Japanese scientists have made a paper-like, polymer-based rechargeable battery.

With recent advances in the technology of portable electronic devices, there is demand for flexible batteries to power them.

The battery, designed by Hiroyuki Nishide, Hiroaki Konishi and Takeo Suga at Waseda University, has an electrode made from a redox-active organic polymer film about 200 nanometres thick. The polymer has nitroxide radical groups which act as charge carriers.

The battery has a high charge/discharge capacity because of its high radical density (two radicals for each repeat unit). According to Nishide, this is just one of many advantages the 'organic radical' battery has over other organic-based materials which are limited by the amount of doping.

'The power rate performance is strikingly high - it only takes one minute to fully charge the battery,' said Nishide, 'and it has a long cycle life, often exceeding 1000 cycles.'

Nishide's team made the thin polymer film by a solution-processable method: a soluble polymer, polynorborene with pendant nitroxide radical groups, is spin coated onto a surface. On UV irradiation, the polymer becomes crosslinked with the help of a bis(azide) crosslinking agent.

A drawback of some organic radical polymers is their solubility in the electrolyte solution which results in self-discharging of the battery; however, the polymer needs to be soluble so it can be spin-coated. The photocrosslinking method used by Nishide overcomes this problem and makes the polymer mechanically tough - this has been a challenging step, said Nishide, as most crosslinking reactions are sensitive to the nitroxide radical.

Peter Skabara, an expert in electroactive materials at the University of Strathclyde, UK, praised the high stability and fabrication strategy of the polymer-based battery. 'The plastic battery plays a part in ensuring that organic device technologies can function in thin film, flexible form as a complete package,' said Skabara. 'However, these materials will not give slow energy release, which limits them to capacitor applications,' he continued. 'The polymers can undergo rapid charging/discharging which will be useful in delivering burst power.'

Nishide envisages that the organic radical battery could be used in pocket-sized integrated circuit cards, used for memory storage and microprocessing, within the next three years.

'In the future, these batteries may be used in applications that require high-power capability rather than high-energy density, such as a battery in electronic devices and motor drive assistance in electric vehicles,' said Nishide.

Alison Stoddart