With respect to stationary applications the demand of suitable storage technologies for electrical energy originates from discontinuous renewable primary energy sources has increased the interest in Li-ion batteries. In order to develop long lasting and cost efficient energy storage systems a comprehensive understanding of battery degradation mechanisms plays a fundamental role. Particularly the formation (and properties) of the cathodic solid electrolyte interface (SEI) under specific cycling conditions and its consequences on the mechanical fatigue has not been fully understood yet. Among the reasons are the limited number of studies published concerning the rudimentary analysis of composite electrodes. In this connection the analysis of the SEI layer on particles inside the electrode has not been performed yet. The main purpose of this work is a surface analysis of particles on the top and inside of the composite cathode after electrochemical cycling following a specific protocol.

This work deals with post mortem analysis of LiCoO₂-composite cathodes by X-ray photoelectron spectroscopy (XPS). The electrodes were taken from 18650 type Li-ion cells, which were galvanostatically cycled under specific protocols. Analysis was performed of the top surface of the cathode and of material buried inside the cathode. To analyze the surfaces of buried material the top part of the cathodes has been mechanically removed. To improve the assignment of the XP-spectral features we used reference materials like LiCoO₂ and SEI related components. We also performed experiments with pure LiCoO₂-cathodes to analyze the SEI layer on the active material without interference from layers formed on other constituents of a composite cathode.

The surface analysis of electrochemically treated LiCoO₂-composite cathodes showed a lower SEI thickness on buried material than on the top layer of the cathode. In both investigated locations of the electrode the same organic and inorganic (e.g. Li_xPO_yF_z-type) surface compounds were observed. We could determine that the proportion of the species in the SEI depends on the applied cycling protocol. Furthermore, the evaluated LiCoO₂ of the composite showed changes in the electronic structure compared to the XP-spectral feature of the LiCoO₂-reference material.