Sodium-air battery offers rechargeable advantages compared to Li-air batteries

Sodium-air battery offers rechargeable advantages compared to Li-air batteries
Scanning electron microscopy image of cubic NaO2 particles on the cathode after cell discharge. Image credit: P. Hartmann

(—Over the past few years, Li-air batteries (more precisely, Li-oxygen batteries) have become attractive due to their theoretical ability to store nearly as much energy per volume as gasoline. The key to this high energy density is the "air" part, since the batteries capture atmospheric oxygen to use in the cathode reaction instead of storing their own oxidizing agent. However, Li-air batteries have conventionally been single-use cells since they cannot be recharged, which significantly limits their applications. Now in a new study, scientists have found that replacing the lithium anode with a sodium anode may offer an unexpected path toward making metal-air batteries rechargeable while still offering a relatively high energy density.

The researchers, led by Professor Jürgen Janek and Dr. Philipp Adelhelm at the Institute of at Justus-Liebig-University Gießen in Gießen, Germany, have published their paper on rechargeable sodium-air batteries in a recent issue of .

Li-air batteries' greatest appeal is their high theoretical (about 3,458 Wh kg-1), which is several times higher than that of Li-ion batteries, the most commonly used in today. However, whereas Li-ion batteries can be recharged many times while retaining most of their capacity, most Li-air batteries cannot be recharged at all. In 2011, several international groups discovered that this irreversibility is due to the instability of the Li-air battery's and other in the presence of the reactive superoxide radical O2-, which forms as a first step during cell discharge. Only recently, improvements in rechargeability have been achieved by using gold electrodes, but the system still suffers from poor energy efficiency and large overpotentials in which some energy is lost as heat.

In the new study, the researchers demonstrated that a sodium-air (Na-air) cell does not suffer from the same negative effects on the electrolyte and energy efficiency as a Li-air cell does. This is because, while lithium and sodium are closely related chemically, they each react very differently with oxygen. When lithium reacts with oxygen, it forms LiO2 (lithium oxide), which is highly unstable and found only as an intermediate species in Li-air batteries, after which it turns into Li2O2. On the other hand, sodium and oxygen form NaO2 (sodium superoxide), a more stable compound. Since NaO2 doesn't decompose, the reaction can be reversed during charging.

Sodium-air battery offers rechargeable advantages compared to Li-air batteries
(Left) When a metal-oxygen battery is discharged, metal A (e.g., lithium or sodium) is oxidized at the anode/electrolyte interface, and the resulting electron is transferred to the outer circuit. At the cathode, oxygen is reduced to a superoxide species that may form a metal superoxide in the presence of the oxidized metal A. (Right) The metal superoxide in a Li-oxygen cell is highly unstable and reacts further. (Center) The metal superoxide in a Na-oxygen cell is much more stable and doesn’t decompose further, allowing the reaction to be reversed. Image credit: P. Hartmann, et al. ©2012 Macmillan Publishers Limited.

The scientists demonstrated the reversibility of Na-air cells in their experiments. Using several techniques, including Raman spectroscopy and X-ray diffraction, the scientists confirmed that NaO2 is indeed produced during discharge, that Na and O2 are separated during charging, and that the cycle can be repeated.

"We could demonstrate that by replacing lithium with sodium, the cell reaction proceeds in an unexpected and beneficial manner," Adelhelm told "The cell discharge and charge process is kinetically favored, which means that the formation and decomposition of NaO2 is very energy-efficient."

Although this evidence of reversibility is a promising step, the reaction is far from ideal. While Na-air batteries have a theoretical energy density of 1,605 Wh kg-1, which is significantly higher than that of Li-ion batteries, it is still only about half that of Li-air batteries. And even though the Na-air batteries can be charged and discharged several times, the capacity decreases after each cycle, with negligible energy storage after eight cycles. The researchers are currently investigating the processes that limit battery lifetime.

Still, Na-air batteries have some attractive characteristics. One advantage of the Na-air battery demonstrated here is its very low overpotential, which is three or four times lower than for any Li-air or Na-air battery previously reported, resulting in fewer losses. In addition, sodium is the sixth most abundant element on Earth, while lithium resources are much more limited.

"Our results are also important from another perspective," Adelhelm said. "NaO2 is chemically very difficult to synthesize. High temperatures, pressures and long reaction times are needed. In our battery, NaO2 forms instantly at room temperature and ambient pressure. Possibly, also other chemical compounds could be prepared this way."

