Thermal Runaway: What It Is and How to Contain It
Thermal runaway is the self-sustaining chain reaction that turns a single failed lithium cell into a 900 °C fire releasing hydrogen fluoride. This engineering guide covers the cascade physics, the trigger mechanisms, the gases involved, and what genuine containment actually requires.
1. What Is Thermal Runaway?
Thermal runaway is a self-sustaining exothermic chain reaction inside a lithium cell, in which the heat produced by internal chemical decomposition exceeds the heat the cell can shed to its surroundings. Once that balance tips, the reaction accelerates itself: more heat drives faster decomposition, faster decomposition produces more heat. No external energy input is required to keep it going, and no external action can stop it.
This is the single most important property to understand about a lithium battery fire, and the one most frequently misunderstood. A thermal runaway event is not a conventional fire. It does not require atmospheric oxygen, because the cathode itself releases oxygen as it decomposes. It cannot be extinguished by smothering, by CO₂, or by removing an ignition source, because there is no ignition source to remove. The reaction is fuelled from within.
Key principle: Thermal runaway cannot be extinguished, only contained and cooled. Every credible safety strategy for lithium battery transport and storage is therefore built around containing the energy release and the gases, not around fighting a fire.
For anyone handling large-format packs (EV modules, BESS strings, defective cells routed to recycling), this distinction drives every downstream decision. Sand, vermiculite and water-based suppression are responses designed for combustion events. They are marginal at best against a reaction that manufactures its own oxidiser. What works is a sealed, thermally resistant enclosure that absorbs the energy, holds the pressure, and neutralises what comes out of the vent.
2. The Cascade: Six Stages from Warm to Catastrophic
Thermal runaway is not a single event but a sequence of overlapping decomposition reactions, each with a characteristic onset temperature. Understanding the sequence is what makes containment design tractable: each stage has different implications for pressure, gas volume and heat flux.
The solid electrolyte interphase, the thin passivation film protecting the graphite anode, begins to break down exothermically. Heat output is still modest and the cell may appear normal externally, but the anode is now chemically exposed to the electrolyte. This is the last stage at which intervention (cooling, isolation) can still arrest the process.
The unprotected lithiated graphite reacts directly with the organic carbonate electrolyte, generating ethylene, ethane and CO₂. Internal pressure rises and the cell visibly swells. In pouch cells this is the point at which the seal typically begins to strain; in cylindrical cells the CID (current interrupt device) may trip.
The polyethylene/polypropylene separator melts and shrinks away from the electrode edges, allowing anode and cathode to make direct contact. The resulting internal short dumps the cell's stored electrical energy as heat in seconds. Past this threshold the cascade is effectively irreversible.
Layered oxide cathodes (NMC, NCA) decompose and liberate oxygen into the cell interior. This is why a lithium fire is self-oxidising: the reaction now has both fuel and oxidiser in intimate contact, sealed inside the cell casing. LFP cathodes are markedly more stable here, which is why LFP packs run cooler and vent less violently, but they still run away.
Internal pressure exceeds the burst disc or seal rating and the cell vents a hot, flammable, toxic aerosol: electrolyte vapour, hydrogen, carbon monoxide, hydrocarbons and hydrogen fluoride, carrying molten particulate. Temperature rise rates in this window routinely exceed 100 °C per second.
Peak temperature of the failed cell. The radiant and conductive heat flux to its neighbours pushes them through Stage 1 in turn, and the module fails sequentially. Without thermal barriers, cell-to-cell propagation intervals of 10–60 seconds are typical, and a full pack can be consumed in minutes.
The critical design insight from this sequence: containment must be engineered for Stage 6, not Stage 1. An enclosure that copes with a single vented cell but not with a full propagating pack offers no meaningful protection, because propagation is the default outcome once Stage 3 is reached in any cell within a module.
3. What Triggers Thermal Runaway?
Four abuse pathways account for effectively all field-observed thermal runaway events. They differ in how they get a cell to Stage 1, but converge on the same cascade thereafter.
