Microstructural Engineering of Li7La3Zr2O12 Solid Electrolytes for Enhanced Stability in H2O/CO2

Abstract

Solid-state batteries with nonflammable inorganic solid electrolytes provide a fundamental solution for resolving safety concerns. Moreover, mechanically robust solid electrolytes can physically block the penetration of Li dendrites, and thus, Li metal can be applied as an anode (negative electrode), boosting the energy density of the batteries. Garnet-type cubic Li7La3Zr2O12 (LLZO) has been considered as a promising candidate for solid electrolytes, due to its high Li+ conductivity and chemical/electrochemical compatibility with Li metal. When exposed to the ambient air, however, LLZO electrolytes are known to react with H2O and CO2 to form Li2CO3. Li2CO3 also grows along the grain boundaries, resulting in the significant degradation in Li+-conducting properties.In an effort to develop LLZO electrolytes with enhanced interfacial stability, here, we investigate the composition–microstructure–stability relationship of LLZO modified with aliovalent elements (Ta and Ga) under controlled H2O and CO2 atmospheres. Dense LLZO electrolytes doped with Ta (LLZTO) and Ta/Ga (Ga-LLZTO) are synthesized via a sol-gel process, followed by high-temperature sintering (1100 °C). Microstructural analyses show that the elemental doping has a crucial role in the grain growth behavior during the sintering process, thereby modifying the density, morphology, and atomic structure of grain boundaries. The interfacial stability of doped LLZO electrolytes is examined under accelerated durability test (ADT) conditions, where the concentration of O2/H2O/CO2 is precisely controlled to promote the interfacial reaction. During the ADTs, the degradation kinetics of Li+-conducting properties is quantitatively determined using in situ AC-impedance measurements combined with ex situ surface characterizations. The Li+ conductivity of Ga-LLZTO is almost retained during the ADT, while LLZTO shows only 27% of its initial conductivity value after 24 h of ADT. The significant degradation of LLZTO is caused by the formation of Li2CO3, which is further verified by Raman mapping and X-ray photoelectron spectroscopy analysis. On the other hand, the compositional and structural characterizations suggest that the enhanced interfacial stability of Ga-LLZTO is attributed to the reduced grain boundary density with enlarged grains and the segregation of H2O/CO2-tolerant Li–Ga–O species in the grain boundaries. The findings of this study would be essential in understanding the degradation of Li+-conducting properties and in developing highly conductive and stable LLZO solid electrolytes.