Researchers are striving to make tomorrow’s batteries charge faster and store more energy. But these conveniences come with safety challenges, like more heat produced in a battery. For the first time, a team of researchers has studied the effects of tiny areas within lithium metal batteries that are much hotter than their surroundings. These hotspots, the researchers find, can make batteries grow spiky tumors of metal called dendrites that could cause short circuits, and potentially lead to fires.