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2013-02-06 11:19:58
The search for better ways of storing electricity is hotting up
Feb 2nd 2013 | LEMONT, ILLINOIS |From the print edition
KRIS PUPEK, an industrial chemist at Argonne National Laboratory in Lemont, near Chicago, waves a tube of white powder in the air emphatically. A mere pinch of the contents is sufficient for his analytical colleagues to work out if it has the potential to be the next whizzy material in battery research. But Dr Pupek does not deal in pinches. His job is to find out whether potential can be turned into practice in other words, whether something that has the right properties can be made cheaply, and in bulk. If it can, it is passed on to industry for testing. The hope is that at least one of the tubes will start a revolution.
Batteries are a hugely important technology. Modern life would be impossible without them. But many engineers find them disappointing and feel that they could be better still. Produce the right battery at the right price, these engineers think, and you could make the internal-combustion engine redundant and usher in a world in which free fuel, in the form of wind and solar energy, was the norm. That really would be a revolution.
It is, however, a revolution that people have been awaiting a long time. And the longer they wait, the more the doubters wonder if it will ever happen. The Joint Centre for Energy Storage Research (JCESR), at which Dr Pupek and his colleagues work, hopes to prove the doubters wrong. It has drawn together the best brains in energy research from America s national laboratories and universities, along with a group of interested companies. It has money, too. It has just received a grant of $120m from the country s Department of Energy. The aim, snappily expressed, is to make batteries five times more powerful and five times cheaper in five years.
Think positive
Most batteries, from the ancient, lumbering lead-acid monsters used to start cars, to the sleek, tiny lithium cells that power everything from e-book readers to watches, have three essential components: two electrodes (an anode and a cathode) and a medium called an electrolyte that allows positively charged ions to move between the electrodes, balancing the flow of negatively charged electrons that form the battery s useful current. The skill of creating new types of battery is to tinker with the materials of these three components in ways that make things better and cheaper. Dr Pupek s white powders are among those materials.
To discover more of them, Argonne will make use of a rapidly growing encyclopedia of substances created by Gerbrand Ceder of the Massachusetts Institute of Technology. Dr Ceder runs the Materials Project, which aims to be the Google of material properties . It allows researchers to speed up the way they search for things with specific properties. Argonne will use the Materials Project as a reference library in its search for better electrodes, and also hopes to add to it.
The first test of any combination of substances that comes out of the Materials Project, or anywhere else, will be to beat the most successful electricity-storage device to emerge over the past 20 years: the lithium-ion battery. Such batteries are now ubiquitous. Most famously, they power many of the electric and hybrid-electric cars that are starting to appear on the roads. More infamously, they have a tendency to overheat and burn. Two recent fires on board Boeing s new 787 Dreamliners may have been caused by such batteries or their control systems. Improving on lithium-ion would be a feather in the cap of any laboratory.
George Crabtree, JCESR s newly appointed director, thinks such improvements will be needed soon. He reckons that most of the gains in performance to be had from lithium-ion batteries have already been achieved, making the batteries ripe for replacement. Jeff Chamberlain, his deputy, is more bullish about the existing technology. He says it may still be possible to double the amount of energy a lithium-ion battery of given weight can store, and also reduce its cost by 30-40%.
This illustrates the uncertainty about whether lithium-ion technology, if pushed to its limits, can make electric vehicles truly competitive with those run by internal-combustion engines, let alone better. McKinsey, a business consultancy, reckons that lithium-ion batteries might be competitive by 2020 but, as the chart below shows, there is still a lot of work to do. Moreover, pretenders to lithium-ion s throne are already emerging.
The leader is probably the lithium-air battery, in which metallic lithium is oxidised at the anode and reduced at the cathode. In essence, it uses atmospheric oxygen as the electrolyte. This reduces its weight and means its energy density is theoretically enormous. That is important. One objection to electric cars is that petrol packs six times more joules of energy into a kilogram than a battery can manage. Bringing that ratio down would make electric vehicles more attractive.
The lithium-air approach has consequently generated a lot of hype. It has problems, though, which will take years of research to resolve. Lithium-air batteries are hard to recharge and extremely temperamental. The chemical reaction which powers them is not far removed from spontaneous combustion. Lithium-air batteries are thus highly inflammable and require heavy safety systems to stop them catching fire.
Luckily, the researchers at JCESR have other irons in the fire. One is the multivalent-ion battery. A lithium atom has but a single electron available for chemical reactions. A magnesium atom, by contrast, has two such valence electrons, and an aluminium atom three.
Theoretically, says Dr Chamberlain, this means it might be possible get two or three times as much energy out of a magnesium or aluminium battery. Though these metals are not as light as lithium (nor as electropositive, to use a piece of chemical jargon that is pertinent to the argument), their extra valence electrons increase the amount of energy they can store, thus pushing them forward in the competition with petrol. They are also cheaper than lithium. And safer. Their ions, however, are harder to move around inside a battery, which is why they have not been used much in the past, and this is where new materials will need to be sought out.
The second transformation, besides electric cars, that better batteries might bring about is what is known as grid-scale storage. If this could be done cheaply enough it would revolutionise the economics of wind and solar energy by making the main problem with such sources that the sun does not always shine and the wind does not always blow irrelevant. To this end, Argonne s researchers are working on what are known as flow batteries.
Go with the flow
In a conventional battery the electrolyte is contained within the cell and serves to transport ions between the electrodes. The battery s charge is held as chemical potential energy in those electrodes. In a flow battery the charge is held in the electrolyte itself, which is stored in a tank and then pumped through the cell to the place where the electrochemical reactions occur.
Unlike batteries based on cells, flow batteries can be made very large indeed, so they can store vast amounts of energy. Hence the idea of using them to collect surplus power from wind turbines and solar panels and squirrel it away for use later. But their water-based electrolytes limit their potential, because of water s tendency to decompose by electrolysis. That restricts the voltage at which they can operate. Replacing their aqueous electrolytes with organic ones would overcome this limitation, and Argonne s researchers are endeavouring to do so.
A battery-driven world, then, would electrify parts of the economy, such as transport, that have been recalcitrant, and would encourage the shift from costly (and polluting) fossil fuels to fuels such as sunlight that cost nothing. As a manifesto for a revolution, that takes some beating. The question is, will the revolutionaries win, or will the ancien r gime prevail?