Extended Data Fig. 1: Schematic and SEM images of the cryo-FIB lift-out sample preparation process, and examples of additional final lamellae.

a, A buried structure or interface is identified for preparation, here a dendrite embedded in frozen electrolyte above the anode (indicated by the red arrow). In our coin-cell batteries, raised regions of electrolyte were used to localize buried dendrites. b, e, f, Trenches are then site-specifically milled around the site of interest, forming a vertical cross-sectional lamella containing the structure or interface. The sample is aligned in the microscope so that the electrode surface normal is parallel to the electron beam direction in e, and tilted by 52° to image the lithium anode–electrolyte interface and the electrolyte-embedded dendrite in f. c, g, A cooled nanomanipulator needle is then attached to the cryo-immobilized lamella by water vapour from a gas-injection system deposited as amorphous ice. The lamella is then cut free from the sample and lifted out. d, h, Finally, the lamella is attached to a TEM grid post with additional ice deposition, cut free from the nanomanipulator and thinned to electron transparency with the ion beam. i, j, Lamellae containing type II dendrites above lithium electrodes. The lamella in i contains a fluorine-rich structure as well. Different electrolyte thicknesses and milling parameters were used to prepare these lamellae, resulting in different final dimensions. k, A lamella produced from an uncycled electrode, used to obtain reference spectra. The increased signal of the uncycled electrode is due to different image-acquisition parameters, not a material difference.