Adsorption of organic carbonate solvents on a carbon surface probed by sum frequency generation (SFG) vibrational spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy was employed to characterize the adsorption structures of organic carbonate solvents, i.e., dimethyl carbonates (DMC) and propylene carbonate (PC), which are used as the solvents in Li-ion batteries, on the surface of a thermally-evaporated carbon thin-film in combination with X-ray photoelectron spectroscopy (XPS). The surface-bound oxygen species on the as-prepared carbon surface could be partially reduced by a pyrolysis process in flowing H₂ at evaluated temperature. Two adsorption geometries on the reduced carbon surface were observed for DMC and PC. PC seems to be preferentially adsorbed on the carbon surface in comparison to DMC from a binary mixture of the solvents. Furthermore, the adsorption of vinylene carbonates (VC) and fluoroethylene carbonates (FEC), which are commonly used as additives in the electrolyte solutions of the Li-ion battery, were also evaluated. It was found that the additives also show a propensity to align on the carbon surface with respect to DMC, but comparable to PC. The different roles of the additives in the formation of the solid electrolyte interphase (SEI) will be discussed in comparison with organic carbonate solvents. The present study is considered to be fundamentally and practically useful to control and understand the interfacial behaviors and the SEI formation on the anode surface of the Li-ion battery.