There is a need for a wide variety of battery technologies in laminar, preferably printed form. Product developers also need to be able to co-deposit batteries with other printed components in order to reduce cost, footprint and failure modes. Japanese scientists have recently made a paper-like, polymer-based rechargeable battery.
The battery was 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 that act as charge carriers.
High discharge capacity
Nishide says, "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 1000 cycles."
It has this 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.
His 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.
Some organic radical polymers suffer from 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 and his team overcomes this problem and makes the polymer mechanically tough. This has been a challenging step 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. However, these materials will not give slow energy release, which limits them to capacitor applications. The polymers can undergo rapid charging/discharging which will be useful in delivering burst power."
Nishide sees the organic radical battery being used in smart cards and for memory storage and microprocessing, within the next three years. However, IDTechEx notes that up market smart cards have largely been commercial failures for thirty years and medical disposables, toys and merchandising may be better outlets.
"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.
There have been several other developments of new battery technologies over the last few years but few seem to be approaching commercialization. In 2005, The first urine-powered paper battery has been created by physicists in Singapore. The credit-card sized unit could be a useful power source for cheap healthcare test kits for diseases like diabetes, and could even be used in emergency situations to power a cellphone, they said. Testing urine can reveal the identity of illnesses, and the new paper battery could allow the sample being tested to also power the diagnostic device.
"We are striving to develop cheap, disposable credit-card sized biochips for disease detection," says Ki Bang Lee, at the Institute of Bioengineering and Nanotechnology in Singapore. "Our battery can be easily integrated into such devices, supplying electricity on contact with biofluids such as urine or blood."
Current biochips need an external reader such as a laser scanner or an external source of power, such as conventional batteries, to perform diagnostic tests. Lee's technology houses both the sensors and the battery on one plastic chip. This design circumvents many of the problems encountered by researchers attempting to shrink down more conventional battery systems for use with bioMEMS - devices utilising bio-microelectromechanical systems.
"Many researchers have tried to design power sources or batteries for systems or bioMEMS devices. However, they have done this by reducing the size of conventional, bulky power systems or batteries," explains Lee. "They faced a lot of problems," including difficulties in getting sufficient electrical energy, he added.
The urine-powered battery was able to generate a voltage of about 1.5 volts - with a corresponding power of 1.5 micro-watts - using just 0.2 millilitres of urine, said Lee. And if a second droplet of urine was added 15 hours after the battery was first activated, the replenished urine could generate still more electricity.
Suitable for disposable devices?
The battery is currently suited for use with disposable devices - it is not yet ready to power laptops or iPods. "But if, for example, we place a small cellular phone or transmitter on a plastic card, the chip will work as a disposable biofluid-activated means of communication in an emergency," Lee declared. "In this case, the size will be less than that of a credit card."
The battery is made of a layer of filter paper steeped in copper chloride, sandwiched between strips of magnesium and copper. This "sandwich" is then laminated in plastic to hold the whole package together. The resulting battery is just 1 millimeter thick and 60 by 30 mm across - slightly smaller than a credit card.
To activate the battery, a drop of urine is added and soaks through the sandwiched filter paper. The chemicals dissolve and react to produce electricity. The magnesium layer acts as the anode, losing its electrons. And the copper chloride acts as the cathode, mopping up the electrons. He added that the voltage, current and capacity of the battery could be improved by different designs or by switching the electrode or electrolyte materials used.
He believes the system could be used in home-based health test kits. "The long-term goal is for people to be able to buy disposable biochips for a disease test from any pharmacy," he says.