Supercapacitor safety: Temperature driven instability and failure of electrochemical double layer capacitors

While supercapacitors are widely considered to be safer than current lithium-ion battery technologies, their reputation for safety, stability, and long cycling lifetimes is primarily based on their testing under highly favourable electrochemical and environmental conditions. However, the impact of extreme conditions on even the most common Electrochemical Double Layer Capacitors (EDLCs) remains unclear, limiting the understanding of their potential failure mechanisms and the risks they could present to individuals and systems into which they are increasingly being integrated. In this study, we investigate the effects of thermal abuse conditions, induced by overheating and overcharging, on a typical commercial EDLC. Our findings reveal that while EDLC cell failures are less extreme than the well-documented failures of Li-ion batteries, they still pose significant risks to the integrity of the cell itself and the direct environment. This is most evident from the fact that between the overheating and overcharging tests, more than half of all the cells tested in this study failed catastrophically, leading to an explosive event. The high cell temperatures induced by these abusive tests led to electrolyte vaporisation and cell gassing that was not effectively mitigated by cell vent designs. This study therefore challenges the perception of intrinsic supercapacitor safety and provides a foundation onto which safer system designs can be built.