In  recent   years,  reports  of   fires  and  even   explosions  caused  by  lithium-ion   batteries  are  common.Lithium-ion batteries  are  mainly composed  of negative  electrode  material, electrolyte  and positive electrode material. The chemical activity of the negative electrode material graphite  in the charged state is a bit like metal lithium. The  SEI film on the  surface would decompose  at high temperature,  and the lithium ions embedded in the graphite would react with  the electrolyte and the binder polyvinylidene fluoride  and finally would release a lot of heat.

Alkyl carbonate  organic solutions  are commonly  used as  electrolytes, which  are flammable.  The positive electrode material is usually a transition metal oxide,  which has strong oxidizing properties in the charged state, and easily decomposes  to release oxygen at  high temperature. The released  oxygen reacts with the electrolyte to oxidize, and then comes out a lot of heat.

Apperately, the lithium ion battery would be unstable when heating with high temperature. However, what exactually would  happen if we keep  heating the battery?  Here we conduceted  a real testing  to a fully charged NCM cell with voltage of 3.7 V and capacity of 106 Ah.

Testing Methods：

1. At room temperature  (25±2℃), the single  cell is first discharged  to the lower  limit voltage with a  current of 1C and left for 15 minutes.  Then use 1C constant current to charge to the upper limit voltage  and switch to constant voltage  charging, stop charging when the  charging current drops to 0.05C,  and put it aside  for 15 minutes after charging;

2. Increase the  temperature from  room temperature  to 200°C  at 5°C/min,  and keep  at 5°C per  liter for  30 minutes;

Conclusion:

Lithium  cells  will eventually  catch  fire  when  the test  temperature  is  continuously  increased.  From the above process  we first see  the exhaust  valve opened, the  liquid ejected;  as the temperature  rises further, the second  liquid  ejection  occured  and start  combustion.  The  battery  cells  failed  at  around  138°C,  which  was already higher than the common standard test temperature of 130°C.