杏吧原创

JO SUTER looks serious. 鈥淗ave you ever visited a battery factory?鈥� he asks.
鈥淲ell I have鈥攁nd it鈥檚 not pleasant.鈥� He talks quickly with a Dutch accent
about a visit he made to a factory which manufactures lithium batteries. These
have electrodes made from lithium and manganese dioxide and are used to power
anything from wristwatches to camcorders.

鈥淟ithium is very dangerous,鈥� he says. A soft, silvery metal, lithium reacts
explosively with water, so the humidity in the factory must be carefully
controlled. 鈥淎nd manganese dioxide is a very fine powder,鈥� he says. 鈥淓xtremely
fine.鈥� Dust is not much use as an electrode, so the tiny particles are bound
together with a resin and then packed tightly into thin films. Although the dust
is not toxic, it gets everywhere. Workers wear masks to prevent them inhaling it
and must take long breaks when they can breathe normally.

Then there are mercury batteries, used to power watches, lead-acid cells for
starter motors in cars, and nickel-cadmium batteries, nicads, that can be
recharged and used in anything from torches to calculators. All contain toxic
heavy metals and handling them safely adds enormously to the cost of
manufacture. In the past, dead batteries have been dumped in landfill sites and
have caused serious contamination. Batteries containing these substances will
soon have to be disposed of more carefully and the cost of this will also push
up their price.

鈥淏ut there鈥檚 nothing like these materials in our batteries,鈥� he says. As a
materials scientist at the applied physics laboratory, part of Johns Hopkins
University in Maryland, Suter is working on the world鈥檚 first all-plastic
battery. He says that the components are nontoxic, making the batteries easy to
manufacture and dispose of safely. They can be recycled to make new batteries,
unlike the components of existing cells.

Although it is still early days, battery makers are queueing up to take a
look. And it鈥檚 not just the environmental benefits they鈥檙e interested in.
Manufacturers also want to know about the amount of energy that can be packed
into a cell, its operating temperatures, the voltage it can produce, whether it
can be recharged and how often. And the real reason why the work is generating
so much interest is that in all these departments, plastic batteries have the
potential to outperform many existing cells. Add to this the fact that plastic
batteries can be moulded into unusual shapes to form part of the structure or
casing of hearing aids, personal stereos and body computers, and the full
attraction becomes clear.

The science of batteries is hugely complex. A simple battery consists of two
electrodes made from materials that interact by swapping ions and electrons. The
trick is to allow the ions to pass through the battery while forcing the
electrons to pass round an external circuit where they power anything from a
mobile phone to a laptop. This is achieved by connecting the electrodes using an
electrolyte, a substance such as a salt solution, which provides a medium for
the exchange of ions. However, the reaction cannot begin until the external
circuit is closed to allow the electrons to flow.

Each electrode performs a fundamentally different job. The electron-producing
electrode is known as the anode. Metals are perfect for the job since they are
easily broken into ions and electrons and few commercial batteries have ever
used anything else. The other electrode, the cathode, has the job of accepting
the electrons at the end of their journey. Cathodes have been made from a
variety of materials including metals, metal oxides and metal sulphides.

So where do plastics fit in? After all, plastics are insulators. Well, not
quite. For more than twenty years, chemists have known that some plastics
conduct electricity. On a molecular level, plastics consist of long chains of
carbon atoms. The feature that all conducting polymers share is that the atoms
in these chains are joined by alternating single and double bonds. By
themselves, these polymers are good insulators, just like other plastics. But
they can also be made to conduct electricity by doping them with other
chemicals.

To understand how these materials conduct, imagine the backbone of a
polypyrrole chain. This is essentially a series of carbon rings in which the
atoms are joined by alternating single and double bonds (see
Diagram).
Ordinarily, the electrons in these rings are locked in place, which prevents a
current from flowing鈥攔ather like gridlocked traffic where nothing can move
because there is no space to move into. Now imagine looking down on a gridlocked
city and removing a single car from the jam below. The car behind moves into
this space thereby creating room for the car behind to move forward, and so on.
While the traffic moves forward, the hole created by the removal of the car
travels backwards against the flow.

Leaping electrons

A similar effect occurs when a polymer is doped. The dopant, an oxidising
agent such as silver perchlorate (AgCl0₄), binds to a polypyrrole
molecule and removes an electron in the process. This allows an electron from
the neighbouring pyrrole ring to jump into its place, leaving behind a hole for
an electron from the next ring, and so on. During this process, the hole created
by the leaping electrons moves in the opposite direction, just like the space in
our gridlocked traffic. The hole, which travels against the flow of electrons,
acts as a positive charge so this kind of doping is known as positive or
p-doping. Of course, although the backbone becomes positively charged, the
material as a whole remains neutral because the perchlorate ion (ClO₄
^鈭�) has an equal negative charge.

P-doped polymers are ideal cathodes. When a battery discharges, the
positively charged polymer chains eagerly accept electrons from the external
circuit. This tends to push the negatively charged perchlorate ions out of the
cathode and into the electrolyte. Because polypyrrole molecules are large, they
form porous structures which allow the ions to drift in relatively easily when
the battery is discharged and back in again during a recharge when electrons
once again leave the polymer chains. This process is remarkably efficient, and
p-doped polypyrroles have already been used as cathodes in some commercial
batteries.

Electronic gridlock can also be broken in another way鈥攂y adding an
electron to the backbone via a reducing agent such as lithium. This creates a
positively charged lithium ion and a negatively charged backbone. Because this
negative charge moves along the backbone, this process is known as n-doping. In
theory, n-doped polymers should react with p-doped polymers in a reversible
process that creates a flow of charge鈥攁n ideal rechargeable battery. But
until now nobody has come up with an n-doped polymer that works.

