XPS analysis of the SEI formed on carbonaceous materials

Two carbonaceous materials were produced by chemical vapour deposition of ethylene and by pyrolysis of dehydrated sucrose. Electrochemical cells assembled from these materials and metallic lithium were cycled between 0.00 and 2.00 V vs. Li/Li⁺ in ethylene carbonate/diethylcarbonate electrolytes containing LiPF₆ or LiAsF₆. The solid electrolyte interphase (SEI) formed on the carbons was characterised by X-ray photoelectron spectroscopy (XPS). We suggest that the carbon matrix has a more marked effect on the composition and thickness of the SEI than does the nature of the electrolyte. The SEI formed on graphite-like soft carbon in both electrolytes proved to be carbonate-free, its inorganic part consisting almost exclusively of LiF, while the SEI formed on hard (non-graphitizable) carbon was found to be considerably thicker and contained, in addition, phosphorus and arsenic compounds. In the bulk SEI, polymer structures (i.e., solvent-polymerisation products) were abundant in all cases, while carbonates were found only on hard carbon in the presence of LiAsF₆.