How could "thermal runaway" be controllable?（1）

How could "thermal runaway" be controllable?

Recently, spontaneous combustion accidents of new energy vehicles have occurred frequently, and lithium batteries have frequently made headlines for "thermal runaway". Data show that in 2021, the number of new energy vehicles in China has reached 7.84 million, and 3,000 fire accidents occurred nationwide.

For pure electric vehicles, the user's mileage anxiety and explosion caused by battery “thermal runaway” have always been the focus inside the industry. The efficiency of thermal management system and the performance of battery thermal management also directly affect the recharge mileage and battery safety.

Calculated according to the national motor vehicle ownership and spontaneous combustion accidents, combustion accident of fuel vehicles is about 2-4 per year of 10,000 units, while that of new energy vehicles is about 0.9-1.2. However, from the perspective of the principle and result of the fire between the two, the "harmfulness" of new energy vehicles is even higher. Pure electric vehicles equipped with lithium batteries release heat after thermal runaway, causing a short circuit that leads to a fire in a short period of time, and the fire inside the battery will trigger a chain reaction, resulting in a very hasty escape time for the people inside the car. In addition, the fire of lithium batteries is also more dangerous. On one hand, the fire temperature is high, so a large amount of water is needed to cool down even after the fire is extinguished; on the other hand, lithium batteries have the danger of re-ignition after being destroyed.

Lithium battery thermal runaway mechanism

The chemical reaction that occurs in the thermal runaway of Li-ion batteries is a complex and continuous process, which is not achieved overnight, but is carried out in stages during the entire process.

The cause of the danger of Li-ion batteries is the runaway of the exothermic reaction inside the battery,which is generally caused by:

(1) Overcharge: Operation behavior that does not conform to the specification. For example, when the Li-ion battery is overcharged, the positive electrode material continues to undergo a lithium deintercalation reaction, causing the electrolyte to be oxidized and decomposed to release a lot of heat, and then the risk of thermal runaway will be greatly increased;

(2) Piercing: Li-ion batteries will produce lithium dendrites on the surface of the negative electrode during long-term cycling, some of which will fall off to form "dead lithium", and the other will continue to grow and pierce the separator, resulting in a short circuit of the battery;

(3) High temperature: The electrolyte solvents are all flammable carbonate and carboxylate organic solvents, which will oxidatively decompose and release a lot of heat under high temperature and high pressure, resulting in thermal runaway;

(4) High voltage: The formation of the solid electrolyte phase interface (SEI) film is unstable, and it decomposes under high voltage, releasing heat;

(5) Damage by external force: problems such as battery structure collapse, SEI film tearing, electrolyte leakage, and internal short circuit may occur when damaged by external forces such as vibration, extrusion, collision, and fire.

It can be said that in addition to the spontaneous combustion of the battery itself, external factors such as violent collision, extrusion, and wading may also cause short-circuits of internal components that result in spontaneous combustion. It is worth noting that the above reactions do not proceed independently, and at some point, multiple reactions are likely to occur simultaneously.