Giovanni Ceccio

In a lithium-sulfur battery, sulfur tends to dissolve and drift toward the other electrode — the battery loses charge within dozens of cycles. To hold the runaway in check, a separator film is placed between the electrodes. A standard polyethylene film lets lithium ions through, but sulfur molecules slip along with them.

A battery film half a micron thick hides a junction that cannot be seen directly. A lithium cobalt oxide cathode and a solid lithium germanium phosphate electrolyte meet so tightly that the boundary between them is invisible.