Advanced Characterization by Using Coupled Operando Mass Spectrometry and Operando Dilatometry

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 cycle life. The focus here is on investigations of ageing processes such as volume changes and electrolyte decomposition during cycling.Combined thickness measurements and gas analytical studies of Li-Graphite half cells with a carbonate-based electrolyte are presented. In addition, the same measurements on Li-O2 cells with an ether-based electrolyte are introduced. The electrolytes consist of 1M LiPF6 in EC:DMC (1:1, wt) and 1M LiTFSI in DEGDME, respectively. All measurements were carried out using specially developed multifunctional test setups and accordingly modified test cells. The different cell specifications were examined by operando mass spectrometry (MS) and operando dilatometry. In addition, post-mortem gas chromatography-mass spectrometry (GC-MS) was conducted.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. 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, operando MS can 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 was performed to identify the individual substances qualitatively.Results of the different cell specifications are shown and can be compared with each other. A detailed relationship can be demonstrated between the change in thickness and the potential profile. Moreover, the cyclic formation of several degradation products is detected. Possible correlations between gas evolution and thickness changes can be shown. In conclusion, these analytical studies provide a new important contribution to get a more detailed insight into ageing processes that take place. The possible coupling of two operando techniques enables an even more holistic examination to analyze cell performance.Parts of the presented work are supported by the Fraunhofer and Max-Planck cooperation program in the project “Clusterbatt” together with Fraunhofer IWS, Dresden and MPIKG, Potsdam. Parts of the presented work are funded by the German Federal Ministry of Education and Research (BMBF) in the joint German-Japanese project “OsabanPlus” (03XP0469C) together with Justus-Liebig-University Giessen, TU Braunschweig and University of Kyoto. Figure 1