The amount of Na-K alloy used in the experiment (5 μL) corresponds to ~70 μm of liquid at the electrode area used (~0.7 cm 2 ) if uniformly distributed. However, the actual liquid layer thickness is greatly reduced upon the application of stack pressure (~1.5 MPa and ~75 kPa), with the excess liquid accumulating at the edges of the electrode. The final layer thickness should be determined by the surface roughness of the solid electrolyte and the lithium metal foil. Since the polished solid electrolyte has sub-micrometer roughness, while the lithium metal foil likely has tens of micrometers initial surface roughness (which may smooth under pressure), we estimate that the thickness of the Na-K liquid metal layer is ~10 𝜇𝜇m.