Solid electrolytes: Polycrystalline Li6.75La3Zr1.75Ta0.25O12 (LLZTO) was obtained from Toshima Manufacturing Inc, Saitama, Japan. All electrolytes had dimensions of ½ inch diameter and ~1 mm thickness. The as-received electrolyte discs were polished using an EcoMet™ 250 Pro Grinder Polisher (Buehler, Lake Bluff, Illinois, USA). To obtain finely polished faces, the samples were sequentially polished using aqueous diamond suspensions of 9 μm and 1 μm particle size, for 25 min and 10 min, respectively. The samples were then ultrasonically cleaned in DI water to remove polishing debris.