Impedance spectroscopy was performed between stepwise increases in the galvanostatic current to further confirm whether a short circuit was present (Fig. ). Upon assembly, all electrochemical symmetric cells exhibited an electrochemical impedance spectroscopy (IS) spectrum (Nyquist plot) that exhibited intercepts at high frequency on the real axis corresponding to the expected total resistance (~150 Ω) of the solid electrolyte disc (thickness ~1 mm) with ionic conductivity of 1-2 mS cm -1 as measured previously . IS spectra measurements (Fig S1 ) after the sudden drop in polarization showed a disappearance of the arcs in the IS spectra, replaced by a single resistive point (<10 Ω) consistent with the formation of a short circuit.