Sandia researchers have developed a new family of liquid salt electrolytes, known as MetILs, that could lead to batteries able to cost-effectively store three times more energy than today’s batteries.
Chemical technologist Harry Pratt synthesizes a copper-based ionic liquid.(Photo by Randy Montoya) Click on thumbnail for a high-resolution image.
The research, published in Dalton Transactions, might lead to devices that can help economically and reliably incorporate large-scale intermittent renewable energy sources, like solar and wind, into the nation’s electric grid.
The grid was designed for steady power sources, making fluctuating electricity from intermittent renewable energy difficult to accommodate. Better energy storage techniques help even out the flow of such fluctuating sources, and Sandia researchers are studying new ways to develop a more flexible, cost-effective and reliable electric grid with improved energy storage.
“The U.S. and the world need significant breakthroughs in battery technology for renewable energy sources to replace today’s carbon-based energy systems,” said Anthony Medina, director of Sandia’s Energetic Components Realization program. “MetILs are a new, promising battery chemistry that might provide the next generation of stationary storage battery technology, replacing lead-acid and lithium-ion batteries and providing significantly higher energy storage density for these applications.”
For the past 20 years, lithium-ion batteries have been at the forefront of energy storage research. Their compact, lightweight design is well suited for cell phones, laptop computers and personal electronics, but lithium-ion batteries are expensive and degradation issues limit their use in stationary, high-capacity application on the nation’s electric grid.
Sandia researcher and inorganic chemist Travis Anderson is leading a team developing the next generation of flow batteries. A flow battery pumps a solution of free-floating charged metal ions, dissolved in an electrolyte — substance with free-floating ions that conducts electricity — from an external tank through an electrochemical cell to convert chemical energy into electricity. Flow batteries are rapidly charged and discharged by changing the charge state of the electrolyte, and the electroactive material can be easily re-used many times. Anderson said flow batteries can sustain more than 14,000 cycles in the lab, equivalent to more than 20 years of energy storage, which would be unusual in a lithium-ion battery.
However, flow battery grid storage systems are roughly the size of a house and can cost more than equivalent lithium-ion batteries. The goal of researchers is to make flow batteries smaller and cheaper, while increasing the amount of energy stored for a given volume, or energy density.
Flow batteries have been fielded in the U.S., Japan and Australia. A number of systems – up to 25 MW – are in the process of being demonstrated under the American Recovery and Reinvestment Act (ARRA) administered by DOE’s Energy Storage Systems Research program. Zinc bromine and vanadium redox systems are among the top contenders. But the materials involved are moderately toxic, and vanadium is subject to major price fluctuations. In addition, the aqueous solution limits the amount of material that can be dissolved and how much energy can be stored, and outside temperature can hurt performance.
Sandia researchers have discovered a new family of liquid salt electrolytes that could lead to batteries with three times greater energy density than other available storage technologies. The MetILs are, from left to right: copper-based compound, cobalt-based compound, manganese-based compound, iron-based compound, nickel-based compound, and vanadium-based compound. (Photo by Randy Montoya) Click on thumbnail for a high-resolution image.
Sandia is pioneering research on flow batteries that avoid these problems by not using water. Anderson assembled a multidisciplinary team of experts from the Labs, including electrochemist David Ingersoll, organic chemist Chad Staiger and chemical technologists Harry Pratt and Jonathan Leonard. What they’ve designed is a new family of electrochemically reversible, metal-based ionic liquids, or MetILs, which are based on inexpensive, non-toxic materials that are readily available within the U.S., such as iron, copper and manganese.
“Instead of dissolving the salt into a solvent, our salt is a solvent,” Anderson said. “We’re able to get a much higher concentration of the active metal because we’re not limited by saturation. It’s actually in the formula. So we can cost-effectively triple our energy density, which drastically reduces the necessary size of the battery, just by the nature of the material.”
The electrochemical efficiency, or ability to reverse charge, in MetILs is far greater than anything else published to date. The team has prepared nearly 200 combinations of cations, anions and ligands, and of those, five outperform the electrochemical efficiency of ferrocene, which has long been considered the gold standard.
A common problem when mixing positively and negatively charged species is that these species will start aggregating together, eventually causing the solution to turn gummy and clog the battery membrane and electrode surfaces. The team addressed that challenge by developing asymmetric cations, or positively charged ions, that resemble a soccer ball. In this analogy, the black pentagons represent negatively charged areas and the white hexagons represent positively charged regions. Such an arrangement lowers the melting point by preventing the ionic liquid constituents from bonding and becoming a solid, while the partial charge still allows electrons to flow freely through the cell to generate a current.