State of Charge is the amplifier. None of these triggers act independently of SoC. A cell at 100% SoC holds the maximum stored energy available to feed the cascade, vents more violently and reaches a higher peak temperature than the same cell at 30%. This is precisely why ADR, IMDG and IATA cap transport SoC at 30% for most lithium consignments. It does not prevent thermal runaway, but it materially reduces its severity.
4. The Gas Problem: Why HF Is the Real Hazard
The fire is visible and dramatic. The gas is what actually harms people, and it is the part of the hazard most often omitted from risk assessments.
When a lithium cell vents, it ejects a mixture whose composition varies with chemistry and SoC but consistently contains flammable, asphyxiant and acutely toxic components. Typical vent gas from an NMC cell is dominated by carbon dioxide and hydrogen, with significant carbon monoxide and light hydrocarbons:
| Component | Typical share of vent gas | Primary hazard | Exposure concern |
|---|---|---|---|
| CO₂ (carbon dioxide) | 30–50% | Asphyxiant | Moderate |
| H₂ (hydrogen) | 20–30% | Explosive (4–75% LEL/UEL) | Severe |
| CO (carbon monoxide) | 5–25% | Acutely toxic | Severe |
| CH₄, C₂H₄, C₂H₆ (hydrocarbons) | 10–25% | Flammable | Moderate |
| HF (hydrogen fluoride) | Trace by volume | Corrosive, systemically toxic | Critical |
| POF₃ (phosphoryl fluoride) | Trace by volume | Highly toxic, hydrolyses to HF | Critical |
4.1 Where HF Comes From
Hydrogen fluoride is not present in a healthy cell. It is produced during the event, when the lithium hexafluorophosphate (LiPF₆) conducting salt in the electrolyte decomposes and reacts with trace moisture and with the organic solvents. Because essentially every commercial lithium-ion chemistry uses a fluorinated salt, HF release is intrinsic to lithium-ion thermal runaway, not a property of defective cells only.
Published measurements place HF yield in the range of roughly 20 to 200 milligrams per watt-hour of cell capacity, varying with chemistry, SoC and whether the vent gas burns on release. The implication for large packs is stark: a 90 kWh module in full runaway can liberate HF on the order of kilograms.
Why this matters operationally: the IDLH (Immediately Dangerous to Life or Health) threshold for hydrogen fluoride is 30 ppm. HF is absorbed through skin and lung tissue, binds calcium and magnesium systemically, and can cause fatal hypocalcaemia and cardiac arrhythmia from exposures that initially seem minor. Effects are frequently delayed by hours. In an enclosed space such as a vehicle trailer, a ship's hold or a warehouse, an unfiltered venting pack can exceed IDLH concentrations long before the fire itself becomes the limiting problem.
This is the specific reason BUNCKER® integrates gas filtration rather than simple pressure relief. Venting a pack into the surrounding atmosphere solves the pressure problem and creates a toxicological one. The Fibercon Altilium® CP Blanket (FACB) chemically neutralises hydrogen fluoride as the vent gas passes through it, so the gas leaving the enclosure is depleted of its most dangerous constituent rather than merely diluted.
5. Containment Strategies Compared
Four broad approaches are used in practice. They are not equivalent, and the distinction matters when specifying packaging for damaged or high-risk cells.
| Approach | Mechanism | Handles full propagation? | Neutralises HF? | Operational cost |
|---|---|---|---|---|
| Filler materials (sand, vermiculite) |
Thermal mass and physical separation between cells | Partial | No | High: slow loading, heavy, messy unloading, disposal burden |
| Water/foam suppression | External cooling of the enclosure | Partial | No, creates HF-laden runoff | High: requires infrastructure, not viable in transit |
| Passive insulated box | Thermal barrier limiting external heat transfer | Depends on rating | No | Low, but gas hazard unaddressed |
| Sealed bell jar + gas filtration (BUNCKER® architecture) |
Pressure-managed sealed enclosure with chemical HF neutralisation | Yes, tested to full pack runaway | Yes, via FACB | Low: no filler, direct loading, reusable |
5.1 The Bell Jar Principle
The BUNCKER® patented bell jar architecture treats the enclosure as a pressure-managed reaction vessel rather than as a box. The battery sits on a pallet base; the containment body lowers over it and seals. During a runaway event the enclosure performs three functions simultaneously:
- Thermal decoupling. The multi-layer wall construction keeps the outer surface below 100 °C even while internal cell temperatures peak in the 600–1,000 °C band. This is what makes the unit safe to remain in place in a trailer, a hold, or a warehouse aisle rather than requiring emergency removal.
