Imagine being able to charge your cell phone in a matter of seconds or your laptop in a few minutes. That might soon be possible, thanks to a new kind of nanostructured battery electrode developed by scientists at the University of Illinois, Urbana-Champaign. The researchers found that their electrode can charge and discharge up to 100 times faster than existing devices while holding the same amount of energy.
High-storage batteries that could charge and discharge quickly might make a number of still-marginal technologies much more attractive. For example, if you could recharge an electric car in minutes rather than hours, filling up your battery at a charging station would take no longer than the amount of time it takes to buy a tank of gas. And batteries that gave up their stored energy quickly could mean uninterrupted solar power, pitching in when the sun goes behind a cloud and solar cells stop producing.
Electrical storage devices known as capacitors charge and discharge very quickly, but because they hold their charge on the surface of metal plates, their storage capacity is limited. Batteries, on the other hand, can store much more energy because they hold their charge inside the bulk of a material, usually an oxide or a phosphate compound located inside the cathode. Unfortunately, because these materials are not good conductors of electric charge, it takes a long time to get the charge in or out of a battery.
Scientists have tried to get around this problem in the past by adding electrical conductors to the energy-storage material, but this reduces the material’s volume, so it tends to lower the battery’s capacity. University of Illinois materials scientist Paul Braun and his colleagues came up with a novel solution, published online this month in Nature Nanotechnology. They built a tiny metal lattice with so many nooks and crannies that when it’s filled with a charge-storage material, the electrons in the material never have far to go before reaching the metal and being conducted away.
The Illinois researchers made their electrode by coating a surface with tiny polystyrene spheres just a few hundred nanometers across, packing the spheres together into a regular lattice structure. Then they filled the gaps between the spheres with nickel and dissolved the polystyrene. That left a three-dimensional metal scaffold, which they thinned down so that the metal made up just 6% of the total volume, and they then coated a thin film of the storage material onto the scaffold. The team used this technique to make cathodes for lithium-ion and nickel-metal hydride batteries, with the former used in many consumer electronics devices such as laptops and cell phones whereas the latter are used in many electric vehicles.
Braun and his co-workers found that the lithium-ion battery could charge and discharge between 10 and 100 times more quickly than the fastest devices on the market today. Its storage capacity was actually slightly larger than normal (by about 10% to 20%). And because every step of their manufacturing process is used in industry today, the researchers say there should be no major problems with incorporating their cathodes into commercial batteries. They just need to show how to scale up their technology—so far they’ve tried it only on watch-size batteries. “I have every reason to believe we can scale up,” says Braun, “and we are looking to partner with the right people to do that.”
However, materials scientist Yury Gogotsi of Drexel University in Philadelphia, Pennsylvania, sounds a note of caution. Although he says the latest work shows the importance of a smart electrode design that uses common materials, he questions how long the batteries can withstand continued charging and recharging over such short intervals. He is also less sanguine about scaling up. "Braun's group provides an advantage in rate performance," he says. "The next step will be to find a simpler way of making three-dimensional electrodes."