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.
References
[1] H.-S. Shin, W. Jeong, M.-H. Ryu, S. W. Lee, K.-N. Jung J.-W. Lee, Chem. Eng. J, published online.
[2] X. Huang, J. M. Su, Z. Song, T. P. Xiu, J. Jin, M. E. Badding, Z. Wen, Ceram. Int., 47(2) (2021) 2123-2130.