The metal oxides often used in lithium-ion batteries decompose at higher temperatures, releasing oxygen. This can react with other materials in the cell – such as organic electrolytes, graphite, or plastics – thus generating even more heat. The resulting hot gases initially cause the battery casing to swell.
Finally, they exit under high pressure in the form of a jet flame, which can also burn for a longer period. The battery can even explode in the process.
In addition to the flames and the risk of explosion, the rapidly forming smoke also poses a risk. It can contain, among other things, hydrogen chloride gas and hydrogen fluoride gas.
Risk factors for thermal runaway
The heat required for thermal runaway can either be supplied externally or generated within the cell itself – for example, due to overload or a short circuit. Important risk factors for thermal runaway are therefore:
- Hot environment and solar radiation: Lithium-ion batteries should therefore not be stored near flames or hot engines, for example, and also not in direct sunlight. This applies especially during operation and charging, as additional heat is generated here. Overheated batteries must first be cooled down to their core before they are recharged or further used.
- Overcharging and deep discharging: If a battery is charged above its final charge voltage or deep discharged, it can lead to damage. For example, dendrites can form. These lithium bridges between the electrodes can create internal short circuits.
- External short circuits: If a short circuit occurs due to accidental contacts or defective devices, it leads to high current flows and excessively rapid discharge of the battery. Both can lead to overheating.
- Mechanical stress and foreign bodies: Mechanical stress or penetrating foreign bodies can lead to electrical contact between the cathode and anode. A lot of heat can be generated at the point of such a short circuit.
Collection points for old batteries are therefore particularly at risk. Lithium-ion batteries that are mechanically damaged or deeply discharged are regularly handed in there. In addition, short circuits can occur in the collection containers. However, tradesmen, who often handle electrical tools roughly and sometimes work in high temperatures, can also be easily affected.
Behavior during thermal runaway
A lithium-ion battery that undergoes thermal runaway and explodes can cause severe injuries. The jet flames can not only cause fires but also directly lead to severe burns. The smoke generated during thermal runaway is also toxic. Even relatively small lithium-ion batteries, such as those in smartphones and e-cigarettes, can therefore lead to serious health damage if they catch fire.
Furthermore, burning lithium ions are almost impossible to extinguish.
This is because the fire originates protectively within the casing and generates its own oxygen. Therefore, such fires cannot be suffocated. At best, the spread of the fire to other cells can only be prevented.
In the event of thermal runaway, the following measures should therefore be taken:
- Protect people: Batteries that heat up significantly, swell, or start to smoke pose an acute danger. They must therefore be kept away from people. For example, they should be taken out of pockets or put down. Rooms where a battery is burning should be evacuated immediately.
- Disconnect from the circuit: If overheating occurs during charging or operation, it can already help to pull the plug or switch off the device. This can sometimes interrupt an incipient thermal runaway.
- Prevent fire spread: If it is still safe to do so, batteries undergoing thermal runaway should be moved as far away as possible from other flammable materials – or vice versa. It is best to remove the battery from the building or at least place it on a non-combustible surface – for example, in a pot or on tiles.
- Cooling: In the early phase of thermal runaway, the chemical reaction can still be interrupted if as much heat as possible is dissipated. Therefore, runaway batteries should be cooled. This even applies to already burning batteries, as this can often prevent thermal runaway in other cells. The most effective method is prolonged immersion in water.
Fireproof boxes and fire-resistant bags for batteries
Thermal runaway in lithium-ion batteries is very rare. However, it can never be completely ruled out, even with careful handling. The charging process is particularly critical. Anyone who wants to be on the safe side can charge and store their batteries in a battery fire protection box or fire protection bag such as the fireproof one. Such containers protect against splinters in the event of explosions and prevent the spread of fire.