Competing Effects in the Hydration Mechanism of a Garnet-Type Li₇La₃Zr₂O₁₂ Electrolyte

Li-ion conducting oxides (Li₇La₃Zr₂O₁₂, LLZO) with a cubic garnet-type structure are among the most promising candidates to be used as solid electrolytes in all-solid-state Li batteries. However, the environmental instability of the electrolyte, induced by interaction between the material and gas molecules commonly found in air, namely, water and carbon dioxide, poses challenges for its manufacture and application. Herein, a combined experimental kinetic and thermodynamic study was performed as a function of temperature to clarify the mechanism of hydration of a garnet-type LLZO electrolyte in moist air. It was found that the kinetics of LLZO hydration is diffusion-limited and the hydration mechanism at room temperature and at higher temperatures differs. The hydration of LLZO increases up to 200 °C. Above this temperature, stagnation of water uptake is observed due to the onset of a competing dehydration process. The dehydration of LLZO takes place up to 400 °C. The partial pressure of water significantly affects the extent of hydration. Expanding this combined kinetic and thermodynamic approach to LLZO materials with a variety of chemical compositions and morphologies would allow prediction of their reactivity in a humid atmosphere and adjustment of the processing conditions accordingly to meet the requirements of technological applications.