Molten-air battery's storage capacity among the highest of any battery type

Sep 19, 2013 by Lisa Zyga feature
This chart compares the characteristics of molten-air batteries that use three different types of materials. Credit: Licht, et al. ©2013 The Royal Society of Chemistry

( —Researchers have demonstrated a new class of high-energy battery, called a "molten-air battery," that has one of the highest storage capacities of any battery type to date. Unlike some other high-energy batteries, the molten-air battery has the advantage of being rechargeable. Although the molten electrolyte currently requires high-temperature operation, the battery is so new that the researchers hope that experimenting with different molten compositions and other characteristics will make molten-air batteries strong competitors in electric vehicles and for storing energy for the electric grid.

The researchers, Stuart Licht, Baochen Cui, Jessica Stuart, Baohui Wang, and Jason Lau, at George Washington University, have published a paper on the new molten-air in a recent issue of Energy & Environmental Science.

"This is the first time that a rechargeable molten-air battery has been demonstrated," Licht told "There have been rechargeable batteries that use molten electrolytes, but not air. For example, molten-sulfur batteries have been widely studied for electric car and grid applications. However, sulfur is twice as massive as oxygen (per electron stored) and its mass needs to be carried as part of the battery (whereas air is freely available). The molten-air batteries are the first rechargeable batteries to use a molten salt to store energy using 'free' oxygen from the air and multi-electron storage molecules."

This ability to store multiple electrons in a single molecule is one of the biggest advantages of the molten-air battery. By their nature, multiple-electron-per-molecule batteries usually have higher compared to single-electron-per-molecule batteries, such as Li-ion batteries. The battery with the highest energy capacity to date, the vanadium boride (VB2)-air battery, can store 11 electrons per molecule. However, the VB2-air battery and many other high-capacity batteries have a serious drawback: they are not rechargeable.

Here, the researchers demonstrated that molten-air batteries offer a combination of high storage capacity and reversibility. The molten-air battery uses oxygen from the air as the cathode material, giving it the benefit of not having to carry this weight. It also has the advantage of not using any exotic catalysts or membranes. Different versions of the battery use different electrolytes, but they are all molten, i.e., melted to a liquid by a high temperature, in this case around 700-800 °C.

The researchers experimented with using iron, carbon, and VB2 as the molten electrolyte, demonstrating very high capacities of 10,000, 19,000, and 27,000 Wh/l, respectively. The capacities are influenced by the number of electrons that each type of molecule can store: 3 electrons for iron, 4 electrons for carbon, and 11 electrons for VB2. In comparison, the Li-air battery has an energy capacity of 6,200 Wh/l, due to its single-electron-per-molecule transfer and lower density than the other compositions.

The researchers explain that they were able to make the battery reversible by using an unusual electrolytic splitting process to function as battery "charging." For example, when the iron molten-air battery is discharged, the iron mixes with the oxygen to produce iron oxide. To charge the battery, the iron oxide is converted back into iron metal, and O2 is released into the air. The carbon and VB2 molten-air batteries recharge in a similar way, although the electrochemical properties of VB2 are not as well understood as the others.

As Licht explained, the molten electrolyte is a key to making the battery rechargeable.

"In the case of molten-air batteries, the molten electrolyte opens a pathway to recharge a wide variety of high-capacity multi-electron storage materials," he said. "These materials, while highest in capacity, are a challenge to recharge (how do you reinsert 11 electrons back into each molecule of vanadium boride?). The molten electrolyte provides an effective media that is compatible with both recharging these materials and 'free' oxygen from the air for storage. The high activity of molten electrolytes allows this charging to occur."

While the molten-air battery's high capacity and reversibility make it an attractive candidate for future energy storage applications, the researchers are continuing to improve other areas of the battery. For example, they plan to investigate other types of molten electrolytes with lower melting temperatures, increasing the voltage (a major contributor to power density and, for electric vehicles, maximum speed), and improving the energy efficiency.

"High temperature for a battery is unusual," Licht said. "However, it is not an impediment. Lower capacity, high-temperature molten sulfur batteries have already been tested without incident in . No weak spot has yet appeared. The discharge current of the molten-air electrode is sufficient to yield high battery voltages, but as described in the study could be even greater when a higher surface area between the cycled air and the molten salt will be achieved."

