By Sarah Swan, Senior Fire Protection Consultant

The last five years of my career have involved one adjustment or another, most of them good. I’ve had two wonderful kids, I work for a new company, and I’ve gotten involved with the SFPE efforts on understanding how sustainability and fire protection are interconnected. But one of the biggest challenges I’ve had to adjust to professionally has been lithium-ion batteries.

I’ve spent the past six years working one lithium-ion battery project or another. I’ve even participated in the efforts to develop the new Battery Safety Code, NFPA 800. All of this work has required me to challenge traditional understandings of how we protect against fires.

Standard fire protection strategies are based on the idea that fires can be controlled by removing one element of the fire tetrahedron. Whether it is by removing the heat (such as sprinklers), removing the oxygen (such as low oxygen environments), removing the fuel source (such as automatic shut offs in flammable liquid supply pipes), or halting the chemical reaction that causes combustion (such as clean agent systems), addressing the fire tetrahedron is the basis of fire protection strategies.

Source: NFPA 921, 2024 Edition

Lithium-ion battery failures break this basic understanding.

This does not mean that detection and suppression systems have no value. It simply means that their objectives and limitations must be understood. Early detection may provide an opportunity to intervene before thermal runaway develops. Once active thermal runaway begins, suppression efforts are generally focused on cooling adjacent batteries, limiting propagation, and protecting surrounding materials. Emergency planning must also account for hazards that can remain after visible flames have been controlled.

The Thermal Runaway Sequence

Before a lithium-ion battery catches fire, it must first fail. Crushing, overcharging, overheating, or an internal defect may create an internal short circuit. The short causes the battery to generate heat. As the temperature rises, the electrolyte can reach its boiling point. The flammable vapors that are generated by this boiling build pressure inside the cell.

The rising temperature also causes a decomposition of the cathode active material. For many of the most prevalent cathode active materials, this decomposition causes the generation of oxygen gas, which also builds within the cell along with the flammable vapors.

When the pressure is high enough, the battery will vent the flammable vapors and oxygen gas that has been generated. Venting usually occurs before visible flames, although the sequence is not guaranteed in every event. If the condition causing the failure can be removed early enough, it may be possible to halt the thermal runaway event before it becomes unstoppable. If the cause cannot be removed, the cell can enter full thermal runaway very quickly.

Source: Spotnitz, R., Franklin, J. Abuse behavior of high‑power, lithium‑ion cells. RSC Advances, 2014. Royal Society of Chemistry.

The image above illustrates how quickly a single-cell failure can escalate into thermal runaway. During testing, the cell was intentionally driven into thermal runaway and experienced two distinct venting events. The first venting event represents a critical opportunity for early detection and intervention. If the thermal runaway event can be detected AND the condition causing the failure can be halted before the second venting event, the cell may be prevented from reaching its maximum temperature and developing into a fire. If intervention is unsuccessful, however, the temperature rises rapidly. When multiple cells are stored together, this temperature rise can cause thermal runaway to spread from one cell to the next, shifting the focus from fire prevention to limiting propagation and damage.

Why Lithium-Ion Battery Fires Behave Differently

The fire risk begins with the construction of the battery. Lithium-ion battery chemistries behave differently, but they share several important fire hazards. Some chemistries may be less reactive or harder to ignite than others, but don’t let this fool you. All lithium-ion battery chemistries can go into thermal runaway, and all can still produce a significant fire event if the hazards are not managed. Reactivity, maximum temperature, state of charge, cell capacity, and heat release all describe different parts of the hazard, so no single measurement establishes that a battery presents a low fire risk.

Lithium-ion batteries also contain a liquid electrolyte that allows ions to move between the cathode and anode. That electrolyte is a flammable solvent. When the cell heats up, the electrolyte begins producing flammable vapors. At the same time, the heat can cause the cathode materials to deteriorate, releasing oxygen.

This means a lithium-ion battery in thermal runaway provides all four elements of the fire tetrahedron on its own. Not only that, it can even start itself because the particulate ejected is often hot enough to ignite the flammable vapors. This is the key reason lithium-ion battery fires do not respond to traditional fire protection methods in the same way as many other commodities.

Why Traditional Detection May Respond Too Late

Lithium-ion battery detection is difficult because the warning signs vary. Batteries often fail quietly at first. Off-gassing may begin before there is flame, visible smoke, or enough external heat to activate conventional detectors.

