Origin of Stability in the Solid Electrolyte Interphase formed between Lithium and Lithium Phosphorus Oxynitride

Lithium phosphorus oxynitride (LiPON) is one of the few solid electrolytes that form a truly passivating solid electrolyte interphase (SEI) when in contact with metallic lithium. Investigations into the origin of this stability may provide the insights needed to replicate it in the SEIs of alternative solid electrolyte materials. In this study, we used in situ lithium plating X-ray photoelectron spectroscopy (XPS) to investigate the formation and evolution of the Li-LiPON SEI. We show that the SEI is chemically and structurally inhomogeneous, with the fully reduced compounds identified in previous studies (Li₂O, Li₃N, and Li₃P) concentrated near the lithium metal side and partially lithiated species, including Li_xP, predominant closer to the LiPON side. Li₃P and Li_xP have recently been suggested as enablers of continuous SEI growth in thiophosphate solid electrolytes. We suggest that the stability of the Li-LiPON SEI is derived from a combination of the LiPON reduction potential (0.68 V vs Li⁺/Li), which is below the oxidation potentials of the fully reduced SEI compounds, and the graded structure of the SEI, which ensures that the most reduced species are not in physical or electrical contact with the LiPON layer.