In reports of lithium battery accidents, "thermal runaway" is the most common keyword. It is not an instant event, but a chain process that gradually evolves. Understanding this chain is the prerequisite for safety protection.

1. The four stages of thermal runaway

The first stage: the trigger appears.Common causes include internal short circuit (metal foreign matter, separator defects, dendrites), external short circuit, overcharge and over-discharge, mechanical damage (extrusion, needle punch), and high temperature environment. At this time, the battery temperature rises slowly.

The second stage: SEI decomposition and side reactions.When the temperature rises to 90~120°C, the SEI film begins to decompose, and the negative electrode and the electrolyte react directly to generate heat; then the separator shrinks and melts (PE is about 130°C, PP is about 160°C), and a short circuit is formed in a large area.

The third stage: spray valve and violent heat release.As the temperature continues to rise, the positive electrode decomposes to release oxygen, the electrolyte undergoes violent oxidation and decomposition, the internal pressure increases suddenly, and the safety valve opens to eject flammable gas and electrolyte vapor, with the temperature reaching 600~800°C.

Stage 4: Spread.If the thermal runaway of a single cell is not controlled, adjacent cells will be ignited through heat conduction and radiation, forming a chain reaction known as "thermal spread."

Two and four layers of protection design

  • Cell layer: Use ceramic-coated separators, cathode materials with better thermal stability, and configure safety valves (CID), PTC and other passive components; strictly control production cleanliness to reduce metal foreign matter.
  • BMS layer: High-precision voltage and temperature sampling, multi-level overcharge, over-discharge, over-current and over-temperature protection, abnormal warning and active power-off; the energy storage system also needs to have insulation detection and fault recording.
  • structural layer: There is a thermal insulation layer (aerogel, mica board) between the cells, the module is designed with pressure relief channels and directional exhaust, and the shell has a flame retardant grade (such as V-0).
  • System layer: Liquid-cooled or air-cooled thermal management maintains uniform temperature, equipped with combustible gas detection, smoke alarm and automatic fire protection (perfluorohexanone, water mist), and blocks the spread through cabin isolation.

3. Standards and testing requirements

Common verifications include GB 38031 (safety requirements for electric vehicle power batteries, requiring no fire or explosion within 5 minutes after thermal runaway), UL 9540A (assessment of thermal runaway spread of energy storage systems), IEC 62619 (battery safety for industrial equipment), etc. Export products need to select corresponding standards according to the target market.

4. Suggestions for engineering practice

Security is a system engineering, and any single point of strengthening is not enough to eliminate risks. The truly effective approach is to conduct FMEA failure analysis in the design stage, conduct acupuncture, extrusion, overcharging and other abuse tests in the trial production stage, strictly control the consistency of incoming materials and processes in the mass production stage, and rely on BMS and operation and maintenance systems for continuous monitoring during the use stage. Only by superimposing four layers of protection can the probability be reduced to an acceptable level.

5. Conclusion

For lithium batteries, safety is not a function, but the bottom line throughout the entire life cycle.