Exothermic just means that more heat is produced than is put in to initiate the reaction.
Lithium batteries which overheat create oxygen themselves, so they are harder to put out. The basic fire triangle tells us we can eliminate any one of the three elements to put it out, but the fuel is the battery itself, and it is creating its own oxygen as part of the failure, so it only leaves heat, so you think that cooling it with water would work. Nope, you really don't want to do that as the heat of the reaction breaks the water down, and provides more hydrogen fuel & oxygen as a result. The same can happen with CO2 if the battery is big enough.
Enclosed lithium batteries where you can't get them out, or properly access them, are pretty much destined to destroy whatever they are in if a thermal runaway happens. I saw a few RC batteries up in flames where someone was dumb enough to overcharge the LiPo in the pursuit of imaginary advantage, the really dumb ones charged in the car... However, these were the same people who blew up NiMH & NiCad cells through abuse as well, it's just that lithium makes the party so much more fun..
