Why are Lithium Batteries in Electric Vehicles Prone to Catching Fire After a Collision? Analysis of Internal Structure and Thermal Runaway Mechanism​

With the popularity of electric vehicles, the safety of lithium batteries has become a focus of public attention. Battery fires caused by multiple collision accidents in recent years have exposed the fragility of lithium batteries under extreme conditions. Why are lithium batteries prone to burning during collisions? The answer is closely related to their unique internal structure, chemical properties, and external impact.

Lithium batteries consist of a positive electrode, a negative electrode, a separator and an electrolyte. During normal operation, lithium ions move between the positive and negative electrodes through the electrolyte. However, in a collision accident:
(1) Separator rupture: External force compression or puncture may cause the separator to tear, the positive and negative electrodes to come into direct contact, and cause a short circuit.
(2) Electrolyte leakage: After the shell is deformed or ruptured, the flammable electrolyte contacts the air, increasing the risk of combustion.

The electrolyte of lithium batteries usually contains organic solvents, which are easy to volatilize and burn under high temperature or short circuit conditions. When a battery short circuits, the internal temperature will rise rapidly, causing the electrolyte to decompose and release flammable gases. These gases will burn rapidly at high temperatures and cause fires.

When the battery is short-circuited or impacted, the local temperature rises suddenly (over 800°C), triggering thermal runaway:
(1) ​​Positive electrode decomposition​​: At high temperatures, the positive electrode materials (such as nickel, cobalt and manganese) of the ternary lithium battery release oxygen to help burn the electrolyte.
​​(2) Negative electrode lithium metal combustion​​: The graphite negative electrode reacts with the electrolyte to generate lithium metal, which has an ignition point of only about 180°C, exacerbating the fire.
(3) ​​Vicious cycle​​: The heat released by thermal runaway further damages the battery structure, causing more short circuits and gas release, and eventually explosion.

Collisions not only directly damage the battery structure, but may also cause the following risks:
​​(1) Shell rupture: The electrolyte leaks and comes into contact with the air, forming a flammable environment.
​​(2) Mechanical stress conduction: The deformation of the battery module may squeeze adjacent cells and cause a chain short circuit.