Standing next to the Ping-Pong table in the offices of the battery startup Ambri, chief technology officer David Bradwell needs both hands to pick up what he hopes will be a building block for a new type of electricity grid. Made of thick steel, it’s a container shaped like a large round cake pan, 16 inches in diameter. Inside it are two metal pucks and some salt powder; a round plate has been welded to the top to make a 100-pound battery cell.

By stringing together a number of these large cells, Ambri plans to make huge batteries, as big as 40-foot shipping containers. It’s not only their size that makes them novel: the chemistry in Ambri’s technology is different from any other currently used in batteries.