Difficult fire suppression / fire extinguishing
It's a tricky proposition to extinguish a battery fire. Water may just accelerate the fire and inert gas would not necessarily stop the battery from burning (but might stop the fire from spreading). In chemistry labs, we'd have buckets of sand to suppress such difficult-to-extinguish fires, but that's a weighty proposition which won't save the battery. Maybe defective batteries should be ejected from the plane, backup batteries could ensure critical flight functions. But the FAA might not like such a solution.