Power/Performance Bits: Nov. 3

Lithium-air batteries gain ground

Scientists at the University of Cambridge have developed a working laboratory demonstration of a lithium-oxygen battery which has very high energy density, is more than 90% efficient, and can be recharged more than 2000 times.

Their demonstrator relies on a highly porous, ‘fluffy’ carbon electrode made from graphene (comprising one-atom-thick sheets of carbon atoms), and additives that alter the chemical reactions at work in the battery, making it more stable and more efficient. While the results are promising, the researchers caution that a practical lithium-air battery still remains at least a decade away.

What the team has developed uses a very different chemistry than earlier attempts at a non-aqueous lithium-air battery, relying on lithium hydroxide (LiOH) instead of lithium peroxide (Li2O2). With the addition of water and the use of lithium iodide as a mediator, their battery showed far less of the chemical reactions which can cause cells to die, making it far more stable after multiple charge and discharge cycles.

False-color microscopic view of a reduced graphene oxide electrode (black, center), which hosts the large (on the order of 20 micrometers) lithium hydroxide particles (pink) that form when a lithium-oxygen battery discharges. Lithium iodide aids in the removal of the particles upon charging, resulting in an efficient and highly energy-dense battery with potential applications in portable electronics, transportation, and grid storage. (Source: Valerie Altounian/Science)

By precisely engineering the structure of the electrode, changing it to a highly porous form of graphene, adding lithium iodide, and changing the chemical makeup of the electrolyte, the researchers were able to reduce the ‘voltage gap’ between charge and discharge to 0.2 volts. A small voltage gap equals a more efficient battery – previous versions of a lithium-air battery have only managed to get the gap down to 0.5 – 1.0 volts, whereas 0.2 volts is closer to that of a Li-ion battery, and equates to an energy efficiency of 93%.

The highly porous graphene electrode also greatly increased the capacity, although only at certain rates of charge and discharge. Other issues that still have to be addressed include finding a way to protect the metal electrode so that it doesn’t form dendrites, which can cause batteries to explode if they grow too much and short-circuit the battery.

“While there are still plenty of fundamental studies that remain to be done, to iron out some of the mechanistic details, the current results are extremely exciting,” said Clare Grey, professor at Cambridge’s Department of Chemistry. “We are still very much at the development stage, but we’ve shown that there are solutions to some of the tough problems associated with this technology.”

Growing graphene nanoribbons

A research team from the University of Wisconsin at Madison (UW) and the U.S. Department of Energy’s Argonne National Laboratory has confirmed a new way to control the growth paths of graphene nanoribbons on the surface of a germainum crystal.

UW researchers used chemical vapor deposition to grow graphene nanoribbons on germanium crystals. This technique flows a mixture of methane, hydrogen and argon gases into a tube furnace. At high temperatures, methane decomposes into carbon atoms that settle onto the germanium’s surface to form a uniform graphene sheet. By adjusting the chamber’s settings, the UW team was able to exert very precise control over the material.

“What we’ve discovered is that when graphene grows on germanium, it naturally forms nanoribbons with these very smooth, armchair edges,” said Michael Arnold, an associate professor of materials science and engineering at UW-Madison. “The widths can be very, very narrow and the lengths of the ribbons can be very long, so all the desirable features we want in graphene nanoribbons are happening automatically with this technique.”

Researchers at Argonne’s Center for Nanoscale Materials have confirmed the growth of self-directed graphene nanoribbons on the surface of the semiconducting material germanium by researchers at the University of Wisconsin at Madison. (Source: Nathan Guisinger et al.)

For use in electronic devices, the semiconductor industry is primarily interested in three faces of a germanium crystal. Depicting these faces in terms of coordinates (X,Y,Z), where single atoms connect to each other in a diamond-like grid structure, each face of a crystal (1,1,1) will have axes that differ from one (1,1,0) to the other (1,0,0).

Previous research shows that graphene sheets can grow on germanium crystal faces (1,1,1) and (1,1,0). However, this is the first time any study has recorded the growth of graphene nanoribbons on the (1,0,0) face.

As their investigations continue, researchers can now focus their efforts on exactly why self-directed graphene nanoribbons grow on the (1,0,0) face and determine if there is any unique interaction between the germanium and graphene that may play a role.

Graphene optical detector

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), working with colleagues from the USA and Germany, have developed a new optical detector from graphene which reacts very rapidly to incident light of all different wavelengths and even works at room temperature. It is the first time that a single detector has been able to monitor the spectral range from visible light to infrared radiation and right through to terahertz radiation. The HZDR scientists are already using the new graphene detector for the exact synchronization of laser systems.

This comparatively simple and inexpensive construct can cover the enormous spectral range from visible light all the way to terahertz radiation. “In contrast to other semiconductors like silicon or gallium arsenide, graphene can pick up light with a very large range of photon energies and convert it into electric signals. We only needed a broadband antenna and the right substrate to create the ideal conditions,” said Dr. Stephan Winnerl, physicist at the Institute of Ion Beam Physics and Materials Research at the HZDR.

The external antenna on the detector captures long-wave infrared and terahertz radiation and funnels it to a graphene flake which is located in the center of the structure on a silicon carbide substrate. (Source: M. Mittendorff)

How it works: the graphene flake and antenna assembly absorbs the rays, thereby transferring the energy of the photons to the electrons in the graphene. These “hot electrons” increase the electrical resistance of the detector and generate rapid electric signals. The detector can register incident light in just 40 picoseconds.

The choice of substrate has now proved a pivotal step in improving the light trap. “Semiconductor substrates used in the past have always absorbed some wavelengths but silicon carbide remains passive in the spectral range,” explained Stephan Winnerl. Then there is also an antenna which acts like a funnel and captures long-wave infrared and terahertz radiation. The scientists have therefore been able to increase the spectral range by a factor of 90 in comparison with the previous model, making the shortest detectable wavelength 1000 times smaller than the longest. By way of comparison, red light, which has the longest wavelength visible to the human eye, is only twice as long as violet light which has the shortest wavelength on the visible spectrum.