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Could a new family of liquid salt electrolytes lead to better batteries?


February 21, 2012
By Anthony Capkun

February 21, 2012 – Researchers at Sandia National Laboratories 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.

The research might lead to devices that can help economically and reliably incorporate large-scale intermittent renewable energy sources, like solar and wind, into the electric grid. The grid was designed for steady power sources, say the researchers, 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 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 re-used many times. Anderson says 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 States, Japan and Australia. A number of systems—up to 25 MW—are in the process of being demonstrated by the U.S. Department of Energy’s (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 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 (MetILs), which are based on inexpensive, non-toxic materials that are readily available, 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 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 Corp., a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy’s National Nuclear Security Administration.