To assemble the cell, ~0.2 grams of a 50 wt% mixture of LiCl and KCl was first dried on a hot plate at ~150°C for 30 minutes. Then the mixture was loaded into a YSZ tube. Separately, lithium metal (~10 mg) was loaded into a smaller LLZO tube along with a stainless-steel current collector inside of an argon glovebox. The LLZO tube with lithium inside was removed from the glovebox, quickly inserted into the YSZ tube and pressurized (<1psi) with argon gas to prevent oxidation of the lithium. To ensure the salt was completely dry, the assembled cell was heated to 130°C for ~20 hours under argon atmosphere before the test began. Even small amounts of water present in the salt could cause the lithium metal to completely oxidize at higher temperatures. After the drying step, the cell temperature was increased to 550°C and maintained at a constant temperature for the test. The geometry of the assembled cell is displayed in Figure .