[News Terms] The Beginning of Dangerous Electric Vehicle Fires: 'Thermal Runaway'

If the separator is damaged, the anode and cathode meet causing heat generation
In case of fire, evacuate quickly and call 119 immediately

'Thermal runaway (TR)' refers to a state where a temperature change causes the environment to change in a way that further accelerates that temperature change. In other words, it means heat generating more heat. Electric vehicle fires start from the battery's 'thermal runaway.'

A cylindrical lithium battery that caught fire due to thermal runaway.
[Photo by Yonhap News]

Recently, awareness of electric vehicle fires has been increasing. The fire incident involving a Mercedes-Benz electric vehicle in the underground first-floor parking lot of an apartment in Incheon on the 1st clearly demonstrated the dangers of lithium-ion battery thermal runaway and the difficulties in firefighting operations. According to the Incheon Fire Department on the 2nd, 20 residents inhaled smoke and were taken to the hospital, and approximately 70 vehicles in the parking lot were estimated to have been damaged. The fire was extinguished after 8 hours and 20 minutes. The enclosed structure of the underground parking lot, combined with the fact that extinguishing fires in electric vehicles is much more challenging than in internal combustion engine vehicles, contributed to the adverse situation.

Lithium-ion batteries generally consist of four components: cathode, anode, electrolyte, and separator. Electricity is generated through a chemical reaction where lithium ions move between the cathode and anode. The cathode is the (+) electrode of the battery, with lithium as the main component. The anode is the (-) electrode, primarily made of graphite. The electrolyte facilitates the smooth movement of lithium ions. The separator prevents the cathode and anode from coming into direct contact. This separator contains tiny pores through which lithium ions pass. In other words, the separator is a key safety material in secondary batteries that prevents fire and explosion by blocking contact between the cathode and anode while allowing ions to move freely. Thermal runaway occurs when overheating or physical damage raises the internal temperature, damaging the separator and causing contact between the battery's cathode and anode, which generates heat.

Principle of Lithium-Ion Battery
[Photo by Fire Insurance Association]

As the internal temperature rises, the electrolyte vaporizes, increasing the pressure inside the battery due to the generated gas. This pressure melts the separator, causing the cathode and anode to meet and resulting in an internal short circuit. A short circuit is a phenomenon where the current changes from its normal path to an unexpected one. Due to the short circuit, all the energy stored inside the battery is released, causing the battery temperature to soar above 800 degrees Celsius. Electric vehicle fires start from the thermal runaway of a single battery cell. This heat then spreads to adjacent battery cells, rapidly causing a fire.

Typically, electric vehicle batteries are installed in a pack form laid under the car floor. A pack is a collection of modules, each made by grouping several battery cells. This arrangement distributes the considerable weight of the battery and considers vehicle stability. However, due to this structural characteristic, when a fire occurs in an electric vehicle battery, it burns adjacent battery cells, causing the fire to spread horizontally. Without wind, unlike internal combustion engine vehicles where fire tends to rise upwards, electric vehicle fires cause greater surrounding damage when they occur.

Additionally, lithium-ion batteries react adversely with water. When lithium meets water, hydrogen gas, which is highly flammable, is produced. Even if a powder extinguisher is used on an electric vehicle engulfed in flames, the powder does not reach inside the lithium-ion battery, and the cooling effect is minimal. Therefore, if a fire breaks out under an electric vehicle, it is crucial to evacuate to a safe place quickly and promptly report to emergency services (119).