Gases can build up in a cell over time or suddenly when the cell fails. At some point, and depending on the cell design, the gas pressure will cause the safety valve in the cell to release or the cell case to fail.
A cell venting doesn’t necessarily mean it will go into thermal runaway, but a thermal runaway will be predated by the cell venting.
The gases being vented will depend of a number of factors:
- chemistry of the cell
- trigger mechanism for gas evolution
- age of the cell
- state of charge of the cell
- temperature of the cell
Typically the vented material will be composed of:
- carbon dioxide
- carbon monoxide
- various VOC’s
- hydrogen fluoride
- Increasing SOC => increasing fraction of hydrogen and carbon-monoxide
- Increasing SoC => decreasing fraction of the inert carbon-dioxide decreases
- as CO2 decreases the overall hazard increases as there is less inert gas
- Hydrocarbons are relatively constant versus SoC:
- 10-15% for NCA and LFP
- 20-25% for LCO
This table only shows the Volume Fraction and not the total amount of gas.
The gas composition, surrounding gas composition, laminar flame speed, lower flammability limit and maximum overpressure all need to be evaluated to understand the likelihood and severity of combustion.
The combustion metrics that were evaluated show that NCA and LCO vented gases produce higher flame speeds and maximum overpressures relative to LFP vent gases. LFP cells also have a higher LFL, which likely reduces the probability of a flammable ignition.Austin R. Baird, Erik J. Archibald, Kevin C. Marr, Ofodike A. Ezekoye, Explosion Hazards from Lithium-Ion Battery Vent Gas, SAND2019-6428J
The volume of gas released is typically 1 to 2 litres per Ah of electrical capacity. This is just a rough estimate.
Sturk et al  measured the release of gas during thermal runaway in an inert gas chamber and got a significant difference in the volume of gas released for NMC/NCO compared to LFP.
- NMC/LMO = 780 litres/kg
- LFP = 42 litres/kg
- David Sturk, Lars Rosell, Per Blomqvist and Annika Ahlberg Tidblad, Analysis of Li-Ion Battery Gases Vented in an Inert Atmosphere Thermal Test Chamber, Batteries, 2019, MDPI
- Austin R. Baird, Erik J. Archibald, Kevin C. Marr, Ofodike A. Ezekoye, Explosion Hazards from Lithium-Ion Battery Vent Gas, SAND2019-6428J