Extended Data Fig. 5: Full spectra recorded from the dendrites (intensities on a logarithmic scale).

The spectra show clear differences in the plasmons and Li K-edges, as well as a large difference in oxygen content between the type I and type II dendrites. The small amount of oxygen on the type II dendrite is probably due to water molecules adsorbed on the surface of the sample in the microscope vacuum, which would typically react with materials such as lithium or sodium at room temperature. No nitrogen was present in either dendrite, confirming that no reaction with nitrogen in the air or liquid N₂ had occurred.