Interaction between Li, Ultrathin Adsorbed Ethylene Carbonate Films, and CoO(111) Thin Films: A Model Study of the Solid Electrolyte Interphase Formation at CoO Anodes

Aiming at a detailed understanding of the formation of the solid electrolyte interphase (SEI) at the electrode–electrolyte interface, which plays a critical role in the performance of Li-ion batteries (LIBs), we studied the interaction between Li, ultrathin films of ethylene carbonate (EC, main electrolyte component in LIBs), and CoO(111) thin films grown on Ru(0001), where the latter serves as a model for a conversion electrode, under ultrahigh vacuum conditions. Employing X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and Fourier transform infrared spectroscopy, we found that vapor deposition of EC on CoO(111) at 80 K results in partial decomposition of EC for films in the monolayer range, most likely because of interaction with defect sites, while it adsorbs molecularly in the multilayer regime. In both cases, desorption sets in at 170 K. Between 220 and 240 K, competing desorption and decomposition take place. To mimic the electrolyte, 0.5–2 ML of Li was stepwise postdeposited on a preadsorbed EC adlayer at 80 K and at 300 K, which leads to EC decomposition, most likely into Li-containing −C═O, −C–O–C–, −C–H, −C–C– species, and Li 2 O 2 or LiOH. This can be considered as the initial stage of the chemical SEI formation (open-circuit conditions). CoO conversion, which is essential for Li storage in the electrode, is observed after postdeposition of Li onto a surface precovered with EC decomposition products at 300 K. In these measurements, we could resolve molecular details on the SEI formation on a CoO model anode and on the conversion of CoO, both of which are important processes in conversion-based LIBs.