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Can Flow Batteries Finally Beat Lithium?

AS SHE DRIVES HER electric vehicle to her mother’s house, Monique’s battery gauge indicates that it’s time to reenergize. She stops at a charging station, taps her credit card at the pump, inserts a nozzle into the car, and in 5 minutes exchanges 400 liters of spent nanofluid for fresher stuff. As she waits, a tanker pulls up to refill the station itself by exchanging tens of thousands of liters of charged for spent fuel. Monique closes her EV’s fueling port and heads onto the highway with enough stored energy to drive 640 kilometers (400 miles).

The battery in her EV is a variation on the flow battery, a design in which spent electrolyte is replaced rather than recharged. Flow batteries are safe, stable, long-lasting, and easily refilled, qualities that suit them well for balancing the grid, providing uninterrupted power, and backing up sources of electricity.

This battery, though, uses a completely new kind of fluid, called a nanoelectrofuel. Compared to a traditional flow battery of comparable size, it can store 15 to 25 as much energy, allowing for a battery system small enough for use in an electric vehicle and energy-dense enough to provide the range and the speedy refill of a gasoline-powered vehicle. It’s the hoped-for civilian spin-off of a project that the Strategic Technology Office of the U.S. Defense Advanced Research Projects Agency (DARPA) is pursuing as part of a drive to ease the military’s deployment of all-electric supply vehicles by 2030 and of EV tactical vehicles by 2050.

Using lithium-based batteries would create its own set of problems. You’d need a charging infrastructure, which for the U.S. military would mean deploying one, often in inhospitable places. Then there’s the long charging time; the danger of thermal runaway—that is, fires; the relatively short working life of lithium batteries; and the difficulties of acquiring battery materials and recycling them when the old batteries are no longer any good. A battery that mitigates these problems is DARPA’s objective. The new flow battery seems to hit every mark. If it works, the benefits to the electrification of transportation would be huge.

Flow batteries are safe and long-lived

Nanoelectrofuel batteries are a new take on the reduction-oxidation (redox) flow battery, which was first proposed nearly a century and a half ago. The design returned to life in the mid-20th century, was developed for possible use on a moon base, and was further improved for use in grid storage.

The cell of a flow battery uses two chemical solutions containing ions, one acting as the anolyte (adjacent to the anode), the other as the catholyte (near the cathode). An electrochemical reaction between the two solutions pushes electrons through a circuit. Typical redox flow batteries use ions based on iron chromium or vanadium chemistries; the latter takes advantage of vanadium’s four distinct ionic states.

On the chemical side of the reaction, each solution is continuously pumped into separate sides of a battery cell. Ions pass from one solution to the other by crossing a membrane, which keeps the solutions apart. On the electrical side, current moves from one electrode into an external circuit, circling around before returning to the opposite electrode. The battery can be recharged in two ways: The two solutions can be charged in place by a current moving in the opposite direction, the way conventional batteries are charged, or the spent solutions can be replaced with charged ones.

Besides beating lithium batteries in performance and safety, flow batteries also scale up more easily: If you want to store more energy, just increase the size of the solution storage tanks or the concentration of the solutions. If you want to provide more power, just stack more cells on top of one another or add new stacks.

This scalability makes flow batteries suitable for applications that require as much as 100 megawatts, says Kara Rodby, a technical principal at Volta Energy Technologies, in Naperville, Ill., and an expert in flow batteries. An example, she says, is the task of balancing energy flows in the power grid.

However, conventional flow batteries pack very little energy into a given volume and mass. Their energy density is as little as 10 percent that of lithium-ion batteries. It has to do with the amount of material an aqueous solution can hold, Rodby explains. There is only so much salt you can dissolve in a glass of water.

Therefore, flow batteries have so far been too bulky for most applications. To shrink them enough to fit in electric vehicles, you need to raise their energy density to that of lithium-ion batteries.

Nanoparticles boost flow battery’s energy density

One good way to add capacity to a flow battery is with nanofluids, which hold nanoparticles in suspension. These particles undergo redox reactions at the electrode surface similar to how the dissolved ions react in conventional flow batteries, but the nanofluids are more energy dense. Importantly, the nanofluids are engineered to remain suspended indefinitely, unlike other suspensions—for instance, sand in water. That indefinite suspension helps the particles move through the system and make contact with the electrodes. The particles can compose up to 80 percent of the liquid’s weight while leaving it no more viscous than motor oil.

Nanofluids suspended in water-based electrolytes were first investigated for this application in 2009 by researchers at Argonne National Laboratory and the Illinois Institute of Technology. The scientists found the nanofluids could be used in a system with an energy-storing potential approaching that of a lithium-ion battery and with the pumpable recharging of a flow battery. What’s more, the nanoscale particles could be made from readily available, inexpensive minerals, such as ferric oxide and gamma manganese dioxide for the anode and cathode materials, respectively.

Additionally, because the nanoelectrofuel is an aqueous suspension, it did not catch fire or explode, nor would the material be hazardous if the battery were to leak. The battery possessed a wide operational range of between -40 °C and 80 °C.

In 2013, the team received a three-year, US $3.44 million grant from the U.S. Department of Energy’s Advanced Research Projects Agency–Energy (ARPA-E) to build a prototype 1 kilowatt-hour nanoelectrofuel battery. The prototype’s success encouraged several of the principal investigators to spin off a company, called Influit Energy, to commercialize the technology. Through additional government contracts, the startup has continued to improve the components of the technology—the nanoelectrofuel itself, the battery architecture, and the recharging and delivery system.

John Katsoudas, a founder and chief executive of Influit, emphasizes the distinction between his company’s design and a conventional flow battery. “Our novelty is in doing what others have already done [with flow batteries] but doing it with nanofluids,” he says.

With the basic science problem resolved, Katsoudas adds, Influit is now developing a battery with an energy density rated at 550 to 850 watt-hours per kilogram or higher, as compared to 200 to 350 Wh/kg for a standard EV lithium-ion battery. The company expects larger versions would also beat old-style flow batteries at backing up the grid because the nanoelectrofuel can be reused at least as many times as a flow battery—10,000 or more cycles—and it will probably be cheaper.

 

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