Let's say you have a battery immersed in a tank of water (aside from the contacts), and you wanted to match the amount of water to the stored energy of the cell, such that, in the event that the stored energy is accidentally released, the last of the water boils away when the battery is empty.

Apparently it takes about 2.37 kwh to boil a gallon of water. That's a lot of energy, but if you have a car with an 80kwh battery pack, you'd need about 33 gallons of water, which is probably too heavy to be a realistic option.

(Granted this is oversimplistic because the energy that comes from a burning battery might be more than the electrical energy you could get out of it just by discharging it the normal way.)

I found a wikipedia article that lists joules per gram required to vaporize various substances:

https://en.wikipedia.org/wiki/Enthalpy_of_vaporization

Water is way up there. Iron and aluminum are even higher, but it's hard to imagine even an EV fire being enough to vaporize large amounts of those. I don't think I'd want to be breathing that either, though given the "battery fire" scenario I guess that's already a given.