© 2020 Li metal anode coupled with solid-state electrolyte is regarded as an important step for next-generation high-energy batteries to boost the performance of electric vehicles. However, commercialization of Li metal anode in all-solid-state batteries have faced challenges, one of which is the interfacial instability originated during battery operation. To overcome the difficulties in characterizing such interfaces due to their sensitivity to beam and ambient air, we applied cryogenic electron microscopy to unravel the stable nature of Li metal/LiPON interface that has exhibited remarkable electrochemical cyclability. An 80-nm-thick interphase with concentration gradients of N and P was observed, with decomposition products embedded in an amorphous matrix and exhibiting a multilayer-mosaic structure. The findings and methodology in this work give rise to a mechanistic understanding of the stability of Li metal/LiPON interface and can then be extended to study other solid-solid interfaces.