Steven Prast is old enough to remember when a mobile phone came in a bag. It was too heavy to carry around, so most people mounted it in the car. Prast argued in a Zoom interview with Fortune that what eventually shrank it into something you wear on your wrist was more the battery than the transistor, holding up his own phone as proof. “What took us from that to this is the battery,” said Prast, a former Tesla manufacturing leader who is now chief operating officer of the battery-materials start...