The continuously advancing technology of portable electronic devices asks for more flexible batteries to power them.

A Japanese team at Waseda University have developed a paper-like rechargeable battery. The battery is made of a redox-active organic polymer film roughly 200 nanometers thick and attached nitroxide radical groups function like charge carriers.

The high radical density induces the battery a high charge/discharge capacity. "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. And it has a long cycle life, often exceeding 1,000 cycles", said Dr Hiroyuki Nishide.

The thin polymer film was made by a solution-processable method: a soluble polymer with the radical groups bound is "spin-coated" onto a surface. After exposure to UV irradiation, the polymer is crosslinked using a bisazide crosslinking agent.

A drawback of some organic radical polymers is the fact they are soluble in the electrolyte solution which results in self-discharging of the battery - but the polymer must be soluble so it can be spin-coated.

However, the photocrosslinking method used by the Japanese team overcomes the problem and makes the polymer mechanically tough. "This has been a challenging step, since most crosslinking reactions are sensitive to the nitroxide radical", Nishide said. "The plastic battery plays a part in ensuring that organic device technologies can function in thin film and flexible form as a complete package", said Professor Peter Skabara, an expert in electroactive materials at the University of Strathclyde.

Within three years, the Japanese team hopes the organic radical battery could be employed in pocket-sized integrated circuit cards, for memory storage and microprocessing. "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," he added.