Explore further: After nuclear phase-out, Germany debates scrapping coal

More information: Stuart Licht, et al. "Molten Air – A new, highest energy class of rechargeable batteries." Energy & Environmental Science. DOI: 10.1039/C3EE42654H
Also at arXiv:1307.1305 [physics.chem-ph]

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2 / 5 (19) Sep 19, 2013
Grid storage would be a fantastic application for this type of battery. I am surprised that this is not a primary goal of the research.
1.5 / 5 (15) Sep 19, 2013
No weak spot has yet appeared.

How about crash safety of carrying around molten metal at 800 degrees C, capable of igniting materials and people on fire upon a leak, or exploding when coming to contact with water? How about losing heat and being unable to start the battery without re-melting it after the car has been standing idle for a couple days? How about the energy efficiency of having to keep the metal constantly molten?

Molten metals, electric cars and practicality are three words that don't really fit in the same sentence.

The second issue is that volumetric energy density is not really the cornerstone of electric car batteries, but energy density per mass. Space is not really at a premium. Still, if the article is correct and the 27,000 Wh/l figure is actually attainable in practice, an electric car would only need one gallon of the active material vanadium boride to match 35 gallons of gasoline, and it would weigh only about 20 kilograms.
1.5 / 5 (8) Sep 19, 2013
Correction, one gallon of the active material to match ten gallons of gasoline.

A small city car like a Nissan Leaf could do away with one liter of the stuff. That would be a 3x3x3 inch cube.

The practical energy density of course depends on how much other stuff you need around it, like the massive layers of insulation to keep the metal molten from +40 to -40 C outside temperatures.
5 / 5 (1) Sep 19, 2013
Another big question is efficiency. How much more energy does it take to charge these batteries versus what you draw out of them?

Yet another question: how much do they weigh? If the energy density is high enough, we could be seeing electric aircraft real soon.
1 / 5 (12) Sep 19, 2013
why not bromine or mercury then?
5 / 5 (2) Sep 19, 2013
@Eikka - a Watt-hour is 3600 kJ so 27,000 Wh is very close to 100 MegaJoules. Gasoline is 36 MJ per liter, so if the article is correct then a liter of the VB2 would be equivalent to ~2.8 liters of gasoline.

I was skeptical of the article's figures because gasoline is hard to beat (let alone by a factor of 2.8). But they list Carbon as 19,000 Wh/L or ~70 MJ per liter, and graphite has an energy density upon oxidation of 73 MJ/L, which is a pretty good match with their figure.

In energy per kg gasoline still beats these; graphite is ~2.8x denser and only holds ~2x the energy per liter, and VB2 is ~6.5x denser an only holds 2.8x the energy per liter. But it is still very impressive for a battery!
1.4 / 5 (11) Sep 20, 2013
so if the article is correct then a liter of the VB2 would be equivalent to ~2.8 liters of gasoline.

But you need 3-4 units of gasoline to turn an engine for one unit of energy out of the crankshaft, so it's actually worth 8.4 - 11.2 liters, or roughly 1:10 as I said.

Gasoline by itself does you nothing.

how much do they weigh?

Vanadium bromide is 5.10 kilograms a liter. Iron is 6.98 kg/l
5 / 5 (4) Sep 20, 2013
Eikka, this is basic research, not a developmental product. You can legitimately criticize their analytical methods, the design of their experiments, their conclusions, and where they are or are not looking for answers, but that's about it.

If you want to complain about insulation, fire hazards, cost, etc. for a particular market niche (like transportation), you'll have to wait until researchers propose a specific product, with performance parameters we can evaluate.

It doesn't look like that will happen soon. They have only just begun to evaluate materials for molten-air batteries. There's a lot of basic research still to be done.
3 / 5 (2) Sep 20, 2013
...a Watt-hour is 3600 kJ...
No, it's 3600 J.

