Electric car sales mean global demand for cobalt could outstrip supply next year Rick Lewis / Alamy Stock Photo

Strings of plastic balls dangled in the ocean could harvest enough cobalt for hundreds of thousands of electric car batteries. The heavy metal is a key battery ingredient, but onshore reserves are running low. So engineers in the US want to mine it from brine.

Maha Haji and Alexander Slocum at the Massachusetts Institute of Technology say the system could catch enough dissolved cobalt from seawater each year to make a battery for every Tesla Model 3 that has …