Can Gas Sensors Detect Lithium-Ion Runaway?

Lithium-ion cells in early-stage thermal runaway emit a characteristic gas signature before the cell vents in any visible way. The chemistry is well-characterised, the sensors exist, and on a sealed bench rig they trip earlier than thermal. The question we get asked is why we do not lead with gas sensing on RoRo decks. The honest answer has three parts.

What runaway emits, and when

Pre-vent off-gassing from a stressed Li-ion cell is dominated by electrolyte VOCs — the carbonate solvents (DMC, EMC, EC) that make up the bulk of the electrolyte — with hydrogen at low concentrations and trace HF appearing as the electrolyte salt decomposes. The sequence depends on cell chemistry but the order is roughly: electrolyte VOCs first, then H₂, then HF as the runaway propagates.

Why it works on a bench rig

A sealed test chamber concentrates the off-gas signature against a clean background. A sensor mounted within a metre of the cell sees a clear ramp from baseline within minutes. Gas detection on the bench is a strong showing — it is genuinely earlier than thermal.

Why we treat it as a complementary layer at sea

1. Background concentration is not zero

A vehicle deck has hundreds of internal-combustion engines that have been driven onto the deck within the last hours. Residual hydrocarbon signatures from cold engines, fuel system off-gassing, and brake system thermal load all sit in the same VOC bands as early Li-ion off-gas. Discriminating signal from cargo background is non-trivial.

2. Ventilation actively works against you

Vehicle decks are mechanically ventilated to clear engine exhaust during loading and to maintain crew safety en route. The same airflow that clears VOC concentrations dilutes early-stage off-gas below sensor threshold within metres of the source. The deck-wide background may rise; the local ramp at the source becomes invisible.

3. Localisation is poor

A gas sensor that trips tells you the deck has a problem. It does not tell you which vehicle. Bridge crew need bay coordinates to dispatch response. Per-vehicle thermal sensing localises natively; gas sensing requires triangulation across a dense mesh that is expensive to install and calibrate.

When gas sensing is the right primary

• Sealed battery storage rooms with low-flow ventilation.
• Lithium-cell spares lockers and engineering stores.
• Battery storage warehouses (the operating environment is closer to the bench rig than to a vehicle deck).

In those environments the bench-rig physics applies. On open vehicle decks, it does not. The architecture has to follow the environment.

Sources

• Larsson, F. et al. — "Toxic fluoride gas emissions from lithium-ion battery fires," Scientific Reports 7, 10018 (2017).
• Golubkov, A. W. et al. — "Thermal runaway of commercial 18650 Li-ion batteries with LFP and NCA cathodes," RSC Advances 4, 3633 (2014).
• EMSA — guidance on the carriage of alternative-fuel vehicles in ro-ro spaces (2023, updated 2024).
• [VERIFY: pre-vent VOC/H₂/HF onset lead-times are RoRoSafe sealed-chamber bench data; cross-check off-gas composition against Essl, C. et al., Batteries 6(2), 30 (2020).]