion control, especially in automobiles, for atmosphere control in furnaces, and as monitors of oxygen concentration in molten metals. Other applications include electrochemical pumps for control of oxygen potential, steam electrolyzers, and high-temperature solidoxide fuel cells (SOFCs). High-temperature SOFCs offer a clean, pollution-free technology to electrochemically generate electricity at high efficiencies. These fuel cells provide many advantages over traditional energy-conversion systems, including high efficiency, reliability, modularity, fuel adaptability, and very low levels of NOx and SOx emissions. The quiet, vibrationfree operation of SOFCs also eliminates the noise usually associated with conventional power-generation systems. Furthermore, because of the high temperature of operation (~1000°C) of SOFCs, naturalgas fuel can be reformed within the cell stack, eliminating the need for an expensive external reformer system. Also, pressurized SOFCs can be successfully used as replacements for combustors in gas turbines; such hybrid SOFC/gas-turbine power systems are expected to reach efficiencies approaching 70%.

The beta-alumina family of materials has played a central role in the field of solid-state ionics. β- and β-alumina have been a principal theme in the field ever since Yao and Kummer reported the extraordinarily high conductivity for sodium β-alumina. The material “came of age” at a time when there was great interest in the science and technology of highconductivity solid electrolytes. A previous MRS Bulletin review introduced this broad family of materials, examining the range of compositions and the two major structures. The beta-alumina family emerged as almost an ideal system in which to explore structure-property relations, as its unusual structure is responsible for its remarkable ion-transport properties. Although the initial interest in this material, and the one which endures to this day, is its rapid sodium-ion transport, the rich ion-exchange chemistry added a dimension to this material that few inorganic systems can match, let alone exceed. β-alumina, in particular, is an almost universal solid electrolyte for cations. It constitutes a broad family of solid electrolytes whose properties are dependent upon the nature of the ion inserted into the conduction plane. As a result, the β-aluminas have served as model systems for a wide range of studies: proton transport and mixed-ion diffusion, order-disorder reactions, and superlattice phenomena. Moreover, these β-alumina compositions demonstrated that fast-ion transport was not limited to a few monovalentions, but could be extended to divalent and even trivalent cations. With the latter materials, the interest was not necessarily ion transport, but optical properties instead. The presence of trivalent cations in this unusual structure, coupled with the ability to control the chemistry of the local environment, enabled the β-aluminas to exhibit a number of novel optical properties.

Polymer electrolytes have been a focus of the scientific community for a quarter century. They extend the realm of traditional solid-state ionics from hard materials to include soft materials and, in doing so, pose issues both fundamental (e.g., how are structure and transport defined in a concentrated electrolyte with an immobile solvent?) and technological (e.g., how can polymer ionics be used to construct electrochemical devices such as sensors, batteries, and fuel cells?)

Mixed conductors show significant mobility of both electronic and ionic species and were the subject of an earlier review in MRS Bulletin.1 The current review is restricted to those mixed conductors of interest for use in lithium batteries, with an emphasis on commercialization. The first lithium batteries were primary cells using pure lithium anodes and carbon monofluoride or manganese oxide as the cathode. Both were developed in Japan, the former for use in fishing floats and the latter for calculators and similar small devices. Such primary cells based mainly on MnO2 or FeS2 cathodes are still extensively used in watches, cameras, and so on. Lithium primary cells are also the main power source for many medical devices, such as pacemakers. In some of these applications, silver vanadate is the cathode.