Chemical stability of Li-stuffed garnet-type Li5 + xBaxLa3 − xTa2O12 (x = 0, 0.5, 1) in water: a comparative analysis with the Nb analogue

In this study, we report the synthesis, chemical stability, and Li ion conductivity of garnet-type Li5 + xBaxLa3 − xTa2O12 (x = 0, 0.5, 1) in water and discuss the chemical stability in comparison to the corresponding Nb analogue. Solid-state (ceramic) synthesis method was used to prepare the investigated compounds which exhibited a cubic garnet-type structure when sintered at 950 °C in air. The ionic conductivity increases with increasing Li and Ba content in Li5 + xBaxLa3 − xTa2O12, and the x = 1 member shows the highest bulk conductivity of about 10− 4 Scm− 1 at room temperature and the lowest activation energy of 0.39 eV (25 °C–125 °C). The Ta-based Li-stuffed Li5 + xBaxLa3 − xTa2O12 garnets show better chemical stability in water compared to the corresponding Nb analogue, as supported by thermo-gravimetric analysis (TGA), and variable temperature powder X-ray diffraction (VT-PXRD). The proton-exchange was found to decrease with increasing Li content in Li5 + xBaxLa3 − xTa2O12 in water, similar to the trend in the Nb series, Li5 + xBaxLa3 − xNb2O12. The slightly improved chemical stability of the Ta garnets (x = 0) in moisture may be due to the lower electronegativity of Ta compared to Nb, resulting in greater covalency of the Ta–O bonds, making them more difficult to break.