Phase evolution and thermodynamics of cubic Li6.25Al0.25La3Zr2O12 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 on 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 calculated. The sample thickness, and hence the surface exposure to atmosphere during calcination, strongly affects the phase evolution of cubic LLZO. The configurational entropy of cubic LLZO is estimated be large and necessary to stabilize cubic LLZO.