Thermal runaway mechanism of lithium-ion batteries

Thermal runaway mechanism of lithium-ion batteries

Fire accidents in electric vehicles and power storage systems are caused by the uncontrollable energy release of lithium-ion batteries, which is essentially a thermal runaway problem of lithium-ion batteries.

Thermal runaway of lithium-ion batteries is generally caused by three factors:

1. Internal short circuit caused by external abuse;

2. Internal short circuit caused by internal defects;

3. Gas crosstalk.

• External abuse can be divided into three categories: mechanical abuse, electrical abuse and thermal abuse, which all correspond to electrochemical abuse at the mechanism level. 
• Mechanical abuse mainly includes damage to the battery structure and deformation of the material due to mechanical extrusion and puncture. For power battery systems, vehicle collision is the most common cause of battery mechanical abuse, while energy storage battery systems rarely experience mechanical abuse.
  
• Electrical abuse mainly includes external short circuit, improper charging, over-discharge, etc.
• Thermal abuse mainly includes local overheating in the battery pack.
• Electrochemical abuse includes internal short circuit, negative electrode lithium deposition, gas crosstalk, etc.
  
  

Among the electrochemical abuse methods that have been proven so far, internal short circuits account for the vast majority; negative electrode lithium deposition accounts for a smaller proportion, but thermal runaway caused by lithium deposition has also begun to appear in actual accident cases.

Gas crosstalk is considered to be the key reaction for high-energy-density batteries to reach the thermal runaway trigger temperature. It has been found at the laboratory research level, but there is no conclusive evidence as the cause of actual accidents.

For cells with manufacturing defects, their thermal runaway is caused by external abuse, as well as internal short circuits and lithium deposition caused by internal defects.

As far as power batteries are concerned, China's domestic battery cell manufacturers and OEMs have adopted big data to monitor the entire life cycle of battery cells. Most quality problems caused by internal defects will be warned in advance by big data and will not develop to the thermal runaway stage. For example, there are pressure difference problems caused by foreign matter, insulation problems caused by tab tearing, and poor welding, etc.