H2O/CO2-Induced Surface Aging of Lithium Borate-Modified Li6.6La3Zr1.6Ta0.4O12 Garnet Electrolytes and Its Impact on Lithium-Metal Interfacial Stability

Vollständiger Abstract

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Abstract Lithium borate glass is an effective sintering additive for reducing the processing temperature of garnet-type Li6.6La3Zr1.6Ta0.4O12 (LLZTO) solid electrolytes owing to its ability to promote densification and improve ionic conductivity; however, its influence on environmental surface stability remains insufficiently clarified. Here, lithium-borate-modified LLZTO ceramics were investigated with emphasis on H2O/CO2-induced surface aging and the resulting impact on Li-metal interfacial stability. The LBO-modified samples retain LLZTO-dominated ionic transport in the 10–4 S cm–1 range while maintaining low electronic conductivity. However, accelerated aging in a H2O/CO2-rich atmosphere induces pronounced surface reconstruction, especially for LBO-containing samples. SEM, XRD, and XPS analyses show extensive surface deposits, Li2CO3-related reflections, carbonate-rich O 1s/C 1s components, and B–O signals, indicating the formation of a chemically reconstructed carbonate/borate-containing surface layer. This aged surface layer severely deteriorates Li/electrolyte interfacial transport: time-dependent impedance measurements show progressively larger and more unstable impedance responses with increasing LBO content, reaching the MΩ to hundreds-of-MΩ range after aging. Supporting DFT calculations using a Li3BO3-rich surface model further indicate strong interaction tendencies with H2O and CO2. These results highlight the importance of considering environmental surface susceptibility when employing lithium-borate-modified LLZTO electrolytes. Although the borate-containing surface is more reactive toward H2O and CO2 and can compromise Li-metal interfacial performance after aging, the dominant aging-induced surface contribution is largely confined to the near-surface region and can be substantially mitigated by mild polishing. Consequently, lithium borate remains a valuable sintering additive, provided that surface conditioning and handling procedures are properly considered during processing and cell assembly.