Recently, Li-LiPON SEIs formation and structure have been investigated with di↵erent microscopy and spectroscopic techniques (Figure ). In situ X-ray photoemission spectroscopy (XPS) probed surface species on LiPON exposed to Li vapor. The XPS spectra indicated that Li 2 O, Li 3 N, Li 3 P, and Li 3 PO 4 form after a short exposure, but then the signals for Li 3 N and Li 3 P become less intense after a longer exposure time. More recent experiments with cryogenic transmission electron microscopy (TEM) and XPS depth profiling revealed the presence of Li 2 O, Li 3 N, and Li 3 PO 4 , as well as a thin multilayer-mosaic structure with N and P concentration gradients (high concentrations near the LiPON side and absent near the Li side) for the Li-LiPON SEI. In situ TEM observed the evolution of LiPON contacting Li under bias. A ⇠60 nm thin layer forms at the Li-LiPON interface, where P species show preferential spatial distribution and do not appear in direct contact with Li metal. While these studies have been insightful in understanding the chemical makeup and spatial distribution of the SEI, the fundamental mechanism that lends to the self-passivating behavior is still unclear. Complementary theoretical modeling can provide further insights into the electronic origin of the self-passivation.