The chemistry behind the battery that could outperform Tesla’s Powerwall

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A rechargeable battery from Harvard just got a little greener . By replacing a corrosive and toxic component with a compound commonly used as a food additive, chemists have come up with renewable energy storage technology that they report is safe to use in homes and businesses. Should the design prove scalable, the researchers say it has the potential to top lithium batteries such as the Powerwall from Tesla Motors TSLA +2.23% in terms of cost as well as length of time it could operate at peak power.
A major stumbling block to getting most of society’s electricity from renewable sources like wind and solar is that these sources are intermittent and unpredictable. And people like predictability when they flip on a light switch or turn on their air conditioning. What’s missing is a reliable, cost-effective way to store the energy that, say, a rooftop solar panel generates in times of plenty for use at night or on cloudy days.
Many companies and research teams are working on building rechargeable batteries to meet this need. The battery announced today in the journal Science is what’s called a flow battery. These kinds of batteries have two main components: hardware that converts chemical energy into electricity, and tanks that contain energy-storing liquids. To increase the amount of energy that the battery can store, all you have to do is make bigger tanks. That’s a potential cost-savings over other battery designs that would require scaling up the whole enchilada. This video produced by Harvard explains more about how flow batteries work:

Thus far, flow battery technology has involved filling those tanks with acidic solutions of metal ions, such as vanadium. That chemistry’s not cheap, and not the safest to work with, either. (Today’s report uses fun scientific euphemisms for this, like “challenges with corrosivity.”) Last year, Harvard University scientists Michael Aziz, Roy Gordon, and their colleagues announced a different kind of flow battery, one that used a family of carbon-based molecules called quinones. Quinones are comparatively cheap because researchers can make them from inexpensive commodity chemicals. And plants use compounds from this family when harnessing the sun’s energy to generate nutrients. Unfortunately, while the negative “tank” of the battery used a quinone, the positive side relied on the chemistry of bromine. The toxicity concerns surrounding bromine make it impractical to use in all but the most specialized, high-security situations.

So Aziz and Gordon’s team went back to the drawing board. That’s when then-postdoctoral fellow Michael Marshak, now an assistant professor at CU-Boulder, thought of a solution for the safety issue. If they could get their flow battery to work in the presence of base (alkaline conditions), they’d be able to use some established chemistry known to be safer than bromine. In the 1970s, Lockheed developed an alkaline flow battery that relied on ferricyanide ion, a food additive. “It is a very well-studied system, safe, and non-toxic,” Marshak says. (Lockheed’s battery used the metal zinc, not a cheap, carbon-based quinone.)

The Harvard team’s quinone had originally been designed for an acidic battery system, but “one advantage of using quinones is that they can be tuned to operate in both acid and base,” Marshak says. Sure enough, by tweaking the chemical structure of their quinone, they were able to make their flow battery work in alkaline conditions (potassium hydroxide solution), with the food additive at the positive side. Battery performance was similar to last year’s bromine-containing counterpart. All told, the battery contains only compounds made from inexpensive elements (carbon, oxygen, nitrogen, hydrogen, iron and potassium) dissolved in water. The researchers point out that because the quinone-ferricyanide battery liquids aren’t as corrosive as last year’s model, they could save money on battery materials for tanks or casings, because it now becomes within the realm of possibility to use inexpensive plastics. (A safety note: Ferricyanide does contain cyanide, but it is not poisonous. The reason is that this particular cyanide is bound so tightly to the iron in the compound that it cannot escape to bind to the iron inside your body.)
OK, so how does all this relate to lithium-ion batteries like the Tesla Powerwall? Recall that a flow battery is designed so that you can scale up the power (the hardware that converts chemical energy to electricity) and the energy storage (the tanks) independently. If you want your charge to last longer, and you have a flow battery, you build a bigger tank. That’s not the case for lithium batteries. “In order for a lithium battery to discharge over a long duration, many separate battery cells are required, rather than one larger battery,” Marshak says. He gave me a hypothetical example to further illustrate this concept:

Suppose for example, you want a battery to provide 2 kilowatts of power for 5 hours (10 kilowatt hours), like the Tesla Powerwall. This would require many smaller lithium battery cells, or one flow battery. If you then want to provide the same 2 kilowatts of power but for 10 hours (20 kilowatt hours), you would need 2x of your lithium battery cells (or two Powerwalls). For the flow battery, however, you just need larger plastic tanks filled with electrolyte. This example illustrates that as need increases for batteries with longer duration discharge, flow batteries can have a lower marginal cost of energy storage than lithium batteries. In our case the marginal cost would be the cost of larger storage tanks, water, alkali (potassium hydroxide), quinone, and ferrocyanide – all of which are inexpensive.

The flexibility with battery tank size also means you can design a flow battery that can last longer than a lithium battery would at peak power, Marshak says. Adds Aziz: “The value proposition of flow batteries is not new: the energy (measured in kilowatt hours) to power (measured in kilowatts) ratio is the number of hours you can discharge your battery before being drained. These two metrics (energy and power) come together in different ratios for different applications. If you want to store your rooftop solar energy so you can run your air conditioning when you get home from work, and run the rest of the electrical appliances in your home through the evening, you might want your battery to discharge over 4 to 16 hours before being drained.” Of course, this battery situation wouldn’t be ideal for powering a vehicle, because you could get heavy tanks pretty quickly. But for stationary storage, the design makes more sense.

Still to be determined is whether the technology will scale, and to what extent regulatory factors will influence the market for this kind of storage. The firm Green Energy Storage negotiated a license for Harvard’s 2014 technology (with bromine) for use in Europe, and is currently in discussions with the university for licensing the new technology. Harvard has not yet chosen a licensee for the U.S. or the rest of the world, Aziz says.

Source: The chemistry behind the battery that could outperform Tesla’s Powerwall, Forbes, September 24, 2015