Unveiling a Critical Exothermic Reaction in the Thermal Runaway of Lithium-Ion Batteries

Battery safety has emerged as a critical issue in the large-scale deployment of Li-ion batteries. While the focus on thermal runaway typically centers on the exothermic reactions associated with the decomposition and interactions of electrode materials and electrolytes, the role of current collectors has been largely overlooked. This study demonstrates that the commonly considered stable aluminum (Al) current collector undergoes severe exothermic reactions with all lithiated cathode materials or air at temperatures exceeding 600 °C, due to the destruction of Al surface passivation film by fluorides. The heat released per unit mass of Al (-22 ~ -33 kJ/g) is nearly double that of the electrolyte (-12 ~ -16 kJ/g). Considering that the weight of Al current collector in batteries accounts for approximately half that of the electrolyte, the heat generated from Al oxidation during thermal runaway is comparable to that from electrolyte combustion. Furthermore, the fluoride-promoted high-energy Al reactions, resembling those observed in Al-based rocket propellants, are resistant to suppression by conventional flame retardants. This finding reveals a prevalent but previously overlooked high-energy reaction mechanism associated with Al current collectors, offering new insights into thermal runaway of uncharged batteries and presenting potential strategies for enhancing safety.