Battery failure is not just an inconvenience. It is a potentially hazardous event marked by the release of toxic and flammable gases. The composition and volume of these gases are influenced by many factors: cell type, failure mechanism, state of charge (SoC), and environmental oxygen levels. Understanding this intricate relationship is paramount for battery pack manufacturers, especially when real-time monitoring is employed to gauge a lithium-ion battery’s (LIB) state of health. Moreover, understanding these gases is crucial for emergency responders attending thermal runaway events.
The Gaps in Current Research
A comprehensive literature review on lithium-ion cell gas analysis has revealed several significant gaps. Notably, there are many experimental setups, each with its unique fixed volumes or air flows. These variables directly impact oxygen availability, subsequently affecting the combustion ratio and, by extension, the gas composition. Many studies, unfortunately, collect gas samples post cell failure. However, gas composition varies from the initial safety vent to the point of thermal runaway. This underscores the importance of real-time gas analysis as a potent technique to delve into the behaviour and composition of battery vent gas.
A Closer Look: The Hiden Analytical Study
In a groundbreaking study, a Hiden HPR-20 mass spectrometer was employed to analyse the real-time battery vent gas emanating from a commercially available NMC-811 21700 cell. The choice of NMC chemistry was predicated on its widespread use in high-performance applications. The experimental setup was meticulously designed, with the mass spectrometer placed in-line with a custom-built rig to scrutinise gases released at varying SoC levels.
The findings were revelatory. During safety venting, methane was predominantly released, and its concentration was directly proportional to SoC. Furthermore, the mass spectrometry detected signs of electrolyte release, evidenced by positive signals for dimethyl carbonate and ethyl methyl carbonate.
However, a marked shift in gas composition was observed during thermal runaway. There was a precipitous increase in hydrogen, carbon dioxide, and carbon monoxide levels. Intriguingly, hydrogen was present in concentrations akin to carbon dioxide, prompting questions about the flammability of such mixtures.
To corroborate these findings, tests were conducted using a 50 L pressure vessel to ascertain gas volumes and compositions in both air and nitrogen atmospheres. The results were consistent: at 100% SoC, hydrogen and carbon dioxide constituted over 50% of the 6.5 L of gas produced. Conversely, at 25% SoC, methane and carbon dioxide emerged as the primary gases.
Implications and Future Directions
The study’s outcomes have far-reaching implications. Battery monitoring systems might benefit from incorporating VOC sensors as potential early warning mechanisms. Moreover, discovering that battery vent gas houses multiple flammable components necessitates further research to determine their flammability thresholds.
In the ever-evolving landscape of battery research, understanding the nuances of battery failure is essential. Residual gas analysers, such as those offered by Hiden Analytical, play a pivotal role in this endeavour. These tools enhance our comprehension of battery failure and pave the way for safer and more efficient battery technologies.For those keen on delving deeper into the intricacies of gas analysers and their applications in battery research, we at Hiden Analytical invite you to explore our range of products, especially our Gas Analysers. Together, let’s drive the future of battery technology to new horizons.