S. Chattopadhyay, A.L. Lipson, H.J. Karmel, J.D. Emery, T.T. Fister, P.A. Fenter, M.C. Hersam, and M.J. Bedzyk
Northwestern University and Argonne National Laboratory

The solid electrolyte interphase (SEI), which forms spontaneously at the electrode-electrolyte interface, plays a critical role in the performance and safety of Li-ion batteries, but little is known about its structure from in-situ observations. A model system, epitaxial graphene on SiC, was used to provide a well-defined surface that is similar to graphite, the most commonly used anode in Li-ion batteries. The formation and growth of one component of the SEI was monitored by in situ X-ray studies: LiF crystallites, which were observed to increase in size with increased charging, are oriented such that the LiF (002) planes are approximately parallel to the graphene sheets. SEI components, other than LiF, did not produce X-ray diffraction peaks and were therefore presumed to be amorphous. High-resolution transmission electron microscopy (HRTEM) showed that the LiF crystallites were located in near proximity to the graphene surface together with additional apparently amorphous material, which can likely be attributed to other SEI components and/or mis-oriented LiF. These observations will assist future efforts to understand and control SEI formation on graphitic materials.

Illustrations

High-resolution transmission electron micrograph of the EG/SiC and SEI interface showing a ~2-nm-thick amorphous SEI region on top of the graphene. Above that layer there is a mix of LiF crystallites and amorphous material or misoriented LiF.

X-ray diffraction pattern of a 0.51 mAh/cm2 lithiated EG/SiC sample at an incident angle of 5°. LiF diffraction rings can be seen. The bright spots in the rings and their angular correlations indicate strong texturing.

Reference

S. Chattopadhyay, A. L. Lipson, H. J. Karmel, J. D. Emery, T. T. Fister, P. A. Fenter, M. C. Hersam, and M. J. Bedzyk, ​“In Situ X-ray Study of the Solid Electrolyte Interphase (SEI) Formation on Graphene as a Model Li-ion Battery Anode,” Chemistry of Materials, 24(15), 3038 –3043 (2012).