In Situ Characterization of Gassing Processes in Lithium-Ion Batteries By Dems-Deirs

Gas evolution has a profound effect on the functioning of state of the art lithium-ion batteries. On the one hand, it is the natural concomitant of SEI formation; on the other hand, because of the demand for high terminal voltages, it is the consequence of electrolyte or electrode material oxidation. This happens on the expense of the efficiency. Furthermore, the presence of gases in the cell raises safety problems. In general it is important to understand gas formation processes. First, to be able to develop novel electrolyte mixtures or tailor electrolyte additives that form a stable SEI. Second, to optimize the surface and/or structure of cathode materials to influence both their bulk oxidative properties upon delithiation and electrocatalytic activities. Differential electrochemical mass spectrometry (DEMS) is one of the most appropriate tools to qualitatively elucidate gas evolution reactions in electrochemical systems. Herein, we report on the in situ gas analysis by means of differential electrochemical mass spectrometry and infrared spectroscopy (DEMS-DEIRS) of various battery systems. We analyze the SEI formation on different anode materials and the influence of various electrolyte additives. The impact of temperature and constant voltage/OCV periods on the first charge cycle (so-called formation process) is investigated. Their effect on both the gassing and electrolyte salt decomposition is described. The stability problem of the most common electrolytes at high voltages is discussed through the example of either NMC (LiNixMnyCozO2) type materials or with high voltage spinels (LiNi0.5Mn1.5O4). The electrocatalytic activity of the individual active material particles seems to play a decisive role in the oxidative decomposition processes. Through the application of modified cathode surfaces a relation between surface activity and the amount of produced gases is revealed. Beyond the decomposition of the electrolyte, oxygen evolution is the crucial factor at high cathode potentials. This effect is particularly apparent for overlithiated and high Ni content materials. Monitoring gassing over 20 cycles uncovers interesting changes being only recognizable in “long term” tests. We demonstrate the importance of potentiodynamic cycling in addition to the traditional galvanostatic test with DEMS-DEIRS in the exploration of reaction mechanisms. Pressure measurements complete the analysis in terms of validation of gas amounts.