H₂, CO, CO₂: The Thermal-Runaway Signature
The gas-phase signature of a stressed Li-ion cell has a specific order and a specific tempo. Knowing it tells you what to sense, where, and when.
Peer-reviewed work on Li-ion off-gas signatures has converged on a consistent picture in the last five years. Electrolyte VOCs come first, then hydrogen as the SEI breaks down, then CO and CO₂ as the runaway propagates, then HF as the electrolyte salt decomposes. The sequence is the detector's map.
The signature in order
- Carbonate solvent VOCs (DMC, EMC, EC) — first to appear, dominated by the electrolyte composition.
- H₂ — released as the solid-electrolyte interphase breaks down, before any visible smoke.
- CO and CO₂ — produced as the organic electrolyte decomposes at higher temperature.
- HF — onset roughly coincident with the first audible vent, often the lethal exposure for crew.
Why H₂ is the most useful primary
Hydrogen is small, fast-diffusing, and not present in significant background concentration on a vehicle deck. Catalytic and electrochemical H₂ sensors with ppm-level sensitivity exist at marine-rated prices. The detection window is wide enough to catch the event before audible venting in bench conditions.
Where gas-only fails on a vehicle deck
Mechanical ventilation, large open volumes, and the residual VOC background from cold ICE engines means a deck-level H₂ trip can be late or absent at typical ventilation rates. The detection architecture has to either localise the sensor close to the source (impractical at scale) or fuse gas signal with thermal signal. We do the second.
Questions, answered
What is the gas signature of early-stage lithium-ion thermal runaway?+
It appears in a consistent order: carbonate-solvent VOCs (DMC, EMC, EC) first, then hydrogen as the solid-electrolyte interphase breaks down, then CO and CO₂ as the organic electrolyte decomposes at higher temperature, and finally HF roughly coincident with the first audible vent. The sequence is effectively the detector's map.
Why is hydrogen the most useful gas to sense first?+
Hydrogen is small, fast-diffusing, and not present in significant background concentration on a vehicle deck. Catalytic and electrochemical H₂ sensors with the required sub-5-ppm sensitivity exist at marine-rated prices, and in bench conditions H₂ leads visible smoke by roughly 2–8 minutes — wide enough to act before venting.
Why does gas-only detection fail on an open vehicle deck?+
Mechanical ventilation, large open volumes, and residual VOC background from cold ICE engines mean a deck-level H₂ trip can be late or absent. The architecture must either place sensors close to the source (impractical at scale) or fuse gas with thermal signal. RoRoSafe fuses the two — gas is information, thermal is localisation.
Continue the thread
Can Gas Sensors Detect Lithium-Ion Runaway?
Yes — and earlier than thermal in some settings. The harder questions: which gases, where to mount the sensor, and why it's a complementary layer at sea.
How Are Thermal Hotspots Detected?
The detection problem on a cargo deck is not measurement — IR sensors are commodity. It is deciding which delta in which cell at which time is real.