What is the primary safety concern associated with thermal runaway in lithium-ion batteries?

The primary safety concern associated with thermal runaway in lithium-ion batteries is the potential for fire and explosion. Thermal runaway is a chain reaction within the battery where internal heat generation accelerates uncontrollably, leading to a rapid increase in temperature. This process can be triggered by various factors, including overcharging, over-discharging, external short circuits, internal short circuits due to manufacturing defects or physical damage, and exposure to high ambient temperatures. As the temperature rises, the battery's internal components begin to decompose, releasing flammable gases such as hydrogen, methane, and ethylene. The increase in temperature can also cause the electrolyte to vaporize and the separator to melt, leading to internal short circuits. Once the temperature reaches a critical point, the flammable gases can ignite, resulting in a fire. In some cases, the rapid build-up of pressure inside the battery can lead to an explosion. Thermal runaway can propagate from one cell to adjacent cells within a battery pack, leading to a cascading failure. Preventing thermal runaway requires robust safety measures, including Battery Management Systems (BMS) to monitor and control battery parameters, thermal management systems to dissipate heat, and safety devices such as vents and fuses to mitigate the consequences of a thermal event.