In Situ Cryogenic X-ray Photoelectron Spectroscopy Unveils Metastable Components of SEI layers in Li-ion Batteries

The solid electrolyte interphase (SEI) layer evolution is a critical yet poorly understood phenomenon in Li-ion batteries. As a multiphasic component evolving from the reactivity of the electrolyte and anode, the SEI comprises many intermediate and metastable constituents, making it challenging to probe without altering chemical and physical states. While X-ray photoelectron spectroscopy (XPS) is widely used, the artifacts and distortions induced by standard pre-sample treatments, ultrahigh vacuum (UHV) conditions, and X-ray beam exposure often prevent a complete and consistent chemical analysis of SEI layers in their native state. In this study, we present a cryogenic X-ray photoelectron spectroscopy (Cryo-XPS) approach, integrated with residual gas analysis (RGA), to investigate the metastable and volatile intermediate constituents of the SEI layer. Our findings reveal that, contrary to previous studies, Li2CO3 and LiF are not the main SEI components but rather relatively stable remanent components that withstood the traditional XPS experimental conditions. Instead, metastable organic carbonates and organic intermediate products predominantly constitute the SEI. We identify lithium fluorophosphate (LiPOxFy) a decomposition product of LiPF6 as one of the primary inorganic constituents, which steadily decomposes to fluoride (LiF) and releases POF₃ upon UHV conditions. Hence, widely observed LiF phases in SEI layer are likely due to the decomposition of residual LiPF6 and LiPOxFy facilitated by UHV and X-ray exposure during traditional XPS analysis. Overall, our Cryo-XPS analysis, performed across varying thermal conditions and complemented by residual gas analyzer insights, unveils the chemical composition throughout the depth of the SEI layers revealing previously invisible components and significantly enhancing our understanding of their chemical evolution in Li-ion batteries.