The beauty of bendable batteries :: The Guardian
The beauty of bendable batteries
Vastly thinner, lighter power sources are destined to make all our electric
dreams come true, writes Alok Jha
Thursday February 24, 2005
Just over 10 years ago, Donald Sadoway went for a drive in his first
electric car. It was an early, clunky design but the experience changed his
world - ever since he has dedicated himself to doing his part in making
electric cars a commercial reality.
"For personal transportation, it's lunacy that we don't have electric
vehicles," says Sadoway, a professor of materials science at the
Massachusetts Institute of Technology. "It's crazy to be burning carbon to
move around short distances. For congested urban environments, the future is
now. The only thing that's missing is a viable battery."
Our increasingly mobile world depends on the ability to store and move
energy around with us in the form of batteries (there's no point in a
top-of-the-range laptop if it needs a cell that costs more than a space
station and is the size of Texas). But lack of academic and commercial
interest in the field, and the concentration of what effort there is on
developing fuel cells and hydrogen cars, means innovation has hit a wall.
Which is why Sadoway is getting excited about crisp packets. Smoothing one
out in front of him, he describes how the thin sandwich of metal and plastic
is set to be the unwitting battering ram to that technological brick wall.
His remarkable idea is a battery which is as thin as a crisp packet and is
as cheap, well, as the crisps in them.
The Slimcell, as Sadoway calls his invention, is a sandwich of lithium and a
special type of Perspex. Because it is so light, it solves the problem that
has kept batteries out of the running for electric cars: it can store a huge
amount of energy per kilogram.
Batteries work because chemical reactions inside them force electrons to
collect at one of the two electrodes. Connect an appliance and the electrons
travel through it to the other electrode, making an electrical circuit.
A traditional lead acid battery (where one electrode is made of lead, the
other of lead dioxide and sulphuric acid floats between the two) has a
capacity of 35 watt hours per kilogram (Wh/kg). The nickel metal hydride
batteries that became available in the early 1990s, and enabled laptop
computing, are about 90Wh/kg. Newer lithium ion batteries, used in mobile
phones and today's laptops, are 125 Wh/kg.
"At 125Wh/kg, you can drive a car 125 miles on a single charge - that's not
good enough," says Sadoway. "You need to go about 250 miles on a single
charge before it's going to have widespread appeal. So you need a battery
that's about 250Wh/kg. We've got batteries in my lab right now that are
300Wh/kg and I can see the possibility of breaking 400Wh/kg."
The Slimcell gets these big energy densities by ditching weight. Sadoway
could take little off the electrodes in a lithium ion battery, because the
metal is so light. Instead, he focused on a component that was crucial but
which didn't contribute to the storage capacity, the liquid electrolyte.
"An ideal battery is maximally electrode and minimally everything else,"
says Sadoway. "The electrolyte doesn't contribute to storage capacity at all
but you have to have it. What you would like is the thinnest electrolyte."
Working with MIT colleague Anne Mayes, Sadoway set out to ditch the liquid.
"The only way we were going to break the 200Wh/kg, I reasoned, was to ask
was it possible to invent a solid polymer electrolyte that had the
mechanical properties of a solid and the electrical properties of a liquid,"
says Sadoway. Many blind avenues later, his team had found his wonder
material and rethought battery design.
"There are various companies that claim to have variants of a solid
separator but, to the best of my knowledge, these companies are all relying
upon infusing some kind of polymer host with something that's an organic
liquid," says Sadoway.
The Slimcell's sandwich of metal electrodes and plastic electrolyte not only
means it is light and easy to make, it also makes it much safer. Because
there is no liquid, the battery can't leak. If it is somehow punctured in
one part, the rest of the battery can carry on working regardless.
Sadoway and Mayes's team have been steadily improving their polymer
electrolyte: by developing a way to evaporate the material on to the lithium
electrodes, they have made it as thin (and light) as physically possible.
The next step is industrial-scale manufacture, something that has yet to be
worked out. Sadoway reckons that the Slimcell is at least five years from
being commercially available.
Remarkable as the battery is, the MIT team behind it see problems in finding
a market, mainly due to resistance from battery manufacturers. In the mid
1990s, just as a lot of companies had invested heavily in NiMH batteries,
the more advanced lithium ion batteries came out of nowhere. Instead of
embracing lithium ion, many producers tried to slow its introduction.
"Until the late Nineties, you could buy laptops fitted with NiMH batteries
and lithium ion was an option, even though it gave superior performance,
because they were saying, 'we need to sell as many NiMH batteries as we can
to try to pay back the capital costs of building these darn plants'," says
"People would like better batteries but they are wary of making investments.
What is required is both a technology push and a market pull. Right now
there are cellphone batteries and the cellphone manufacturers are
comfortable with them."
It will take a specialised niche, then, to make the Slimcell viable. Sadoway
says that whoever develops it commercially needs to find applications for
which dedicated batteries don't yet exist - medical devices, for example.
"In a wristwatch, imagine the battery is in the strap and there's a medical
sensor in there connected to the internet," he says. "If someone is
monitoring that, they could phone up if the user has forgotten to take some
medication. This could save hundreds of dollars in medical fees later.
What's missing? It's a stable battery."
The Slimcell's big advantage is its versatility. "You can do something with
this you can't do with any other battery - fold it up, make it conform to
the shape," says Sadoway. "You can put the battery not in a compartment but
behind the screen of a computer, or in the fender of a car, so you can have
distributed power. You don't have it in one big block." After these
applications, Sadoway's dream of powering electric cars will be a step
closer. Which is quite a feat for something that is, basically, just like a