Flow What: A Battery Designed For Grid Storage

Even if you accept that storage is a prerequisite to increased renewable energy deployment – a proposition that might deserve more debate – there are many technologies beyond batteries. Still, batteries are among the possibilities, so when a researcher publishes work in a top journal that offers up a battery that “may be the best yet designed to regulate the natural fluctuations of … alternative energies,” it deserves a look.

The researcher is Yi Cui, from the U.S. Department of Energy’s SLAC National Accelerator Laboratory and Stanford University. His work, published in Energy & Environmental Science, emerges from one of outgoing Energy Secretary Steven Chu’s pet projects, an Energy Innovation Hub, this one the Joint Center for Energy Storage Research.

flow battery slac

image via SLAC/Greg Stewart

So what are Cui and his team offering? It’s a new take on “flow” batteries, aka “redox flow” batteries, which don’t look much like what the average person thinks of as a battery. Flow batteries store energy as liquid electrolytes in separate tanks, with various different chemistries employed in today’s research, including vanadium, iron/chromium and zinc/bromide. When the electrolytes are pumped into a chamber separated by a membrane, an electrochemical reaction produces electricity. During the charge, the flow of electrons is reversed.

Cui and Co. have recongifured the concept in a fundamental way, using just one storage tank and no membrane, and introducing the use of the less pricey elements lithium and sulfur. As explained in the SLAC release:

(Lithium and sulfur) 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.

According to the researchers, the benefit here is the use of cheaper materials, and the lack of need for a costly membrane. That makes sense, as the cost of flow batteries has been seen as one of the major challenges to overcome in putting them to use.

In addition, the researchers said, “(i)n initial lab tests, the new battery also retained excellent energy-storage performance through more than 2,000 charges and discharges, equivalent to more than 5.5 years of daily cycles,” Cui said.

flow battery slac

Yi Cui holds a lab demo of the new lithium-polysulfide flow battery contained in a simple flask. (image via SLAC/Matt Beardsley)

Meanwhile, the system would theoretically have the big advantage of redox flow batteries: the ability to be sized to fit an applications needs, which in turns allows for scaling up to grid-storage dimensions.

“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,” Cui said, and he clearly thinks he’s on track with this design. The next step, according to SLAC: “Cui’s group plans to make a laboratory-scale system to optimize its energy storage process and identify potential engineering issues, and to start discussions with potential hosts for a full-scale field-demonstration unit.”

Pete Danko is a writer and editor based in Portland, Oregon. His work has appeared in Breaking Energy, National Geographic's Energy Blog, The New York Times, San Francisco Chronicle and elsewhere.