The team is funded by the Department of Energy’s Office of Electricity Delivery and Energy Reliability. Imre Gyuk, energy storage systems program manager for that office, has been a champion of Sandia’s efforts and provided the necessary funding.
“The MetILs approach represents an ingenious, out-of-the-box solution to the cathode/electrolyte paradigm. Because it is based on readily available, inexpensive precursors, it may well lead to innovative, cost-effective storage systems with major impacts on the entire U.S. grid,” said Gyuk.
The findings apply to new flow battery cathode materials. The next step for the Sandia team is to find similar materials for flow battery anodes, and researchers are encouraged by their progress.
“There are three things you’re juggling at the same time, and they aren’t always related: viscosity, electrical conductivity and the fundamental electrochemical efficiency,” Anderson said. “The excitement of having all three things go right at the same time, it’s like finding the treasure, but without the map. We’re creating that map, and we’re very excited by the possibilities.”
Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy’s National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.
Sandia news media contact: Stephanie Hobby, shobby@sandia.gov, (505) 844-0948
http://www.sandia.gov/ess/docs/pr_conferences/2010/anderson_snl.pdf
http://www.sandia.gov/eesat/2011/papers/Anderson_Tuesday_EESAT_Manuscript_Final.pdf
http://energy.sandia.gov/wp/wp-content/gallery/uploads/New-MetILS-SAND-2012-4435P.pdf
A team of Harvard scientists and engineers has demonstrated a new type of flow battery that could fundamentally transform the way electricity is stored on the grid, making power from renewable energy sources such as wind and solar far more economical and reliable.
Under the OPEN 2012 program, the Harvard team received funding from the U.S. Department of Energy’s Advanced Research Projects Agency–Energy (ARPA-E) to develop the innovative grid-scale battery and plans to work with ARPA-E to catalyze further technological and market breakthroughs over the next several years.
The paper in Nature (see footnote) reports a metal-free flow battery that relies on the electrochemistry of naturally abundant, inexpensive, small organic (carbon-based) molecules called quinones, which are similar to molecules that store energy in plants and animals.
The mismatch between the availability of intermittent wind or sunshine and the variability of demand is the biggest obstacle to getting a large fraction of our electricity from renewable sources. A cost-effective means of storing large amounts of electrical energy could solve this problem.
The battery was designed, built, and tested in the laboratory of Michael J. Aziz, Gene and Tracy Sykes Professor of Materials and Energy Technologies at the Harvard School of Engineering and Applied Sciences (SEAS). Roy G. Gordon, Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science, led the work on the synthesis and chemical screening of molecules. Alán Aspuru-Guzik, Professor of Chemistry and Chemical Biology, used his pioneering high-throughput molecular screening methods to calculate the properties of more than 10,000 quinone molecules in search of the best candidates for the battery.
Flow batteries store energy in chemical fluids contained in external tanks—as with fuel cells—instead of within the battery container itself. The two main components—the electrochemical conversion hardware through which the fluids are flowed (which sets the peak power capacity), and the chemical storage tanks (which set the energy capacity)—may be independently sized. Thus the amount of energy that can be stored is limited only by the size of the tanks. The design permits larger amounts of energy to be stored at lower cost than with traditional batteries.
By contrast, in solid-electrode batteries, such as those commonly found in cars and mobile devices, the power conversion hardware and energy capacity are packaged together in one unit and cannot be decoupled. Consequently they can maintain peak discharge power for less than an hour before being drained, and are therefore ill suited to store intermittent renewables.
“Our studies indicate that one to two days’ worth of storage is required for making solar and wind dispatchable through the electrical grid,” said Aziz.
To store 50 hours of energy from a 1-megawatt power capacity wind turbine (50 megawatt-hours), for example, a possible solution would be to buy traditional batteries with 50 megawatt-hours of energy storage, but they’d come with 50 megawatts of power capacity. Paying for 50 megawatts of power capacity when only 1 megawatt is necessary makes little economic sense.
For this reason, a growing number of engineers have focused their attention on flow battery technology. But until now, flow batteries have relied on chemicals that are expensive or difficult to maintain, driving up the energy storage costs.
The active components of electrolytes in most flow batteries have been metals. Vanadium is used in the most commercially advanced flow battery technology now in development, but its cost sets a rather high floor on the cost per kilowatt-hour at any scale. Other flow batteries contain precious metal electrocatalysts such as the platinum used in fuel cells.
