We demonstrate a computational approach to assessing the electronic passivation of SEIs and stability of solid-solid interfaces in ASSBs. Specifically, we used firstprinciples calculations to determine the concentrations of point defects and charge carriers in the Li-LiPON SEIs, and demonstrated that the interfacial stability arises from electronic passivation of the SEI layer. In calculating the defect and charge carrier concentrations, we considered di↵erent (electro)chemical conditions in the 300-500 K temperature range. Combining the experimentally observed spatial distribution of the di↵erent SEI components, we identified their relevant chemical conditions in the SEI layer. Under the relevant moderate Li chemical potential condition, Li 3 N shows high electronic conductivity up to 10 6 ⇠ 10 3 S/cm in the temperature range of interest. Li 3 P is e↵ectively under the Li-poor condition in the SEI layer, and it shows considerable electronic conductivity of 10 10 ⇠ 10 5 S/cm. Across the chemical conditions from Li-rich (reducing) to Li-poor (oxidizing), Li 2 O and Li 3 PO 4 show negligible carrier concentrations and electronic conductivity arising from the defects. We reveal that the overall SEI layer is electronically insulating because the stoichiometrically abundant and uniformly distributed Li 2 O is insulating, while the electronically conducting Li 3 N and Li 3 P were previously found to be absent near the Li-metal anode or non-uniformly distributed in the SEI. These findings provide evidence for the hypothesis that the Li-LiPON interface is stabilized due to the formation of electronically passivated SEI that prevents further LiPON decomposition. While we have focused on the Li-LiPON interface, our case study demonstrates a computational approach that can e↵ectively complement experimental investigations of interfacial stability in ASSBs. We expect this study to stimulate future investigations on the interfacial stability for other solid electrolytes and interphases.