Lithium-ion batteries, which power devices from mobile phones to electric vehicles, present a unique and complex fire hazard that defies standard firefighting methods. The chemistry and physical design mean that fires are not only difficult to extinguish but can also reignite hours or even days later. This challenge stems from the highly reactive materials contained within the battery and a self-perpetuating heat cycle known as thermal runaway.
The Fundamental Chemical Reactivity
Traditional fires are suppressed by removing heat, fuel, or oxygen. Lithium-ion battery fires involve a unique chemical composition that defeats this strategy. The organic liquid electrolyte, which allows lithium ions to travel between the electrodes, is composed of highly volatile and flammable solvents like alkyl carbonates. This liquid is a significant fuel source, often having a flash point as low as 25°C.
A chemical complication arises when water is applied to a compromised battery. If cells are damaged and water penetrates the casing, a reaction involving the highly reactive alkali metal occurs. This highly exothermic reaction generates lithium hydroxide and hydrogen gas. The hydrogen gas is extremely flammable and can ignite instantly, intensifying the fire and potentially leading to a small explosion.
The fire is further sustained because it does not require external atmospheric oxygen to burn. The positive electrode, or cathode, is typically made of a lithium metal oxide compound. When subjected to high heat, this cathode material decomposes, releasing oxygen internally. This process means the fire is self-oxidizing, allowing combustion to continue even if the battery is completely submerged or smothered.
The Mechanism of Thermal Runaway
The physical process making these fires persistent is thermal runaway, a self-sustaining cycle where internal heat generation rapidly exceeds heat dissipation. This cycle can be triggered by internal short circuits, physical damage, overcharging, or external heat. Once the internal temperature rises, a series of exothermic decomposition reactions begin.
The sequence starts with the breakdown of the solid electrolyte interface (SEI) layer on the anode around 80°C. This is followed by the decomposition of the flammable organic electrolyte, releasing volatile gases. As the temperature escalates past 130°C, the separator material melts, causing a catastrophic internal short circuit that releases stored electrical energy as heat. This rapid temperature increase causes the cathode material to release its internal oxygen, leading to rapid gas venting.
This violent chain reaction can propel the internal cell temperature to over 600°C within minutes. In battery packs, intense heat from one failing cell transfers to adjacent cells, triggering a cascading failure known as thermal propagation. Even if visible flames are suppressed, the internal temperature of the entire pack remains dangerously high. The stored heat is enough to restart the entire sequence hours or days later, necessitating prolonged monitoring.
Specialized Suppression Techniques
Since lithium-ion battery fires are driven by internal chemistry and heat, the primary goal of modern suppression techniques is rapid and sustained cooling, rather than simple smothering. The goal is to break the thermal runaway cycle by forcing heat dissipation to exceed heat generation. Large volumes of water are recommended for firefighting, used for thermal management rather than extinguishing the chemical reaction itself.
Firefighters use a steady application of water to cool the surrounding environment and the battery pack’s casing. This slows the internal exothermic reactions and prevents thermal runaway from spreading to adjacent cells. This process requires significantly more water than a typical fire and can take hours to fully cool a large battery system.
For smaller-scale fires, specialized suppression agents are available. These include encapsulator agents that rapidly absorb heat and create a barrier to inhibit the release of flammable vapors. Specialized cooling gels or fire blankets are also used to contain the flames and slow the spread of heat until the fire burns itself out. Traditional Class D fire suppressants are generally ineffective for the complex, multi-class hazard of a lithium-ion battery pack.
Isolation and Monitoring
Due to the high risk of delayed reignition, any burning battery must be isolated and monitored for an extended period. This monitoring often lasts 24 hours or more, even after the visible fire has been put out.