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Betting on a Metal-Air Battery Breakthrough

A government-funded start-up claims it can make ionic liquid energy storage feasible.
November 5, 2009

A spinoff from Arizona State University says it can develop a metal-air battery that dramatically outperforms the best lithium-ion batteries on the market, and now it has the funding it needs to prove it.

Liquid salt: This image shows ionic liquids (the blue globules) in a beaker of mineral oil.

The U.S. Department of Energy last week awarded a $5.13-million research grant to Scottsdale, AZ-based Fluidic Energy toward development of a metal-air battery that relies on ionic liquids, instead of an aqueous solution, as its electrolyte.

The company aims to build a Metal-Air Ionic Liquid battery that has up to 11 times the energy density of the top lithium-ion technologies for less than one-third the cost. Cody Friesen, a professor of materials science at Arizona State and founder of Fluidic Energy, says the use of ionic liquids overcomes many of the problems that have held back metal-air batteries in the past. “I’m not claiming we have it yet, but if we do succeed, it really does change the way we think about storage,” says Friesen, who was named one of Technology Review’s top innovators under 35 in 2009.

Metal-air batteries, such as those that use a zincanode, typically rely on water-based electrolytes. Oxygen from ambient air is drawn in through a porous “air” electrode (-cathode) and produces hydroxyl ions on contact with the electrolyte. These ions reach the anode and begin to oxidize the zinc–a reaction that produces current through the release of electrons.

But like any aqueous solution, the water in the electrolyte can evaporate, causing the batteries to prematurely fail. Water also has a relatively low electrochemical window, meaning it will begin to decompose when the cell exceeds 1.23 volts. These were two problems researchers at the U.S. Air Force Academy began tackling about 25 years ago. In the early 1980s they experimented with ionic liquids–salts that are a liquid at room temperature, and which often can remain a liquid in sub-zero temperatures or above the boiling point of water.

“They’re wonder fluids. They’re remarkable,” says John Wilkes, an ionic liquids expert who heads the academy’s chemistry department. “If you look at these liquids in a bottle, they look like water, except they’re viscous. They’re not volatile, they don’t evaporate, they’re physically stable and they conduct electricity fairly well.”

Friesen, whose Arizona State research team has spent the past few years experimenting with various ionic liquids, says a metal-air battery using an ionic liquid as its electrolyte not only functions significantly longer–because drying out is no longer a problem–but it also gets a big boost in energy density. “These liquids have electrochemical stability windows of up to five volts, so it allows you to go to much more energy-dense metals than zinc.” He says his research team will target energy densities of at least 900 watt-hours per kilogram and up to 1,600 watt-hours per kilogram in the DOE-funded project.

The problem with ionic liquids is that they’re still made in small quantities, making them expensive compared to many other solvents used to dissolve salts. “But some people are making ionic liquids now out of things that are already known and produced in high quantities, like detergents,” says Wilkes.

Robin Rogers, a professor of chemistry at the University of Alabama, says the challenge is finding “commodity ionic liquids” with the right set of properties that can completely change the economic equation for metal-air batteries. “It’s not impossible,” he says. “I look at ionic liquids and say, take a step back, because you need to do it in a completely different way.”

Friesen downplays the cost concern, pointing out that the liquids become quite economical when developed in-house in large volumes. He’s careful, however, not to say too much about the ionic liquids his team has developed, revealing only that there are “several contenders that seem to work well.”

Friesen is also cautious when talking about the other key component of Fluidic Energy’s research: a metal electrode structure that overcomes the problem of dendrite formation. These branch-like structures can grow on, for example, a zinc electrode and cause a metal-air battery to short-circuit. Dendrite formation happens in rechargeable batteries when the chemical reactions are reversed, limiting the number of charging cycles. Fluidic Energy has developed an electrode scaffold with multi-modal porosity, meaning it has a range of pore sizes down to as small as 10 nanometers. The scaffold surrounds the metal, in this case zinc, and can prevent dendrites that form during charging.

With the ability to eliminate evaporation, boost voltage and eliminate dendrites, “we’re working now on taking it to the next level,” says Friesen. “It’s about taking everything we’ve done over the last four years and leveraging that work into a battery that looks and feels just like a lithium battery, but has energy densities far beyond that.”

This would mean that energy storage would no longer be a limiting factor for renewable energy, and electric vehicles that could travel 400 to 500 miles on a single charge, he says, “at a cost just a little over lead-acid batteries.”

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