Development of Oxide-Based Solid-State Batteries

Solid-state batteries they provide high reliability to lithium-ion batteries are regarded as promising next-generation batteries. Solid-state batteries with sulfide electrolytes were reported to attain higher performance than lithium-ion batteries, and they are under development aiming at vehicle application. However, sulfide electrolytes are unstable and hygroscopic materials, and the batteries should be assembled under strictly-controlled atmosphere. Therefore, replacement of the sulfides with stable oxides is strongly required; however, performance of the oxide-based solid-state batteries is much lower than that of the sulfide systems.<br/>Although the highest conductivities having reported for oxides are of the order of 10^-3 S cm^-1, which is lower than that of sulfides by one order of magnitude, they are not the main reason for the poor battery performance: the conductivities are almost comparable to lithium-ion conductivities of organic-solvent liquid electrolytes used in the current lithium-ion batteries. High resistance of oxide-based solid-state batteries originates from the interfaces.<br/>Oxide-based solid electrolytes often show high grain boundary resistance. The grain boundary resistance can be lowered by increasing sintering temperature, indeed; however, it does not lead to high performance in the solid-state batteries. The solid electrolytes are in contact with active materials in the batteries. Sintering at high temperatures induces mutual diffusion between the solid electrolytes and active materials, which forms resistive impurity layers at the interfaces. Sulfide-based solid-state batteries have achieved the high performance not only by the high ionic conductivities of sulfide-based solid electrolytes but also by their deformability. Since sulfide solid electrolytes are deformable, they can be closely contacted to active materials only by cold pressing, where mutual diffusion takes place. Therefore, it is necessary to lower sintering temperature of the oxide solid electrolytes for realizing oxide-based solid-state batteries. I have been leading some research projects on oxide-based solid-state batteries. Achievements in the projects will be presented at the meeting.