Lithium-ion batteries, despite being one of the most important pieces of modern technology, have always had a particularly grievous flaw: They're highly flammable. If you puncture a lithium-ion battery, or you charge/discharge them improperly, you'll usually have a pretty bad fire on your hands -- or worse, if the conditions are just right, an explosion. Most notably, the Boeing 787 Dreamliner was grounded in 2013 due to its lithium-ion battery packs causing electrical fires mid-flight -- but there have also been a fair number of stories about exploding smartphone batteries recently, too. Now, long overdue, researchers at the University of North Carolina at Chapel Hill say they've built a nonflammable lithium-ion battery.