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Battery may benefit renewables

Researchers from the US Energy Department’s SLAC National Accelerator Laboratory and Stanford University have designed what they say is a low-cost, long-life “flow” battery that could enable solar and wind energy to become major suppliers to the electrical grid.

The research, led by Yi Cui, a Stanford associate professor, is a product of the new Joint Center for Energy Storage Research (JCESR), a DOE Energy Innovation Hub.

Led by Argonne National Laboratory, with SLAC as major partner, JCESR is one of five such Hubs created by the Department to accelerate energy research and was established last November.

While solar and wind make a substantial contribution to the nation’s energy supply, they also create significant power fluctuations, which can sometimes exceed the tolerances of the electrical grid.

“Flow” batteries, such as developed by Cui’s group, can smooth those fluctuations. It is also relatively simple to scale their tanks, pumps and pipes to the sizes needed to handle large capacities of energy.

The new battery uses a simplified, less-expensive design than other batteries, which may improve its scalability and cost-effectiveness. In laboratory tests, it demonstrated excellent energy-storage performance through the equivalent of more than 5 ½ years of daily charge and discharge cycles, according to DOE.

"For solar and wind power to be used in a significant way, we need a battery made of economical materials that are easy to scale and still efficient. We believe our new battery may be the best yet designed to regulate the natural fluctuations of these alternative energies," says Cui.

Today's flow batteries pump two different liquids through an interaction chamber where dissolved molecules undergo chemical reactions that store or give up energy. The chamber contains a membrane that only allows ions not involved in reactions to pass between the liquids, while keeping the active ions physically separated.

This battery design has two major drawbacks: the high cost of liquids containing rare materials such as vanadium – especially in the huge quantities needed for grid storage – and the membrane, which is also very expensive and requires frequent maintenance.

The new battery design by Cui’s group uses only one stream of molecules and does not need a membrane at all. Its molecules mostly consist of the relatively inexpensive elements lithium and sulfur, which interact with a piece of lithium metal coated with a barrier that permits electrons to pass without degrading the metal.

When discharging, the molecules, called lithium polysulfides, absorb lithium ions; when charging, they lose them back into the liquid. The entire molecular stream is dissolved in an organic solvent, which doesn't have the corrosion issues of water-based flow batteries.

Cui's group now plans to make a laboratory-scale system to optimize its energy storage process and identify potential engineering issues. It also plans to start discussions with potential hosts for a full-scale field-demonstration.

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