In early 2012, Envia Systems, a small Silicon Valley start-up headed by an ambitious battery engineer, announced that it had discovered the key to an electric-car revolution. For decades, materials scientists across the globe had labored like modern-day alchemists, mixing and matching elements from the periodic table — some lithium here, a little nickel and cadmium there — in search of some magic combination that would produce gold: a battery light, powerful, reliable and cheap enough to rival the gasoline-powered internal-combustion engine.