Thermal runaway

Example

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Learn it in Lesson 1 · Cell anatomy →

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Exothermic and endothermic

A reaction that releases heat is exothermic; one that absorbs it is endothermic. Thermal runaway is a chain of exothermic reactions; a few cell processes, such as graphite lithiation at some stages, are mildly endothermic and cool the cell.

Onset temperature

The temperature at which a cell begins to generate heat by itself, as the SEI starts to decompose, typically 80–120 °C. Measured in an accelerating rate calorimeter, it marks the start of the path to thermal runaway, not the runaway itself.

Separator shutdown

A built-in safety feature of polyolefin separators: around 130 °C the polyethylene layer melts and closes its pores, stopping ion flow and the current with it. It helps only if the separator does not also shrink and let the electrodes touch, so shutdown behavior is not universal.

Thermal propagation

Thermal runaway spreading from one cell to its neighbours through conducted heat, hot gas and flame. Pack designs are judged by whether, and how slowly, a single failing cell takes the rest with it.

Thermal-runaway energy

The heat a cell releases when it fails, which exceeds its stored electrical energy because decomposition reactions add heat of their own. Calorimetry on 18650 cells has measured decomposition energy up to about twice the electrical energy, and combustion of the vented contents up to about six times, so a battery fire carries far more energy than its kWh rating.

Venting

The controlled release of gas and vapour from a cell whose internal pressure has risen too far, through a vent designed to open before the case bursts. The vented gas is flammable and toxic, so packs route it away from occupants.

References & further reading

Technical references

1. Thermal runaway mechanism of lithium ion battery for electric vehicles: A review

   Xuning Feng et al. · Energy Storage Materials · 2018

   Technical reference · Electrical, mechanical and thermal abuse; thermal-runaway mechanisms and propagation.