Despite numerous efforts to elucidate interface-related phenomena of Li ion battery cathodes, the exact nature of cathode/electrolyte interfaces is still not fully resolved. Key factors for the properties of semiconducting ionic electrodes are band bending and energy level alignment at the interface, which have not been given much attention in the past. In this contribution, we investigate the formation of the electrochemical interface for a LiCoO 2 electrode in contact with a solvent adsorbate phase by a surface science approach. Diethyl carbonate (DEC) was adsorbed stepwise onto a LiCoO 2 thin film electrode and the electrode surface analyzed with X-ray photoelectron spectroscopy (XPS) after each adsorption step. Adsorption results in the formation of a charged layer in the electrode, which we attribute to the transfer of lithium ions from the electrode to the adsorbed phase. The offset between the LiCoO 2 valence band and HOMO of the adsorbed DEC is large (4 eV) under the experimental conditions, which renders solvent oxidation unlikely.

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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.

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