The increasing demand of suitable storage technologies for electrical energy 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) layer under specific cycling conditions and its consequences on the fatigue of the cells has not been fully understood. Furthermore the nature of the SEI and its effects on particles inside the electrode has not been investigated yet. The composition of the SEI layers has been mostly investigated with X-ray photoelectron spectroscopy (XPS).
The highly inhomogeneous composition of the SEI makes the interpretation of spectral features and therefore an unambiguous assignment difficult. The assignment of XPS peaks can be principally improved by using reference materials and model systems. Some reference measurements have been performed in the past, but they are scarce, not always complete, and do not consider the interaction between the different components in a multicomponent system like the SEI and the composite electrode itself.
In this contribution we present the XPS characterization of particle surfaces, located on the outside and inside of cathodes of 18650 type Li-ion cells. The analysis was performed after electrochemical cyclings, which followed protocols that simulate charge-discharge cycles used in the application of batteries for temporary energy storage. To analyze the surface of buried materials the top part of the cathodes was mechanically removed. Particular attention was given to improve the XP-spectral features assignment, including the interference among the different components within the SEI and the constituents of the composite electrode. To do that, we used reference materials which relate to the SEI components. We also performed experiments with pure LiCoO2-cathodes to analyze the nature of the SEI layer on the active material without contribution of other cathodic constituents like in composite cathodes.
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. LixPOyFz-type) surface compounds were detected. We found out that the SEI thickness and the proportion of the containing species, as well as the changes of the oxidation state of Co-ions in LiCoO2, depend on the applied cycling protocol.
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