My greater concern is not that these batteries catch fire, but that they fail! On a fly-by-wire aircraft, that gives a whole new meaning to the term "dead stick". Yes, I know almost everything else has to fail before this matters. But consider Sully's celebrated ditching:
He lost both engines. His APU was not running. If his batteries had failed, and his aircraft had been a 787, he would have had no flight controls. At all.
The 787 also has individual back up batteries for the three ACEs (Actuator Control Electronics-part of the fly by wire system) they are also Li-Ion. The 777 is similar.
I note that the a/c needs 2 GPUs to provide engine starting, I have been to numerous outstations where you are lucky to get even one doubtful unit.
Boeing have said this a/c is a game changer, I agree, but it is very complex, and as such I can see lots of down line problems.
It's worse than that. Two GPUs is the absolute minimum-engine start cycle can be up to 3 mins due to the lack of power.
For optimum engine starting a third GPU is required-plugged in at the aft EE bay area.
As has been said, getting two 90Kva GPU supplies is hard enough, but three? FORGET IT!