I wonder what would happen if you tried to extinguish a lithium battery array with liquid nitrogen in a typical traffic accident situation? The fire engine of the future would have an N2 cryo tank on board that is charged at the station. Perhaps 50 gallons. When arriving at the scene, the firefighter dons full coverage gear and an O2 tank. Whether or not water or foam has been used already, the N2 team sprays the cabin or battery with all the N2 in the tank, then sprays water (mist setting) on top to insulate it. This is much colder than water, and if N2 contacts cells at all, it stops them from burning. They can't generate enough ions for heat. They would warm slowly and lose their charge before even reaching their operating temperature. While it would help battery fires, it would also suppress liquid hydrocarbon fires. Only 'fumes' actually ignite, liquids don't. Once you gel fuel, it doesn't burn easily if at all. It must first get warm enough to vaporize to burn. It would plug up any liquid fuel in lines and tubes by gelling it. Non-toxic, not insanely expensive, evaporates cleanly. Water will be frozen, which will make controlling runoff of toxic chemicals easier. And it's not a GHG. BTW - CO2 is fairly lethal even in low concentrations.