That said surely halon might be useful in the boeing battery box.
If it is certain that there is no way for the contents of the box to
generate oxygen, then an inert atmosphere of any kind should
also work. If the batteries have a major short there still remains
the problem of the heat generated.
Unfortunately, LiCoO2 reacts with the electrolyte above c. 130C and decomposes approaching 200C, giving off oxygen in the process. This becomes self-sustaining, i.e. thermal runaway. Halon will suppress naked flames by interfering with the chemistry but can't do much about what's happening inside the cell.
The total energy release from an electrical short and combustion of the materials that make up the battery is enough to raise the temperature by *thousands* of degrees. This would actually be made worse by insulation - disrupted cells get hot enough to melt steel...
From the FAA 787 "special conditions" for Li-ion:
(2) Design of the lithium ion batteries must preclude the occurrence of self-sustaining, uncontrolled increases in temperature or pressure
Note that it doesn't say that it's okay if they are in a box, or if it only happens every now-and-then. LiCoO2 batteries are vulnerable to the above due to their chemical composition - whether it is triggered by over/under-charging, excessive current, manufacturing defects, cosmic rays or a change in interest rates, doesn't really matter. So far, there has been no (public) confirmation of *why* these failures occurred, so pushing a solution seems ever so slightly premature.
I do wonder how they're going to get round this provision: either the FAA will have to retract the offending paragraph or Boeing will have to use a different type of battery. I suppose the third way will be to argue "we don't know what caused it but we've changed lots of things so it's OK now"; depends on how much spine the regulator has that morning, I suppose...