The formation energy of defects depends on the chemical potentials of the elements associated with a defect (Eq. 2). For a given compound, the accessible range of chemical potentials is set by the phase stability in the compound phase diagram obtained through a convex hull analysis. We investigate the thermodynamically stable phases in the quaternary Li-P-O-N chemical space. Figure shows the quaternary phase diagram containing the SEI binaries. We use Li, LiP, Li 2 O 2 , and N 2 as the end members. Elemental oxygen and phosphorous are not used because they are less relevant in either the synthesis of LiPON or the interfacial reactions. Notably, the ternary SEI component Li 3 PO 4 is outside of this phase diagram. Due to its complex phase equilibria, the phase diagram of Li 3 PO 4 is discussed separately in Section II E.