Operando GCMS Monitoring of gaseous Decomposition products from electrolytes containing SEI forming additives in Lithium-ion-Batteries

With electric vehicles reaching an all-time high in popularity, the safety of lithium-ion-batteries (LIB) is an increasingly pressing matter. However, various failure mechanisms such as overcharging, overheating, and decomposition reactions which take place during thermal runaway all lead to the emission of highly flammable and toxic gases. Therefore, the investigation of the chemical composition of the emitted gases is crucial for understanding the reactions that occur in the battery cell and assessing their impact on safety during battery failure. This knowledge can not only be used to produce safer batteries but can also help to improve their performance. The development and tailoring of electrolyte additive chemistries has been shown to improve cell performance by stabilizing the electrode/electrolyte interfaces. The most popular anode additives strengthen the solid electrolyte interface (SEI) by improving its mechanical integrity. The SEI, which is produced at the anode/electrolyte interface during the formation cycles, is composed of decomposition products that originate from reduction of the electrolyte. The reactions that lead to the SEI formation are also accompanied by gaseous side products, which can be analysed with gas chromatography-mass spectrometry (GC-MS). The GC permits the separation of the complex gas mixture, whereas the MS can be used to identify the separated analytes. In this work, the gaseous decomposition products of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) as SEI forming additives were investigated with an operando GCMS. The operando method allows the volatile decomposition products to be monitored during electrochemical cycling. The operando technique not only allows the gases that evolve during SEI formation to be investigated, but also give valuable information on decomposition phenomena during overcharge. Our results show that the gas mixture evolving from the overcharge experiments contain a large amount of carbon dioxide due to the decomposition of the electrolyte solvents. In addition, various fluoroalkanes were observed, which can be attributed to the decomposition of the conducting salt lithium hexafluorophosphate (LiPF6). The findings are supported by cyclic voltammetry and additional electrochemical experiments.