Traditional photoelectric smoke detectors operate by sensing how smoke affects a beam of light. Battery off-gas has a different composition from conventional smoke and may not block or scatter light in the expected way. In addition, although some failures produce smoke large amounts of smoke prior to ignition, others may not produce enough smoke to activate detection until nearby materials have also become involved. See below image for an example of when automatic smoke detectors activated vs. when thermal runaway occurred.

Energy Safety Response Group (2021). Nexceris Li‑Ion Tamer Testing Summary Report. Prepared for Nexceris Inc.

Heat detection presents a similar timing problem. Heat generated inside a cell may take time to reach a detector installed elsewhere in the room. By the time a traditional smoke or heat detector activates, thermal runaway will likely be unstoppable and the available response will be limited to protecting nearby exposures.

This helps explain why some lithium-ion battery incidents appear to occur without warning. Warning may have been present in the form of off-gas, but the installed system was not designed to recognize it.

The Value of Earlier Off Gas Detection

Detection technology designed specifically for lithium-ion battery off-gas may provide an earlier indication of failure than traditional smoke detection. These systems are intended to recognize gases released during the early stages of battery failure, before enough smoke or external heat has developed to activate conventional detectors. The image above shows the difference in warning time between the Li-Ion Tamer sensor, a detector specifically designed for lithium-ion battery off-gas, and traditional smoke detectors.

That additional warning time can be significant, but detection does not stop the failure by itself. A useful early warning system must be connected to a response capable of addressing the cause of the failure. If the failure mode cannot be removed, thermal runaway may still continue after the alarm activates.

Specialized detection also does not eliminate the need for conventional fire alarm systems. Conventional fire alarm systems provide value by providing early detection of fires in ordinary combustibles in the areas in which lithium-ion batteries are stored or used. This early detection enhances the chances that the fire can be extinguished or controlled before the lithium-ion batteries become involved.

What Suppression Can and Cannot Do

Suppression systems traditionally control fire by removing heat, separating the fuel, reducing oxygen, or interrupting the chemical chain reaction. A lithium-ion cell in thermal runaway complicates each of these methods because the reaction is occurring inside the cell.

Water cannot stop the internal chemical reaction once a cell is in active thermal runaway. Water can, however, still serve an essential purpose by cooling adjacent cells and surrounding materials. Sufficient cooling may prevent additional cells from reaching the temperature at which they begin venting, reducing the likelihood that thermal runaway will propagate through an entire battery arrangement.

Systems that suppress fire by reducing or displacing oxygen also have limitations. Lithium-ion cells can release their own oxygen during thermal runaway, so oxygen-reduction, carbon dioxide, and inert-gas systems may not control the internal reaction as they would an ordinary fire.

Clean-agent systems may extinguish visible flames by interrupting the chemical chain reaction or providing some cooling. However, the battery will continue to heat and vent flammable vapors after the flames are gone. With no flame to ignite them, they will continue to accumulate. When a door is opened to access the batteries to ensure final extinguishment, the introduction of fresh air can create a flammable mixture and lead to deflagration.

Emergency Response After Suppression

Lithium-ion battery events create hazards for emergency responders beyond the visible fire. Off-gas may create a toxic and flammable atmosphere. Cylindrical cells and other battery components may become projectiles, carrying burning material beyond the original battery arrangement and involving distant combustibles.

Reignition is another concern. A battery fire that appears controlled may reignite hours or even days later if damaged cells remain hot or continue to fail. Response plans must account for atmospheric monitoring, cooling, isolation, and extended observation. Decisions about entering or ventilating a space should reflect the possibility that flammable vapors remain even when no flames are visible.

Fire Protection Objectives for Lithium-Ion Batteries

Lithium-ion battery fire protection requires realistic objectives. Early detection should identify abnormal conditions before thermal runaway whenever the battery and facility allow it. Suppression should provide cooling and control the spread of fire to adjacent cells and surrounding commodities. Emergency procedures should anticipate toxic off-gas, deflagration, flying debris, and reignition.

These objectives work together. Earlier detection may create time to remove the failure condition. If prevention is no longer possible, suppression can limit propagation and protect exposures. Emergency planning addresses the hazards that remain during and after those actions.

The central lesson is that lithium-ion battery protection cannot rely on the same assumptions used for ordinary combustibles. Battery chemistry, cell configuration, spacing, enclosure conditions, detection technology, and the planned response all affect the outcome. Understanding how the battery fails is the foundation for determining what the fire protection systems must accomplish.

About the Author

Sarah Swan is a Senior Fire Protection Consultant with RAN Fire Protection Engineering. She has worked extensively on lithium-ion battery manufacturing and storage projects since 2021 and participated in the NFPA 800 storage and manufacturing workgroups.