By capitalizing on recent advancements in cryo-EM and cryo-ET, we elucidated the structural and compositional changes in calendar aged Li metal anodes where we reconstructed a 3D representation of the deposited Li metal dendrites. During the calendar aging process, we observed two aging phenomena: the growth of LiH and the growth of extended SEI. We identified both temporal and spatial separation of these two phenomena in Li metal anode aging. In shorter-term calendar aging, we only noted the growth of LiH; in prolonged calendar aging, we observed more extensive LiH growth accompanied by the growth of the extended SEI. Specifically, a majority of the extended SEI was found to be spatially associated with the Li metal portion within the dendrite, while only limited extended SEI was observed on the LiH portion. By employing a combination of multiscale imaging and diffraction capabilities, we achieved atomic-level resolution images of LiH intraparticle heterogeneities and of the interfaces between Li and LiH. Although we cannot exclude the possibility of SEI dissolution and precipitation mechanisms contributing to extended SEI growth on the LiH portion, the predominant presence of extended SEI on the Li metal portion suggests that its growth occurs mostly locally. Our temporal and spatial differentiations of LiH and extended SEI growth provide valuable insights into understanding reaction kinetics in distinct Li metal anode aging modes. Moreover, these findings offer potential guidelines for mitigation strategies, such as electrolyte design, aimed at controlling and optimizing Li metal batteries. With improved instrumentation, we will be able to further increase the resolution to resolve the spatial arrangements of organic and inorganic domains in the SEI and their corresponding 3D spatial distributions. The establishment of correlation between intraparticle heterogeneity development, interfacial structures, and battery performance will provide tremendous opportunities to decipher otherwise inaccessible spatial and temporal scales of battery degradation mechanisms. The integration of multiscale imaging and three-dimensional tomography has significantly expanded the scope of our analysis, enabling these crucial insights.