Following the electrical detection of the short circuit, the cells were disassembled and imaged under an optical microscope to examine the surfaces of the positive and negative current collector as well as both surfaces of the solid electrolyte (Fig. ., Supplementary Fig. ). In all cases the metal electrode remained confined within the boundary of the solid electrolyte disc, ruling out the possibility of an external electronic short circuit around the edges of the electrolyte disc. This supports the inference from the IS measurements that the sudden drop in cell impedance is due to metal penetration through the solid electrolyte. It was observed that the Li electrode increases somewhat in area during cell assembly due to spreading of the metal. The electrode area may further grow during cycling. Therefore, we calculated the final current density and areal capacity for the experiments based on the measured post-mortem electrode areas from disassembled cells after cycling to failure. This methodology yields the most accurate current densities and areal capacities at the point of short circuit. In some instances, opaque, semi-circular regions at the edge of the alkali metal electrode were observed in the solid electrolyte that appear to be metal filled, as indicated in Fig. . These regions likely indicate a location of crack propagation. Due to field concentration effects elucidated in prior work on single crystal solid electrolytes , crack and dendrite initiation near such electrode edges may be favored. Fig. exhibits all of the critical current density measurements over the relevant areal capacities (3 -30 mAh cm -2 ) performed in this study. Numerical data are listed in Supplementary Table . Each datum in Fig. represents one critical current density measurement derived from a single cell. The areal capacities used in our experiments (3 -30 mAh cm -2 ) are up to nearly a hundred times higher than those at the high end of the range used in previous studies (0.25 mAh cm -2 ) [4-10], except for a recently published study from our group in which areal capacities of ~3 mAh cm -2 were used . as the solid electrolyte. The composition of the NaK alloy is the eutectic (0.68K-0.32Na, by mole) in all cases except for two data points marked with a cross overlay (where the composition is 0.25K -0.75Na, this alternative composition was used to test whether the CCD depended on the NaK composition). The liquid alloy is present at the interface between Li metal foil and LLZTO. All the experiments were conducted at 20°C under a nominal stack pressure of ~1.5 MPa, except for circled points, which are from experiments conducted at < 75 kPa. The term "warm-pressed" means that the cell was heated at 170°C for 3 hours under 1.5MPa stack pressure post-assembly, after which the cell was cooled to room temperature and the stack pressure removed if specified (see Methods). The term "as-assembled" means that no such pretreatment was performed, and that cycling was carried out immediately after assembly. Each datum represents an independent cell measurement. The total span of the error bars represents the variation in CCD for a single increment by which the current density was stepped in the galvanostatic protocol.