Venting

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

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.

  • NMC/LMO = 780 litres/kg
  • LFP = 42 litres/kg

Rupture Discs

In a pack design the vent gases will need to be released in a controlled manner.

OsecoElfab DualGard breather and vent disc

Pressure Equalization vs. Pressure Relief

Pressure equalization, commonly referred to as “breathing,” aims to maintain pressure balance within a battery enclosure due to environmental or elevation changes. Think of a plastic gas can in the garage in the summer months vs winter (it can be expanded or contracted in shape). However, pressure equalization alone does not mitigate overpressure risks. Rupture discs provide active pressure relief by opening at predetermined thresholds, exhausting excess pressure, and preventing catastrophic explosions.

Soft and Hard Venting

We tend to use the description “Soft Venting” for a gas release from a cell that is not alight, “Hard Venting” is when a cell is in thermal runaway and the gases are combusting. These terms can be misinterpreted and hence best to fully describe the event.

Thermal Runaway

If you heat a battery cell to somewhere above 130°C then exothermic chemical reactions inside the cell will increase the temperature and further reactions will take place. The result is an uncontrolled runaway and increase in temperature. The cell should vent in a controlled manner with fire and molten material. In severe cases the cell may explode. The energy released from one cell failing is likely to heat neighbouring cells that again could be triggered into thermal propagation.

References:

  1. 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
  2. Austin R. Baird, Erik J. Archibald, Kevin C. Marr, Ofodike A. Ezekoye, Explosion Hazards from Lithium-Ion Battery Vent Gas, SAND2019-6428J
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