Insights Into Thermal Runaway of Li–Ion Cells by Accelerating Rate Calorimetry Coupled with External Sensors and Online Gas Analysis

Abstract Besides high energy density, fast‐charging capability, and costs, safety of Li‐ion batteries is fundamentally important – even after long‐term usage or abusive conditions. In this paper, a new combination of accelerating rate calorimetry (ARC) coupled with a mass spectrometer (MS), as well as cell resistance and audio recording was applied to study commercial 18650‐type Li‐ion cells. This novel ARC‐MS setup allows following the electrochemical and thermal behavior simultaneously to the evolved gases during cell venting and thermal runaway. The tested cells were (i) unaged, (ii) aged by low temperature cycling (main mechanism: Li deposition), as well as (iii) overcharged. The aged and the overcharged cells show an early onset‐of‐self‐heating at 36 °C whereas the self‐heating started at 96 °C for the unaged cells. The rank of the time until explosion of the cells is in the order of overcharged cell<aged cell with Li deposition<unaged cell. The simultaneous recording of 13 individual values allows drawing conclusions on processes like separator pore closure, cell venting, explosion, and gas formation. Due to electrolyte decomposition by anodic and cathodic exothermic reactions, the amount of formed gases such as CO₂, ethylene, and POF₃ are depending on the history of the cell, i. e., aging mechanism or overcharging. Additional gas sensors in the battery pack for improved battery safety are indicating venting of the cells and therefore might be a low cost solution.