Adsorption of Diethyl Carbonate on LiCoO2 Thin Films: Formation of the Electrochemical Interface

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 LiCoO2 electrode in contact with a solvent adsorbate phase by a surface science approach. Diethyl carbonate (DEC) was adsorbed stepwise onto a LiCoO2 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 LiCoO2 valence band and HOMO of the adsorbed DEC is large (4 eV) under the experimental conditions, which renders solvent oxidation unlikely.