Unveiling the roles of alumina as a sintering aid in Li-Garnet solid electrolyte

The superiority of garnet-type solid electrolyte makes it one of most promising candidates for all-solid-state lithium batteries. Several studies show that introduction of alumina during synthesis can greatly improve the density and ionic conductivity of garnet electrolyte Li₇La₃Zr₂O₁₂ (LLZO), but the reason of poor sinterability of LLZO is still unclear. In this study, we reveal that lithium carbonate, which has a high decomposition temperature and covers on the particle surface of LLZO, is the underlying reason that handicaps the sinterability of Li-Garnet electrolyte in air. The addition of alumina promotes the decomposition of Li₂CO₃ (down to 400°C) and the concomitant product LiAlO₂, as a fast Li-ion conductor, facilitates the sintering process and bulids a fast Li-ion conducting network along the grain-boundaries, significantly increasing the ionic conductivity of Li-Garnet electrolyte. By the conventional solid state sintering, the 10 mol% Al₂O₃ modified Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZT-10Al₂O₃) electrolyte reaches a relative density of 96% and shows a conductivity of 0.31 mS cm⁻¹ at room temperature. The prepared LLZT-10Al₂O₃ electrolyte exhibits a good wetting property toward metallic Li electrode with an interfacial resistance of 59 Ω cm² compared to 1270 Ω cm² for LLZT/Li. This work provides a fundamental understanding and a valuable strategy for developing high performance garnet-type electrolyte for all-solid-state lithium batteries.