Additive-Specific SEI Nanostructures on Silicon Anodes Revealed by Cryo-TEM and EELS under Suppressed Bulk Alloying

Silicon anodes suffer from unstable solid electrolyte interphases (SEI) that drive capacity fade. Here, using low-dose cryogenic TEM and EELS under suppressed bulk Li–Si alloying (0.1 V vs Li/Li+ cutoff, 10 cycles), we resolve the atomic-scale SEI nanostructures induced by fluoroethylene carbonate (FEC), ethylene sulfite (ES), and lithium difluorophosphate (LiPO2F2). FEC forms a dense ∼20 nm LiF-rich nanocrystal scaffold, ES produces a 10–20 nm heterogeneous mosaic of LiF/Li2SO4 within an organic-rich matrix, and LiPO2F2 yields an ultrathin (∼10 nm) inorganic-dominated but brittle layer. Despite being the thickest, the FEC-derived SEI delivers the best long-term cycling stability. Mechanistically, optimal performance arises from a balanced architecture that combines strong electronic insulation, efficient Li+ transport across grain boundaries, and mechanical coherence rather than minimized thickness alone. These findings identify composition and nanostructural continuity as key regulators of interfacial stability in silicon anodes.