Chapter 3: History of Solid Electrolyte

Ion conduction in solids has been known for more than a century [1]. The first electrical conductive solid was discovered by Michael Faraday who, in 1883, reported that the electrical conductivity of Ag₂S was largely increased with increase in temperature [2]. Additionally, he discovered similar behavior in several other inorganic solids such as PbF₂, in 1838 [3]. This behavior was difficult to explain at that time because the behavior was opposite to that in metallic phase. Later, Hittorf investigated the conductivity of Ag₂S and Cu₂S and concluded that these materials had an electrolytic conduction mechanism [4]. Then, in 1884, Warburg proved that sodium ion could migrate through the solid, which overturned the conclusion of Arrhenius that neither pure salt nor pure water can be a conductor but that only salt dissolved in water…