Lithium-ion batteries (LIBs) are the primary power source for devices ranging from handheld electronics and electric bicycles to electric vehicles and grid-scale energy storage systems. Their widespread adoption is due to high energy density and long cycle life. However, this high energy density also represents a stored chemical hazard that, when released uncontrollably, poses a significant thermal risk. Understanding the severity and behavior of a lithium-ion battery fire is necessary for safety in a world increasingly reliant on these cells.
Understanding Thermal Runaway
A lithium-ion battery fire is the result of a rapid, self-sustaining chemical process known as thermal runaway. This phenomenon begins when the internal temperature of a cell rises above a critical threshold, often due to an internal short circuit, overcharging, or external damage. Once the temperature causes internal components to decompose, the process becomes unstoppable. Initial exothermic reactions, such as the decomposition of the solid electrolyte interphase (SEI) layer, generate heat that accelerates further reactions.
The escalating heat causes the separator material between the electrodes to melt, leading to a widespread internal short circuit. This short circuit generates a massive surge of electrical heat, causing the flammable, organic electrolyte solvent to rapidly degrade and vaporize. The breakdown of the cathode material also releases elemental oxygen, which feeds the combustion of the electrolyte vapor. This cascade creates a chain reaction of escalating heat generation.
The temperature required to trigger thermal runaway depends heavily on the battery’s specific chemistry. Nickel-Manganese-Cobalt (NMC) cells, common in electric vehicles, often trigger at 160°C to 210°C. Lithium Iron Phosphate (LFP) cells are generally more thermally stable, requiring a higher trigger temperature, often 220°C to 270°C. Once initiated, the speed of the temperature rise can be extremely rapid, sometimes increasing by hundreds of degrees Celsius per second.
Peak Temperatures During a Thermal Event
Determining how hot a lithium battery burns requires differentiating between the cell’s exterior temperature and the actual internal reaction temperature. The external surface temperature of a failing cell can quickly rise to approximately 500°C (932°F) as internal reactions vent hot gases and flames. However, the internal temperature of the electrode materials during the peak of thermal runaway can be far more extreme.
During the most intense phase, internal temperatures can exceed 1,000°C (1,832°F), with some tests recording temperatures reaching 1082.1°C on the positive electrode side. The cell-face temperature, which measures heat radiating outward, also differs based on chemistry. NMC cells have been observed to peak at around 800°C, while LFP cells typically peak at a lower temperature of 620°C.
The maximum temperature reached is significantly influenced by the cell’s State of Charge (SOC), or the amount of energy stored at the time of failure. A battery at 100% SOC contains maximum stored chemical energy, intensifying the exothermic reaction and leading to the highest peak temperatures. The size of the battery pack also plays a role, as heat from a single cell’s failure can spread to adjacent cells via thermal propagation, creating a larger fire. Tests on large battery packs have shown that the surface temperature can be raised up to 1000°C due to the combined effect of multiple cells combusting.
Associated Hazards Beyond Heat
While extreme temperatures pose an immediate threat, a thermal event releases additional dangers, primarily highly toxic and flammable gases generated by the decomposition of the electrolyte and internal components. The most concerning gas is Hydrogen Fluoride (HF), a colorless gas formed when fluorinated lithium salts, such as lithium hexafluorophosphate, react with water vapor.
HF is highly corrosive and can cause severe respiratory damage and systemic toxicity, as it forms hydrofluoric acid upon contact with moisture in the eyes, skin, and lungs. Other gases released include Carbon Monoxide (CO), an odorless asphyxiant, and flammable hydrocarbons like methane, which can accumulate in enclosed spaces. The gas release is often forceful, building up internal pressure that is eventually released through a safety vent or by rupturing the cell casing.
This forceful venting of hot, pressurized gas and burning material is referred to as cell jetting or ejections. Ejected materials include molten battery components and burning particles, which spread the fire and present a physical projectile hazard. The gases themselves are combustible; if not immediately ignited upon release, they can accumulate and create the risk of a secondary gas explosion. Even after visible flames subside, risk remains as heat can propagate to adjacent cells or toxic byproducts leave behind harmful residue.
Preventing and Managing Battery Fires
The most effective way to manage lithium-ion battery hazards is through proactive prevention and careful handling.
Prevention
Always use charging equipment specifically designed for the device and avoid damaged or uncertified chargers, which may lack safety controls. Avoid overcharging by not leaving devices plugged in and unattended for extended periods after they reach full capacity.
Proper storage involves keeping batteries in a cool, dry environment, ideally between 5°C and 20°C (41°F and 68°F), and away from direct sunlight or heat sources. Any battery showing signs of physical damage, swelling, or excessive heat during normal operation should be immediately stopped from use and safely disposed of through a dedicated recycling program.
Management
In the event of a fire, the immediate response is to evacuate the area and contact emergency services due to the significant toxic gas hazard. Water is the primary extinguishing agent for lithium-ion battery fires, despite the misconception that it should not be used. This is because the chemistry involves a lithium salt electrolyte, not pure lithium metal. Water works to cool the cells and stop the spread of thermal runaway, though extinguishing the fire may require large volumes and time due to the high stored energy.