Fuel cells and flow batteries don't have this problem. The energy-storing material--a fuel such as hydrogen or a liquid electrolyte, respectively--can be flowed past a membrane, which makes it easier to get the energy out.

The problem with a fuel cell is that it can't be recharged by applying electrical current--you need to refill the fuel tank. That's fine if the fuel is widely available, but right now hydrogen can be hard to come by. Flow batteries require vast amounts of electrolyte because their energy density is low. "It's like managing a large swimming pool full of corrosive liquid," Chiang says. As a result, flow batteries are not practical for cars.

As with fuel cells, the new battery can store large amounts of energy without also needing large amounts of supporting materials to extract it, Chiang says. Yet it retains the rechargeability and energy density of lithium battery electrode materials. The result is that the battery can store a relatively large amount of energy at low cost. But he's purposefully vague about the mechanisms involved, saying only, "The final version of the device will look very different from both a conventional battery and a flow battery."

Chiang says the new design could work with a range of battery chemistries. So far, he's developed a proof-of-concept device--which was needed to get the Arpa-e grant. But, he says, "there's a lot of work to do." He's setting a goal of five years to get the first systems out in the field.