Dozens of worldwide regulations and norms are emerging to ensure the safety of electric vehicles. Amongst them, thermal propagation studies and how to mitigate them are part of the cornerstone. This process is extremely dangerous not only due to the risk of end up with flames/fire, but also due to the presence of dangerous gases emitted during the cell’s combustion. This paper aims to shed some light on the emissions generated (composition and volumes) during a battery fatal failure, or rather, thermal runaway of one cell as product of overheating trigger methodology. Cell emissions during thermal runaway, unlike most processes in the industry, is not a continuous process, and these emissions have a limited duration of a few minutes. Within the framework of this study, different methodologies of hydrofluoric acid (HF) measurement have been compared: ion capture (washing bottles), which is used for environmental agencies as standard measurement technique and provides the total amount of HF generated in a process (single point measurement); versus Fourier-transform infrared spectroscopy (FTIR) technology, which provides concentration of HF over time (continuous measurement). In addition, other gases likely present in combustion process were determined according to variety of methods. This study has been based at cell level and not at vehicle level. The fact of extrapolating cell results at battery pack level results in different scenarios concerning gas emissions: the worst case, considering all cells suffer thermal runaway at the same time; and a more realistic scenario, assuming a phased or graduated thermal propagation on the complete battery pack. The novelty in this research is the type of cell object of study. Last generation of Lithium-ion batteries have been investigated, more specifically, NMC 120Ah cells which constitute a high voltage battery pack of one of 2022 electric vehicles. Several pollutants, not only HF, were monitored during the cell’s thermal runaway of commercially available EVs. Those include gases like CO, CO2, NOx, TOC, HCl as well as O2. The results indicate presence of pollutants such as CO, CO2, TOC and HF, but not NOx or HCl. KEYWORDS: Electric vehicles / Cell emissions / Safety

Dr. Miquel Planells, Project manager, IDIADA