These are sodium ion batteries, not metallic sodium half or full cells. Sodium chloride is a sodium ion, so it’s sodium Iron chloride, so is sodium sulfate , so is sodium floride. None of those are flammable. Sodium ion batteries store Sodium as ions on each of the anode and cathode never in metallic form. Depending on the electrolyte almost assuredly another sodium ion compound it won’t be flammable either. Sodium ions also has the ability to pass through glass and ceramic membranes opening up solid state cells. With nano tech and micron thin glass separators the surface area can be huge for very fast current loads with low joule heating.
Sodium half cells have been made with metallic sodium film like foil as the anode those would be flammable if oxygen or water was allowed to get to the foil layer but those cells are always inside a nitrogen or argon enclosure and only for.stationary use.
There is also molten sodium sulfur 55 gallon barrel sized cells were both sodium and the sulfur are molten here again they are only for bulk use and enclosed usually underground. These types of cells us a ceramic separator and can do 50,000+ cycles no other battery tech can reach that at the commercial level.
Thanks for the clarification