Off-gas

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Gas detection

Sensors in a pack or storage container that detect electrolyte vapour or hydrogen released before thermal runaway. Off-gas appears minutes before heat and smoke, which makes it the earliest practical warning.

Hydrofluoric acid

Formed when LiPF₆ electrolyte meets water, or when a fluorinated cell burns. Inside a cell traces of it attack the positive electrode and SEI; in a fire it is the most dangerous component of the vented gas.

Thermal runaway

Self-accelerating heating in a cell: heat speeds up reactions that release more heat, until the cell vents, catches fire or ruptures. An internal short, overcharge or external heating can trigger it.

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

Further reading on this topic

These sources provide broader context; they are not presented as citations for every statement in this entry.

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

   Xuning Feng et al. · Energy Storage Materials · 2018

   Topic background · Electrical, mechanical and thermal abuse; thermal-runaway mechanisms and propagation.