The development of new materials leads to the invention of new devices. The exploitation of high ionic conductivity materials has facilitated the emergence of a new category of energy storage devices, including the all-solid-state battery. This paper reviews the history of the development of lithium solid electrolytes and their application in all-solid-state batteries. Particular focus is given to the development process of Li 10 GeP 2 S 12 , which surpasses the conductivity characteristics of liquid-electrolyte systems targeted by lithium-ion conductors, and its application to solid-state batteries is described. Furthermore, this review describes new science that will be born when batteries become solid-state.