Short answer: Thermal propagation happens when a cell in thermal runaway heats its neighbours past their own runaway onset temperature. Heat reaches them by conduction through contact surfaces and busbars, by hot vent gas and flame, and by ejected particles. A cell-to-cell barrier must keep the neighbouring cell cool enough for long enough, while surviving compression, flame and high voltage.
How thermal runaway starts in one cell
A lithium-ion cell goes into thermal runaway when the heat produced by internal chemical reactions exceeds the heat it can lose, and the rising temperature speeds the reactions further. The usual sequence, as summarised by Feng et al. (2018), is:
- SEI decomposition at relatively low temperatures (roughly the 80–120 °C range). The passivating film on the anode breaks down and the anode starts reacting with the electrolyte.
- Separator melting and shrinkage. Polyethylene separators soften at around 130 °C and polypropylene ones at higher temperatures. When the separator fails, a large internal short circuit can form.
- Cathode decomposition and oxygen release. Layered oxide cathodes such as NMC release oxygen at elevated temperature, which reacts with the electrolyte and anode. This is the step that drives the steep temperature rise.
- Venting. Pressure builds up and the cell vents hot, flammable, toxic gas. Hydrogen, carbon monoxide, carbon dioxide and hydrocarbons are typical. The vent stream often carries burning electrolyte and solid particles.
Triggers include overcharge, internal short circuits from manufacturing defects or dendrites, mechanical damage, and external heating.
Three routes from one cell to the next
| Route | What happens | What blocks it |
|---|---|---|
| Conduction | Heat flows through the cell faces that touch the barrier, and through busbars, cooling plates and the module frame | Low through-thickness thermal conductivity at high temperature; thermal breaks in busbars and cooling plates |
| Vent gas and flame | Hot gas and burning electrolyte flow over neighbouring cells and can ignite inside the pack | Directed venting channels, flame-resistant covers, gas-tight separation between cells and modules |
| Particle ejection | Hot, electrically conductive particles hit nearby surfaces, erode thin barriers and can cause arcing | Barriers with high mechanical and erosion resistance; mica and ceramic facings |
In practice, engineers who design only for conduction are often surprised in testing by hot gas and particles.
The four jobs of a cell-to-cell barrier
- Thermal insulation at high temperature. What matters is the thermal conductivity at several hundred °C, not the room-temperature value quoted on page one of a datasheet. A good barrier keeps the far face of the barrier below the neighbouring cell’s runaway onset temperature for the required time.
- Mechanical compliance. Cells swell during cycling and ageing. A barrier between prismatic or pouch cells has to accept that compression over life without losing thickness, or thermal performance, permanently.
- Electrical insulation. In high-voltage packs the barrier often also has to provide dielectric insulation between cells, including after it has been exposed to flame.
- Fire and erosion resistance. The barrier itself must not burn, melt through or crumble when hit by flame and particles.
No single material is best at all four, which is why many commercial barriers are laminates, such as an aerogel core with mica or ceramic facings. See the materials data page for typical values.
What standards ask for
Most modern standards no longer only ask whether a single cell fails safely. They also require a propagation test: one cell is forced into thermal runaway and the response of the whole module, pack or system is observed. Examples include China’s GB 38031-2025 for EV traction batteries and UL 9540A for energy storage. The exact trigger methods, observation times and pass criteria differ; each standards page sets them out.
References
- Feng X. et al., "Thermal runaway mechanism of lithium ion battery for electric vehicles: A review", Energy Storage Materials 10 (2018) 246–267. doi:10.1016/j.ensm.2017.05.013
- Wang Q. et al., "Thermal runaway caused fire and explosion of lithium ion battery", Journal of Power Sources 208 (2012) 210–224. doi:10.1016/j.jpowsour.2012.02.038
Frequently asked questions
What is the difference between thermal runaway and thermal propagation?
Thermal runaway is the self-heating failure of a single cell. Thermal propagation is the spread of that failure to neighbouring cells, modules or racks. Barriers cannot prevent the first; they are there to stop the second.
Can a thermal barrier stop a cell from going into thermal runaway?
No. Barriers slow heat transfer to the neighbouring cells so that those cells stay below their own runaway onset temperature. The failing cell itself will still vent and heat up.
Why do LFP and NMC cells need different barrier designs?
NMC and other nickel-rich cells usually reach higher peak temperatures and release heat faster than LFP cells, so they demand barriers with lower high-temperature thermal conductivity and better resistance to flame and particle jets. LFP cells release large volumes of flammable gas, so gas management matters as much as heat blocking.