The glassy lithium phosphorus oxynitride Li x PO y N z (with unknown stoichiometry), termed as LiPON, is one of the few Li-ion solid electrolytes considered to be electrochemically "stable" against lithium(Li)-metal anode. The chemical composition of LiPON, let aside its structure, has been the subject of many studies. A cycle life of 10,000 with 90% capacity retention has been claimed for ASSBs incorporating LiPON electrolyte paired with high-energy density Li-metal anode and a spinel LiNi 0.5 Mn 1.5 O 4 cathode. Despite the good interfacial stability indicated by the device performance, first-principles calculations have shown that the decomposition of LiPON and the consequent formation of Li-LiPON SEIs are thermodynamically favorable. LiPON reacts with Li and 4 are the primary decomposition products present in the Li-LiPON SEI. The spatial distribution of the SEIs' components shown in this schematic are only for representative purposes, and inspired by existing detailed experimental characterizations. decomposes to binary Li 2 O, Li 3 N, and Li 3 P and ternary Li 3 PO 4 . The fact that LiPON reacts with Li seems contradictory to the interfacial stability, but the reaction shows self-passivating behavior, which stabilizes the interface. It is believed that progressive LiPON decomposition is prevented after a "thin" SEI layer forms and serves as a passivating layer that limits electron transfer, while still allowing facile Li-ion transport.