World's First Battery Fueled By Air

Hugh Pickens writes "The Telegraph reports on the revolutionary 'STAIR' (St Andrews Air) battery could now pave the way for a new generation of electric cars, laptops and mobile phones. The cells are charged in a traditional way but as power is used an open mesh section of battery draws in oxygen from the surrounding air that reacts with a porous carbon component inside the battery, which creates more energy and helps to continually 'charge' the cell as it is being discharged. The battery has a greater storage capacity than other similar-sized cells and can emit power up to 10 times longer. 'The key is to use oxygen in the air as a re-agent, rather than carry the necessary chemicals around inside the battery,' says Professor Peter Bruce of the Chemistry Department at the University of St Andrews. 'Our target is to get a five to ten fold increase in storage capacity, which is beyond the horizon of current lithium batteries.'"

Comments

Re:What's the output?

[confused one]

Ok, second time I've had to do this; but, clearly the articles are pooorly written and do not describe the technology well, if at all.

It's a lithium-air battery in a carbon matrix. One electrode is lithium metal, one electrode is carbon. The oxygen, supplied by air and entering through the porous carbon electrode, reacts with the lithium to create lithium oxide. When the battery is recharged, the oxygen is liberated, returned to the atmosphere, and the lithium ions are returned to (plated on) the lithium metal electrode.

No CO2.

The output is electricity during discharge and oxygen during charging.

Re:Any side-effects or drawbacks?

[confused one]

The carbon is only used as a reaction matrix and an electrode. It's a lithium air battery. The oxygen reacts with the lithium during discharge to create lithium oxides. During recharge the oxygen is liberated and returned to the atmosphere.

More informative article

[Tweenk]

There is next to no information in the first article... this one is much more informative:
http://www.theregister.co.uk/2009/05/19/lithium_oxygen_stair_battery/

The concept (taking one of the reagents from air) is not new. There were zinc-air batteries for decades, and they are widely used. They have one of the highest energy densities of all types of commercially available batteries.
http://en.wikipedia.org/wiki/Zinc-air_battery

Seems like four years ago somebody even figured out how to make them electrically rechargeable (before that, the usual method of recharge was to replace the zinc plates and remove oxide waste, which was facilitated by cell design).
http://www.eetimes.com/news/semi/showArticle.jhtml?articleID=164903727

However, if those new batteries use carbon instead of zinc, they might have a higher theoretical upper bound on energy density. It looks like they're using graphite-lithium intercalate for the negative electrode (a standard thing), and the positive electrode is essentially a combined catalyst/adsorbent for Li2O2 which forms during electricity generation.

CnLi ---> Li+ + Cn + e
2Li+ + 2e + O2 --cat.-> Li2O2

Note that the first article is rather bogus: O2 does not "recharge" the battery, it is only a reagent.

I'm not familiar with the cost breakdown for the components of Li-ion batteries, but lithium seems like a major contributor, so this might not be much cheaper than the traditional Li-ion.

Re:Powered by Air?

[morgan_greywolf]

Agreed. It's not even the first battery powered by air in this manner. As Taco ever heard of zinc-air batteries? These are commonly used in, among other things, hearing aids.

Re:Is it rechargable?

[Tweenk]

Rechargeable zinc-air battery, 4 years ago:
http://www.eetimes.com/news/semi/showArticle.jhtml?articleID=164903727

I wonder whether they managed to take it anywhere. rechargeable zinc-air would be cool, because there's much more zinc than lithium on Earth.

Re:Question

[Tweenk]

Addendum: the same amount of oxygen would be liberated when charging. It could be a problem if you decided to charge it in an air-tight box, but under normal conditions it won't be a problem.

Re:Question

[Rei]

I don't think it's that awesome. Air batteries (which are nothing new, BTW), tend to suffer from poor cycle lifes, poor power density, and very low efficiency. Often these "breakthroughs" aren't as impressive as they at first sound.

Now, that said, the other recent battery breakthrough -- on the Li-S front -- really does look as impressive as it sounds. I read through the paper on the research the other day as "light reading" at the dentist's office ;) Li-S's big problem has long been its really atrocious cycle life. It has great energy density, good power density, and very good efficiency, but the cycle life is a killer. And the variants they tried to improve cycle life really shot the energy density.

The reason it has these cycle life problems is because of how it works: you have sulfur in a carbon matrix (needs a conductive matrix because sulfur is an insulator) on one side of a separator film and metallic lithium on the other. The lithium ions migrate across the membrane and bond with the (insoluble) sulfur cathode to form (insoluble) Li2S; then, when running the cell in the other direction, the Li2S is split and the ions migrate back to the metallic anode. But there are intermediary reaction products -- various lithium polysulfides -- and these *do* tend to be very soluble. So, some of the polysulfides dissolve into the electrolyte, migrate across the membrane, and precipitate out on the other side and are rendered useless.

The new technique is pretty clever. They start by making a form of mesoporous carbon. This is made kind of like aerogel, via nanocasting, and it's covered in really deep pits. They then mill and then heat together the sulfur and carbon. The sulfur, having low surface tension, wicks into all of the pore space, with only a small amount of room left over to account for expansion. They then bake the composite at 155C, which boils all of the sulfur off the surface, leaving it only in the pits. So when the polysulfides form, they have a lot of trouble migrating out of the carbon.

That alone is a big improvement, but they took it a step further. The polysulfides are hydrophobic, so they bonded polyethylene glycol to the exposed surface of the carbon to make it repel the polysulfides. So now they have even more trouble migrating out of the pore space. To show how well they have them trapped, they took a traditional Li-S cathode and used a worst-case electrolyte -- something that loves to dissolve polysulfides. After 30 cycles, 96% of the sulfur was gone. With their cathode in the same worst-case situation, only 26% was lost.

In normal coin cells, their tests showed an initial capacity of around 80% of the theoretical maximum, falling about 15% in the first few cycles and then plateauing, nice and stable. The theoretical maximum for Li-S, if you discount everything but the sulfur, is 2,500Wh/kg (the best li-ion batteries on the market are 200Wh/kg). Now, obviously, you can't discount everything but the sulfur. The sulfur:carbon:lithium ratio, by weight, works out to something like 7:3:2. So, excluding the electrolyte, separator, and casing (which should be small components on large-format prismatic cells), they should get about 950Wh/kg. I imagine in a large format cell, they could probably get 800-850 -- over 4x the best current li-ion. Also, it's quite convenient that all of the raw materials are cheap and have low toxicities.