- Pressure management. Vent gas is directed along a controlled path rather than rupturing the enclosure, so the structure stays intact for the duration of the event.
- Chemical gas treatment. The FACB layer neutralises hydrogen fluoride in the vent stream before the gas exits.
Eliminating filler material is not merely a convenience feature. Sand and vermiculite add substantial mass, must be loaded and disposed of as contaminated waste after an event, and, critically, make visual inspection of the battery impossible once loaded. A filler-free design means a pack can be loaded in minutes, inspected at any point, and the container reused after certified service.
BUNCKER® containment performance is established by real fire testing with live lithium batteries in full thermal runaway, not by computational modelling or single-cell extrapolation. The units are certified to UN LP906 for all lithium battery types including defective, damaged and end-of-life cells, and are valid across road (ADR), sea (IMDG) and rail (RID) transport as well as static storage, indoor and outdoor.
6. Stopping Propagation: The Decisive Variable
Single-cell containment is a solved problem and a largely irrelevant one. The engineering challenge that determines real-world outcomes is propagation: whether one failed cell takes the rest of the pack with it.
Propagation is driven by heat transfer from the failed cell to its neighbours through three paths: conduction through busbars, module structure and cooling plates; radiation across internal gaps; and convection via the hot vent gas itself. A containment system that addresses only one path does not stop propagation, because the remaining paths simply carry the heat.
At the pack level, propagation resistance is designed into the module (cell spacing, intumescent barriers, vent routing). At the transport and storage level, that design is no longer under your control. You receive the pack as built, often damaged, often of unknown internal condition. The containment enclosure therefore has to assume worst-case propagation: that the entire energy content of the pack will be released.
Specification rule of thumb: size containment by the full energy content of the pack in kWh, not by the mass of a single cell or by the probability of propagation. A 90 kWh pack should be specified against a 90 kWh runaway event. This is why the BUNCKER® range is indexed by kWh capacity, from SDR at 45.8 kWh through XXL at 200 kWh, rather than by volume alone.
BUNCKER® Pro Line XL: 100 kWh Containment
Patented bell jar architecture with integrated Fibercon Altilium® CP Blanket HF filtration. UN LP906 certified for all lithium battery types (stable, unstable, damaged and end-of-life) with no sand, vermiculite or filler material required. Outer surface stays below 100 °C throughout a full thermal runaway event. Engineered and manufactured in the Netherlands by VDL.
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7. Operational Response: What to Do During an Event
If a pack in your custody enters thermal runaway, the objective is not to extinguish it. It is to isolate the event, protect people from the gas, and let the energy discharge under containment.
Do not apply water to an unsealed lithium pack in an enclosed space. Water contacts fluoride species and produces HF-laden runoff, converting a localised gas hazard into a contaminated liquid one. Water is used in lithium incidents as a cooling measure on surrounding structure by trained responders, not as an extinguishing agent on the cells.
8. Regulatory Context: Where Containment Is Mandatory
Thermal runaway risk is the reason the dangerous goods framework treats lithium batteries as Class 9 and imposes escalating requirements as cell condition deteriorates. The regulatory trigger points that most directly require certified containment are:
- ADR Special Provision 376. Batteries identified as damaged or defective, or as liable to rapidly disassemble, react dangerously or produce a dangerous evolution of heat, must be transported in packaging meeting specified containment criteria. Cells assessed as high-risk cannot move in ordinary packaging at all.
- Packing Instruction P908 / LP904. The dedicated instructions for damaged and defective cells and batteries, requiring protection against short circuit, thermal management and containment of any release.
