New battery-boosting recipe

A common problem with portable electronic devices is that their
rechargeable lithium batteries deliver power for only a short time, and
lose their ability to be fully recharged as the battery gets older. Now,
Italian chemists have added tin and sulfur to the rechargeable recipe to
overcome these problems in next generation batteries.

Bruno Scrosati and Jusef Hassoun of the University of Rome point out
that theoretically at least, lithium-sulfur batteries would be the
energy source of choice. They would, after all, have much higher energy
density than lithium-ion batteries. However, the electrodes in such
batteries slowly dissolve and the lithium metal forms branching, or
dendritic, deposits that lead to electrical short circuits. Commercial
“lithium” batteries contain graphite through which lithium ions can
diffuse, which means no disintegrating lithium metal electrodes, but
lower energy density.

Comparison of battery energies (Credit: Wiley)

The Italian team has combined the advantages of lithium metal with the
longevity of lithium ion by developing a new type of lithium-metal-free
cell that has a negative electrode composed of a carbon/lithium sulfide
composite. The battery’s charge carrying electrolyte solution is
replaced by a lithium-ion-containing liquid enclosed in a polymer gel
membrane. The polymer protects the electrodes from corrosion. For the
anode (positive electrode), Scrosati and Hassoun chose nanoscopic tin
particles embedded in a protective carbon matrix.

The electricity generation process involves the lithium sulfide cathode
splitting into elemental sulfur and lithium ions, which releases
electrons. The lithium ions migrate through the electrolyte membrane to
the anode, where they take up electrons to become uncharged lithium
atoms, which are then bound into an alloy by the tin nanoparticles. The
process is reversible by applying a current (from the mains supply) in
the opposite direction, so that the battery can be charged repeatedly.
The new battery surpasses all previous attempts at a lithium-metal-free
battery with its specific energy of about 1100 Watt-hours per kilogram.
Such a high energy could not only be useful for portable music players
and mobile phones but is substantial enough for electric vehicles.

Links

Angew Chem Int Edn, 2010, in press
Scrosati

Non-carbon nanotubes

Carbon nanotubes rose to prominence on the back of the buckyball chemistry revolution in the 1990s and are now emerging from prototype applications across academic and industrial laboratories. They have potential in microelectronic circuits, novel sensor devices, special light conductors, and light-emitting nanotubes for display technology.

Indeed, applications as diverse as medical technology, for fibres with ultrahigh tensile strength, in hydrogen storage, for rechargeable batteries, in catalysis, and in nanotechnology are being developed. There are even applications for antifouling coatings for ships.

With this in mind, chemists in Germany who work with inorganic materials have now developed an approach to synthesising tin sulfide nanotubes which could expand the nanotube concept much further still and open up yet more avenues for applications. After all, carbon does not have a monopoly on nanotubes. Early in the development of tubular fullerene structures, inorganic chemists opted to make their nanotubes from metals and non-carbon atoms: tungsten sulfide, nickel chloride, vanadium sulfide, titanium sulfide, and indium sulfide. Many others have been produced.

Wolfgang Tremel and colleagues, Aswani Yella, Martin Panthoefer, Helen Annal Therese, Enrico Mugnaioli, Ute Kolb, of the Johannes Gutenberg-Universitaet in Mainz, Germany, have now developed a new process for the production of tin sulfide nanotubes, which they report in the Wiley journal Angewandte Chemie, the researchers found they could “grow” tin sulfide nanotubes from a drop of metal using a bismuth catalyst.

The team were faced with one of the fundamental problems of synthesising sulfidic nanotubes in that they require a high temperature to force the planar layers of material to bend and fuse into tubular structures. For tin sulfide, the situation is complicated still further by an unstable intermediate that is almost impossible to trap because it decomposes at a lower temperature.

The researchers used a different approach. First, they employed a vapour-liquid-solid (VLS) process, a technique borrowed from semiconductor scientists for producing nanowires as opposed to nanotubes. The process involved mixing bismuth metal powder with minute flakes of tin sulfide and heating this mixture in a tube furnace under a stream of the relatively unreactive noble gas argon. The product of the reaction forms a deposit at the cooler end of the tube.

