Kiskaloo,
Agreed, in that Boeing don't design batteries. They would have looked at the energy density values and at the design stage, Lithium-Ion would have been a no-brainer.
Yuasa's expertise in battery design is beyond reproach, however at this moment it's uncertain what caused the battery pack to ignite. While it may have been a faulty battery, we don't know if the cell spacing exacerbated the problem or if the charge/discharge algorithms were faulty or if it was an as yet unknown factor.
What is clear is that Boeing pretty much went all in on 'Electric plane' concept and this made the Lithium battery a commitment rather than a contribution.
I'm still slightly surprised that there were no indications of potential battery problems during the testing/flight testing regime, these normally take each system to the edge of the envelope, yet no issues reported?