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A way to substantially reduce corrosion from occurring in metal-air batteries has been developed by MIT

Aluminium dipped into 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

While typical rechargeable, lithium-ion batteries only lose about 5% of their charge after a month of storage, they are too costly, bulky, or heavy for many applications. Primary (nonrechargeable) aluminium-air batteries are much less expensive and more compact and lightweight, but they can lose 80% of their charge a month.

MIT says its design overcomes the problem of corrosion in aluminium-air batteries by introducing an oil barrier between the aluminium electrode and the electrolyte — the fluid between the two battery electrodes that eats away at the aluminium 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% a month.

While several other methods have been used to extend the shelf life of metal-air batteries (which can use other metals such as sodium, lithium, magnesium, zinc, or iron), these methods can sacrifice performance, former MIT graduate student, Brandon J. Hopkins says.

Most of the other approaches involve replacing the electrolyte with a different, less corrosive chemical formulation, but these alternatives drastically reduce the battery power.

Other methods involve pumping the liquid electrolyte out during storage and back in before use. These methods still enable significant corrosion and can clog plumbing systems in the battery pack. Because aluminium is hydrophilic (water-attracting) even after electrolyte is drained out of the pack, the remaining electrolyte will cling to the aluminium electrode surfaces. “The batteries have complex structures, so there are many corners for electrolyte to get caught in,” which results in continued corrosion, Hopkins explains.

A key to the MIT system is a thin membrane placed between the battery electrodes. When the battery is in use, both sides of the membrane are filled with a liquid electrolyte, but when the battery is put on standby, oil is pumped into the side closest to the aluminium electrode, which protects the aluminium surface from the electrolyte on the other side of the membrane.

The new battery system also takes advantage of a property of aluminium called “underwater oleophobicity” — that is, when aluminium is immersed in water, it repels oil from its surface. As a result, when the battery is reactivated and electrolyte is pumped back in, the electrolyte easily displaces the oil from the aluminium surface, which restores the power capabilities of the battery. Ironically, the MIT method of corrosion suppression exploits the same property of aluminium that promotes corrosion in conventional systems.

The result is an aluminium-air prototype with a much longer shelf life than that of conventional aluminium-air batteries, claims MIT.

The researchers showed that 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 aluminium-air battery packs, they are still five times lighter and twice as compact as rechargeable lithium-ion battery packs for electric vehicles, the researchers report.

Professor Douglas P. Hart of MIT explains that aluminium, besides being very inexpensive, is one of the “highest chemical energy-density storage materials we know of” — that is, it is able to store and deliver more energy per pound than almost anything else, with only bromines, which are expensive and hazardous, being comparable. He says many experts think aluminium-air batteries may be the only viable replacement for lithium-ion batteries and for gasoline in cars.

Aluminium-air batteries have been used as range extenders for electric vehicles (EVs) to supplement built-in rechargeable batteries, to add many extra miles of driving when the built-in battery runs out. They are also sometimes used as power sources in remote locations or for some underwater vehicles. But while such batteries can be stored for long periods as long as they are unused, as soon as they are turned on for the first time, they start to degrade rapidly.

Such applications could greatly benefit from this new system, Prof. Hart explains, because with the existing versions although you can flush it and delay the process, you can’t really shut it off. However, Prof. Hart continues, if the new system were used, for example as a range extender in a car, “you could use it and then pull into your driveway and park it for a month, and then come back and still expect it to have a usable battery. I really think this is a game-changer in terms of the use of these batteries.”

According to Hopkins, with the greater shelf life that could be afforded by this new system, the use of aluminium-air batteries could extend beyond current niche applications. The team has already filed for patents on the process.

Bethan Grylls

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I did a lot of work on Al-air batteries for the aluminum industry back in the 1980s. Our senior executives had great hopes for the battery for vehicle propulsion --- but it never fulfilled that dream. True, aluminum possesses a lot of theoretical energy --- but if all of that energy were to be released it would be too violent for consumer use. That is why Al is used in explosives !
I would be interested to know what electrolyte MIT are using. With aqueous alkaline electrolytes, aluminum reacts to produce lots of hydroxide which quickly clogs up the electrolyte space and fouls the air electrode. In most acids, aluminum is attacked (corroded) and can be quickly consumed. So, in additional to the useful electrochemical reaction, the aluminum suffers useless parasitic corrosion. I can appreciate how the presence of oil could possibly reduce that aqueous corrosion reaction
Al-air batteries can be useful "reserve" batteries, i.e., they are stored "dry" and then filled with electrolyte when needed. To "recharge" them involves physically replacing the corroded aluminum metal electrode(s). A messy and time-consuming business. Al-air cells with aqueous electrolytes are, to my knowledge, not electrically rechargeable. So, not very consumer-friendly.
Also, in terms of cell volume, they do not make very compact batteries.
I honestly don't see Al-air batteries with aqueous electrolytes being suitable for vehicle propulsion.

Posted by: Dr. Frank N. Smith, 16/11/2018
A very interesting article. Aluminum-Air batteries have long been acknowledged as the most likely replacement for combustion engines in transport because they are effectively an electric fuel tank. They don’t need recharging and have a very long range capability, plus they’re safe, cheap and recyclable.

MAL Research & Development Ltd. in the UK has developed and patented the most practical and efficient ‘Stop/Start’ functionality in its ‘MAAPS’ rotary Aluminium-Air battery which is suitable for transport applications.

Posted by: Trevor Jackson , 12/11/2018

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