Understanding the SEI Formation at Pristine Li‐Ion Cathodes: Chemisorption and Reaction of DEC on LiCoO2 Surfaces Studied by a Combined SXPS/HREELS Approach

Reactions and solid electrolyte interface (SEI) formation at electrode–electrolyte interfaces are crucial for the stability and performance of Li-ion batteries, but are still not fully understood on a fundamental level. For improving battery properties, a detailed understanding of these degradation processes is needed. In this contribution, the interface formation between a thin film LiCoO2 cathode material and diethyl carbonate (DEC) as typical battery electrolyte solvent is presented. A surface-science approach is used performing a stepwise adsorption of DEC onto LiCoO2 at low temperatures. The interface is studied after each step by synchrotron-based X-ray photoemission spectroscopy (SXPS) and high-resolution electron energy loss spectroscopy. The results demonstrate that the decomposition of carbonate solvents in contact with fully lithiated cathode materials as observed in adsorption experiments is complex, including the reduction of solvent, subsequent decomposition reactions, and also catalytic effects. In the present case, lithium ethyl carbonate, lithium ethoxide, and lithium oxides are assigned as reaction products. The spectra provide indications for partial electron transfer coupled to covalent interaction involving surface oxygen O2p orbital and DEC lowest unoccupied molecular orbital (LUMO) states.