Solid state ionics: electrochemistry meets materials science

Abstract

Most chemists and physicists, when hearing the term ‘‘electrochemistry’’, think of aqueous electrolytes and reactions such as oxidative dissolution of Zn or electrolysis of water. Modern electrochemistry, however, has a much broader view, and many electrochemical systems employ non-aqueous electrolytes, including solid electrolytes such as ceramics and polymers. For example, all technologically important types of fuel cells, many modern batteries, gas sensors, permeation reactors, and other electrochemical devices are based on non-aqueous ion conductors. Of particular importance are ion-conducting solids since they can offer substantial advantages compared to liquid systems, such as improved mechanical, thermal, or chemical stability. Accordingly, the number of research groups dealing with ion conducting solids has strongly increased recently (and still is increasing). The corresponding solid state electrochemical activities represent a substantial part of the research field generally known as ‘Solid State Ionics’. Despite the breadth of electrolytes used in electrochemical systems, also most introductory textbooks on ‘‘Electrochemistry’’ are more or less restricted to aqueous solutions. Solid ionic conductors, for example, are either only briefly introduced or neglected. This is definitely not caused by a minor significance of solid state ion conduction in nature and technology. Rather, the often missing generalized approach to ‘‘Electrochemistry’’ partly originates from the very different and hardly overlapping scientific communities dealing with aqueous systems and solid ion conductors; the latter field has much stronger links to materials science (e.g. ceramics) and solid state chemistry than to ‘‘traditional’’ aqueous (electro-) chemistry. Also the phenomenon of ion transport itself raises different questions to aqueous and solid state electrochemists: in aqueous solutions a large number of ions is mobile in a single medium (namely water) while solid state electrochemistry often deals with a single mobile ion, but a huge number of different media where this ion can move (e.g. O in oxides). In order to allow other colleagues in chemistry to become more familiar with novel aspects of solid ionconductors, this special issue entitled Solid State Ionics: Electrochemistry meets Materials Science comprises 17 articles covering various aspects of the field. First, in three review-like papers applications of non-aqueous electrochemical systems are in the focus: Progress and challenges of micro solid oxide fuel cells (SOFCs) and materials design aspects for SOFC cathodes are discussed and the status of soft matter electrolytes for Li-ion batteries is reviewed. Then three key aspects of research in Solid State Ionics are considered in more detail.