Fig. 3. Lithium metal–LLZO interface at 7.4 MPa.

Polarization test of Li (s)–LLZO interface at 7.4 MPa stack pressure conducted in a Li/LLZO/Li cell. After a few hours of initial mechanical equilibration, 0.2 mA/cm² is applied galvanostatically until the voltage cutoff of 5 V, followed by a 25 h OCV rest step, a second polarization step in the same direction, and a second OCV rest step. a Normalized acoustic amplitude. b Voltage profile showing an increase in overpotential to voltage cutoff of 5 V during the galvanostatic current step of 0.2 mA/cm². Vertical guidelines (dotted blue lines) are shown as a visual aid to delineate start and stop of galvanostatic current. c Zoomed in region of second polarization step which is accompanied by a sudden attenuation of acoustic amplitude. d 1D slices of ex situ ⁷Li CSI highlighting the lithium metal region between 200 and 280 ppm, as indicated by the dashed region in e. e Ex situ ⁷Li CSI contour plot showing microstructural lithium signal at the Li (s)–LLZO interface for both electrodes, as well as a small quantity of lithium metal signal extending through the body of the electrolyte. f Nyquist plot from potentiostatic electrochemical impedance spectroscopy conducted during initial OCV, after galvanostatic current to the voltage cutoff, after the 25 h OCV, and after the second plating step to the voltage cutoff. The open circle indicates the high-frequency start point (7 MHz), and the cross indicates the low-frequency end point (0.5 Hz). g Simplified cartoon illustration of interface behavior implied by the experimental data (not drawn to scale).