A piece of chromium exposed to air gets covered in invisible armor, an oxide film, within a minute. That film protects the metal from rust, but for a battery it's a disaster: the film blocks chromium ions, so no current flows.
Scientists mixed five metals — chromium, bismuth, copper, tin, and nickel. The result was an alloy that, when oxidized, forms not a solid armor but a mosaic of different oxides. At the seams of this mosaic, "doors" appeared: chromium ions slip through them like commuters through subway turnstiles, while oxygen gets stuck. Some seams let chromium through, others keep oxygen out, and the battery lives.
The test was harsh: the battery was charged and discharged five thousand times in a row: that's ten thousand hours. The voltage barely changed. And when the chromium alloy was paired with a sulfur electrode, it made a full-fledged battery that also worked.
Now chromium has a shot at becoming the basis for cheap, high-capacity batteries — without lithium or cobalt.
The chromium film is so thin you can't see it, yet it reliably protects the metal for decades: that's why machine parts are chrome-plated.