The team explains that tiny droplets on the nanometre scale are form within the oven. These nanodroplets act as local points of contact for the tin so that the reactants become concentrated within the metal droplet and nanotubes can then grow from these seeds.

“In this process, the metal drop is obtained as a sphere at the end of the tube, and the nanotubes grow out of the sphere like a hair out of a follicle,” explains Tremel. “Catalysis by the metal droplet makes growth possible at low temperatures.”

The team has successfully grown nanotubes comprising multiple layers of tin sulfide with few defects. The nanotubes have diameters of between 30 and 40 nanometres and are 100 to 500 nm in length.

Tin sulfide nanotubes grow from droplets (Credit: Tremel et al/Wiley-VCH)

Angewandte Chemie, in press

Group of Prof. Dr. Wolfgang Tremel

Chips are down

Graphene is a modified form of the all-carbon pencil lead material graphite and is being touted as the material of choice for a future generation of computer chips to augment, or even usurp, silicon. Now, three research teams have devised new approaches to handling graphene that could accelerate development of this material.

Carbon has several allotropes – same element, different forms. Graphite is the stuff of pencil lead and exists as layer upon layer of hexagonally patterned chicken wire type sheets with a carbon at each vertex. Diamond is the hardest known materials and exists as a robust tetrahedrally bonded network of carbon atoms. Fullerenes and nanotubes are tiny spheres, spheroids, and tubes. Amorphous carbon, which has a mixture of the trivalent and tetravalent bonded carbon atoms. Graphene is akin to single layers of graphite.

Nitin P. Padture

Nitin P. Padture

Andre Geim and colleagues at The University of Manchester and colleagues focused on experimental measurements of the intriguing electronic properties of graphene after theoreticians had predicted them. Now, research teams around the globe are further investigating this intriguing substance. However, processing graphene is not without limitations. As such, various efforts have focused on ways to simplify the handling of the material.

Rod Ruoff and his colleagues at the University of Texas at Austin have found a way to disperse chemically modified graphene in a wide variety of organic solvents. This could open the door to developing graphene in conductive films, polymer composites, ultracapacitors, batteries, paints, inks and plastic electronics, the team says.

Rod Ruoff

Rod Ruoff

By using ‘solubility parameters’ ubiquitously applied by industry to determine the solvents most likely to dissolve certain materials or to create good colloids, we have developed a set of solubility parameters for chemically modified graphenes, explains Ruoff. We believe that this approach will have exceptional utility for technology transition in use of colloidal suspensions of graphene sheets.

Tomás Palacios

Tomás Palacios

In parallel, but unconnected work, a team at Ohio State University, led by Nitin Padture are developing a technique for mass producing computer chips made from graphene. Graphene has huge potential, it’s been dubbed the new silicon, says Padture, but there hasn’t been a good process for high-throughput manufacturing it into chips.

Graphene’s chickenwire structure

Graphene’s chickenwire structure

He and his colleagues have found a way to mesh the graphene fabrication process with standard microelectronics manufacturing methods. In their first series of experiments, the team stamped high-definition features just ten graphene layers thick on to a silicon oxide substrate, making this a potential mass-production method.

A graphene frequency multiplier (Photo by Donna Coveney)

A graphene frequency multiplier (Photo by Donna Coveney)

In other work to be published in the May issue of Electron Device Letters, MIT researchers, led by Tomás Palacios, have built an experimental graphene chip known as a frequency multiplier. Frequency multipliers are widely used in telecommunications and computing applications. However, current technology suffers from noise interference that requires energy-intensive filtering. The graphene frequency multiplier system has but a single transistor and so, these researchers say, efficiently produces a very clean output that needs no filtering.

Further reading

Nano. Lett., 2009, in press
http://dx.doi.org/10.1021/nl803798y

Adv Mater, 2009, 21, 1243-1246
http://dx.doi.org/10.1002/adma.200802417

Nanoscience and Technology Lab
http://bucky-central.me.utexas.edu/

Nitin P. Padture homepage
http://www.matsceng.ohio-state.edu/faculty/padture/padturewebpage/

Tomás Palacios homepage
http://web.mit.edu/tpalacios/

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