Professor Paul Braun's research group has created a 3-D nanostructure for battery cathodes that allows for super-fast charging and discharging without sacrificing energy storage capacity.
"This system that we have gives you capacitor-like power with battery-like energy," said Braun, a professor of materials science and engineering.
"Most capacitors store very little energy. They can release it very fast, but they can't hold much," Braun explained. "Most batteries store a reasonably large amount of energy, but they can't provide or receive energy rapidly. This does both."
The performance of typical lithium-ion or nickel metal hydride rechargeable batteries degrades when they are rapidly charged or discharged.
Paul Braun, professor of materials science and engineering, center, graduate student Xindi Yu, left, and postdoctoral researcher Huigang Zhang developed a 3-D nanostructure for battery cathodes. (Photo by Brian Stauffer courtesy U. Illinois)
Braun's group has found that making a thin film the active material in the battery allows for very fast charging and discharging, but reduces the capacity to nearly zero because the active material lacks volume to store energy.
Then the researchers discovered that wrapping a thin film into three-dimensional structure, achieves both high capacity and large current.
This kind of performance could lead to phones that charge in seconds or laptops that charge in minutes, as well as high-power lasers and defibrillators that need only split-seconds to power up before or between pulses.
Batteries that store a lot of energy, release it fast and recharge quickly also are desirable for lasers and military applications.
Braun sees electric vehicles as natural application for the new batteries because short battery life and long recharging times are limitations of today's electric vehicles.
"If you had the ability to charge rapidly, instead of taking hours to charge the vehicle you could potentially have vehicles that would charge in similar times as needed to refuel a car with gasoline," Braun said. "If you had five-minute charge capability, you would think of this the same way you do an internal combustion engine. You would just pull up to a charging station and fill up."
The key to the group's novel 3-D battery structure is self-assembly of the nano-scale spheres incorporated into the thin film.
The researchers begin by coating a surface with the nano-spheres, packing them tightly together to form a lattice. Trying to create such a uniform lattice by other means is time-consuming and impractical, but the inexpensive spheres settle into place automatically.
Then the researchers fill the spaces around the spheres with metal. The spheres are melted or dissolved, leaving a porous 3-D metal scaffolding. Electropolishing etches away the scaffold surface to make an open framework. This frame is then coated with a thin film of the active material.
Braun explains that the result is a bicontinuous electrode structure with small interconnects, so the lithium ions can move rapidly; a thin-film active material, so the diffusion kinetics are rapid; and a metal framework with good electrical conductivity.
The group demonstrated both lithium-ion and nickel metal hydride batteries, but the structure is general, so any battery material that can be deposited on the metal frame could be used.
All the processes are also used at large scales in industry, so Braun says the technique could be scaled up for manufacturing.
"We like that it's very universal, so if someone comes up with a better battery chemistry, this concept applies," said Braun, who is also affiliated with the Materials Research Laboratory and the Beckman Institute for Advanced Science and Technology at Illinois.
"This is not linked to one very specific kind of battery, but rather it's a new paradigm in thinking about a battery in three dimensions for enhancing properties."
The U.S. Army Research Laboratory and the Department of Energy supported this work. Visiting scholar Huigang Zhang and former graduate student Xindi Yu were co-authors of the paper.
The research is published in the March 20 advance online edition of the journal "Nature Nanotechnology."
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