Structure Formation and Thermal Stability of Mono- and Multilayers of Ethylene Carbonate on Cu(111): A Model Study of the ElectrodelElectrolyte Interface

In this work, we aim at a molecular scale understanding of the interactions and structure formation at the electrode|electrolyte interface (EEI) in Li-ion batteries. Therefore, the interaction of the key electrolyte component ethylene carbonate (EC) with Cu(111) was investigated under ultrahigh vacuum conditions. Scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIRS), and dispersion-corrected density functional theory (DFT-D) calculations were employed. After vapor deposition of EC (sub-) monolayers on Cu(111) at 80 K, STM measurements (100 K) reveal a well-ordered commensurate superstructure, in which EC molecules assume different configurations and whose total adsorption energy is mainly governed by van der Waals interactions, as demonstrated by DFT-D. In the temperature range between 150-220 K, competing desorption and decomposition into -C = O, -C-O-C-, -C-H, and -C-C- compounds, as derived by XPS and confirmed by FTIRS, result in distinct changes of the adlayer composition. Similar heating of an EC multilayer film from 80 K to room temperature results in a surface that is almost completely covered with adsorbed, carbon-containing decomposition products. This can be interpreted as the initial stage of chemical EEI formation, and the relevance of these results for battery applications is discussed.