(Invited) Epitaxial Thin Films of Solid-State Battery Material

Abstract

Solid-state batteries are regarded as next-generation batteries, which will play important roles in the coming low-carbon society; however, they have been suffering from their low power densities. Although low ionic conductivities of solid electrolytes had been the reason for the low power densities, intense studies have given solid electrolytes with high ionic conductivities: The highest conductivity exceeded 10−3 S cm−1 in 1990s and has reached 10−2 S cm−1 recently among sulfides. These conductivities are comparable to, or higher than, those of organic-solvent liquid electrolytes; however, even such high conductivities do not lead to high power densities without fast ionic conduction at the interfaces between the battery materials. In spite of the importance, studies on interfacial transport phenomena are still pioneering stage. For example, although high interfacial resistances between high-voltage cathodes and sulfide solid electrolytes have successfully reduced [1], mechanism of the high interfacial resistance is still in an argument; and although oxide systems have been suffering from high grain boundary resistance [2], origin of the grain boundary resistance is still unclear.Thin-film systems are ideal for such fundamental studies on the interfaces owing to their simple geometry, in which the interface can be a plane, and the ionic conduction can be regarded as one-dimensional. The thin films should be single crystal with controlled crystal orientation in order to construct interfaces formed between defined crystal planes and free from any defects including grain and domain boundaries.We have been focusing our attention on making such thin films by epitaxial growth. We have successfully formed some battery materials into single-crystal-like thin films with controlled crystal orientation [3–5]. An epitaxial film of perovskite-type solid electrolyte shows the influence of domain boundaries on ionic conduction, while anisotropic ionic conduction expected in LiCoO2 is clearly observed in a high-quality epitaxial film for the first time.