Dopant Concentration-Porosity-Li-Ion Conductivity Relationship in Garnet-Type Li_5+2xLa₃Ta_2-xY_xO₁₂ (0.05 ≤ x ≤ 0.75) and Their Stability in Water and 1 M LiCl

Highly Li-ion conductive Y-doped garnet-type Li_5+2xLa₃Ta_2-xY_xO₁₂ (0.05 ≤ x ≤ 0.75) were studied to understand the effects of yttrium- and lithium-doping on crystal structure, porosity, and Li-ion conductivity using ⁷Li MAS NMR, electrochemical ac impedance spectroscopy, and scanning electron microscopy (SEM), as well as ex situ and in situ powder X-ray diffraction (PXRD) to further explore the potential application of garnets in all-solid-state Li-ion batteries. Solid-state ⁷Li MAS NMR studies showed an increase in the Li-ion mobility as a function of Y- and Li-doping in Li_5+2xLa₃Ta_2-xY_xO₁₂, which is consistent with the results from ac impedance spectroscopy. The SEM studies on sintered pellets indicated a systematic decrease in porosity and an increase in sinterability as the Y- and Li-doping levels increase in Li_5+2xLa₃Ta_2-xY_xO₁₂. These results are consistent with the calculated porosity and densities using the Archimedes method. Using the variable-temperature in situ PXRD in the temperature range of 30-700 °C, a thermal expansion coefficient of 7.25 × 10^-6 K^-1 was observed for Li₆La₃Ta_1.5Y_0.5O₁₂. To further explore the possibility of a new application for the Li-stuffed garnets, the stability of these materials in aqueous LiCl solution was also studied. A high degree of structural stability was observed in these materials upon 1 M LiCl treatment, making them suitable candidates for further studies as protective layers for lithium electrodes in aqueous lithium batteries. (Graph Presented).