Venting - Battery Design

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.

Gas Composition

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:

electrolyte
carbon dioxide
carbon monoxide
hydrogen
various VOC’s
hydrogen fluoride

Baird et al [2] have reviewed the published experimental data and pulled this together into a table showing the gas composition by chemistry and SOC:

This shows:

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

Gas Volume

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 [1] 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.