Solid state ionics: a Japan perspective

The 70-year history of scientific endeavor of solid state ionics research in Japan is reviewed to show the contribution of Japanese scientists to the basic science of solid state ionics and its applications. The term 'solid state ionics' was defined by Takehiko Takahashi of Nagoya University, Japan: it refers to ions in solids, especially solids that exhibit high ionic conductivity at a fairly low temperature below their melting points. During the last few decades of exploration, many ion conducting solids have been discovered in Japan such as the copper-ion conductor Rb₄Cu₁₆I₇Cl₁₃, proton conductor SrCe_1- Y O₃, oxide-ion conductor La_0.9Sr_0.9Ga_0.9Mg_0.1O₃, and lithium-ion conductor Li₁₀GeP₂S₁₂. Rb₄Cu₁₆I₇Cl₁₃ has a conductivity of 0.33 S cm^-1 at 25 °C, which is the highest of all room temperature ion conductive solid electrolytes reported to date, and Li₁₀GeP₂S₁₂ has a conductivity of 0.012 S cm^-1 at 25 °C, which is the highest among lithium-ion conductors reported to date. Research on high-temperature proton conducting ceramics began in Japan. The history, the discovery of novel ionic conductors and the story behind them are summarized along with basic science and technology.