The electrodes of lithium ion batteries are submerged in a liquid called an electrolyte, which allows for the movement of ions and consists of lithium salt and organic solvents. It is these organic solvents which are the leading fire hazard in Li-ion batteries.
for a lithium cell the electrolyte is a solution of lithium salts and organic solvents. When the battery is charged, lithium ions are driven from the electrolyte into a carbon anode. ... This can trigger a thermal runaway in which the battery overheats and can burst into flame.
The chemicals inside the battery begin to heat up, which causes further degradation of the separator. The battery can eventually hit temperatures of more than 1,000 F. At that point the flammable electrolyte can ignite or even explode when exposed to the oxygen in the air.
Lithium-ion batteries have built-in safeguards designed to stop them from exploding if they're left charging while at maximum capacity.
Some Chinese cheapies do not have this safety built in. These batteries can swell, and become hot while charging and the fire risk is high....Moral is; Don't buy Chinese made cheapie lithium ion batteries.