In Situ Solid Conversion into Mechanically Adaptive LiF‐Rich Solid Electrolyte Interphase via MgF ₂ Precursor on Si Surface in Lithium‐Ion Batteries

Abstract Silicon (Si) anodes hold exceptional promise for high‐energy‐density lithium‐ion batteries (LIBs) due to their ultrahigh theoretical capacity (∼4200 mAh g⁻¹). However, their commercialization is severely hindered by the significant volume expansion (∼300%) and unstable solid electrolyte interphase (SEI). Conventional SEI, predominantly composed of organic species, suffers from low ionic conductivity, low electronic insulation, and poor mechanical robustness, leading to rapid capacity decay. Herein, we propose an interface engineering strategy by decorating Si nanoparticles with an in situ conversed MgF 2 layer (with coating integrity of 94.2%). During initial lithiation, the applied MgF 2 layer is in situ conversed into SEI film with high ionic conductivity, electronic insulation, and better mechanical adaptability. The prepared Si@MgF 2 ‐1 anode achieves a high initial coulombic efficiency (91.7%), superior rate capability (2000 mAh g⁻¹ at 10 C), and remarkable cycling stability (1794.9 mAh g −1 after 500 cycles). Full‐cell based on the Si@MgF 2 ‐1 anode and NCM811 cathode further validate the practicality of this approach. The robust conversion strategy for the construction of a mechanically adaptive LiF‐rich SEI layer holds significant promise for the advancement of durable silicon‐based LIBs.