One of the approaches to improve the performance and ensure safe operation of lithium-metal batteries is the use of solid polymer electrolytes (SPE) that demonstrate relatively low reactivity towards metallic lithium. However, when lithium comes into contact with SPE, a solid electrolyte interphase (SEI) film is formed at the interface, although its composition, properties, and formation mechanism have not yet been sufficiently investigated. The present paper focuses on the issue of the lithium metal - polymer electrolyte interface stability. We used cyclic voltammetry and impedance spectroscopy to show that a passivating SEI layer forms at the working electrode in contact with SPE (PEO₂₀LiTFSI) both during the cell cycling and at an open circuit potential, with the SEI thickness increasing to a certain point depending on the temperature. The chemical reaction that corresponds to the SEI formation was studied by XPS and NEXAFS spectroscopy for model experiments, including lithium deposition on polyethylene oxide (PEO) under ultrahigh vacuum. Based on the spectroscopy data, we propose a reaction mechanism that features reductive cleavage of a PEO molecule by lithium atoms, which yields not only lithium alkoxides (ROLi) but also polyethylene fragments. Although such kind of SEI should generally reduce the ionic conductivity of SPE, it adds stability to the system at the same time.