Section 2 · Topic 7
Thermal Runaway Response and Fire Mitigation
Thermal runaway resists conventional suppression because the failing cell does not depend entirely on the surrounding atmosphere for fuel or, in some chemistries, for oxygen.
As a cell in runaway heats, it vents a mixture of hydrogen, carbon monoxide, hydrocarbons, and vaporised electrolyte — all flammable, all toxic, and all able to burn in atmospheric oxygen regardless of what the cathode does. Smothering the visible flame therefore does not arrest the exothermic reactions driving the event from inside the cell.
Why smothering fails — and what oxygen release depends on
The original framing that runaway is universally “self-oxygenating” overgeneralises. Layered-oxide cathodes — LCO, NMC, and NCA — release lattice oxygen at runaway temperatures and are largely immune to oxygen exclusion. LFP does not: the strong phosphorus–oxygen bond of its olivine structure holds oxygen in the lattice, the same property that gives LFP its thermal-stability advantage. The distinction governs hazard assessment, not response.
Whether or not the cathode supplies its own oxygen, the vented gas burns in atmospheric air for every chemistry. The objective is to extract heat and stop propagation — cool, don’t smother — not to extinguish the failing cell.
Response: cool and contain
A cell that has entered runaway can vent and reignite hours or days later, and the immediate risk is that its heat triggers its neighbours and cascades through the pack. Visible white-grey venting, hissing, or swelling signals an event in progress, and the released hydrogen fluoride and carbon monoxide make evacuation and ventilation the first actions, with emergency services told explicitly that a lithium-ion fire is involved. Water, applied in volume to the cells adjacent to the failing one, is the most effective agent for pulling heat out of the pack; a standard extinguisher serves only to knock down open flame for egress.
A cell that has undergone runaway can restart its reactions without warning. The damaged pack is held in an approved quenching medium and handled as hazardous waste — not stored or transported as a normal cell.
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