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 LiCoO 2 thin film model electrodes in the electrolyte solvent diethyl carbonate (DEC) and the LiPF 6 -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 Co 3+ oxidation state and thus a surface corrosion of LiCoO 2 was not observed. In contrast, the solid electrolyte interface (SEI) film of the LiPF 6 -electrolyte soaked electrode showed an inner layer, containing predominantly corroded LiCoO 2 , i.e. Co(II,III) x O y (OH) z and LiF as well as aliphatic fluoroorganic species. The outer SEI layer consists mainly of a poly-organic network and randomly distributed Li x PO y F z 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 LiCoO 2 surface, which is passivated by reaction products such as LiF originating from HF induced processes.