Phase-selective SEI formation dynamics in Si–graphite composite electrodes

Understanding how each phase within a silicon (Si)–graphite composite anode forms its solid–electrolyte interphase (SEI) is essential for advancing interphase engineering in next-generation lithium-ion batteries. However, this question has remained unresolved because conventional electrochemical measurements provide only a volume-averaged response that obscures the contributions of individual phases. Here, by combining phase-resolved operando current measurements with complementary surface-sensitive analyses — X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and scanning spreading resistance microscopy (SSRM) — we reveal that SEI formation in Si–graphite composites is inherently phase-selective. Fluoroethylene carbonate (FEC) decomposes predominantly on Si, producing a fluorine-rich and highly resistive interphase, whereas the graphite surface remains weakly passivated. This imbalance is not a trivial consequence of surface-area differences but arises from the stronger interaction of Si surfaces with electrolyte molecules, as supported by density functional theory (DFT) calculations. The resulting passivation asymmetry highlights a fundamental constraint on interphase engineering in composite electrodes: the SEI on each phase must be optimized through a phase-resolved approach. Importantly, we show that this intrinsic phase selectivity can be turned into a design advantage rather than merely mitigated. As a proof of concept, we employ a co-additive strategy using FEC and vinylene carbonate (VC). Because both additives preferentially reduce on Si, their concurrent decomposition rapidly passivates the Si surface; once this passivation suppresses further additive reduction on Si, the reduction current redistributes toward graphite, ultimately producing a compositionally uniform interphase — both fluorine-rich and polymer-rich — across both phases. This uniform interphase effectively suppresses Li-inventory loss during calendar aging. Our findings establish phase-selective SEI formation as an intrinsic characteristic of composite electrodes and demonstrate that this selectivity can be exploited as a design principle for phase-resolved interphase engineering in multicomponent battery electrodes.