Overall, Adelhelm hopes that Na-air batteries may serve as one more option to turn to for future energy storage applications.

"A broad variety of electrochemical devices with different properties is needed for future mobile and stationary applications," he said. "In short, sodium's abundance could be an important cost advantage over lithium; however, for otherwise identical batteries, the 'lithium version' will always provide the higher energy density. But any working metal/air battery will provide a higher energy density than current Li-ion batteries."

Explore further

Lithium-air batteries' high energy density could extend range of electric vehicles

More information: Pascal Hartmann, et al. "A rechargeable room-temperature sodium superoxide (NaO2) battery." Nature Materials. DOI: 10.1038/NMAT3486
Journal information: Nature Materials

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User comments

Jan 02, 2013
Really good work! Who nows, a Na-air battery with ~1 kWh/kg and a few charges may allready be suitable some applications?

Jan 02, 2013
So while the battery is charging (or charged), there is pure Na on the cathode. That should react violently with moisture in the air. Or they supplying this battery with specially-prepared low-humidity air?

Jan 02, 2013
Can this process also pull sodium out of salt water?

Jan 02, 2013
That should react violently with moisture in the air.

Just like lithium (which also reacts rather violently with moisture). You'll want to dehumidify the air in any case before feeding it to the battery. Even with zinc air batteries prior dehumidification is a good idea (though for a different reason).

Jan 03, 2013
The figure they gave for the NA-air's theoretical energy density is about 4 times that of LI-ion. So an electric car that has a range of 75-100 miles now would go 300-400? About the same as a tank of gasoline.

If you could double it, 600-800 miles is about as far as the average automobile driver on a long trip would want to drive in a day, making an overnight charge a non-issue. Too bad they can't get the LI-air to work.

Also using sodium instead of lithium should make them much cheaper assuming the limitations they mentioned can be surmounted without resorting to some Rube-Goldberg design.

The new insight they think they have found in the chemical processes involved may be the most valuable part of this.

Jan 03, 2013
The sodium/lithium air battery is not only lighter, because it doesn't contain oxidization agent, but it's inherently safer, because it doesn't contain oxidizer in concentrated state. The sodium/air battery will simply burn during impact, but it cannot explode. Not to say, the sodium is abundant nontoxic element which everyone can afford in his kitchen.

Jan 03, 2013
The volumetric energy density of neither lithium-air nor sodium-air batteries is provided in the article, so the editor's claim that "their theoretical ability to store nearly as much energy per volume as gasoline" cannot be evaluated. Gasoline has a gravimetric energy density of more than 12,600 Wh/kg, which is nearly four times the theoretical best value for lithium-air of 3,458 Wh/kg provided in the article. Of course there are offsetting advantages in the thermal efficiencies and power-train mass of electric versus gasoline-powered vehicles. Today's real world lithium-ion batteries muster only 128 Wh/kg in published manufacturer specs I have found. Anybody out there have the volumetric energy density for lithium-air and sodium-air batteries, or better data on current lithium-ion batteries?

Jan 03, 2013
As to safety, any battery containing sodium or lithium must be protected from unintended exposure to air or water. The danger of normal lithium-ion batteries is when the case is cracked and they become unintentional lithium-air batteries, with oxygen and moisture intruding and causing a very energetic chemical reaction that leads to intense heat and fire. No one should confuse highly reactive pure reduced sodium metal for inert sodium chloride (table salt).

Jan 03, 2013
The volumetric energy density of neither lithium-air nor sodium-air batteries is provided in the article, so the editor's claim that "their theoretical ability to store nearly as much energy per volume as gasoline" cannot be evaluated

You can get it from wikipedia. Theoretical limit for Lithium air is about the same as gasoline (12kWh/kg vs 13kWh/kg...also both are about the same volume). This excludes the oxygen mass (air batteries get heavier during discharge)

But since the electric motors are more efficient both come out to about 1.7kWh/kg at the wheels.
Theoretical limit for sodium air batteries is about half as good as for Lithium air batteries.

Jan 13, 2013
There is the law of diminishing returns! A 400 W/kg battery for less than $150/kWh makes ICE-cars obsolete. Great number of eCars brings about electrification of roads, obviating the need for cars with range of more than, say, 200 or even 100mi. Further developments are needed just to further reduce the cost, it's just a question of optimization. So Na-air is good enough for most cars, leaving more expensive Li-air batteries to aircraft, where every pound counts. Let 1000 batteries bloom!

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