Study of highly oriented pyrolytic graphite as a model for the graphite anode in Li-ion batteries

The mechanisms of oxidation of the basal plane and of the cross-sectional face of highly oriented pyrolytic graphite (HOPG) and the formation of a solid electrolyte interphase (SEI) on HOPG samples that were cycled in ethylene carbonate:diethyl carbonate (EC:DEC 1:2) solutions containing 1 M LiAsF₆ were studied. X-ray photoelectron spectroscopy, energy dispersive spectrometry, and scanning electron microscope techniques were used for the analysis of the surface layer formed on the basal plane and cross section of HOPG. The analysis indicates that the oxidation mechanisms of the basal plane and the cross section are entirely different. The SEI formed in the LiAsF₆ solution is thinner on the basal plane than on the cross section and its composition is different. The SEI formed on the cross section is rich in inorganic compounds whereas the SEI formed on the basal plane is rich in organic compounds. Thus it can be concluded that on the basal plane, the greatest contribution to SEI formation is solvent reduction (EC and DEC), whereas on the cross-sectional face, it is electrolyte salt (LiAsF₆) reduction.