LAUSR

Summary To shed light on the formation process and structure of the solid electrolyte interphase (SEI) layer on native oxide-terminated silicon wafer anodes from a carbonate-based electrolyte (LP30), we combined in situ synchrotron X-ray reflectivity, linear sweep voltammetry, ex situ X-ray photoelectron spectroscopy, and first principles calculations from the Materials Project. We present in situ sub-nanometer resolution structural insights and compositional information of the SEI, as well as predicted equilibrium phase stability. Combining these findings, we observe two well-defined inorganic SEI layers next to the Si anode—a bottom-SEI layer (adjacent to the electrode) formed via the lithiation of the native oxide, and a top-SEI layer mainly consisting of the electrolyte decomposition product LiF. Our study provides novel mechanistic insights into the SEI growth process on Si, and we discuss several important implications regarding ion and electron transport through the SEI layer.