Let me add some comments about the 2013 grounding of the Boeing 782 Dreamliner due to safety concerns about the battery that Boeing chose.

This comment from CNN at the time is partly revealing but doesn't get down to "the real nitty gritty":

"Boeing chose a particularly risky type of chemical makeup in its lithium ion battery, one that provides more power but does not stand up well to overheating,"

The battery that grounded Boeing (cnn.com)

So, pulling out a quote from a very old radio show and a still old, but not as old, TV show:  "Who was that Masked Man?"  Answer to the radio and TV query is easy:  "The Lone Ranger!"

However, as that question relates to the fiasco of Boeing's choice for a battery for the 787 Dreamliner, the answer is quite different... no masked man, just a bad choice when it came to selecting a battery.  Boeing chose the most power dense battery available at that time.  So what was wrong with that?  It was a lithium ion battery that had Cobalt incorporated into the Cathode. And what was wrong with that?

"Engineers designed many parts to be radically different. For example, the 787 would rely much more on electricity. But to do so, the plane needed a more powerful battery.

Boeing chose the design that it says best met the needs of the 787, including all federal and international safety standards. “We chose the lithium-ion battery because it was superior in performance to other battery technology at the time,” Boeing 787 Chief Engineer Mike Sinnett told accident investigators. “We were looking for power density,” he said.

The company had chosen one of the most volatile chemistries available, lithium-cobalt. The battery produced nearly 10 times the current of its nickel-cadmium equivalent on the Boeing 777."

Source of the above quote is an article from 2014: Boeing’s 787 battery: Problems from the start | Aviation | Al Jazeera

The highest possible power density was in that lithium ion cobalt battery, but so was the propensity for thermal runaway.