Physics of Thermal Runaway

A cell rarely fails in a single step. Once a trigger event (see Triggers of Thermal Runaway) pushes a cell past its stability point, thermal runaway unfolds as a chain of exothermic reactions, each one supplying enough heat to set off the next. Understanding that chain, rather than just the trigger that starts it, is what explains why thermal runaway is so hard to stop once it begins, and why it produces heat, gas, ejected material and, in some cases, fire or explosion.

The Reaction Chain

The first link is usually the solid electrolyte interphase (SEI), the thin passivation layer that forms on the anode surface. This layer is only stable up to roughly 90-120°C. Above that range it starts to break down, and because the breakdown reaction is itself exothermic, the heat it releases pushes the cell further into the temperature range where decomposition accelerates. The fresh anode surface exposed by a broken-down SEI then reacts directly with the surrounding electrolyte, adding a second source of heat on top of the first.

If nothing intervenes, the rising temperature eventually reaches the point – typically above around 130°C – where the polyethylene or polypropylene separator loses mechanical integrity (see the Separator section of the main Thermal Runaway page, and Shutdown Separators for how some designs try to blunt this step). A melted or shrunk separator lets the anode and cathode touch directly, producing an internal short circuit that adds a sudden, large heat spike on top of the gradual heating from the earlier reactions.

The final major link is decomposition of the cathode itself. At high enough temperature a delithiated cathode structure breaks down and releases oxygen, which both adds heat and, because oxygen is what combustion needs, changes the character of everything that follows. This step is covered in more depth in Oxygen Release & Chemical Crosstalk, including how it can carry heat between electrodes even without a physical short circuit.

Gas Generation and Venting

Every reaction in the chain above also produces gas, mostly from electrolyte solvent breaking down under heat, alongside further gas from the anode and cathode reactions themselves. As gas builds up faster than it can escape through normal seals, internal pressure rises until the cell vents through a designed weak point or, in an uncontrolled failure, ruptures. The composition and quantity of that gas depends heavily on state of charge and chemistry – see the Gas Composition and Gas Volume sections of the main Thermal Runaway page for the data by chemistry.

Particle Ejection

The gas leaving a venting cell moves fast enough to physically carry solid material with it. Multiphase modelling of cell venting [1] shows this high-speed flow stripping fragments of electrode material out through the vent path as it goes, which is part of why a cell that has been through thermal runaway is structurally collapsed inside, not just electrically discharged.

Secondary Combustion and Explosion Risk

Whether the vented gas mixture ignites depends on its composition, how much it dilutes with the surrounding air, and whether an ignition source is present – the cell’s own venting temperature is often source enough. Large-scale FAA testing gives a sense of the scale involved: gas vented from 400 18650 cells into an enclosed 10.8 m³ chamber, then deliberately ignited with a spark igniter, produced an explosion with a recorded pressure rise of 29 psig [2]. The same test programme found the vented gas mixture’s flammability window widened with state of charge, which is one of the reasons a pack’s charge state at the time of a fault changes how severe the resulting event is.

This is also why gas composition and containment/venting design in a pack matter as much as preventing the trigger event in the first place – by the time gas is venting in volume, the reaction chain above has already run past the point where it can be stopped at the cell.