Thermal runaway

Thermal runaway occurs when a cell has reached the temperature at which the temperature will continue to increase on its own and it becomes self-sustaining as it creates oxygen which feeds the fire (literally). Once the temperature of the cell reaches about 80°C the SEI layer on the anode begins to decompose and break down in an exothermic reaction (generating heat) due to the reaction of the lithium with the solvents used in the electrolyte. At about 100°C–120°C the electrolyte begins to break down in another exothermic reaction, which in turn generates various gases within the cell. The gases that may be created during this reaction, depending on cell chemistry, include carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), ethane (C2H6), ethylene (C2H4), and hydrogen (H2) (Ohsaki et al., 2005; Wang et al., 2012).

As the temperature nears 120°C–130°C the separator finally melts allowing the anode and cathode electrodes to make contact and cause an internal short circuit and generating more heat. As the temperature continues to rise, at about 130°C–150°C, the cathode begins breaking down in another exothermic chemical reaction with the electrolyte which also generates oxygen. It is this release of oxygen along with the carbonate LiPF6 electrolyte that ultimately allows the cell to burn and catch fire. The breakdown of the cathode active material is a highly exothermic reaction generating a lot of heat and continuing to drive the cell toward ultimate failure and fire.

When temperatures rise above 150°C–180°C the reaction may become self-sustaining if the cell is not able to rapidly dissipate the heat being generated. At this point the cell is in what is referred to as “thermal runaway” as the oxygen generation makes the fire self-sustaining—at least until all the fuel has been used. If the gases continue to build up within the cell the cell may rupture or vent through a safety valve. The cell may rupture or vent the flammable hydrocarbon gases and hydrofluorocarbon electrolytes at this point and the introduction of a spark could ignite the electrolyte and the gases causing flame, fire, and potentially an explosion. But if the pressure continues to build up it is also possible that the cell will split open and eject the jellyroll from the housing (Wang et al., 2012)..

#snsinstitution

#snsdesignthinkers

#designthinking