H₂, CO, and CO₂ — The Gas Signature of Early-Stage Thermal Runaway

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.