(Invited) Development of Bulk-Type Solid-State Batteries with Oxide-Based Solid Electrolytes

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ALCA-SPRING of Japan Science and Technology Agency (JST) was a 10-year project aiming at development of next-generation batteries. About 170 researchers in 70 laboratories all over Japan joined the project, and we had been working on solid-state batteries with oxide-based solid electrolytes.Solid-state batteries are regarded as promising next-generation batteries because they provide high reliability to lithium-ion batteries. Batteries for vehicles and energy storages of renewables are necessary to realizing a low carbon society, where long lifetime is required. High performance has been achieved in solid-state batteries with sulfide-based solid electrolytes, and they are in the final stage toward vehicle applications. However, sulfide-based solid electrolytes have a drawback: they are unstable and hygroscopic materials. Thus, the batteries should be assembled under strictly-controlled atmosphere, which raises the production cost. In addition, sulfide electrolytes generate harmful and corrosive H2S, when they are decomposed in ambient air. Therefore, replacement of the sulfides with stable oxides is strongly required; however, performance of oxide-based solid-state batteries is much lower than that of the sulfide systems.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; however, they are almost comparable to lithium-ion conductivities of organic-solvent liquid electrolytes used in the current lithium-ion batteries: although the conductivity of organic-solvent liquid electrolytes is of the order of 10-2 S cm-1, the transport number for lithium ion is less than 0.5, which suggests that the ionic conductivity is not a dominant reason for the low performance of oxide-based batteries.High resistance of oxide-based solid-state batteries originates from the interfaces between the battery materials. Sulfide-based solid electrolytes are deformable substance, and thus connection between the battery materials can be achieved only by cold pressing in sulfide-based batteries. On the other hand, oxide-based solid electrolytes are hard materials, and sintering process is necessary to connect the electrolyte particles with each other. In addition, oxide-based solid electrolytes, especially those showing conductivities around 10-3 S cm-1, often show high grain boundary resistance even after the sintering. 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, because the solid electrolytes are in contact with active materials in the batteries, and the high-temperature sintering induces mutual diffusion between the solid electrolytes and active materials, which forms resistive impurity layers at the interfaces. Therefore, it is necessary to lower sintering temperature of the oxide solid electrolytes for realizing oxide-based solid-state batteries.We employed three approaches to lower the sintering temperature of garnet-type solid electrolytes: reducing particle size of the solid electrolyte, control the electrolyte composition to form molten phases during sintering, and forming a composite electrolyte with a sinterable solid electrolyte. These approaches have lowered the sintering temperature and enabled us to assemble bulk-type solid-state batteries that are operatable at room temperature.