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?