With low-dose cryo-EM, we further examined the compact SEI, the SEI portion that closely interfaced with the anode particle, on LiH surfaces. For the 24-hour calendar aged sample, the compact SEI on the LiH surface was fully amorphous and uniform (Fig. ). It also has a thickness of approximately 10 nm (Fig. ), similar to that of the SEI on the fresh Li metal surfaces. This indicated that the preferential nucleation of LiH in certain parts of the Li metal dendrite was unlikely the result of a shorter SEI diffusion length for reactive hydrogen containing species to reach the Li metal surface. The interface between SEI and LiH remained sharp and smooth. For the one-week calendar aged sample, the thickness of compact SEI on LiH surface increased by about 2-3 nm, to ~12-14 nm. The SEI, after one-week aging, became rougher while the LiH-SEI interface remained sharp (Fig. ). Notably, a similar amount of increment in SEI thickness was observed for both LiH and Li metal surfaces. Still, the aged SEI on both Li metal and LiH remained amorphous (Fig. ). The extended SEI showed lighter contrast compared to the compact SEI, which is consistent with the previous report that extended SEI were mostly organic species from electrolyte decomposition .