charge faster, last longer, and pack more power into a smaller package. They revolutionized communications and transportation, enabling pocket-sized cell phones and long-range electric cars. They're now widely used in smartphones, laptop computers, cameras, and power tools. Most of today's modern lithium batteries operate safely, says New York University mechanical engineer Nikhil Gupta. But some of them are cheaply made and catch fire, like the one in the New York City e-bike shop. Lithium-battery chemistry differs from that of most other common batteries because it uses a flammable organic electrolyte to help generate a current, part of what creates a fire risk if the battery gets too hot.??