Thermal Runaway and How Li-ion Cells are designed to prevent it

Thermal runaway occurs when  a Li-ion cell gets so hot that it starts to generate its own heat. This happens because the separator – the polymer film that separates the anode and cathode – melts around 80 degrees C. This can cause an internal short, allowing current to run unabated within the battery. This uncontrolled current leads to further heating. As the cell gets hotter the electrolyte starts to evaporate at 100 C, leading to a high pressure inside the cell, further increasing the temperature. At a critical temperature, the cathode begins to shed oxygen, making the cell combustible due to rapid reaction between the oxygen and electrolyte. For an LCO cathode, oxygen breaks away from the cobalt at 150 C. For an LFP cathode, the oxygen-phosphorus bond is not broken until 350 C. This is the primary argument for the safety of LFP cells. It is actually hard to get to 350 C inside the cell because the rate of heat transfer out of the cell increases with cell temperature.

Of course, Li-ion cells have a number of safeguards in place to prevent any increase in temperature, and the onset of thermal runaway.  The first line of defense is the Battery Management  System, which prevents overcharging, overdischarging, and external shorting. All of these things could cause an internal short or a rise in cell temperature.  Cells are also equipped with a “polyswitch” that prevents excessively high currents out of the cell.

In the case that there is an internal short or the battery is exposed to heat externally, cells are equipped with a burst valve that opens when the pressure gets too high. This immediately reduces the temperature and prevents thermal runaway.