Lithium is the lightest metal on the periodic table and possesses the highest specific heat capacity of any solid element, making it valuable in various industrial and technological applications. Elemental lithium is highly reactive, especially when exposed to air and moisture, which leads to a significant fire hazard. The temperature required for combustion is surprisingly low, often lower than what people might expect for a metal. Understanding this high reactivity and low ignition point is necessary due to the risks associated with both the pure metal and the compounds used in modern battery technology.
The Ignition Point of Pure Lithium
The temperature at which pure, elemental lithium spontaneously ignites in a normal atmosphere is known as its auto-ignition temperature. For lithium metal, this point is quite low, generally observed at approximately 179°C (354°F). This temperature is nearly identical to its melting point, which is 180.5°C. This means that once solid lithium begins to melt, it is already at the temperature threshold where it can react vigorously with the surrounding air without an external ignition source.
The ease of ignition stems from its placement as an alkali metal, which readily loses its single outer electron to form chemical bonds. Many common structural metals require significantly higher temperatures to ignite, making lithium’s low auto-ignition point a unique safety concern. Due to this flammability, the pure metal must be stored under an inert atmosphere or submerged in a hydrocarbon sealant, such as mineral oil, to prevent contact with air.
The Chemistry of Lithium Combustion
Once lithium ignites, the resulting fire burns with extreme intensity because the combustion reaction is highly exothermic, releasing a significant amount of heat. The metal primarily reacts with the oxygen in the air to produce lithium oxide (\(\text{Li}_2\text{O}\)). This reaction generates a substantial amount of thermal energy, which feeds back into the process and sustains the combustion.
Lithium is also one of the few metals that reacts with nitrogen gas, which makes up nearly 78% of the atmosphere. This secondary reaction forms lithium nitride (\(\text{Li}_3\text{N}\)) and contributes to the difficulty of extinguishing a lithium fire, as the metal can draw a reactant from an otherwise inert gas. The combination of these two reactions produces a dense, corrosive smoke and drives the fire to extremely high temperatures, which can exceed 1200°C in an unlimited air environment.
Lithium-Ion Batteries and Thermal Runaway
The fire hazard associated with consumer electronics and electric vehicles comes not from pure elemental lithium but from lithium-ion (Li-ion) batteries, which contain lithium compounds and salts. These batteries do not have the same simple auto-ignition point as the metal, but instead are threatened by a process called “thermal runaway”. Thermal runaway is a self-sustaining cycle where heat generation within a single battery cell exceeds the rate of heat dissipation.
This event is typically initiated by electrical, thermal, or mechanical damage, such as overcharging, an internal short circuit, or physical penetration. The internal heat causes the solid electrolyte interphase (SEI) layer on the anode to decompose, triggering a chain reaction of exothermic events. The internal temperature can rapidly climb above 300°C and reach up to 600°C, causing the battery’s flammable liquid electrolyte to vaporize and ignite. The fire is a result of the entire cell structure burning, not just the lithium compounds themselves, and the process can quickly propagate to adjacent cells in a battery pack.
Safe Handling and Extinguishing Lithium Fires
Extinguishing a fire involving pure lithium metal requires specialized knowledge and equipment, as conventional fire suppression methods can exacerbate the situation. Water, for instance, reacts violently with elemental lithium, producing flammable hydrogen gas and intensifying the fire. Therefore, fires involving pure lithium metal are classified as Class D fires, and they must be fought using a specific Class D dry powder fire extinguisher.
In contrast, the strategy for controlling a lithium-ion battery fire focuses primarily on cooling the cells to halt the thermal runaway process. Since the fire is fueled by the flammable electrolyte, large amounts of water can be used to cool the structure and prevent the runaway from spreading to other cells. Specialized extinguishing agents are also available that are designed to rapidly cool the battery and break the chain reaction. Due to the risk of re-ignition, the affected battery must be continuously cooled and monitored for an extended period, even after the visible flames have been suppressed.