where i is the current, d is the effective diameter of the lithium-LLZO contact, σion is the ionic conductivity of LLZO, j is the areal current density, and j0 is the exchange current density (see the Supplemental Information for more details on the model). We employ this model to calculate the lithium whisker diameter (d) from the electrochemical data, and Figure shows a histogram of failure diameter values. From d, we then determine the current density and working electrode overpotential. The geometrically averaged current density at failure is extremely high with wide variation (Figure ), significantly higher than reported for macroscopic lithium metal cells, which is consistent with previously reported observations . Figure presents the analysis for three typical experiments. As we discuss later, d and the current density are not sensitive to either the choice of the charge-transfer mechanism (i.e., Butler-Volmer) or the exchange current density, as η WE is much smaller than η Ohm in our experiments. Specifically, neglecting the charge-transfer overpotential (second term) in Equation translates only to a difference in the estimated whisker diameter of < 10%.