"Lithium ion" (LiIon) is a term covering a number of battery chemistries, packaging & mechanical configurations. Advantages they share are: high energy density & low physical density (weight for volume). These are very attractive properties for aircraft where weight & space are at a premium.
On the other hand, some LiIon chemistries are unstable. Once an energy source becomes uncontrollable, high energy density also becomes a problem.
One suggested solution is replacing LiIon with nickel metal hydride (NiMh). Problem: lower energy density & high physical density. It has been suggested that a NiMh replacement will add 200 kg to the weight of a 787. This is equivalent to 2 to 4 passengers.
http://news.cnet.com/8301-11386_3-57564393-76/faa-grounds-boeings-787-dreamliner-after-battery-fires/ / Federal Aviation Administration ordered airlines ground fleet fleets fleet of Boeing 787 Dreamliner plane onboard batteries proven safe operate Japanese carriers All Nippon Airways Japan Airlines grounding battery fire forced evacuation ANA flight in-flight Boeing 787 battery incident emergency airworthiness directive AD address potential battery fire risk operators to temporarily cease operations FAA flight operators U.S. US USA U.S.A. registered aircraft demonstrate Federal Aviation Administration batteries are safe ground grounded Boston Logan International Airport caught fire cause traced back battery pack auxiliary power unit United Airlines mishaps recent weeks oil fuel leaks cracked windshield false warnings electrical panel composite materials Boeing Company /
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