Formation mechanism of sol-gel synthesized Li7-3xAlxLa3Zr2O12 and the influence of abnormal grain growth on ionic conductivity

The development of solid-state batteries requires the optimization of ceramic electrolyte such as garnet type cubic Li7La3Zr2O12(LLZO). Exploration of powders formation mechanism and the sintering behavior of ceramics are crucial to acquire high performance LLZO electrolytes. This study adopts quantitative analysis based on X-ray adiabatic principle to investigate the formation mechanism of Li7-3xAlxLa3Zr2O12 (x = 0, 0.1, 0.15, 0.2, 0.25, LALZO) powders synthesized by sol-gel method. The whole calcination procedure could be divided into three stages as follows, La2Zr2O7 (LZO) formation ( < 750 degrees C), transformation from LZO to LLZO (750 degrees C-850 degrees C) and LLZO decomposition ( > 850 degrees C). The phase, morphology and conductivity of LALZO ceramics demonstrate abnormal grain growth (AGG) caused by over-sintering leads to random orientation and conductivity deterioration. The optimal sintering condition could not only make grains bound tightly but also avoid AGG. Ceramics consist of LALZO (x = 0.25) exhibit better total ionic conductivity of 3.08 x 10(-4) S cm(-1) (E-a = 0.27 eV) with density of 92.5% after sintering at 1100 degrees C for 15 h. This research provides guidance for quickly acquiring pure LLZO powders and excellent performance ceramics.