Polycrystalline LLZTO was obtained from Toshima Manufacturing Inc. (Saitama, Japan) as 1 mm thick, 12.7 mm diameter pellets. The phase purity of these pellets was confirmed via X-ray diffraction, and the bulk conductivity was measured as 1.03 mS/cm in the previous work of Park et al. LLZTO electrolytes were then mechanically polished to the end thickness specified within the text, using oil-based 1 µm diamond suspension for the last polishing step. Immediately after polish, the electrolyte discs were transferred into an oven within an Ar-containing glovebox. The discs were heat treated at 500°C for 3 hours.