Microstructure of the Li-Al-O Second Phases in Garnet Solid Electrolytes

The microstructure of the Li7La3Zr2O12 (LLZO) garnet solid electrolyte is critical for its performance in all-solid-state lithium-ion battery. During conventional high-temperature sintering, second phases are generated at the grain boundaries due to the reaction between sintering aids and LLZO, which have an enormous effect on the performances of LLZO. However, a detailed structure study of the second phases and their impact on physical properties is lacking. Here, crystal structures of the second phases in LLZO pellets are studied in detail by transmission electron microscopy. Three different crystal structures of Li-Al-O second phases, gamma- LiAlO2, alpha-Li5AlO4, and beta-Li5AlO4 were identified, and atomic-scale lattice information was obtained by applying low-dose high-resolution imaging for these electron-beam-sensitive second phases. On this basis, the structure-property relationship of these structures was explored. It was found that sintering aids with a higher Li/Al ratio are beneficial to form Li-rich second phases, which result in more highly ionic conductive LLZO.

Automated summary

The microstructure of the Li7La3Zr2O12 (LLZO) garnet solid electrolyte plays a critical role in the performance of all-solid-state lithium-ion batteries. Second phases generated at the grain boundaries during high-temperature sintering have a significant impact on the properties of LLZO. However, the detailed structure of these second phases and their effect on physical properties have not been thoroughly studied. In this study, the crystal structures of Li-Al-O second phases in LLZO pellets were examined using transmission electron microscopy. Three different crystal structures were identified, and their structure-property relationship was explored. It was found that sintering aids with a higher Li/Al ratio promote the formation of Li-rich second phases, which enhances the ionic conductivity of LLZO.