Thermal runaway is a chain reaction inside a battery where rising temperatures trigger chemical reactions that generate even more heat, creating a self-accelerating cycle that can end in fire or explosion. It happens when a battery cell’s internal temperature climbs past a critical threshold, typically starting around 150–170°C, and the heat produced by decomposing materials outpaces the heat the battery can shed to its surroundings. Once this tipping point is crossed, the process feeds itself and can reach temperatures above 800°C in a matter of seconds.
How the Chain Reaction Unfolds
Thermal runaway doesn’t happen all at once. It progresses through a series of internal chemical breakdowns, each one releasing heat that pushes the battery closer to the next. The first layer of protection to fail is a thin film on the battery’s negative electrode, called the SEI layer, which normally keeps the electrode stable. When heat breaks this film down, the exposed electrode begins reacting directly with the liquid electrolyte, generating more heat and gas.
As temperatures continue climbing, additional reactions pile on: the positive electrode decomposes and releases oxygen, the electrolyte itself breaks down, and gases build inside the sealed cell. Once internal pressure gets high enough, the cell’s safety vent pops open and releases hot, flammable gases. In lab tests with standard cylindrical cells, this venting typically occurs around 170°C. If heat keeps building, the thin plastic separator between the electrodes melts, allowing a direct internal short circuit. At that point, around 260°C in one study of common cylindrical cells, the full thermal runaway event kicks off: combustible gases ignite, sparks and molten particles eject from the cell, and temperature spikes to its peak within roughly a minute.
After the violent energy release, the reaction slows simply because there’s little material left to burn. The cell begins cooling passively.
What Triggers It
The triggers fall into three categories: mechanical damage, electrical abuse, and excessive heat.
- Mechanical damage covers impacts, punctures, or crushing forces that deform the cell internally and push electrodes into contact with each other. Even a dent that doesn’t break the outer casing can create a tiny internal short circuit.
- Electrical abuse includes overcharging and over-discharging. Overcharging causes metallic lithium to plate onto the negative electrode, and this plated lithium can grow into needle-like structures called dendrites. These dendrites can physically pierce the separator and short the cell from inside. Over-discharging can cause copper dendrites to grow from the current collector toward the opposite electrode with the same result.
- Excessive heat from external sources, like a nearby fire, direct sunlight on a damaged pack, or a neighboring cell already in runaway, can push a healthy cell past its stability limits without any electrical fault at all. Research on battery storage systems shows that higher ambient temperatures cause thermal runaway to begin noticeably earlier and burn for shorter, more intense periods.
Manufacturing defects are a special concern because they create latent risks invisible to the user. A microscopic metal particle trapped between electrode layers during production can slowly work its way through the separator over months or years of normal use, eventually causing an internal short circuit with no external warning.
Battery Chemistry Changes the Risk
Not all lithium-ion batteries behave the same way in thermal runaway. The cathode material, the positive side of the battery, is the biggest factor in how hot and how violent the event becomes.
NMC (nickel manganese cobalt) cells, common in electric vehicles for their high energy density of around 230 Wh/kg, produce the most severe runaway events. Cell surface temperatures can reach 800°C, and the hot gases venting from the cell can hit 1,000°C. They also release the most gas per unit of capacity: roughly 0.07 mol per amp-hour, with carbon monoxide making up about 36% of that gas.
LFP (lithium iron phosphate) cells are significantly more stable. Their surface temperatures peak around 480°C during thermal runaway, with vent gas temperatures reaching 446°C. They produce about a third as much gas as NMC cells (0.02 mol/Ah), and the gas contains much less carbon monoxide. This is a major reason LFP batteries are increasingly popular in applications where safety margins matter more than maximum range.
Toxic Gases Released During Venting
The gases released during thermal runaway are both flammable and toxic, which is why battery fires are treated differently from ordinary fires. The vented mixture contains carbon monoxide (which displaces oxygen in the blood), carbon dioxide, and hydrogen, all of which are hazardous in enclosed spaces.
The fluorine-containing compounds inside the battery pose a particular danger. The electrolyte salt and electrode binders can decompose into hydrogen fluoride, an extremely toxic gas that causes severe burns to the lungs and skin even at low concentrations. Hydrogen fluoride is a well-known industrial hazard, and its presence in battery fire smoke is one reason firefighters treat these incidents with extra caution and specialized protective equipment.
How Battery Packs Are Designed to Contain It
Modern battery management systems (BMS) monitor every cell in a pack continuously, watching for the earliest signs of trouble. The primary signals are temperature, voltage, and in some systems, pressure. A typical alarm strategy uses absolute temperature thresholds (warning above 70°C, automatic shutdown above 100°C) combined with rate-of-change monitoring. A cell temperature climbing faster than 1°C per second is a red flag even if the absolute temperature is still moderate.
Voltage monitoring is trickier. The subtle voltage fluctuations caused by early-stage micro-short circuits are often too small to distinguish from normal electrical noise, which is why engineers are increasingly looking at impedance-based methods that combine voltage and current data for better sensitivity. Some systems also detect the volatile organic compounds that vent from a cell before full runaway begins, essentially sniffing for the earliest stage of electrolyte breakdown.
Even with early detection, a cell can still fail. That’s why physical barriers between cells are critical. The goal is to prevent one failing cell from overheating its neighbors and triggering a cascade through the entire pack. Aerogel sheets are one of the most effective options: even a 1 mm layer of aerogel between cells delayed the spread of thermal runaway from about 6 minutes to over 21 minutes in testing, buying enough time for active cooling or passenger evacuation. Other barrier materials include ceramic fiber, intumescent coatings that expand when heated to form an insulating char, and composite firewalls made from aluminum and fiberglass.
How Common Are Battery Fires
Despite the intensity of thermal runaway when it does occur, the overall failure rate for modern battery systems is low. U.S. National Transportation Safety Board data shows roughly 25 fires per 100,000 electric vehicles sold, compared to about 1,530 fires per 100,000 gasoline-powered vehicles sold. Similar ratios have been reported in Norway, Sweden, and Australia. The difference is partly because EV batteries are engineered with multiple layers of protection, and partly because gasoline is inherently more flammable than a sealed battery pack under normal conditions.
The concern with battery fires isn’t frequency but behavior. A thermal runaway fire can reignite hours or even days after it appears to be extinguished, because heat can linger deep inside the pack and restart reactions in cells that hadn’t yet failed. This makes battery fires harder to declare fully out and is why damaged EVs are sometimes quarantined in water-filled containers after an incident.