In situ X-ray study of the solid electrolyte interphase (SEI) formation on graphene as a model Li-ion battery anode

The solid electrolyte interphase (SEI) plays a critical role in the performance and safety of Li-ion batteries, but the crystal structure of the materials formed have not been previously studied. We employ the model system of epitaxial graphene on SiC to provide a well-defined graphitic surface to study the crystallinity and texture formation in the SEI. We observe, via in situ synchrotron X-ray scattering, the formation and growth of LiF crystallites at the graphene surface, which increase in size with lithiation dose and are textured such that the LiF (002) planes are approximately parallel to the graphene sheets. Furthermore, X-ray photoelectron spectroscopy (XPS) reveals the composition of the SEI formed in this system to consist of LiF and organic compounds similar to those found previously on graphite. SEI components, other than LiF, do not produce X-ray diffraction peaks and are categorized as amorphous. From high-resolution transmission electron microscopy, the LiF crystallites are seen in near proximity to the graphene surface along with additional apparently amorphous material, which is likely to be other SEI components detected by XPS and/or misoriented LiF. This new understanding that LiF crystallites grow on the graphene surface with strong texturing will assist future efforts to model and engineer the SEI formed on graphitic materials.