Abstract:
The field of Solid State Ionics is concerned with the understanding and tailoring of defects, diffusion and reactions in solids. It has nowadays wide technological applications in energy conversion and storage, data storage, sensors etc. Thus, Solid State Ionics and its technological implications are inevitable for a future sustainable development of our world.
In this contribution I will, however, focus on some fundamental questions and still unresolved problems in the Science of Solid State Ionics. For this purpose, I will start with a brief history of Solid State Ionics showing the foundations of the field. Then I will focus on two major topics: The first is concerned with the role of defect interactions. This topic is of particular importance in materials with high defect concentrations where defect interactions are unavoidable. Interestingly, nearly all materials with technological importance belong to this class of materials. In contrast, the theoretical treatment of interactions is mostly limited to diluted systems. I will show a possible route to solve this problem by combining ab initio calculations with Monte Carlo simulations [1]. In this way, not only the problem of defect interactions can be solved, but also the link between the microscopic energetics and dynamics and the macroscopic thermodynamics and kinetics can be made. As examples, I will discuss our results for oxygen ion conductors and proton conductors [2,3].
The second topic is concerned with the number of components in a material. Nowadays, most materials in Solid State Ionics are multicomponent materials containing, e.g., three or more chemical elements. Thermodynamically, this is a challenge as the phase diagrams become rather complicated and are mostly unknown. On the other hand, there is another subtle problem which is concerned with the number of mobile species. Historically, in solid state kinetics only two mobile species were considered, e.g., two mobile cations during interdiffusion or one mobile anion and electrons in mixed conductors. The situation becomes, however, more complicated if there are three mobile species, e.g., two ionic defects and one electronic defect. I will discuss corresponding examples and the thermodynamic and kinetic implications [4,5].
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