Lithium-ion batteries catch fire when a process called thermal runaway turns the energy stored inside a cell into uncontrolled heat. The battery’s electrolyte, a liquid that shuttles charge between the two electrodes, is flammable. When something goes wrong inside the cell, a chain of chemical reactions can raise temperatures faster than the heat can escape, eventually igniting that electrolyte and potentially spreading to neighboring cells. Understanding what triggers this chain reaction helps explain why everything from phones to electric vehicles can, in rare cases, go up in flames.
How Thermal Runaway Works
A lithium-ion battery stores energy by moving lithium ions back and forth between two electrodes through a liquid electrolyte. That electrolyte typically contains a fluorine-based lithium salt dissolved in organic solvents, and the whole mixture is flammable. Under normal conditions, a thin barrier called the separator keeps the two electrodes apart. If that barrier is breached, or if the cell overheats for any reason, a self-reinforcing cycle begins.
First, a protective film on the electrode surface starts to break down, releasing a small amount of heat. That heat accelerates the breakdown of the electrolyte itself, which releases more heat and generates flammable gas. If the temperature keeps climbing, the electrode materials begin to decompose. At this point the reactions are feeding themselves: each one raises the temperature enough to trigger the next. The cell vents hot, flammable gas, and if that gas ignites, you get an open flame. In a battery pack with many cells, the heat from one failing cell can push its neighbors into the same spiral, which is why battery fires can reignite repeatedly and are so difficult to extinguish.
What Triggers It From Inside the Cell
The most common internal trigger is a short circuit, where the two electrodes make electrical contact through the separator. This can happen in a few ways.
During charging, lithium ions are supposed to slot neatly into the electrode material. Sometimes, especially during fast charging or charging in cold temperatures, the lithium deposits as metallic spikes called dendrites instead. These dendrites grow over repeated charge cycles and can eventually puncture the separator, creating a direct path between the electrodes. Researchers have found that dendrites tend to nucleate in cracks on the electrode surface where charge distribution is uneven, making the problem worse as batteries age and their internal structures degrade.
Manufacturing defects are another path to an internal short circuit. Microscopic metal particles, particularly copper, are the most common contaminants found in defective cells. A copper fragment smaller than a grain of sand, trapped inside during production, can slowly work its way through the separator over weeks or months of use. This is why some battery fires seem to happen spontaneously in devices that have been working fine for a long time: the defect was there from the start, but the short circuit took time to develop.
External Causes: Overcharging, Heat, and Damage
Physical damage is the most straightforward trigger. A puncture, crush, or hard impact can breach the separator instantly, creating an internal short circuit with no warning. This is why swollen or visibly deformed batteries are dangerous, and why airlines restrict lithium batteries in checked luggage.
Overcharging is another major risk. Lithium-ion cells are designed to operate within a narrow voltage window, and the safety cutoff for a single cell is typically around 4.2 volts. Charging beyond that threshold destabilizes the electrode materials and can cause the electrolyte to decompose, generating heat and gas. Every lithium-ion battery pack includes a battery management system (BMS) that monitors voltage in real time and cuts off charging when cells approach that limit. When that system fails, or when a user pairs a battery with a charger that doesn’t communicate properly with it, overcharging becomes possible.
External heat matters too. Leaving a device in direct sunlight, near a heat source, or in a hot car can raise the cell temperature into the range where the protective film on the electrodes starts to break down. Once that first reaction kicks off, the cell can generate enough internal heat to sustain the cascade on its own.
Why Some Battery Chemistries Are Safer
Not all lithium-ion batteries carry the same fire risk. The difference comes down to how the cathode (positive electrode) behaves at high temperatures. Nickel-rich cathode materials, commonly used in batteries labeled NMC (nickel-manganese-cobalt), release oxygen from their crystal structure as they heat up. That released oxygen feeds the fire from within the cell itself, which is part of what makes lithium-ion battery fires so hard to smother with water or foam. Research has shown that the higher the nickel content in an NMC cathode, the lower the temperature at which oxygen release begins.
Iron phosphate cathodes (LFP) behave very differently. The phosphorus-oxygen bonds in their crystal structure are strong covalent bonds that hold together even at elevated temperatures. Comparative testing has confirmed that LFP cells exhibit significantly better thermal stability than NMC cells. This is a major reason why LFP batteries have become popular in applications like home energy storage and many electric vehicles, despite storing somewhat less energy per kilogram. They are not immune to thermal runaway, but the threshold is higher and the reaction is less violent.
Toxic Gases Released During a Battery Fire
The fire itself is only part of the danger. When a lithium-ion cell vents or burns, it releases a cocktail of toxic gases that can be harmful even in a well-ventilated space. Carbon monoxide and carbon dioxide are produced in significant quantities. But the fluorine-containing salts in the electrolyte create an additional hazard: hydrogen fluoride, a highly toxic and corrosive gas. Testing published in Scientific Reports measured hydrogen fluoride emissions ranging from 20 to 200 milligrams per watt-hour of battery capacity. For a laptop battery, that might be a modest amount. For an electric vehicle battery pack or a warehouse full of e-bike batteries, it represents a serious inhalation risk to anyone nearby.
Some tests also detected phosphoryl fluoride, another toxic fluorine compound, at concentrations of 15 to 22 milligrams per watt-hour. Based on analogies with similar chlorine compounds, phosphoryl fluoride may be even more toxic than hydrogen fluoride. These findings are a key reason why firefighters now treat lithium-ion battery fires with specialized protocols and why burning e-bikes in apartment hallways have been so deadly.
Warning Signs Before a Fire
Battery fires don’t always strike without warning. The most reliable physical sign is swelling or bulging. When internal reactions produce gas, the cell’s casing expands. If your phone, laptop, or power tool battery looks puffy or is pushing against its housing, stop using it and keep it away from flammable materials.
Unusual heat is another precursor. Batteries generate some warmth during normal charging and use, but if a device feels too hot to comfortably hold, something is wrong internally. A sudden drop in how long your battery holds a charge can also indicate internal degradation that increases fire risk, though this alone is less specific.
A hissing sound or the smell of solvents (sometimes described as sweet or chemical) means the cell is actively venting gas, and a fire may follow within seconds to minutes. At that point, move away from the device and get it away from anything flammable if you can do so safely.