Application-Oriented Analysis of Ageing Processes Using the Example of Li-C Half Cells

Abstract

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 and electrolyte decomposition during cycling. Thickness measurements as well as gas analytical studies of Li-C half cells with a carbonate-based electrolyte are presented. Graphite and Hard Carbon, respectively, were 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 over the entire electrode stack as well as of the separate working electrode only.Furthermore, in-operando MS could be used to detect gaseous substances produced by (electro)chemical processes as a function of the state of charge. With the GC-MS, a post-mortem analysis could be performed to identify the individual substances qualitatively. Results of the different half cells are shown and can be compared with each other. A detailed relationship can be demonstrated between the change in thickness and the potential curve. Moreover, any emerging Li-plating is clearly detectable. Among others, carbon dioxide, ethane as well as ethene were identified as degradation products of the carbonate-based electrolyte and are formed during the charging process. In addition, possible correlations between gas evolution and thickness changes can be shown. These analytical studies make an important contribution to get a more detailed insight into ageing processes that take place. The possible combination of in-operando dilatometry with in-operando mass spectrometry opens up new interesting perspectives to analyse cell performance. Finally, the shown results lead to a better understanding and help to develop appropriate countermeasures in order to reduce the negative effects and thus to ensure a higher cycling stability. This work is supported by the Fraunhofer and Max Planck cooperation program (Germany) in the project “ClusterBatt” with Fraunhofer Institute for Material and Beam Technology IWS, Dresden and Max Planck Institute of Colloids and Interfaces MPIKG, Potsdam. Figure 1