The LLZO used for the cantilever bending experiment was mechanically polished and heat treated before cutting into cantilevers. The polishing and heat treatment processes are the same as those used for sample preparation for the EIS measurements of ionic conductivity (see Supplemental Information). A LLZO cantilever of 15 mm × 3 mm × 1 mm was cut using a custom scoring and fracturing jig, and then hand-pressed onto a thin (~100 µm) lithium-oncopper foil. Due to the significantly higher elastic modulus and thickness of LLZO than lithium and copper, the contribution of the lithium and copper was neglected in strain calculation. The sample was installed on a 3D printed electronically conductive fixture made of polylactic acid.