Utilities would love to be able to store the power that wind farms generate at night—when no one wants it—and use it when demand is high during the day. But conventional battery technology is so expensive that it only makes economic sense to store a few minutes of electricity, enough to smooth out a few fluctuations from gusts of wind.

Harvard University researchers say they’ve developed a new type of battery that could make it economical to store a couple of days of electricity from wind farms.