Enter Peter Searson and his colleague Ted Poehler at the main Johns Hopkins
campus in Baltimore, who head a small team of materials scientists and chemists.
鈥淭he trouble with most conducting polymers is that the backbone becomes unstable
when you try to add an electron,鈥� says Searson.

While Suter is concentrating on developing plastic batteries for commercial
applications, Searson and Poehler have carried out the basic research that is
making it possible. Working with a team of four researchers, they have spent the
past three years studying conducting polymers from a windowless lab in the
basement of the university鈥檚 engineering department. Their goal has been to
develop a stable polymer anode and use it to construct the world鈥檚 first
all-polymer battery.

Fooling the system

Their initial approach was to combine polypyrrole with polystyrenesulfonate,
a nonconducting polymer which can hold a negative charge and remain stable. When
lithium is also added, the polystyrenesulphonate chains become negatively
charged while the polypyrrole remains neutral. 鈥淏y doing this, you fool the
system into behaving like an anode,鈥� says Searson. Last year, the team used this
approach to synthesise a stable polymer anode which they used with a p-doped
polypyrrole cathode and a polymer gel electrolyte containing lithium perchlorate
to build the first all-polymer battery.

The cell generated a potential of 1 volt and, amazingly, could be charged and
discharged more than a hundred times without degrading鈥攁bout the same as
today鈥檚 commercial nicads.

But this battery had several drawbacks. The main problem was the low voltage
which limited the cell鈥檚 power. The amount of power that can be packed into a
battery鈥攊ts energy density measured in Watt hours per kilogram鈥攊s
the all-important factor that researchers look for when deciding to develop a
battery further.

Although the cell was a significant step forward, Searson and Poehler knew
that the voltage could not be raised. Even in theory, the performance of the
battery could never come close to those already on the market. 鈥淲e decided we
needed to look for a new system,鈥� says Searson, and in September last year, they
went back to the drawing board.

Since then, their work has been hugely successful. To their surprise, the two
researchers found a promising alternative remarkably quickly. By the beginning
of this year, they had synthesised a stable, n-doped polymer from a class of
plastics known as polythiophene, a polymer made up of carbon rings similar to
polypyrroles.

Searson won鈥檛 give away any details about how he has manipulated the polymer
to make his electrode until patents have been filed, but he will drop the odd
clue. He says that by adding chemical groups that help to store electrons, the
polythiophene backbone can be made stable even when it is n-doped. At the same
time, the team has come up with a non-metallic鈥攁nd
anonymous鈥攔educing agent that takes the place of lithium as a dopant. 鈥淲e
consider this a major breakthrough,鈥� he adds.

In July, the team produced the first all-plastic battery using the new anode.
Suter says the cell鈥檚 energy density is around 45 Watt hours per kilogram and
can still be improved. Nicads, by comparison, produce 35 Wh/kg. The new cell can
be discharged and charged more than a hundred times without degrading and
produces a potential difference of 2.9 volts, about the same as lithium
batteries. It also works in temperatures ranging from 鈥�20 掳C to 45 掳C.

As if that were not good enough, the cell is made of components that are
environmentally benign compared with today鈥檚 batteries. 鈥淥ne would not swallow
the components but they are nothing compared with lithium or cadmium,鈥� says
Suter.

There is one drawback, however. The battery loses its charge at a rate of 2
per cent per week compared with less than 0.2 per cent for commercial batteries.
Suter says this self-discharge is not necessarily a problem for batteries that
are in constant use, such as those used in cell phones or laptops, because they
must be regularly recharged anyway. Even so, the team鈥檚 next step will be to
work out why the self-discharge rate is so high.

Self-discharge can be caused by unwanted reactions between the electrodes and
chemicals inside the cell, says Yossef Gofer, a materials scientist in the team.
Over the next few months, Gofer and colleagues Jeff Killian, Jennifer Giaccai
and Harri Sarker hope to identify all the chemicals created in the electrolyte
during charging and discharging. Any that shouldn鈥檛 be there is a sign that
unwanted reactions have occurred.

There are other challenges, too. The battery components must be protected
from water which reacts with them to destroy the cell. 鈥淥ther batteries are
packaged in metal but that would violate our goal of an all-plastic cell,鈥� says
Suter. Creating plastic waterproof packaging is suprisingly difficult. Because
polymer molecules tend to be big, water molecules can pass between them
relatively easily. 鈥淥nly the food industry and toothpaste manufacturers have
really perfected the art of plastic packaging,鈥� says Suter, waving a prototype
cell sealed in a plastic food bag. 鈥淏ut we鈥檙e working on it,鈥� he adds.

The next stage for the group is to find an industrial backer to develop the
battery for manufacture. To demonstrate the battery鈥檚 potential, Suter is
building a one-off cell that will power his portable phone. Such batteries
require relatively high power over extended periods. 鈥淚t鈥檚 a tough spec. That鈥檚
why it will create real interest.鈥�

In the long run, both Suter and Searson are realistic about the promise of
plastic batteries. For example, the new cells will never match the energy
density of lithium-based cells which have energy densities in excess of 90
Wh/kg. Consequently, there will be no weight-saving when it comes to batteries
for portable machines.

But plastic batteries will find their niche and environmental issues will
play a big part in their success, says Suter. 鈥淚n future, there is no way that
landfill sites will accept the batteries that are on the market today,鈥� he
predicts, and he points to recycling schemes that battery manufacturers have
already set up in Europe and to legislation that is being drawn up in the US to
control their disposal. If he is right, batteries may never be the same
again.