There is immense promise in it. It is rechargeable, non-toxic and flexible. It is the new plastic battery
A VISIT to a factory that manufactures
batteries is riot a very pleasant experience. A factory manufacturing lithium
batteries uses electrodes made from
lithium and manganese dioxide (mo)
Which are used to run anything from
wristwatches to camcorders. Lithium
reacts explosively with water, therefore
the humidity in the factory must be
carefully controlled. mo being extremely
fine is not supportive either. Though MO
dust is not toxic and is bound together
with a resin, it gets everywhere. Workers
wear masks to prevent inhaling these
particles and must take long breaks
when they can breathe normally.
There are other kinds, like mercury
batteries for watches, lead-acid cells for
starter motors in cars, and nickle-cadmium batteries that can be recharged
and used in anything from torches to
calculators. All contain toxic metals and
handling them safely adds enormously
to the cost of manufacture. Dead batteries have been dumped in landfill sites
and have caused serious contamination
in the past.
Now an option is coming up. "There
is nothing toxic in our batteries," says Jo
Suter, a materials scientist at the applied
physics laboratory, part of Johns
Hopkins University in Maryland, us.
Suter is developing the world's first all-plastic battery. He says that the components are non-toxic, making the batteries easy to manufacture and dispose off
safely. They can be recycled to make new
batteries. Although it is too early to predict anything, battery makers are queuing up to take a look. Not just for environmental benefits, but manufacturers are interested in knowing 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
main reason for all this, interest is
the fact that plastic batteries have
the potential to outperform man),
existing cells. With all this there is
another advantage of flexibility.
These batteries can be moulded
into unusual shapes to form part of
the structure or casing of hearing
aids, personal stereos and body
computers.
The science involved is quite
complex. A simple battery consists
of two electrodes made from materials that exchange 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 done
by providing a medium, called electrolyte, for the exchange of ions. The
medium is usually a salt solution.
Both the electrodes perform a diametrically opposite 'job. The electron producing electrode is known as the
anode. Metals perform this job perfectly
and there are few commercial batteries
that have ever used anything else. The
other electrode, cathode, performs 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.
Although plastics are great insulators, it has been known for nearly two decades that some plastics conduct electricity. These conducting plastics consist
of long chains of carbon atoms joined
by alternating single and double bonds.
These polymers are made to conduct
electricity by doping them with other
chemicals. To understand how these
materials conduct electricity, take a look
at a gridlocked traffic where nothing can
move because there is no space. Imagine
looking down on such a city and remove
a single car from the jam below. The car
behind moves into this space, thereby
creating room for another car to take its
place, and so on. While the traffic moves
forward, the hole created by the removal
of the car travels backwards against the
flow.
A similar effect occurs when a polymer is doped. Take the backbone of a
polypyrrole chain, which is essentially a
series of carbon rings in which the
atoms are joined by alternating single
and double bonds. The dopant, an oxidising agent, binds to the 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. The 'hole' created by the leaping
electrons moves in the opposite direction acting as a positive charge, and
so this kind of a doping is known as
positive or p-doping. The material on
the whole remains neutral. P-doped
polymers are ideal cathodes.
Electronic gridlock can also be broken in another way - by adding an
electron to the polypyrrole 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 -- an ideal
rechargeable battery. But until now
nobody has come up with an n-doped
polymer that works.
In comes 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. "The trouble with most
conducting polymers is that the backbone becomes unstable when you try to
add an electron," says Searson. While
Suter is working to develop plastic batteries for commercial applications,
Searson and Poehler have carried out
the basic research that is ma4ing it possible. In the beginning of the year, the
two researchers synthesised a stable, n-
doped polymer from a class of plastics
known as polythiophene, but have shied
from giving any details before patents
have been filed.
In July, the team produced the first
all-plastic battery using the new anode.
Suter says the cell's energy density is
around 45 watt-hours per kilogram and
can be improved. Thg only drawback is
that the battery loses charge at the rate
of two per cent per week as compared
to less than 0.2 per cent in the case
of commercial batteries. The team is
working on why the self di9charge rate is
so high.
The next stage is to find an industrial backer to develop the battery for
manufacture. In the long run, both
Suter and Searson are optimistic about
plastic batteries. "In future, there will be
no way that landfill sites will accept the
batteries that are on the market today,"
Suter says (New Scientist, Vol 152,
No 2058).
We are a voice to you; you have been a support to us. Together we build journalism that is independent, credible and fearless. You can further help us by making a donation. This will mean a lot for our ability to bring you news, perspectives and analysis from the ground so that we can make change together.
Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.