A few cell phones ago, back when the batteries were still removable, I had a battery that was swelling, to the point that it would no longer fit properly in the phone. It was a long shot, but I wondered if I carefully pieced the outer cover, if it might let some gas pressure out, allowing the battery to shrink back to its proper size.
I was treated to a rather spectacular pyrotechnic display. Fortunately, I was able to get the battery out to a concrete patio, away from anything flammable, before it set me or my home on fire.
From what I observed of this cell phone battery, I have to imagine that if you scale it up to the size that would be necessary to power an electric car, and managed to set it off, the results could very easily be quite catastrophic.
I have a swollen laptop battery from my wife's laptop computer, that I've been saving with the intent of setting my camera up to record a video, and setting off in a more controlled manner than what happened with the aforementioned cell phone battery.