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. Display omitted •Multi-property study of the structure and composition of the SEI•Potential-dependent growth of two well-defined inorganic SEI layers on Si/SiO2 anode•Unraveling of Li+ and e− transport properties through the SEI Despite the electronic revolution initiated by lithium-ion batteries (LIBs) three decades ago, one aspect of these energy storage devices still puzzles researchers. This is the solid electrolyte interphase (SEI) that forms on electrodes because LIBs operate outside the electrolyte stability window and can effectively passivate the electrode. Experimentally, the SEI is challenging to study with the desired atomic resolution as it is buried at the electrolyte-electrode interface. In this article, we provide fresh insights into the nature and transport properties of the SEI, via a multi-property combined experimental and simulation approach utilizing well-defined model systems. We unraveled the structure and composition, as well as the formation mechanism of the SEI on silicon anodes. Our findings are discussed with regard to understanding possible SEI-induced bottlenecks in LIBs and the relevance for their optimization. The solid electrolyte interphase (SEI) is a passivation layer naturally formed on battery electrodes. It protects electrodes and electrolytes from degradation and dictates charging time capabilities and lifetime. Despite its importance, it remains a poorly understood battery component. This study provides novel insights into the formation, morphology, and composition of the SEI on Si anodes through a multi-modal approach. The findings show a layered SEI and the ion and electron conductivities, as well as their relation to performance, are discussed.