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Thursday, June 28, 2012

Edison's battery goes ultrafast, charges in 2 mins

Thomas Edison designed the rechargeable nickel-iron battery in the early 1900s to power electric vehicles. It was used in electric cars until about 1920. The battery's long life and reliability made it a popular backup power source for railroads, mines and other industries until the mid-20th century. Scientists at Stanford University said they have dramatically improved the performance of Edison's nickel-iron battery.

"The Edison battery is very durable, but it has a number of drawbacks," said Hongjie Dai, professor of chemistry. "A typical battery can take hours to charge, and the rate of discharge is also very slow."

The Stanford team has created an ultrafast nickel-iron battery that can be fully charged in about two minutes and discharged in less than 30 seconds. They managed to increase the charging and discharging rate by nearly 1,000 times.

The high-performance, low-cost battery could someday be used to help power electric vehicles, much as Edison originally intended, Dai said.

Edison created the nickel-iron battery as an inexpensive alternative to corrosive lead-acid batteries. Its basic design consists of two electrodes – a cathode made of nickel and an anode made of iron – bathed in an alkaline solution.

"Importantly, both nickel and iron are abundant elements on Earth and relatively nontoxic."

Carbon has long been used to enhance electrical conductivity in electrodes. To improve the Edison battery's performance, the researchers used graphene – nanosized sheets of carbon that are only 1-atom thick – and multi-walled carbon nanotubes, each consisting of about 10 concentric graphene sheets rolled together.

In conventional electrodes, people randomly mix iron and nickel materials with conductive carbon. Instead, the team grew nanocrystals of iron oxide onto graphene, and nanocrystals of nickel hydroxide onto carbon nanotubes.

This technique produced strong chemical bonding between the metal particles and the carbon nanomaterials, which had a dramatic effect on performance, the researchers said.

"Coupling the nickel and iron particles to the carbon substrate allows electrical charges to move quickly between the electrodes and the outside circuit. The result is an ultrafast version of the nickel-iron battery that's capable of charging and discharging in seconds."

The 1-Volt prototype battery developed in Dai's lab has just enough power to operate a flashlight. Now, the researchers aim to make a bigger battery that could be used for the electrical grid or transportation.

Most electric cars, such as the Nissan Leaf and the Chevy Volt, run on lithium-ion batteries, which can store a lot of energy but typically take hours to charge.

"Our battery probably won't be able to power an electric car by itself because the energy density is not ideal," Graduate student Hailiang Wang said. "But it could assist lithium-ion batteries by giving them a real power boost for faster acceleration and regenerative braking."

The enhanced Edison battery might be especially useful in emergency situations. There may be applications for the military, for example, where you have to charge something very quickly.

The prototype battery has one key drawback – the ability to hold a charge over time. "It doesn't have the charge-discharge cycling stability that we would like," Dai said. "Right now it decays by about 20 per cent over 800 cycles. That's about the same as a lithium-ion battery. But our battery is really fast, so we'd be using it more often. Ideally, we don't want it to decay at all."

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