Cubic garnet phases based on Al-substituted Li₇La₃Zr₂O₁₂ (LLZO) exhibit superior ionic conductivity and outstanding electrochemical stability against metallic lithium anode, making them promising solid electrolyte for next-generation rechargeable battery systems. However, high interfacial impedances of LLZO have prevented their wide applications. The interfacial resistances of lithium cells containing Al-substituted LLZO solid electrolytes are sensitive to their microstructures and histories of exposure to air. Air exposure of LLZO samples with large grain sizes results in dramatically increased interfacial impedances in cells containing them, compared to those with pristine large-grained samples. In contrast, a much smaller difference is seen between cells with small-grained pristine and air-exposed LLZO samples. A combination of soft X-ray absorption (sXAS) and Raman spectroscopy, with probing depths ranging from nanometer to micrometer scales, revealed that the small-grained LLZO pellets are more air-stable than large-grained ones, forming far less surface Li₂CO₃ under both short- and long-term exposure conditions. Surface sensitive X-ray photoelectron spectroscopy (XPS) indicates that the better chemical stability of the small-grained LLZO is related to differences in the distribution of Al and Li at sample surfaces. Density functional theory thermodynamic calculations show that LLZO can react via different pathways to form Li₂CO₃. These observations have important implications for the operation of solid-state lithium batteries containing LLZO because the results suggest that the interfacial impedances of these devices is critically dependent upon specific characteristics of the solid electrolyte and how it is prepared.