A solar installation in Papua, Indonesia, that uses Fluidic Energy’s batteries as backup power storage.
ENERGY STORAGE
Batteries that breathe air Rechargeable zinc-air batteries are going mainstream, but other metalair chemistries will have to wait ALEXANDER H. TULLO, C&EN NEW YORK CITY
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CREDIT: FLUIDIC ENERGY
ast year, zinc-air battery start-up Fluidic Energy installed backup batteries for an array of solar panels in Welay Selatan, a tiny village on the Indonesian island of Alor. The location is remote, a 77-hour drive from Jakarta, including intermittent ferry voyages. The new solar power setup can bring electricity to 52 homes not currently connected to the grid. The scene isn’t unique. So far, the Scottsdale, Ariz., firm has deployed 22 MWh of battery storage for 96 rural solar electrification projects on some of the Indonesian archipelago’s remoter islands, providing power to 110,000 people. The company signed an agreement with the Indonesian government to provide backup power for a total of 1.2 million inhabitants. Ramkumar Krishnan, Fluidic’s chief technology officer, says many people from the world’s most underdeveloped regions are getting power for the first time from the sun, photovoltaic cells, and batteries locally rather than waiting for long-distance transmission lines to reach them from coal-fired power plants. “It is very similar to what happened in the telecom industry, where essentially all of the people in those regions skipped land lines and went to mobile communication,” he says.
It is also a demonstration of the real-world potential of rechargeable metal-air batteries. The current state-of-the-art technology, the Li-ion battery, brought the world portable electronics and is powering the modest fleet of electric vehicles on the road. But the limitations of Li-ion batteries may make them insufficient to provide the energy storage needed to wean the world off fossil fuels. Metal-air batteries, in which oxygen from the atmosphere reacts with metals to generate power, may be the next-generation battery technology the world needs. Li-air and Na-air batteries have energy density potential far beyond that of Li-ion. This could lead to increased success of electric vehicles. Other chemistries, such as Fluidic’s zinc-air batteries, made of inexpensive raw materials, may prove to be the answer for stationary storage. The batteries could
allow people to use renewable energy when the sun isn’t shining or the wind is calm. Fluidic is perhaps the most commercially advanced rechargeable metal-air battery developer. The firm sprang up in 2006 from the laboratories of Cody Friesen, a materials science associate professor at Arizona State University. Since then, it has raised nearly $200 million, with backing from investors including the venture capital firm TN2, equipment maker Caterpillar, International Finance Corp., and Asia Climate Partners. Fluidic has 100,000 batteries in the field already, and it aims to bring reliable power to 100 million people worldwide by 2025. Single-use zinc-air batteries have been around for decades and are the battery of choice for hearing aids. These aqueous alkaline batteries use zinc as their anode material. At their cathodes, water and oxygen form hydroxide ions, which react with zinc to make zinc oxide, releasing electrons in the process. As in all batteries, the electrons flow from the anode through the circuit—powering the device—and return to the cathode. Fluidic’s Krishnan says the same chemistry underpins its batteries. The challenge was making it work in both directions to create a rechargeable battery. The biggest obstacle, he says, is that metal-air batteries form dendrites on the metal anode. The dendrites grow like icicles with each charge and can eventually short-circuit the cell. The company solved this problem in three ways. One was to use ionic liquids as additives in the aqueous electrolyte to help suppress dendrite formation. The company FEBRUARY 27, 2017 | CEN.ACS.ORG | C&EN
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CREDIT: C&EN
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e– – + also came up with a scaffold electrode 500 program in 2009. The name design that promotes bottom-up derives from the company’s goal to growth of the zinc anode. Additionaldevelop a battery that can power a O2 Li ly, electronic controls preferentially car for 500 miles (800 km) on a sindeposit zinc where it needs to be. gle charge. A few advantages make the batIn 2012, the company signed on Li O2 teries perfect for competing with Japanese chemical firms Asahi Kasei lead-acid batteries and diesel gento work on membranes and Central Li O2 erators for rural electrification and Glass to work on electrolytes for air backup power for telecom networks. batteries. Since then, however, IBM Electrolyte The electrode raw material cost of has released few official pronouncezinc is cheap, at $1.00 per kWh, verments about the program, though Li metal Carbon air sus $6.00 for Pb-acid and $17 for Lithe company says it is still active. anode cathode ion. Air is free. The batteries also are PNNL’s Zhang says with all the safe, give off no emissions, and can challenges, he doesn’t see practical Blue = charge Red = discharge be remotely monitored. applications for rechargeable LiFluidic is getting 35 Wh/kg from air batteries within the next 15 to – Li2O2 2Li + O2 + 2e its Zn-air batteries, which exceeds 20 