Application-oriented analysis of ageing processes in Li-C half cells

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Mechanical modifications or harmful side reactions are undesirable ageing effects that can occur during cycling. These phenomena have a negative impact on cell performance and consequently limit cycling stability. The focus here is on investigations of ageing processes such as volume changes during cycling.

Thickness measurements of Li-C half cells with a carbonate-based electrolyte are presented. Graphite was used as carbon materials. The electrolyte applied consists of 1M LiPF6 in EC:DMC (1:1, wt). All investigations were carried out using specially developed multifunctional test setups and accordingly modified test cells.

Application-oriented analysis such as gas analysis, thickness measurements or both combined are suitable methods to define operating parameter, to benchmark and to identify decomposition reactions. In-operando electrochemical dilatometry was conducted to check whether an irreversible increase in thickness occurred. On the corresponding cell configurations, the dilatation was recorded of the separate working electrode only.

A detailed relationship can be demonstrated between the change in thickness and the potential curve. The characteristic dilatometer profile for Graphite is recognizable and the relative reversible thickness change per cycle is roughly 4%. Moreover, the different areas of the potential profile can be distinguished in the dilatometer curve which illustrates the intercalation of the Li-ions in Graphite. Finally, the SEI-formation is identified by considering the irreversible increase in thickness after the first cycle.

In conclusion, high-resolution dilatometer measurements on electrode level can be realized with the presented multifunctional test setup. This analytical operando method enables a good insight into mechanical modifications that take place.

This work is supported by the Fraunhofer and Max-Planck cooperation program in the project “ClusterBatt” with Fraunhofer Institute for Material and Beam Technology IWS, Dresden and Max Planck Institute of Colloids and Interfaces MPIKG, Potsdam.

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