XPS-Surface Analysis of SEI Layers on Li-Ion Cathodes: Part I. Investigation of Initial Surface Chemistry

In this contribution, we investigate the initial surface chemistry on fully lithiated LiCoO2 thin film model electrodes in the electrolyte solvent diethyl carbonate (DEC) and the LiPF6-electrolyte by means of soaking experiments. The interfacial layer composition is analyzed by X-ray photoelectron spectroscopy (XPS), and possible layer morphologies and spontaneous formation mechanisms are discussed in detail. Upon decomposition of DEC a layered system of surface-bound semi-organic components (inner layer) and cross-linked organic moieties (outer layer) is formed, while a change of the Co3+ oxidation state and thus a surface corrosion of LiCoO2 was not observed. In contrast, the solid electrolyte interface (SEI) film of the LiPF6-electrolyte soaked electrode showed an inner layer, containing predominantly corroded LiCoO2, i.e. Co(II,III)xOy(OH)z and LiF as well as aliphatic fluoroorganic species. The outer SEI layer consists mainly of a poly-organic network and randomly distributed LixPOyFz domains. The thickness of the deposit on the electrolyte soaked electrode surface was only half as thick due to the significantly lower amount of organic and semi-organic compounds. Our investigation indicates that the solvent decomposition is related to the catalytically active LiCoO2 surface, which is passivated by reaction products such as LiF originating from HF induced processes.