Putting the car in a large, fire-proof steel container (specialty engineering, they exist) for a few weeks (so you are at least somewhat sure it won't start cat

Putting the car in a large, fire-proof steel container (specialty engineering, they exist) for a few weeks (so you are at least somewhat sure it won't start catching fire RFN). Then dismantling the battery by hand, inspecting each cell individually. You can't really put anything around it (like a nitrogen atmosphere) that prevents fires, because the oxygen is already in there in the cathode. And so is the energy to ignite it.