The new flow battery developed by the Harvard team already performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious metal electrocatalyst.
Michael J. Aziz, Gene and Tracy Sykes Professor of Materials and Energy Technologies (Credit: Eliza Grinnell, Harvard SEAS Communications)
“The whole world of electricity storage has been using metal ions in various charge states but there is a limited number that you can put into solution and use to store energy, and none of them can economically store massive amounts of renewable energy,” Gordon said. “With organic molecules, we introduce a vast new set of possibilities. Some of them will be terrible and some will be really good. With these quinones we have the first ones that look really good.”
Aspuru-Guzik noted that the project is very well aligned with the White House Materials Genome Initiative. “This project illustrates what the synergy of high-throughput quantum chemistry and experimental insight can do,” he said. “In a very quick time period, our team honed in to the right molecule. Computational screening, together with experimentation, can lead to discovery of new materials in many application domains.”
Quinones are abundant in crude oil as well as in green plants. The molecule that the Harvard team used in its first quinone-based flow battery is almost identical to one found in rhubarb. The quinones are dissolved in water, which prevents them from catching fire.
To back up a commercial wind turbine, a large storage tank would be needed, possibly located in a below-grade basement, said co-lead author Michael Marshak, a postdoctoral fellow at SEAS and in the Department of Chemistry and Chemical Biology. Or if you had a whole field of turbines or large solar farm, you could imagine a few very large storage tanks.
The same technology could also have applications at the consumer level, Marshak said. “Imagine a device the size of a home heating oil tank sitting in your basement. It would store a day’s worth of sunshine from the solar panels on the roof of your house, potentially providing enough to power your household from late afternoon, through the night, into the next morning, without burning any fossil fuels.”
“The Harvard team’s results published in Nature demonstrate an early, yet important technical achievement that could be critical in furthering the development of grid-scale batteries,” said ARPA-E Program Director John Lemmon. “The project team’s result is an excellent example of how a small amount of catalytic funding from ARPA-E can help build the foundation to hopefully turn scientific discoveries into low-cost, early-stage energy technologies.”
Team leader Aziz said the next steps in the project will be to further test and optimize the system that has been demonstrated on the bench top and bring it toward a commercial scale. “So far, we’ve seen no sign of degradation after more than 100 cycles, but commercial applications require thousands of cycles,” he said. He also expects to achieve significant improvements in the underlying chemistry of the battery system. “I think the chemistry we have right now might be the best that’s out there for stationary storage and quite possibly cheap enough to make it in the marketplace,” he said. “But we have ideas that could lead to huge improvements.”
By the end of the three-year development period, Connecticut-based Sustainable Innovations, LLC, a collaborator on the project, expects to deploy demonstration versions of the organic flow battery contained in a unit the size of a horse trailer. The portable, scaled-up storage system could be hooked up to solar panels on the roof of a commercial building, and electricity from the solar panels could either directly supply the needs of the building or go into storage and come out of storage when there’s a need. Sustainable Innovations anticipates playing a key role in the product’s commercialization by leveraging its ultra-low cost electrochemical cell design and system architecture already under development for energy storage applications.
“You could theoretically put this on any node on the grid,” Aziz said. “If the market price fluctuates enough, you could put a storage device there and buy electricity to store it when the price is low and then sell it back when the price is high. In addition, you might be able to avoid the permitting and gas supply problems of having to build a gas-fired power plant just to meet the occasional needs of a growing peak demand.”
This technology could also provide very useful backup for off-grid rooftop solar panels—an important advantage considering some 20 percent of the world’s population does not have access to a power distribution network.
William Hogan, Raymond Plank Professor of Global Energy Policy at Harvard Kennedy School, and one of the world’s foremost experts on electricity markets, is helping the team explore the economic drivers for the technology.
Trent M. Molter, President and CEO of Sustainable Innovations, LLC, provides expertise on implementing the Harvard team’s technology into commercial electrochemical systems.
“The intermittent renewables storage problem is the biggest barrier to getting most of our power from the sun and the wind,” Aziz said. “A safe and economical flow battery could play a huge role in our transition off fossil fuels to renewable electricity. I’m excited that we have a good shot at it.”
Brian Huskinson, Michael P. Marshak, Changwon Suh, Süleyman Er, Michael R. Gerhardt, Cooper J. Galvin, Xudong Chen, Alán Aspuru-Guzik, Roy G. Gordon, Michael J. Aziz (2014). A metal-free organic–inorganic aqueous flow battery Nature, 505, 195-198 DOI: 10.1038/nature12909
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