For more than two decades, lithium-ion batteries have powered almost everything around us. They are inside smartphones, laptops, electric vehicles, drones, and even many medical devices. Batteries have improved a lot over the years. But they can still overheat, use expensive materials like cobalt, and are becoming harder to improve. Instead of developing a completely new battery, researchers at the California Institute of Technology (Caltech) are focusing on the one we already use.