Prior to assembly the polished LLZTO discs were heat treated at 500°C in an argon-filled glove box at oxygen and water content below 0.1 ppm and 0.1 ppm, respectively, which has been shown to remove surface impurity layers (LiCO3 and LiOH) on the electrolyte and reduce the interfacial impedance. Electrochemical cells incorporating plain Li metal electrodes were assembled by placing a disc of flat lithium foil (0.375" diameter, 0.25 mm thickness, 50 mAh⋅cm -2 , ~20 μL lithium) on the LLZTO solid electrolyte with a stainless-steel cylinder acting as the current collector. Before electrochemical cycling, the cells were heated to 170°C for 3h at an applied stack pressure of 1.5 MPa (calibrated independently with a compression force gauge), which reduced the interfacial impedance of the cell. The area-specific interfacial impedance of the cells was measured to be on the order of 25-35 Ω⋅cm -2 (sum of both interfaces).