LiON batteries have the potential to explode or catch fire. But how often does this happen, and is it really a problem we need to worry about?
Short answer: no.
I have two justifications for this answer. First is the "do you know anyone?" test. We all have many LiON batteries in our phones, laptops, high-powered flashlights, vaping pens (some people) and, of course, electric cars. With all that equipment and with hundreds of friends and acquaintances, I do not know one single person who has ever had a battery catch fire or blow up.
My second justification for my statement is from basic research. I was easily able to find sensational news stories about "spontaneous" LiON fires or explosions, but a little reading showed almost all of those to not be spontaneous at all, but instead were caused by some sort of accident (dropping, crushing, shorting, etc.). This includes the Consumer Reports article which quoted the FAA study about multiple LiON fires aboard aircraft. Most (not all) of those, when you read the report, involved dropping boxes or having the batteries in a box where the battery terminals were allowed to touch a metal object packed in the same box.
So, compared to gasoline, which most of us store in the garage in cans and in our cars, I think these batteries are pretty darned safe, and I think are extremely unlikely to cause a problem.
As for long-term storage, DJI includes circuitry that will discharge the battery to under 60% if you let it stand for about ten days. This is more for extending the life of the battery than for safety, but it obviously reduces the impact of any overheating, no matter how remote that possibility. Since the DJI circuits take care of the discharge, you don't need to do it.
To avoid anything touching the battery terminals, I created some terminal covers. However, if you have a fireproof bag (which I do), that will suffice.
So, get the bags, let the DJI circuits manage the discharge, don't store the batteries next to oily rags or old newspapers, and then go and find something else to worry about.