The flames may go out from lack of oxygen, but that doesn't mean that the internal thermal runaway has halted. There are two processes going on in a flaming li

The flames may go out from lack of oxygen, but that doesn't mean that the internal thermal runaway has halted. There are two processes going on in a flaming lithium battery - the internal exothermic runaway, which heats up the electrolyte and various other things enough to gas and escape, and then the combustion of those gasses in atmospheric oxygen. The vacuum chamber will terminate the flaming combustion part, as there isn't any oxygen left to burn the gasses with (leaving a rather more toxic cocktail of stuff including HF in many cases), but it won't do a thing for the internal runaway. To stop that, you've got two options: Cool the pack sufficiently to get everything below the exothermic point, or wait for it to run out of stuff to react. Also, I've no interest in "just trying" lithium pack runaways. I've rebuilt a number of lithium packs for people over the years, and I've tried very, very hard (and so far successfully) to avoid that sort of thing.