Why do lithium batteries sometimes catch fire?

Lithium batteries sometimes catch fire because of thermal runaway: heat makes the chemistry inside run faster, the faster chemistry makes more heat, and the loop feeds itself. Thermal runaway is a process accelerated by rising temperature that releases energy and so raises the temperature further. Once a cell is trapped in that loop, it can rupture, leak, explode or burn.

Lithium-ion batteries are especially prone to it, for two reasons. First, they have a flammable liquid electrolyte, so a faulty cell has fuel on board. Second, the trigger can be small. A fire can start from thermal abuse such as poor cooling, electrical abuse such as overcharge or an external short circuit, mechanical abuse such as a crash, or an internal short circuit from manufacturing flaws or aging. Once it starts, a cell can stay above 500 °C and ignite things around it.

One well-known trigger is lithium plating. If charging is too fast or the battery is too cold, lithium metal deposits on the negative electrode, and the resulting needle-like dendrites can pierce the separator between the electrodes. That short-circuits the cell from inside, causing high current, heating and ignition.

Real-world proof came in 2016, when Samsung suspended sales of the Galaxy Note 7 after a manufacturing defect in its batteries caused some phones to overheat, catch fire or explode. In the original batteries, a design flaw let electrodes bend and weakened the separation between the positive and negative tabs.

Engineers fight back with protection circuits that disconnect a battery outside a safe voltage range, pressure-triggered switches, and safer chemistries.