To improve the Edison battery's performance, the Stanford team 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," Wang explained. "Instead, we grew nanocrystals of iron oxide onto graphene, and nanocrystals of nickel hydroxide onto carbon nanotubes."
Schematic drawing of the ultra-Ni–Fe battery made from inorganic/carbon hybrid materials. A Ni(OH)2/MWNT hybrid was used as the cathode and a FeOx/graphene hybrid was used as the anode. 1 M aqueous KOH solution was used as the electrolyte. On charging, Ni(OH)2/MWNT and FeOx/graphene were converted to NiOOH/MWNT and Fe/graphene
Nature Communications - An ultrafast nickel–iron battery from strongly coupled inorganic nanoparticle/nanocarbon hybrid materials
This technique produced strong chemical bonding between the metal particles and the carbon nanomaterials, which had a dramatic effect on performance.
"Coupling the nickel and iron particles to the carbon substrate allows electrical charges to move quickly between the electrodes and the outside circuit," Dai said. "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. The researchers' goal is 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," 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, Dai added. "There may be applications for the military, for example, where you have to charge something very quickly," he said.
"It's definitely scalable," Wang said. "Nickel, iron and carbon are relatively inexpensive. And the electrolyte is just water with potassium hydroxide, which is also very cheap and safe. It won't blow up in a car."
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 percent 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."
Dai said the use of strongly coupled nanomaterials represents a very exciting approach to making electrodes.
"It's different from traditional methods, where you simply mix materials together. I think Thomas Edison would be happy to see this progress," he said.
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