Thermal runaway – the thermal escalation of a lithium-ion cell – is particularly dangerous because it is not a “normal” fire. It is a self-reinforcing chemical-thermal reaction that occurs within the battery and is difficult to control from the outside. Once this process begins, the battery generates more heat than it can dissipate. This additional heat further accelerates the chemical reactions – creating a vicious cycle.

A central issue is the extremely high temperatures. While many typical household fires occur at just a few hundred degrees Celsius, a runaway lithium-ion system can reach temperatures of over 800 to 1000 °C. This heat is sufficient to spontaneously ignite surrounding materials, weaken metals, and significantly strain structural integrity.

Additionally, the release of toxic and corrosive smoke gases poses a danger. The decomposition of the electrolyte and other cell components produces irritating and highly toxic gases, among others. In enclosed spaces, these can rapidly reach life-threatening concentrations. At the same time, aggressive residues may settle on surfaces, often causing severe secondary damage to electronics, controls, and metal parts.

Another risk is the potential for re-ignition. Even if a fire appears to be extinguished, there may still be sufficient residual energy within the battery to restart the reaction. This means that an incident is not necessarily over just because flames are no longer visible.

Explosive pressure events are also possible. The formation of large amounts of flammable gases causes a significant increase in pressure within the cell casing. If the casing ruptures, these gases can ignite suddenly – resulting in a flash flame or shock wave.

This situation becomes particularly critical in larger battery systems, such as in home storage or electric vehicles. Here, many cells are arranged closely together. When one cell undergoes thermal runaway, the heat quickly transfers to adjacent cells. This so-called cell-to-cell propagation can convert a local defect into a widespread fire incident.

Firefighting is also challenging. Conventional extinguishing agents can suppress flames but do not automatically halt the chemical processes inside the battery. Intensive cooling is often required, and even after that, the system may need to be monitored for an extended period.