Investigating the Electrolyte Degradation of Lithium-Ion Cells with Silicon-Containing Anodes

Abstract

Silicon with a high gravimetric capacity of 3579 mAh g-1 of the pure material becomes increasingly common in the anode of lithium-ion batteries to increase energy density on the full cell level. However, silicon changes its volume excessively during (de-)lithiation making it prone to ageing. In commercial cells, it is typically applied as SiOx in small quantities in a composite anode together with graphite.Gas analytical studies of Gr/SiOx||NMC622 cells with varying silicon content and carbonate-based electrolyte are presented. The electrolyte consists of 1M LiPF6 in EC:DEC (1:1, v/v) + 5 wt-% FEC. The focus of this study is on investigations of ageing processes such as electrolyte decomposition during formation cycling. 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 post-mortem gas chromatography-mass spectrometry (GC-MS).Strong gas formation leads to a loss of electrolyte, which can cause the cell to dry out and consequently limit the cycle life. Gas analysis is a suitable method to identify decomposition products as well as to illustrate degradation reactions. Operando MS can be used to detect gaseous substances produced by (electro)chemical processes as a function of the state of charge. Furthermore, a post-mortem GC-MS analysis was performed to identify the individual substances qualitatively.The cyclic formation of several degradation products can be determined. Among others, carbon dioxide, hydrogen as well as ethane are identified as characteristic decomposition products of the electrolyte. In addition, the decomposition path of the conducting salt can be detected. Another main result is the evidence of the presence of silicon-containing gaseous fragments as a function of the Si-content in the composite anode.These gas analytical studies of silicon graphite composite anodes provide an important contribution to the understanding of the degradation reactions taking place. The results shown help to illustrate them and give information about the influence of the silicon content on electrolyte degradation. Figure 1