Tailoring grain boundary structures and chemistry of Li7La3Zr2O12 solid electrolytes for enhanced air stability

Solid-state batteries with inorganic solid electrolytes provide a fundamental solution for resolving safety concerns. Garnet-type Li7La3Zr2O12 (LLZO) is considered a promising candidate for solid electrolytes because of its high Li+ conductivity and superior chemical/electrochemical stability against metallic Li. However, when exposed to ambient air, LLZO electrolytes react with H2O and CO2 to form Li2CO3, resulting in significant degradation of Li+ conductivity. In this study, we propose a simple but effective approach to enhance air stability of LLZO via tailoring grain boundary structures and chemistry. The interfacial stability of the solid electrolytes is examined under accelerated durability test (ADT) conditions, where the concentrations of O2, H2O, and CO2 are precisely controlled to promote interfacial reactions. We show that Ga incorporation into Ta-doped LLZO (LLZTO) plays a crucial role in governing the grain growth behavior during the sintering process to modify the density, morphology, and composition of the grain boundaries. Furthermore, Ga-incorporated LLZTO (Ga-LLZTO) exhibits remarkably improved stability over LLZTO upon ADTs with high H2O and CO2 concentrations and enables stable cycling of metallic Li electrodes. The combined microstructural/compositional analyses and theoretical simulations suggest that the enhanced air stability of Ga-LLZTO can be attributed to the remarkably reduced grain boundary density with enlarged grains and segregation of H2O/CO2-tolerant lithium gallate (LiGaO2) in the grain boundaries. The findings of this study are critical for understanding the role of microstructural engineering in mitigating the degradation of Li+ conductivity and developing highly conductive and stable LLZO electrolytes.