To understand how LiH intraparticle heterogeneity develops as aging progresses, we further extended the calendar aging to a week for the Li metal anode sample before cryo-ET sample preparation. The same tomographic reconstruction and segmentation pipeline was used. The tilt series and 3D reconstruction are shown in Fig. . There were three major differences compared with the 24-hour aged sample. First, the SEI surface of the dendrite became rougher and thicker. Second, there were porous clusters of SEIs attached to the dendrite surface (Fig. ). Both observations are consistent with previous reports about the growth of SEI on aged carbonaceous anodes and Li metal anodes, where these agglomerates make up the extended SEI . These porous clusters that make up the extended SEI expanded up to over 100 nm in thickness. Third, in the one-week aged sample, the LiH took up a much larger portion of the dendrite compared with the 24-hour aged sample, which indicated a continuous growth of LiH upon aging (Fig. ). The LiH remained single-crystalline after extended aging for a week. Note that the growth of extended SEI is another source of lithium inventory loss aside from the LiH formation during the aging process. The reactive Li metal might lose electrons to partially reduced species in the swollen SEI and such processes would generate reactive hydrogen containing species to either react with Li metal to form LiH or diffuse into bulk electrolyte and initiate extended SEI growth. However, we observed a clear spatial separation between extended SEI growth and LiH growth. Although some extended SEI growth occurred on the LiH surface, the majority of the heavy extended SEI growth was on the remaining Li metal surface. This spatial separation was interesting. On one hand, it suggested that the extended SEI growth was unlikely the result of partially soluble SEI species reprecipitation from the electrolyte, otherwise we would expect more uniform extended SEI distribution on both Li metal and LiH. In addition, it seemed to suggest that extended SEI growth was more of a local phenomenon at the proximity of Li metal surface, and the fact that substantial extended SEI growth was observed only after prolonged calendar aging suggested that there existed also temporal separation between extended SEI growth and LiH growth aside from the spatial separation (Fig. ). We postulate that a likely scenario would be that LiH nucleation and growth happened faster in the beginning to convert local active Li metal in the dendrite, and progressed towards the other end. At the same time, the original Li metal portion that was far from the LiH nuclei reacted with electrolyte to form extended SEI with a slower rate. As the LiH growth progressed, the extended SEI grew thicker and made it difficult for reactive hydrogen containing species to diffuse to the Li metal surface and to react.