The diameters of the dendrites were around 100-200 nm. Dendrites came in various morphologies, including "rod" and "bent" shapes. Distinct 2D shapes were observed for the same "bent" Li metal dendrite imaged at various tilting angles as shown in Supplementary Movie 1 and 2. In the tomography reconstruction, to show only the surface SEI layer, a threshold on pixel intensity was imposed to show only the surface of the dendrite. However, it is still challenging to determine the exact shape of the dendrite due to the limited tilt range of the stage. The reconstructed tomograms have elongation and ghost tail artifacts. Fig. | A schematic of cryo-ET, cryo-ET tilt series and corresponding 3D reconstruction of freshly deposited, half-stripped, and calendar aged Li metal dendrites. a, The schematic of cryo-ET data collection and image processing procedures. b, Cryo-ET tilt series of a Li metal dendrite deposited in 1M LiPF6 in EC/DEC electrolyte and calendar aged for 24 hours, sampled images shown here for every 10 o between -60 o to + 60 o . The scale bar is 200 nm. Supplementary Movie 5 shows corresponding full tilt series. c, A single image from the tilt series of freshly deposited Li metal dendrite (left panel) and the corresponding 3D representation of reconstructed freshly deposited Li metal dendrite (right panel). The sample was on carbon support. The SEI layer is located on the surface of the Li metal and is highlighted in this display with its color based on the height in z-direction in the reconstruction. d, A single image from the tilt series of half-stripped Li metal dendrite (left panel) and the corresponding 3D representation of reconstructed half-stripped Li metal dendrite (right panel). The hollow shell of SEI and remaining Li metal sits on the carbon support. The SEI is colored and highlighted the same way as the freshly deposited sample. e, A single image from the tilt series of 24-hour calendar aged Li metal dendrite (left panel) and the corresponding 3D representation of reconstructed calendar aged Li metal dendrite (right panel). The darker contrast region in the cryo-EM image corresponds to LiH. The interface between Li and LiH looks sharp and clear in this projection. The SEI is colored and highlighted the same way as the freshly deposited sample. The LiH particle was annotated separately in white color and overlaid in this dendrite reconstruction to illustrate its spatial disposition. Supplementary Movie 6 shows the reconstructed calendar aged Li mental dendrite.