Progress in solid electrolytes toward realizing solid-state lithium batteries

Solid-state lithium batteries have become more important than ever, because their high reliability as well as high energy density meet the requirements for energy storage in many aspects of the coming low carbon society. Solid electrolytes with high ionic conductivities are inevitable for realizing the solid-state lithium batteries, and the studies have been focused mainly on sulfide and oxide-based electrolytes. Sulfide-based solid electrolytes are advantageous to batteries owing to their high ionic conductivities and deformability. Although they show high resistance at the interface to cathode materials, interposing thin films of oxide-based solid electrolytes into the interface has successfully reduced the interfacial resistance. Combination of the highly-conductive sulfide electrolyte and the interface design have made performance of the sulfide-type solid-state batteries comparable or superior to current lithium-ion batteries. On the other hand, oxide-based electrolytes show higher chemical stability than sulfides, which is beneficial for manufacturing process. Although the highest conductivities have reached 10−3 S cm−1 also in oxides, practical performance has not been achieved in the oxide system due to the high grain boundary resistance.