Metal-air batteries are light, compact power sources with a high energy density, but they have had a major limitation: They corrode.

A new design from the Massachusetts Institute of Technology uses oil to reduce the corrosion and extend the shelf life of single-use metal-air batteries.

The key: Oil.

To prevent the deterioration of the metal, the MIT researchers placed an oil barrier between the aluminum electrode and the electrolyte — the fluid between the two battery electrodes that eats away at the aluminum when the battery is on standby.

The oil is rapidly pumped away and replaced with electrolyte as soon as the battery is used.

As a result, the energy loss is cut to just 0.02 percent a month — more than a thousand-fold improvement, according to the MIT team.

The findings were reported last week in the journal Science by former MIT graduate student Brandon J. Hopkins ’18, W.M. Keck Professor of Energy Yang Shao-Horn, and professor of mechanical engineering Douglas P. Hart.

How does a metal-air battery work exactly?

A metal-air battery uses some type of metal (like aluminum) for the anode, air as the cathode, along with a liquid electrolyte.

In the case of aluminum, oxygen from the air then combines with the metal to create aluminum hydroxide, which activates the electrolysis process and creates a current.

Because aluminum attracts water, the remaining electrolyte often clings to the aluminum electrode surfaces, even after electrolyte is drained out from the cell.

“The batteries have complex structures, so there are many corners for electrolyte to get caught in,” said Hopkins.

The many corners lead to many opportunities for corrosion.

Hopkins and his team, however, placed a thin membrane barrier between the battery electrodes; both sides of the membrane are filled with a liquid electrolyte when the battery is in use.

When the battery is put on standby, oil is pumped into the side closest to the aluminum electrode, which protects the aluminum surface from the electrolyte on the other side of the membrane.

Aluminum, when immersed in water, repels oil from the surface. When the battery is reactivated and electrolyte is pumped back into the cell, the electrolyte easily displaces the oil from the aluminum surface, which restores the battery’s power.

The result is an aluminum-air prototype with a much longer shelf life than that of conventional aluminum-air batteries. When the battery was repeatedly used and then put on standby for one to two days, the MIT design lasted 24 days, while the conventional design lasted for only three.

Even when oil and a pumping system are included in scaled-up primary aluminum-air battery packs, they are still five times lighter and twice as compact as rechargeable lithium-ion battery packs for electric vehicles, the researchers reported.

Currently, aluminum-air batteries are used as backup power sources. Professor Hart spoke with Tech Briefs about why he believes the new design will someday find its way beyond niche applications and into electric vehicles.

Tech Briefs: Why are metal-air batteries valuable?

Douglas Hart, Professor of Mechanical Engineering: They are extremely high-energy-dense batteries. These are considered primary batteries, meaning they’re not rechargeable. In this case the aluminum gets consumed.

And aluminum is extremely abundant, unlike a lot of other metals that are made to make batteries. Aluminum is one of the most abundant materials on Earth, and it’s distributed throughout the world, so it’s not something that one country owns.

Tech Briefs: Where are metal-air batteries being used currently?

One of the problems with backup generators is that they take a while to come online and they use diesel fuel, which can go bad. So, many hospitals have aluminum-air batteries as backup systems; when the power goes down, they can go very quickly back online, at least long enough for a secondary power system to come online.

Phinergy, a company in Israel, is making aluminum-air batteries for range extenders on cars. There’s a plan for them to be included, so if you run out of electrical power from a battery in an electrical vehicle, the aluminum-air battery should kick in and get you through the extra miles to get you to a charging station. They’re basically a battery system that can be replaced, just because they have so much higher energy than a lithium-ion battery.

Tech Briefs: What are the limitations of a metal-air batteries?

Prof. Hart: Once you turn them on, you can’t turn them off. The only way to stop the reaction is to drain the electrolyte out of the system. And when you do that, each time there’s a little bit of electrolyte that stays on the battery’s metal surface and corrodes it. After a while, you can put the electrolyte back in, and it won’t start up again; the battery becomes corroded, and on the surface this byproduct plugs it up. Some people have found that you can flush it with water, but the water gets contaminated with electrolytes.

To demonstrate the ability of aluminum to repel oil underwater, the researchers plunged this sample of aluminum into a beaker containing a layer of oil floating on water. When the sample enters the water layer, all the oil that clung to the surface on the way down quickly falls away, showing its property of underwater oleophobicity. (Credit: MIT)

Tech Briefs: Why is mitigating the corrosion effect so important?

