Solid-State NMR Investigation of Electrolyte Effects on Silicon–Graphite Composite Anode: Solid Electrolyte Interphase Formation and Failure Mechanisms

Silicon (Si) is a promising anode material due to its high specific capacity (∼3580 mAh g^-1), far exceeding that of graphite (∼372 mAh g^-1). However, its large volumetric expansion (∼300%) during lithiation induces mechanical stress, fracturing particles, and repeatedly exposing fresh surfaces to the electrolyte. This leads to continuous SEI growth, consuming lithium and electrolyte, and causing rapid capacity fading. To address these issues, strategies such as incorporating Si into graphite (Gr) composites and optimizing electrolytes have shown promise in improving the stability and performance of Si-based anodes. NMR spectroscopy offers element-specific sensitivity and can probe local chemical environments, making it a powerful tool for examining both the surface and bulk properties of battery materials. In this work, we use solid-state NMR spectroscopy to investigate Si/Gr anodes in two systematically chosen electrolytes: one EC-based (known to form organic-rich SEI) and one FEC-based (inorganic-rich SEI). We conducted 1D ⁷Li, ¹⁹F, and ¹H NMR experiments to elucidate the lithiation mechanism and identify SEI components in Si/Gr composite anodes during the first cycle and after extended cycling in the fully lithiated state for these two electrolyte systems. Additionally, we performed cross-polarization (CP) and two-dimensional exchange spectroscopy (EXSY) NMR experiments to gain deeper insight into Li⁺ coordination within different SEI components and to probe dynamic exchange processes between the SEI and lithiated Si/Gr phases (Li _x Si/Li _x C₆). ¹H/¹⁹F → ⁷Li CP-MAS EXSY NMR was employed to selectively probe Li⁺ exchange originating from either the organic or inorganic fraction of the SEI. These NMR results were correlated to the electrochemical performance of the Si/Gr anode in both electrolyte systems.