Development of Li<sub>10</sub>GeP<sub>2</sub>S<sub>12</sub>-type Solid Electrolytes for the Understanding of Superionic Conduction, and Their Application in All-solid-state Batteries

The lithium superionic conductor Li10GeP2S12 (LGPS) and its crystal structure analogues can be applied as solid electrolytes for all-solid-state lithium-ion batteries due to their exceptionally high ionic conductivities, which exceed 10 mS cm−1 at room temperature. This paper reviews the author’s publications into LGPS and its derivatives. Initially, its unique crystal structure is described with a particular focus on the ionic conduction mechanism. Subsequently, the syntheses of various structural analogues of LGPS are summarized, focusing on the phase diagram containing LGPS, as well as highlighting Li9.54[Si0.6Ge0.4]1.74P1.44S11.1Br0.3O0.6, which was discovered during high-entropy material design. Finally, the development and understanding of all-solid-state batteries that incorporate the developed solid electrolytes are described. This review is notable due to the importance of discovering novel solid electrolytes for further clarification of the superionic conduction mechanism and for improving battery performances.