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Finding energy storage ‘solutions’ in MetILs

Finding energy storage ‘solutions’ in MetILs

By Stephanie Hobby

On the molecular scale, it resembles a traditional black-and-white soccer ball, but that molecule holds the promise of being a big player in the nation’s energy storage efforts.

CHEMICAL TECHNOLOGIST Harry Pratt (2546) synthesizes a copper-based ionic liquid. (Photo by Randy Montoya)

Sandia researchers have discovered a new family of liquid salt electrolytes, which could lead to batteries packed with three times the energy density of other available storage technologies. The findings, featured on the Nov. 21 cover of Dalton Transactions, offer a possible new solution to the vexing challenge of incorporating intermittent renewable energy sources into the nation’s electric grid.

Energy demand is at an all-time high; as of Oct. 31, the UN Population Fund estimates that there are more than 7 billion of us roaming the planet, and exploring every energy option has become the only option to meet the energy demands of all these people. Renewable energy sources are a likely part of the solution, but incorporating them into the current electrical grid on a large scale is problematic. The world’s grids were built to accommodate steady power sources and aren’t equipped to deal with the peaks and valleys produced by intermittent power sources.

A long-term, high-capacity option

Energy storage technologies are one way to even out the flow of electricity from intermittent renewable sources of energy. Sandia is researching new storage technologies and materials that will help in the design of a more flexible and reliable electric grid with higher storage capacity. For the past 20 years, lithium-ion batteries have been at the forefront of energy storage research. The compact, lightweight, and affordable design is ideal for cell phones and laptop computers, but there’s a nagging problem. No matter how new your cell phone is, during every charge and discharge, lithium physically moves from the cathode to the anode and back again. All this motion degrades the battery, and over time, it will just give out.

Such an option might be adequate for electronics that come and go, but the nation’s electric grid needs a long-term, high-capacity option.

Sandian Travis Anderson (2546), an inorganic chemist with nine years of experience, is leading a team that’s developing the next generation of flow batteries. A flow battery pumps a solution of charged metals dissolved in an electrolyte 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.

“The system is simple, it performs very well, lasts a long time, and has a high cycle efficiency. In a lab, it can do well over 14,000 cycles, which is equivalent to about 20-plus years. That’s unheard of in a lithium-ion battery,” says Travis. “But these batteries are huge — about the size of a building — so they’re expensive. The goal is to make them smaller and cheaper, and we do that by increasing the energy density.”

Flow batteries are not easy to find — only one has been built in the US — but they are more common in Japan and Australia, where the first patented flow battery originated. Of the existing flow batteries, the highest performers use vanadium, which is moderately toxic and fluctuates in price. Furthermore, temperature impacts the performance of aqueous flow batteries. Finally, water limits how much material can be dissolved, which ultimately limits how much energy can be stored.

Non-aqueous flow battery research is largely unchartered territory, and Sandia is leading the way.

“We’re not trying to reinvent the wheel. We want a liquid that flows from storage tank to cell, just like vanadium,” Travis says. “But we’re trying to generate a new fuel.”

Tripling energy density

Travis pulled together a multidisciplinary team of experts from around the Labs to find that fuel, including electrochemist David Ingersoll (2546), organic chemist Chad Staiger (6124), and chemical technologists Harry Pratt and Jonathan Leonard (both 2546). What they’ve built is a new family of electrochemically reversible, metal-based ionic liquids (MetILs), and it’s generating a lot of attention from the energy storage community.

“So instead of dissolving the salt into a solvent, our salt is a solvent,” Travis says. “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 triple our energy density just by the nature of the material we have.”

The ionic liquids the team prepared use readily available, inexpensive, nontoxic materials that can be found in the US, such as iron, copper, and manganese.

A common problem when mixing positively and negatively charged species is that these species will want to start aggregating together, eventually causing the solution to turn gummy, crash out, and clog the battery components such as the 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 keeps the melting point low enough to prevent the ionic liquid from bonding to itself and becoming a solid, while the partial charge still allows electrons to flow freely through the cell to generate a current.

Exceeding the ferrocene standard

Another desirable property is high electrochemical efficiency, or reversibility of the charge.

“The ease at which you can change the charge state of MetILs is by far better than anything that’s ever been published,” says Travis. The gold standard for determining reversibility is to measure it against ferrocene. If the compound has better reversibility, it’s considered a top performer.

“We’ve prepared nearly 200 combinations of cations, anions, and ligands, and of those, there are five that exceed the ferrocene standard,” Chad says.

The family of compounds the team discovered with reasonably desirable properties is growing; last year, there were 11, this year, there are more than 30.

“There are so many parameters we can try, and you hit more bad ones than you can hit good ones, so when you find one that’s better than ferrocene, the data get very exciting,” Harry says. “This is new research. We aren’t following on anyone else’s coattails.”

The current research will apply to new flow battery cathode materials; the next step is to find similar materials for flow battery anodes.

The team is in its last year of a Sandia Laboratory Directed Research and Development project, but has also received funding from the DOE’s Office of Electricity Deliverability and Energy Reliability. Imre Gyuk, an energy storage systems program manager for that office, has been a champion of Sandia’s efforts and provided the necessary funding.    

“There are three things you’re juggling at the same time, and they aren’t always related: viscosity, electrical conductivity, and the fundamental electrochemistry efficiency,” Travis says. “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." – Stephanie Hobby

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