Solid-state cells would be faster and safer than today’s lithium-ion equivalentsIllustration: Mark PerniceLIKE ANY champion who spends too long at the top, the lithium-ion battery is stagnating. Over decades as the battery of choice in everything from smartphones to electric cars and drones, its design has been tweaked countless times to improve its energy density and performance. But, some scientists say, those improvements are approaching their theoretical limits. Even the best models are pron...