Thermal runaway in lithium-ion batteries

A lithium-ion cell normally produces some heat during charging and discharging, but in ordinary operation it can release that heat to its surroundings. Thermal runaway describes a failure state in which a cell produces heat faster than it can dissipate it, leading to an uncontrolled rise in temperature and further reactions. The term is used for the cell-level event; propagation describes a failure spreading from a cell to other cells in a module or pack.

Initiating faults can be internal or external. A damaged or defective separator can allow an internal short circuit between electrodes. External stresses identified by UL Research Institutes include overcharge, repeated excessive discharge followed by charging, an external short circuit, and high- or low-temperature environments. Mechanical damage can also be a trigger in tests and real incidents. The precise response depends on cell chemistry, design, state of charge, and the way the cell is abused; a single list of triggers does not predict how every battery will fail.

A cell undergoing thermal runaway can release heat and hot gases and may eject material. If enough heat reaches neighboring cells, those cells may enter runaway too, creating a chain reaction through a pack. Laboratory work on small modules found that propagation differed with cell form, physical contact, and electrical configuration: pouch-cell modules in that study propagated more readily than the tested cylindrical modules. These particular test results illustrate the importance of pack design; they should not be treated as a universal comparison for all batteries.

Possible consequences include high temperatures, vented gases, smoke, fire and, in some cases, violent cell rupture. The US National Transportation Safety Board identifies uncontrolled increases in temperature and pressure in damaged vehicle cells as a thermal-runaway hazard, with potential battery reignition. Bulging, cracking, hissing or popping, visible venting, and a rising temperature are warning signs cited by the U.S. Fire Administration. These signs indicate a potentially dangerous failure, not a reliable way for an untrained person to diagnose a cell safely.

Battery safety relies on controls at cell, pack and system levels: manufacturing quality, protection against off-nominal charging and discharging, thermal management, monitoring, and design that limits heat transfer between cells. Such measures reduce risk but cannot guarantee that every failure is prevented or contained. A damaged or overheating battery can present fire, chemical and electrical hazards; emergency responders use device- and vehicle-specific guidance and established procedures. A battery in distress should not be handled casually.