"Gas Generation' during a thermal runaway event is a complex process in lithium-ion batteries. Thermal runaway refers to a scenario where an increase in internal temperature initiates a set of exothermic reactions. These reactions further amplify the temperature, triggering even more reactions, much like a domino effect. The result of this can be gas generation, swelling of the battery, and in extreme cases, even explosions.
Various Gases Released During Different Stages of Thermal Runaway:
Starting at the lower end of the temperature spectrum, around 90°C, the Solid Electrolyte Interface (SEI) begins its decomposition. The SEI is a crucial component, acting as a protective layer between the electrode and the electrolyte. Its decomposition can lead to the release of gases such as C₂H₄ (Ethylene), CO₂ (Carbon Dioxide), and O₂ (Oxygen).
As we approach the 100°C mark, another significant reaction takes place. The intercalated lithium, which is lithium stored within the anode's material structure, begins to react with the electrolyte. This results in the generation of gases like C₃H₆ (Propene), C₂H₄, and CH₄ (Methane).
Progressing further, at around 130°C, the separator starts to melt. This melting can cause internal short circuits, intensifying the thermal runaway process. Reaching 200°C, the cathode begins to decompose, releasing O₂. This oxygen can further react with the electrolyte, resulting in the emission of O₂ and CO₂ gases.
Between 200-300°C, the electrolyte undergoes decomposition, releasing a cocktail of gases such as CO₂, HF (Hydrogen Fluoride), and C₂H₄. HF is particularly concerning as it poses significant health risks upon exposure. Near 228°C, the anode undergoes a re-decomposition process, releasing CO₂. One of the more violent reactions occurs around 260°C, when the intercalated lithium reacts with the binder. This results in the explosive release of hydrogen gas, or H₂.
Lastly, at extremely high temperatures, carbon-containing materials present in the battery can burn violently, emitting CO (Carbon Monoxide) and CO₂ gases.
As the battery undergoes thermal runaway, a series of chemical reactions take place at various temperature thresholds, each contributing to gas generation and further heating. By understanding the temperature thresholds at which these reactions occur, researchers can develop innovative strategies to mitigate the risks and enhance the battery's safety profile.