Phase transformation of the garnet structured lithium ion conductor: Li7La3Zr2O12

Phase stability of the Garnet-structured lithium ion conductor Li7La3Zr2O12 (LLZ) was investigated using high-temperature X-ray diffraction (HT-XRD). The tetragonal LLZ containing CO2 transformed to the cubic phase around 150 °C. However, once the tetragonal LLZ heated up to 1000 °C, the cubic phase transformed back to the tetragonal phase again, because of the CO2 loss during the heating process. After the CO2 loss, the phase transformation temperature shifted to 650 °C. The HT-XRD study for Al-doping process was also performed. The addition of γ-Al2O3 to the tetragonal LLZ eliminated the formation of impurity phase such as La2Zr2O7 and La2O3, resulting in the formation of a pure cubic phase. A comparison of the HT-XRD results suggested that the Li loss is necessary to stabilize the cubic phase with Al.