Phase Evolution and Thermodynamics of Cubic Li _6.25 Al _0.25 La ₃ Zr ₂ O ₁₂ Studied by High-Temperature X-ray Diffraction

The cubic garnet Li₇La₃Zr₂O₁₂ (LLZO) is a prototypical ceramic electrolyte for solid-state Li-ion batteries. While the electrochemical performance of LLZO is well studied, the thermodynamics of the formation of LLZO is not fully understood, and reliable synthesis of phase-pure cubic LLZO requires such knowledge. Here, we report a high-temperature X-ray diffraction (HTXRD) study of the crystallization of Al-doped LLZO from an amorphous gel with different amounts of excess Li. Based on the phases identified in the precursor powders before and during heating, a net chemical reaction for the formation of LLZO is proposed, and its thermodynamic properties are calculated. The sample thickness, and hence the surface exposure to the atmosphere during calcination, strongly affects the phase evolution of cubic LLZO. The configurational entropy of cubic LLZO is estimated to be large and necessary to stabilize cubic LLZO.