Analysis of fluoride compounds from Li-ion battery fires

Sammanfattning

Lithium-ion batteries are widely used and remain the dominant energy storage technology for modern electronics and electric vehicles. Despite their widespread adoption, these batteries pose safety risks, including fire hazards and thermal runaway. One aspect that has received relatively little attention in current research is the formation and impact of fluorinated compounds during battery degradation and thermal runaway events. This project aims to improve understanding of the role and formation of these compounds particularly hydrofluoric acid (HF) in lithium-ion batteries, through a comprehensive literature review complemented by a small scale simulation study. The main takeaways from the literature review are that thermal decomposition of LiPF6 is central to HF formation, producing PF5, \ce{H2O}, \ce{POF3}, and HF. The formation of HF induces a feedback cycle in which HF both forms from and accelerates further electrolyte breakdown. Electrolyte composition strongly affects these reactions. Solvent properties, especially permittivity, influence HF formation, with DMC showing higher thermal stability than EC, though EC is critical for solid electrolyte interphase formation. LiPF6 modifies decomposition temperatures and gas evolution patterns. The main gas oberved in thermal runaway are carbon oxides, hydrocarbons, fluorohydrocarbons, and oxygenated species. Radical reactions involving HF further contribute to fluorohydrocarbons formation, highlighting the complexity of these processes. Simulation results suggest that HF has no effect on ignition delay time but decreases flame speed, while CH3F reduces ignition delay time and increases flame speed. Together, these findings emphasize the need for targeted strategies to control HF formation and mitigate its impact on battery safety.