Edit: OK, that would still be 100 MJ
1 / 5 (5) Sep 20, 2013
So, rust as energy - oxidizing - is it necessary to have it as a molten salt for anything other than recharging? Does it have to be molten to discharge? If this were the case, the recharging would add the cost of melting it in the first place, rather than an ongoing way of keeping it molten. Is molten necessary to keep oxygen circulating in what would increasingly become an oxidized electrolyte?
1 / 5 (6) Sep 20, 2013
You can legitimately criticize...

I'm criticizing their claims beyond the research, as Mr. Licht makes an unfounded comment about there being no "weak spots" in molten metal batteries for electric cars so far.

But for example the ZEBRA battery has numerous flaws, including the fact that if you leave it sitting for two days, even with the insulation in place it turns solid and you have to spend considerable amount of time and energy to get it working again.
1 / 5 (7) Sep 20, 2013
Take iron for example. It takes 2 kWh to melt one liter of iron. A reasonably sized battery would of course have several liters of it, so you're looking at spending on the order of 10 kWh just to make your battery work. That's enough energy to drive 40-50 miles.

If you have to do that every other morning because your battery has solidified, it becomes a huge waste of time and energy. Alternatively, you have to keep your battery heater connected at all times, which wastes almost as much energy because a heater as small as a typical car block heater (200W) will consume 5 kWh in a day. A heater as small as a lightbulb will still consume 2-3 kWh a day - enough to drive you 10 miles.

So the act of keeping the battery molten obviously and greatly hurts its efficiency. Cars can't afford extensive insulation because of weight and space issues. If that's not a weak point then I don't know what is.
not rated yet Sep 20, 2013
But you need 3-4 units of gasoline to turn an engine for one unit of energy out of the crankshaft, so it's actually worth 8.4 - 11.2 liters, or roughly 1:10 as I said.

@Eikka - excellent point on the efficient usability of the energy- your ~10x figure for usable energy is much more appropriate than my 2.8x figure for total energy, and I stand corrected!

@atomsk - thanks - I originally had 3.6 kJ, then meant to edited it to 3600 J but missed deleting the 'k'.
1 / 5 (12) Sep 20, 2013
MIT guys have thought high temp liquid metal batteries were the way to go for a few years now.
this seems to be engineered specifically for scaling the battery in sheer capacity to store per $.

this doesn't seem to be geared for cars, but specifically for bulk industrial energy storage.

not rated yet Sep 20, 2013
Like the MIT work, this should probably be geared toward larger scale energy storage for the reasons that have already been outlined by others here. Unfortunately, aiming for EVs is the best way to draw funding, whether or not it is the best application.
1 / 5 (13) Sep 20, 2013
Just like the Earths inner generates electricity and a magnetic field. The molten air battery should mean stronger Magnets.
1 / 5 (4) Sep 24, 2013
A quick back of the envelope calculation.

Polystyrene foam (styrox) insulates at 0.033 W/(m-K). Let's suppose we have an insulating ball around the molten material to maximize thickness and minimize surface area. The wall thickness of that ball shall be 30 cm and the radius should be approximately 50 cm, so you have a roughly 1 meter scale object which is fairly large. It wouldn't fit in the trunk of an economy car.

Let's further suppose that the styrox doesn't melt.

The surface area of the ball is 3.14 sq-m so the heat loss through the insulation becomes 0.35 Watts per Kelvin. If it's 20 degrees C outside and 800 C on the inside of the ball, the continuous heat loss is therefore 270 Watts, or 6.5 kWh a day, which is fairly significant.

That's why I stressed the importance of practical consideration when it comes to electric vehicles. The necessary insulation won't be simple or very practical, and you can never completely do away with the gradual energy loss.
1 / 5 (5) Sep 24, 2013
The thermal conductivity of glass or mineral wool is about 0.04 W/(m-K) so the result would be approximately the same, if slightly larger.

To effectively insulate the molten metal within a reasonably small volume of insulation, one would need some kind of vacuum layer insulation, like in a thermos flask where you have one glass bottles one inside another with a silvered heat reflecting mirror on the inside surface and a strong vacuum in the space between the two to stop heat conduction.

The problem then becomes what happens when the bottles break.

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