If the discussion was to halt now, and a spontaneous ignition solution had to derive, dendrites due age and lack of monitoring would be my posit....Maybe an MRI of each cell periodically? (I jest)...sort of
in the scheme of things, I give Boeing an A+ for the composite solution to efficiency, an A+ for its elegant electrical system, an A+ for their part in powerplant development, and an F for the ironic and more or less last resort choice of Lithium chemistry.
A two hundred million dollar ship grounded by regulated, demanded back up batteries
Probably unnecessary, so long as all served aerodromes have a start cart. Maybe Airbus can get the archaic requirement for batteries extinguished in time for the 350?
Dendrites as "Fossils"? Likewise FAA regs?