years. In fact, a technology other the capabilities of Pb-acid. It aims than Li-air is the focus of PNNL’s Li-air batteries tap into the reaction between O2 and to improve this to 100 Wh/kg in the consortium, also called Battery 500, Li. next two to three years. which aims for a 500 Wh/kg battery. But for electric vehicles, Li-air batteries the weight of components such as the elecThe Li-metal batteries that PNNL is workmay offer the greatest potential because trolyte, current collectors, and packaging ing on has cathodes made of lithium nickel they can store the most energy in the small- materials. manganese cobalt oxide or sulfur. est space. They generate power in a very Li-air batteries would be so prodigious Li and Zn aren’t the only chemistries for different way than their Li-ion cousins do. for a few reasons. Li is more densely packed metal-air batteries. For example, researchCommon Li-ion batteries use carbon in the metal anode than it is in the intercaers have been experimenting with Na-air intercalated with lithium as the anode, lated carbon of the Li-ion cell. Li is a lightbatteries, which are potentially appealing lithium cobalt oxide as the cathode, and an weight metal to boot. Moreover, Abraham because Na is plentiful. In 2012, BASF electrolyte of lithium hexafluorophosphate says, the reduction of the oxygen to form researchers cowrote a paper describing dissolved in an organic carbonate solvent. Li2O2 involves more electrons than does a rechargeable Na-oxygen cell that genWhen the battery discharges, Li ions mithe cathode reaction for Li-ion batteries erated NaO2 at the cathode(Nat. Mater. grate from anode to cathode through the and thus yields more electricity. DOI:10.1038/nmat3486). electrolyte. The operation reverses when Ji-Guang (Jason) Zhang, a laboratory felAbraham says metallic Na might offer the battery is charging. low at Pacific Northwest National Laborato- better recharge efficiency than Li, but he Li-air batteries use Li metal as the anry, points to another advantage. “We could notes that its high reactivity with water—it ode. The active material of the cathode is take oxygen from the ambient air,” he says. explodes when it is thrown into a pond, for oxygen, which enters the battery cell via a Never the less, technologists have to instance—would pose a challenge. porous carbon substrate. When the battery overcome a number of obstacles before The Israeli start-up Phinergy is develis discharged, the Li ions migrate through they can make real-world Li-air batteries. oping a battery from the most common the electrolyte and react with the oxygen For example, Zhang says finding battery metal in Earth’s crust, aluminum. Its Al-air at the cathode, forming lithium peroxide materials that can stand up to oxygen is battery isn’t rechargeable, however. The Al (Li2O2). Charging the battery breaks down tricky. Early models suffered from electroanode plates are converted into aluminum the Li2O2. Li is plated again on the anode, lytes that degenerated and cathodes that hydroxide as the battery discharges. The and the oxygen is released. lost conductivity after reacting with oxygen. plates are replaced as they are depleted. Li-ion batteries on the market today can Abraham says potential solutions to The company is eyeing applications such store 265 Wh/kg, according to K. M. Abrathese problems include the use of catalysts as secondary batteries for electric vehicles. ham, principal of the battery consulting and improved cathode materials together Phinergy says its batteries, which pack firm E-KEM Sciences and a professor at with advanced electrolytes. 400 Wh/kg, can extend range by 1,600 km. Northeastern University. With that technolUsing ambient air for the battery brings The company is partnering with Al firm ogy, a Chevy Bolt can go 383 km on a single even more problematic molecules into the Arconic, which is apparently keen on a charge. The upcoming Tesla Model 3 travels mix, Zhang says. Carbon dioxide reacts with technology that consumes Al like fuel. 346 km. But car companies and consumlithium to form lithium carbonate. And Li is E-KEM’s Abraham thinks a similar ers want at least double that range, which highly reactive with water. Most batteries in scheme might work for Li-air batteries would require more energy than traditional labs today run on pure oxygen. should they prove too difficult to recharge Li-ion batteries can even theoretically hold. Future cars could have air purification anytime soon. Lithium would essentially Working Li-air batteries would be a equipment as part of the battery system, be used as fuel, with waste Li2O2 recycled big improvement. They can theoretically but that adds bulk. A membrane on the outside the vehicle. store 5,200 Wh/kg, about 20 times as much cathode to allow only oxygen to enter To tap into the storage power of Li metal, as traditional Li-ion cells. Abraham, who would be ideal, Zhang says. “But that kind the world may need to take such measures published the first patent for a nonaqueous of membrane still doesn’t exist,” he notes. if it wants better electric vehicles and Li-air cell in the 1990s, predicts practical Researchers are pressing on despite the greener electricity. Abraham says: “If you cells would likely have a capacity of 30 to challenges. Perhaps the most famous effort look at the periodic table, you don’t have 50% of the theoretical value, accounting for is IBM’s. The company launched its Battery many options.” ◾ e–
