Chemical stability of cubic Li7La3Zr2O12 with molten lithium at elevated temperature

Cubic garnet of nominal composition Li 7 La 3 Zr 2 O 12 (LLZO) is of interest as a possible electrolyte in a Li–metal halide battery composed of molten Li and an insoluble metal halide as electrodes operating in the temperature range of ~300–350 °C. As a consequence, the chemical stability of LLZO with molten Li in this temperature range was investigated. After heating in molten Li, the LLZO surface had undergone chemical coloration. X-diffraction, X-ray photoemission spectroscopy, electron paramagnetic resonance, and heat-treatment experiments suggest that chemical coloration is a result of the formation of color centers, composed of an electron trapped at an oxygen vacancy. It was also observed that after immersion in molten Li that LLZO exhibited intergranular cracking. It is believed that cracking results from stresses generated from an increased Li + content, gained during immersion.