To identify these heterogeneities inside the lithium metal dendrites, we used high-resolution cryo-EM to obtain atomic resolution crystalline lattice images of these regions down specific axes. For the darker contrast region closely interfaced with Li, the lattice spacings were 2.05 Å and 2.35 Å, matching well with LiH (200) and (111) planes, respectively (Fig. ). The LiH heterogeneity developed here was a single crystal (Fig. ). The darker contrast of LiH compared to Li in the bright field imaging mode is in accordance with a higher gravimetric density (LiH: 0.78 g cm -3 , Li: 0.53 g cm ). Furthermore, we captured the atomically sharp interface between Li metal and LiH. Note that this sharp interface does not imply that the entire interface extends toward the viewer in 3D as a sharp plane. In fact, this interface is irregular in 3D (Fig. ). The interface between Li and LiH is along Li (110) with lattice spacing of 2.44 Å and LiH (111) with lattice spacing of 2.35 Å (Figu. 2c). We conclude that LiH is the intraparticle heterogeneity that develops inside the Li metal dendrites upon calendar aging but is absent in the freshly deposited samples. This was consistent with our previous reports on not finding the LiH in freshly deposited samples , and suggested a LiH growth mechanism where LiH nucleation and growth happened after initial Li metal deposition, due to gradual accumulation of active hydrogen containing species and its slow diffusion through the SEI layer. If excessive water contamination exists in the electrolyte, such a process could potentially be accelerated to form more LiH during Li metal deposition due to high reactivity of hydrogen in water molecules . To put the atomic details in the context of the aged dendrites, we examine the reconstruction of the LiH clearly showing an irregular "swiss-cheese-like" shape when viewed in 3D from different orientations (Fig. , Supplementary Movies 7, 8). There is an irregular surface facing the Li metal portion in the dendrite, while the outer surface of LiH extends to the dendrite surface. This LiH particle inside the Li dendrite metal was roughly 100×200 nm in 2D. More interestingly, in the reconstruction, we were able to identify a smaller third region inside the dendrite that was different from Li and LiH, as another heterogeneous particle. This region was closely interfaced with both Li and LiH as shown in the 3D reconstruction (Supplementary Movies 7, 8). The lattice spacing of this third region was 1.8 Å which corresponded to Cu (200). This encapsulation indicates that the Cu nanoparticle was picked up during the nucleation and growth of Li metal. The Cu impurity could also be found in the half-stripped case (Supplementary Movie 3). Such Cu nanoparticle detachment from the current collectors could lead to inferior battery performance, because metal particle contamination can cause an internal short circuit between the anode and cathode, which leads to a thermal runaway inside the battery . The fact that Cu nanoparticles resided in the middle of the dendrite, instead of the tip or the base, supported the coexistence of the basal and the tip growth modes in the Li metal dendrite growth. Otherwise, the particle would be either at the tip in the case of basal growth or at the base in the case of tip growth . Cu nanoparticles can be found in either fresh or calendar aged dendrites. However, there is not a necessary causal relationship between Cu nanoparticle existence and LiH growth as can be seen from a later case where there was no Cu nanoparticle but still significant LiH growth inside Li metal dendrites. The spatially close interface between LiH and Cu suggests that the strained area inside the Li metal can be a preferred location for heterogeneous phase nucleation, and the convex surface of the Cu nanoparticle further lowered the nucleation barrier of LiH.