Bradwell's battery is based on an electrolyte that can dissolve a compound consisting of two metals, such as magnesium and antimony. Applying a current in one direction splits the compound, and the two metals are deposited onto opposite electrodes. When no electricity is delivered, a voltage difference between the electrodes drives a current in the other direction. That generates electricity and causes the metals to recombine in the electrolyte.
The system could eventually cost less than $100 per kilowatt-hour for a new installation--about the same as pumping water up a hill to be released later to spin a turbine (the cheapest conventional approach for large-scale energy storage), says Arun Majumdar, the director of ARPA-E. The battery, however, would have the advantage of working in places without hills or large amounts of water, where many renewable power resources are located.