Manipulating Li₂O atmosphere for sintering dense Li₇La₃Zr₂O₁₂ solid electrolyte

Cubic Li-Garnet (c-Li₇La₃Zr₂O₁₂, c-LLZO) is a promising solid electrolyte, however, it is very hard to prepare highly densified LLZO ceramics under air ambient condition. In this work, sintering in the presence of, but without direct contact with, “mother powder” as a lithium-compound vapor control strategy has been proposed. The existence of volatile Lithium compounds (mainly Li₂O gas) are proved and the impact of the Li₂O(g) atmosphere on LLZO sintering has been elucidated. At a low level, ceramics shows all trans-granular fracture microstructure; while at a high level, abnormal grain growth (AGG) occurs. Microstructure evolution under these two sintering conditions is evaluated, where a very rapid densification speed is observed for LLZO. A gas-liquid-solid sintering mechanism is proposed, where Li₂O gas condenses to liquids on the surfaces of LLZO grains to promote the solid material transportation. Thence, the sintering duration for high ionic conductive Ta doped LLZO ceramics can be greatly slashed to 30 min with low-cost conventional solid-state reaction and air ambient sintering techniques. In addition, this work provides a universal and efficient strategy for production of high ionic conductive Ta-LLZO ceramics with relative densities of 97–98%, conductivities of ~1 × 10⁻³ S cm⁻¹ at 25 °C, critical current density of 0.9 mA cm⁻² and highly-densified microstructure, which is fundamental for further researches on solid-state batteries.