- Packing Instruction P911 / LP906. The highest containment tier, for batteries liable to rapid disassembly or dangerous heat evolution. Certification requires demonstrated performance under a real thermal runaway event, including limits on external surface temperature and on release of hazardous substances.
- IMDG and RID. The sea and rail regimes apply parallel provisions, with the enclosed-space gas hazard weighing particularly heavily in maritime carriage.
The practical consequence is that any organisation handling end-of-life, warranty-return, accident-damaged or unknown-condition packs will encounter the LP906 threshold. Certification at that tier is not an incremental upgrade over ordinary packaging. It is a different category of product, validated by live fire testing rather than by drop and stack tests alone.
Related reading: for a full treatment of the certification protocols, see Understanding UN P911 & LP906 Certification. For the classification framework that determines which tier applies to a given pack, see ADR 2026: What Changes for Lithium Battery Transport.
Frequently Asked Questions
Frequently Asked Questions
Not in the conventional sense. Because the decomposing cathode supplies its own oxygen, the reaction does not depend on atmospheric oxygen and cannot be smothered. Suppression agents can cool surrounding structure and slow propagation to adjacent cells, but they cannot stop a cell that has passed the separator-melt threshold. The realistic objective is containment and controlled burn-out, which is why certified containment packaging, rather than suppression equipment, is the primary control for transport and storage.
The first exothermic decomposition, the SEI layer breaking down, typically begins between 80 and 120 °C. However, the practical point of no return is separator melt at roughly 130–150 °C, after which an internal short circuit is essentially inevitable. Exact thresholds vary by chemistry: LFP cells are more thermally stable than NMC or NCA and both initiate later and peak lower, but they are not immune.
The fire is localised and visible; the gas is neither. HF is immediately dangerous to life or health at 30 ppm, is absorbed through skin as well as lungs, and acts systemically by binding calcium and magnesium, which can produce fatal cardiac effects from exposures that cause little immediate pain. Symptoms are often delayed by hours, so personnel may not realise they have been exposed. In enclosed spaces such as trailers, ships' holds and warehouses, HF concentration can reach hazardous levels across a far larger volume than the fire itself affects. This is why BUNCKER® units incorporate the Fibercon Altilium® CP Blanket to chemically neutralise HF rather than simply venting it.
No. It reduces severity but does not prevent the event. A cell at 30% SoC still contains sufficient stored energy and reactive material to run away if triggered. What changes is the magnitude: lower peak temperature, smaller gas volume, less violent venting and reduced propagation energy. This is why dangerous goods regulations cap transport SoC at 30% for most lithium consignments while still requiring certified packaging. SoC reduction and containment are complementary controls, not alternatives.
Filler materials are a workaround for enclosures that cannot themselves manage the thermal and gas load: they act as thermal mass and as a crude barrier between cells. The BUNCKER® bell jar architecture handles thermal decoupling structurally and handles the gas chemically through the FACB layer, so the filler has no function to perform. Removing it eliminates the loading and unloading time, the added mass, the contaminated-waste disposal burden after an event, and the inability to visually inspect a loaded pack.
The outer surface remains below 100 °C throughout, even while cells inside peak in the 600–1,000 °C range. This threshold is what allows a unit in thermal runaway to remain in situ, in a trailer, a hold or a warehouse, without igniting adjacent cargo or structure, rather than requiring emergency removal to a safe area. It is verified by real fire testing with live batteries, not by modelling.
A unit that has contained an event must be inspected, decontaminated and recertified through the BUNCKER® service programme before returning to service. The FACB filtration layer in particular is consumed by neutralising HF. Do not open or attempt to assess a post-event unit without appropriate PPE and a documented procedure; fluoride residues remain corrosive and toxic. The certified service programme includes regular inspection intervals for units in normal service as well as post-event recertification.
Contain It Before It Starts
BUNCKER® units are UN LP906 certified for every lithium battery type and condition, with patented HF gas filtration and no filler materials required. Our specialists can size the right model for your packs and your operation.
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