Prof. Hart: You’d like to be able to use these batteries in something like an automobile; you want to park it in your driveway, leave it there for a week, come back, and expect it to start again. These batteries are slowly eating themselves way, so you lose a lot of your energy. The energy density becomes pointless then because it’s consuming itself.

People have looked at all types of ways to mitigate this corrosion process. They’ve looked at better chemistries for the surface of aluminum and alloys. We discovered a very simple approach: Instead of flushing it with water, we simply displace the electrolyte with oil.

Tech Briefs: What was the reaction to this idea?

Prof. Hart: The first reaction everybody had was: “Are you kidding me? The oil is going to plug up everything and destroy it.” It turns out that in the presence of the electrolyte, the aluminum prefers to work with the electrolyte rather than oil. The oil actually does not foul things. It simply displaces the electrolyte, shuts the reaction down (because it’s non-conductive), and as soon as you put the electrolyte back in, it starts right back up. But even better, we can flush it with the same oil over and over again and never contaminate the system.

Tech Briefs: Is this an easy design feature to incorporate?

Prof. Hart: The membrane is actually a very easy thing to put in place. It actually can be built on the cathode itself before being installed. It’s a very simple modification to existing battery tech. It’s a thin membrane to protect the cathode, because the cathode is a high surface contact material. The membrane provides a long-term longevity to the cathode material. It also allows the use of oils that are not as stable as other oils.

Tech Briefs: In what kinds of applications do see this new design being used?

Prof. Hart: Range extenders for cars is certainly a good one. One reason that people are afraid to buy electric cars is because they’re scared to death of running out of power. And [for vehicle applications], this would be used mostly as a backup system to get over that fear of not having enough to get to the next charging system.

Tech Briefs: Will they still be used as backup power sources?

Prof. Hart: Right now, many people have small generators in their houses, but these produce carbon monoxide, so they’re very dangerous to use. Aluminum-air batteries are a far safer device to have sitting in your basement than a backup generator. If the power goes off, you can turn it on. If the power comes back on, you can turn it off. And an aluminum-air battery is certainly great for hospital use, and backup power systems for data servers.

Tech Briefs: Are metal-air batteries a viable option now compared to, say, the lithium-ion battery?

Prof. Hart: Right now, if you wanted to make our transportation system and convert it all to electric vehicles, people have pointed to lithium-ion batteries; certainly, Tesla is using lithium-ion batteries. But lithium-ion batteries require lithium, which is owned by a subset of the countries in the world. That makes it a politically difficult situation.

The worst part is that there’s simply not enough cobalt to make enough batteries for all the cars in the world. They have to find an alternative to cobalt. Some experts say they’ll be able to replace cobalt with nickel. We have to find an alternative battery system to make things like storage systems viable, because we simply don’t have enough cobalt and nickel.

Aluminum is a great energy source for any type of transportation system. I could see it being used in aircraft and other places where standard batteries might be used. Again, you can’t recharge these. They are more of a fuel than a pure energy storage device.

A metal-air battery aluminum air-battery from Massachusetts Institute of Technology Professor Douglas Hart, MIT graduate student Brandon J. Hopkins, and Professor of Energy Yang Shao-Horn
The proof-of-concept battery built by the research team to demonstrate how their system could work in practice. The battery lasted through 24 days of use and standby cycles, compared to just three days for a comparable battery without the new protective system in place. (Image Credit: MIT)

Tech Briefs: What’s next for your team regarding this research?

Prof. Hart: I’m hoping that it gets picked up by one of the commercial battery manufacturers. I think it has great potential, and I’d love to see it being put to use. We’ve shown about all we need to in terms of research in the lab, and I think it now needs to be implemented in a real system and proven out for commercial application.

Tech Briefs: What have the results shown? How well does the battery perform?

Prof. Hart: Phenomenally. Brandon has been able to show that you can turn it on and off for the full lifespan of the battery, and there’s almost no degradation at all, unlike previous systems. Essentially this work has given it the ability to shut off like a normal battery, so it doesn’t sit there and corrode away while it sits in your driveway, if you will.

That means, for something like a hospital, when the power goes out, you can truly turn this thing on, and if you don’t use all the energy that’s in the battery, you can shut the battery off and use it again next time. Normally, you might have a power failure that happens for a few minutes, then the power comes back on. You’ve used up this very expensive battery because, while it sits there, it corrodes away. Now, you can turn it on and off at will.

What do you think? Will metal-air batteries find a place in electric vehicles? Share your comments and question below.