Composite electrodes, particularly silicon/carbon (Si/C) anodes, hold significant potential for enhancing the energy density of lithium-ion batteries (LIBs). However, performance issues arise when silicon content exceeds 8-10 wt.%. A detailed understanding of the solid electrolyte interphase (SEI) in Si/C composites is crucial for addressing these challenges and improving energy density. Traditional techniques such as ex-situ X-ray photoelectron spectroscopy and cryogenic electron microscopy have limitations, including exposure to ultrahigh vacuum and solvent washing, which can alter the SEI layer. In contrast, nano-FTIR (Fourier transform infrared near-field spectroscopy) offers a non-destructive method to probe the SEI layer's chemistry and structure at room temperature within an inert atmosphere.

In this study, we designed custom-patterned electrodes with amorphous Si on atomically flat highly ordered pyrolytic graphite (HOPG) to serve as model composite electrodes. Using nano-FTIR spectroscopy, near-field nanoscale white-light imaging, and atomic force microscopy, we discovered that the SEI on HOPG is rich in organic lithium ethylene dicarbonate (LiEDC), while the SEI on lithiated silicon contains inorganic Li₂CO₃. Additionally, we identified a unique "mixed" SEI layer at the Si/HOPG interface, comprising SEI species from both lithiated Si and HOPG. This mixed SEI layer likely has distinctive physicochemical properties, forming along Si-C interfaces in Si/C composite electrodes. These findings offer a comprehensive approach to studying model composite electrodes and provide valuable insights for optimizing the surface passivation of Si/C composite electrodes to achieve high-performance LIBs.

This research was supported by the U.S. Department of Energy, Vehicle Technologies Office (DOE-VTO) under the Silicon Consortium Project, directed by Nicolas Eidson, Carine Steinway, Thomas Do, and Brian Cunningham, and managed by Anthony Burrell.