Plastic wonders

All-plastic batteries are lighter and even flexible

Published: Thursday 31 July 1997

a battery has been developed by researchers at John Hopkins University that is made entirely of plastic. This is being considered a significant breakthrough. An all-plastic battery was considered an impossibility a few years ago. Plastic is an electrical insulator and previous attempts at producing all-plastic batteries were marred by several technical hurdles ( Scientific American , Vol 276, No 4).

The heavy weight of batteries seriously limits the performance of every application they are put to. The greatest advantage of the all-plastic batteries would be their light weight. Another advantage is that they are flexible. This makes it possible to fit them into awkward places. They are also safer and ecologically less harmful than batteries containing lead, cadmium or lithium, says the inventor, Joseph J Suter of the Applied Physics Laboratory. The project was sponsored by the us Air Force to develop a plastic battery capable of powering a two-way radio for an hour. Suter is negotiating with manufacturers to roll-up the technology to make aa -size cells. A panel of plastic cells is slated to be tested on a satellite in 1998.

The greatest obstacle in producing the plastic batteries -- the fact that plastic is an insulator -- was overcome by incorporating 'dopants' into certain types of polymers. Dopants are substances that either supply extra electrons to conduct the charge or, alternatively, take electrons away to create 'holes' -- places in a molecule that conduct charge by accepting electrons. Compounds called polypyrroles are now in use in commercial cells in combination with metals but have not achieved voltages high enough to be commercially useful.

Theodore O Poehler and Peter C Searson of the John Hopkins University have succeeded in making thin-sandwich cells that can produce a voltage up to three. Carefully chosen combinations of plastics, identified as fluorophenylthiophenes, were used as electrodes. A polymer gel containing a boron compound was used to connect the electrodes. It was found that the cell could store more electricity than lead-acid or nickel-cadmium batteries, although not yet as much as lithium batteries. New electrode material, now in early testing, may store 10 times more energy than fluorophenylthiophenes, says Poehler.

But plastic batteries are not without drawbacks. Special electronics is required to charge them optimally. They are difficult to seal properly. But the most important drawback is that such batteries can be used by terrorists to make undetectable letter bombs. Suter turned down inquiries from an Iraqi researcher because of this fear. Batteries produced till date have been deliberately incorporated with metal grids to make them visible on x -ray machines.

But none of the disadvantages can prevent the commercial production and success of plastic batteries in the near future, says Suter. The team at John Hopkins expect to be issued patents covering all types of polymer batteries.

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