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Roger Arnold's picture
Director Silverthorn Institute

Roger Arnold is a former software engineer and systems architect. He studied physics, math, and chemistry at Michigan State University's Honors College. After graduation, he worked in...

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  • Oct 18, 2021
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As the website summary for the link below indicates, aluminum-air battery technology has been making headlines. This video does a pretty good job of explaining the pros and cons.

The only point that Joe fumbles, I believe, is in talking about the ease of recycling aluminum as a mitigating factor for the non-rechargeability of aluminum-air batteries. Aluminum, as a metal, is easily recycled. But when an aluminum-air battery discharges, the end product is not aluminum metal. It's an aluminum hydroxide gel -- hydrated aluminum oxide. If you're not into chemistry, skip the next paragraph to get to the bottom line.

Hydrated aluminum oxide from spent aluminum-air batteries is the same thing as what's made in the Bayer process for refining bauxite ore. The hydrated aluminum oxide is calcined to drive off the water and produce aluminum oxide powder. That powder is then dissolved in molten cryolyte to be reduced to molten aluminum via the Hall-Heroult process. The Hall-Heroult process is an electrolytic process that uses a consumable carbon anode. Oxidation of the anode emits a mixed stream of CO2 and CO. If the overall process is to be green, it must be paired with fuel synthesis or other processes to utilize the waste stream and keep carbon emissions out of the atmosphere. In principle, it's possible to use inert anodes that produce oxygen rather than CO2 and CO. The cell voltage and hence to energy cost per ton of aluminum would be higher, and AFAIK, a zero-carbon alternative to the Hall-Heroult process has never been commercially deployed.

The bottom line is that not only are aluminum-air batteries not rechargeable, but the process for recycling a spent aluminum-air battery to produce a new replacement would be quite expensive. It's energetically equivalent to producing fresh metal for the aluminum anodes from ore. If aluminum-air batteries were to be routinely used to power EVs, it would require at least an order of magnitude increase in new aluminum production capacity.

That said, I can see a couple of applications where aluminum-air batteries could make a lot of sense. One would be the hybrid EV application that Joe refers to near the end of his video. An EV could carry a swappable Aluminum-air batter with a capacity for 3 or 4 hundred miles of range, paired with a somewhat reduced lithium-ion battery pack good for 200 miles of range. 200 miles is ample for most driving needs; for occasions when it isn't the aluminum-air batter could be tapped for an additional 3 or 4 hundred miles. That disposes of the "range anxiety" issue from having only 200 miles in the rechargeable battery pack.

The other application would be for power backup in hospitals, commercial facilities, and micro-grids. The aluminum-air batteries would replace the diesel powered generators currently used. They'd be cleaner, more reliable, and probably a good deal more economical than a diesel-powered backup generator. The key to feasibility is that they would only be used for power emergencies. Once their charge was used up, they'd have to be replaced. However, the energy density of aluminum-air is high enough that trucking in a megawatt-hour replacement battery would be no big deal.

 

 

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Matt Chester's picture
Matt Chester on Oct 18, 2021

But when an aluminum-air battery discharges, the end product is not aluminum metal. It's an aluminum hydroxide gel -- hydrated aluminum oxide.

I'm also curious when it comes to this type of recycling what the energy intensity required is and how much that cuts into the circulare economy of it all. 

Roger Arnold's picture
Roger Arnold on Oct 19, 2021

I'm curious about that too. I expect to be doing some digging in that area.

 

When I first heard the buzz about aluminum-air batteries, I was skeptical. I assumed that the cost of "remanufacturing" the batteries -- as opposed to simply recharging them -- would mean that the effective  round trip energy storage.efficiency for aluminum-air batteries would be very low. I guesstimated 10%.

 

After thinking about it, I realized that recycling could be a pretty simple process. Just a matter of opening the battery case, sucking out the aluminum hydroxide gel, removing what's left of the spent aluminum anode, replacing it with a fresh anode, closing the battery case, and refilling it with fresh electrolyte. Something that could probably be done at local swap stations.

 

The collected aluminum hydroxide gel would need to be shipped off to a specialized aluminum smelter for manufacturing aluminum anodes. New anodes would need to be distributed back to swap stations. Collection and distribution are low energy operations. Round trip efficiency then largely reduces to the efficiency of the aluminum electrolysis operations at smelters. I've learned that those can be pretty efficient. Quantifying "pretty high" is where I need to do more digging. But maybe up to 80%?

 

An intriguing fact is that aluminum has an energy density, as fuel, that's more than 50% better than diesel. That's thermal energy. Figuring in the much higher conversion efficiency of aluminum to electricity in an aluminum-air battery, the effective energy density of aluminum would be three or four times higher than diesel. Shipping it around as an energy carrier should be quite practical. And stockpiling it for long periods is dead easy. So it might actually be a better solution than hydrogen for long duration energy storage.

 

Mark Silverstone's picture
Mark Silverstone on Oct 20, 2021

I think the shipping application may be attractive for some purposes.  Also, the  Lithium Ion/Aluminum Air hybrid may well have a future.

Jim Stack's picture
Jim Stack on Oct 20, 2021

Being able to recharge is very important. Have a long life is also key as well as safety. I think the new Tesla 4680 cells will be the best choice. 

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