Century-old battery technology given new life
Nickel-iron batteries, a rechargeable technology developed by Thomas Edison more than a century ago, have been largely out of favor since the 1970s - until now.
Designed in the early 1900s to power electric vehicles, they're today used in just a few niche applications, primarily the storage of surplus electricity from solar panels and wind turbines.
"The Edison battery is very durable, but it has a number of drawbacks," says Hongjie Dai, a professor of chemistry at Stanford. "A typical battery can take hours to charge, and the rate of discharge is also very slow."
But Dai and his colleagues have now created an ultrafast nickel-iron battery that can be fully charged in about two minutes and discharged in less than 30 seconds - nearly 1,000 times faster than before.
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.
To improve the Edison battery's performance, the Stanford team used graphene 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," says graduate student Hailiang Wang. "Instead, we 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 gives the dramatic effect on performance.
Unfortunately, the one-volt prototype 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," says Wang. "But it could assist lithium-ion batteries by giving them a real power boost for faster acceleration and regenerative braking."
It could also be useful in emergency situations, where batteries need to be charged very quickly.
There is one drawback - the ability to hold a charge over time.
"It doesn't have the charge-discharge cycling stability that we would like," says Dai. "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."