The interfacial reactions between Li and LiPON and the stable nature of the Li-LiPON SEI have been investigated by various microscopy and spectroscopy techniques. Schwöbel et al. studied the interface layer formation between Li and LiPON using in situ XPS. LiPON was exposed to Li vapor, and the reaction products after di↵erent exposure time were revealed by the XPS spectra. The authors proposed two reaction routes to describe the reactions between Li and two typical structural units in amorphous LiPON. The two proposed reaction routes can be added up to get Eq. 1. Li 6 P 3 O 9 N in the reaction formula is only an exemplary chemical structure in the amorphous LiPON, and it does not represent the exact stoichiometry of LiPON, which is presently unknown. Eq. 1 gives an approximation of the ratio of the Li-LiPON reaction products. Assuming Eq. 1 has a yield of 100%, the number of moles of Li 2 O significantly exceed the other three products, which are expected to only make up a small fraction of the Li-LiPON SEI. In this sense, one can see that the insulating Li 2 O (Table ) alone contributes significantly to the nature of the SEI electronic conductivity.