Other cells, cycled under nominal areal capacities of 15 and 30 mAh cm -2 per half cycle (75 and 150 um lithium plating thickness respectively), were subjected to galvanostatic cycling at nominal current densities from 0.5 to 3 mA cm -2 using 0.5 mA cm -2 increments. Li/LLZTO/NaK/Li asymmetric cells and their respective Li/LLZTO/Li controls were subjected to 1 galvanostatic cycle at nominal current densities of 0.25 or 0.5 mA cm -2 at a nominal areal capacity of 6 mAh cm - 2 . As discussed in the main text, actual current densities were determined by measurement of electrode areas in the disassembled cells. The process by which the final critical current density as well as its associated error is determined is described in Supplementary Methods, using data from Figure as an example.