Lithium Ion batteries have had their fire problems in computers, cars, drills, just about in everything in which they have been used. I remember several years ago a regional carrier on which I often flew wouldn't let us have our carry-on computer bags that wouldn't fit go into the hold due to battery fire danger. We had to take the computer and spare batteries if we had them out and keep them with us at our seat where a fire hazard would be easily identified. Back then it was suspected contamination in the battery material. By the way Airbus does use some Lithium-ion batteries on current planes but nowhere to the extent of the 787.