The prototyped rechargeable battery (photo courtesy of NEC)
The prototyped rechargeable battery (photo courtesy of NEC)
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NEC Corp announced that it has prototyped an "organic radical battery" with a thickness of 0.3mm.

An organic radical battery is a battery that uses an organic radical compound for its electrode active material. A gel-like flexible electrode is formed by impregnating a composite positive electrode consisting of an organic radical compound and carbon fibers with electrolyte. Charging and discharging are done by oxidation-reduction reaction at the radical part.

Though organic radical batteries are expected to be applied to smart cards, electronic paper devices, etc, it has been difficult to reduce their thicknesses to less than 0.7mm, making it difficult to apply them to 0.76mm-thick standard-size smart cards, NEC said.

This time, NEC reduced thickness by 50% or more by using printing technologies to integrate a circuit board and a battery. As a result, it became possible to add a display, a data communication function, an advanced encryption function and other functions that consume a large amount of power to smart cards, the company said.

NEC used a 0.05mm-thick polymer film for the package of the battery, and the film can also be used as a circuit board. And it realized the thickness of 0.3mm by directly forming negative electrodes by using printing technologies on the circuit board and forming separators and radical polymer positive electrodes on the negative electrodes. The aluminum layer that is used for the package of the company's previous battery is 0.2mm in thickness.

On the circuit board, small electronic components can be mounted, too. With a circuit board having a battery, antenna and chip, it is possible to make a standard-size smart card equipped with a battery, NEC said.

NEC prototyped a 3 x 3cm battery with a thickness of 0.3mm and a capacity of 3mAh and confirmed that the battery has an output density of 5kW/L. When fully charged, the battery can either refresh a screen 2,000 times, set off a flash 360 times in succession, transmit location information 35 times, etc.

Also, in a charge-discharge cycle test, the fully charged capacity of the battery was reduced by 25% after 500 cycles. It is equivalent to the capacities of lithium-ion (Li-ion) rechargeable batteries for mobile phones, the company said.