The Li/NaK/LLZTO/NaK/Li electrochemical cells were constructed as described in Methods. Details of the LLZTO sample preparation and the subsequent surface, mechanical, and electrical property characterization is available elsewhere . The critical current density (CCD) was measured by reversibly plating Li metal in both directions at stepwise increasing galvanostatic currents until a discontinuous change in current and polarization across the cell signaled the onset of a short circuit (Fig. ). We use the example of the Li-NaK/LLZTO/Li-NaK symmetric cell in Fig. to illustrate how the short circuits were identified. Here, the short circuit occurred at the beginning of the 12th cycle after a cumulative plated lithium thickness of ~700 μm (60 μm per cycle, 30 μm per half-cycle corresponding to 6.0 mAh cm -2 in each direction). The elapsed time of the experiment was about 150 hours. Fig 1a . shows the voltage-time trace for this experiment, where the sudden drop to zero voltage in the 12th cycle corresponds to the growth of a metal filled crack completely through the electrolyte disc. (bottom), here for a Li/NaK/LLZTO/NaK/Li symmetric cell reversibly plating 6 mAh cm -2 of Li metal (based on the initial assumed area) at a stack pressure of 1.5MPa. The composition of the NaK alloy is the eutectic (0.68K-0.32Na, by mole). The onset of cell failure, which serves as the measurement of the CCD, is indicated by the arrow. The onset of an electrical short circuit is detected as a voltage drop across the cell at 2.75 -3 mA cm -2 (corresponding to a true critical current density of 2.17 ± 0.09 mA cm -2 and a final measured areal capacity of 4.53 mAh cm -2 ) b) Optical digital microscopy images of the LLZTO solid electrolyte cycled in a) after post-mortem assembly. The site of metal penetration and the subsurface dendrite is labelled by an arrow. Inset: magnification of observed dendrite. Scale bar denotes 5 mm.