If I found myself boarding a B787 unexpectedly, I would be reassured by the fact that before it was allowed back into service a great many people who understood such things will have made damn sure that the batteries are now safe.
I would be even more reassured by the fact that it is the first large mass-production passenger aircraft (OK, first equal with A350) to have a fully-effective fuel tank inerting system built into it as part of the original design.