Li-on batteries can be dangerous, and have started/almost started fires on aircraft. The FAA wants passengers to have them with them in case something goes wrong with them and they can work with crew members to prevent anything serious happening. There have been airplane crashes where the exact cause of fire was never exactly determined, but there were Li-on batteries listed as cargo ( industrial quantities ). I have never had any catch fire, but I had one which got hot enough to discolor the info sticker on it.
Unfortunately, it would not be too difficult for someone who knew what they were doing to modify a "smart" device into a timer controlled incendiary device...
The world we live in.