Formation Mechanism of Garnet-Like Li7La3Zr2O12 Powder Prepared by Solid State Reaction

The formation mechanism of garnet-like Li7La3Zr2O12 (LLZO) phase was investigated using LiOH·H2O as raw materials by XRD and DTA/TG measurements. It is found that the reaction mechanism of the LLZO formation is as follows: 7Li2O+3La2O3+4ZrO2=2Li7La3Zr2O12. The LLZO phase forms at about 680 °C according to the XRD patterns, which is in agreement with the thermic peak at 700 °C in DTA/TG curves, and the peak arises from the formation of LLZO phase. The temperature range of stable LLZO phase is wider, from 720 to 1000 °C. However, LLZO is thermally unstable at the high temperatures (<1000 °C) due to a serious loss of lithium element, and it discomposes into pyrochlore phase La2Zr2O7. And the amount of the decomposed products increases, and LLZO decreases gradually with the increase of the calcination temperature. When the reactant mixture is calcined at low temperature, the produced phases are identified to be lanthanum compounds because La2O3 absorbs moisture and CO2 very easily. The chemical reactions at lower temperature are mainly related with the reactions among lanthanum compounds. LLZO powder could be synthesized by solid state reaction at 800 °C. It is found that the LLZO crystal belongs to the tetragonal symmetry. SEM observation shows that the obtained LLZO powder is nano-sized.