Li-stuffed garnet solid electrolytes: current status, challenges, and perspectives for practical Li-metal batteries

Solid-state Li-metal batteries have gained considerable attention for next-generation energy storage because of their potential high energy densities and improved safety. Solid electrolytes are critical to the development of solid-state Li-metal batteries. While various solid electrolytes exhibit fast-ion conductivity, garnet-type oxides are among the few that show good chemical stability against Li metal. In addition, their high oxidation stability allows the use of high-voltage cathodes. However, the practical application of garnet solid electrolytes faces severe challenges: 1) difficulty in sintering thin and large-area garnet solid electrolytes, 2) large interfacial resistance between garnet electrolytes and electrode materials, and 3) Li dendrite growth. This review summarizes recent advances in garnet-type solid electrolytes and emphasizes the key challenges hindering their practical application in Li-metal batteries. Based on a comprehensive literature survey and our studies, the optimization of crystal structure and ionic conductivity in Li₇La₃Zr₂O₁₂ (LLZO) is nearly complete. The focus of the field is shifting from high-temperature sintered thick pellets to low-temperature processed thin and flexible LLZO-based organic/inorganic sheet electrolytes, which are more promising for commercialization. Additional research is needed to fully understand the mechanics, interface behavior, Li-ion pathway, and manufacturability of castable LLZO-based sheet electrolytes. In terms of cell energy density, the gravimetric energy density of polycrystalline LLZO-based all-solid-state Li-metal pouch cells is estimated to reach only 272 Wh kg^-1 under ideal conditions.