Have a look at the You tube video of the Al Jazeera programme about the 787
https://m.youtube.com/watch?v=rvkEpstd9os ( interesting in itself) .
8mins in, they talk about the batteries and there is a test they do where they shoot a bullet into a lithium battery. Now that may be a bit unrealistic ( shooting a bullet into a battery) but when you see the result you will understand the concern!!!!!