the wiki article on Li-ion batteries is not particularly pleasant reading
to quote
"
These safety issues present a problem for large scale application of such cells in Electric Vehicles; A dramatic decrease in the failure rate is necessary."
Lithium-ion battery - Wikipedia, the free encyclopedia
the rest of the safety information is quite worrying for an airliner.
they might have to go back to something less sexy for battery tech. NiMH like the hybrid cars?