Inside the world’s largest battery plant, delicate robot arms coat sheets of aluminum and copper foil—each only 5 micrometers thick, about a 20th the diameter of a human hair—with an electrode slurry, a process that resembles nothing so much as spreading jam on bread. The coated material, along with a thin separator film, is guided along by massive steel rollers, precisely calibrated to ensure each strip is neither too tight nor too loose. The next step is to wind the strips into tightly packed spirals known as jelly rolls and seal them into casings. More robotic arms then inject an ...