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

A radical approach to understanding polymers

Polymerization is used to make a whole range of materials but understanding exactly what happens during synthesis when it involves free radicals is difficult. Now, New Zealand chemists have uncovered important clues by following the rates of reaction and the termination steps involved.

Radical polymerization is commonly used to make half the world’s polymers and now Greg Russell and his colleagues at the University of Canterbury have investigated the kinetics of the process that shuts off polymerisation, the termination rate coefficient, for some very common reactions. They have revealed that the diffusion behaviour of short polymer strands, oligomers, in the reaction system is the critical factor.

“The majority of chemists simply try to bring about reactions by mixing different chemicals together under different conditions,” Russell explains. “However it is also important, especially for those who make chemical products on a large scale, to have precise quantitative descriptions of the speeds at which reactions occur. Chemical kinetics is the field of work that develops such descriptions. It is therefore an area where chemistry and mathematics intersect.

Greg Russell
Greg Russell

He adds that the most difficult problem in radical polymerization is the termination step that stops the polymer growing longer. Diffusion of the growing polymers is affected by their size, concentration, temperature and so on, which makes termination a complicated process to study, one that even after 60 years of intensive investigation is still not fully understood.

Russell, who is on sabbatical at the University of Goettingen, Germany, worked with Philipp Vana there and have found that although highly specialized techniques for measuring termination rate coefficients under precisely controlled conditions have been employed the results they found on attempting to replicate earlier studies were inconsistent. “I have taken this information and attempted to see whether it is consistent with systems where many different termination reactions occur at once, as is the case in commercial processes,” Russell says. “For the monomer styrene [used to make polystyrene] I find there is consistency, but for methyl methacrylate [used for polymethylmethacryalate, PMMA] there is not.”

In trying to explain this result, he eliminated most of the conventional views, and came to the conclusion that the answer lies with the oligomers in each system, which seem to have slightly different diffusional behaviour.

LINKS

Macromol. Chem. Phys. 2010
http://dx.doi.org/10:1002/macp.200900668

Greg Russell
http://www.chem.canterbury.ac.nz/people/russell.shtml

Philipp Vana
http://www.fpm.chemie.uni-goettingen.de/pvana.htm

Tubes in space

Carbon nanotubes form in space but use a metal-free chemistry until now unavailable to chemists on Earth. The discovery is a surprising outcome of laboratory experiments designed by Joseph Nuth at NASA’s Goddard Space Flight Center, in Greenbelt, Maryland, and his colleagues. They were hoping to understand how carbon atoms are recycled in stellar nurseries, the regions of space where stars and planets are born, but the finding could have applications in nanotechnology, as well as help explain some characteristics of supernovae.

Writing in the journal Astrophys J Lett, Nuth and colleagues explain how astrochemistry makes carbon nanotubes without requiring a metal catalyst. Nanotubes are produced, they say, when graphite dust particles are exposed to a mixture of carbon monoxide and hydrogen gases, conditions that exist in interstellar space.

The finding corroborates the discovery of graphite whiskers, bigger than nano nanotubes, in three meteorites. The meteoric discovery hinted at why some supernovae appear dimmer and farther away than they ought to be based on calculations using current models. Nuth’s approach is a variation of a well-established way to produce gasoline or other liquid fuels from coal. It’s known as Fischer-Tropsch synthesis, and researchers suspect that it could have produced at least some of the simple carbon-based compounds in the early solar system. Nuth proposes that the nanotubes yielded by such reactions could be the key to the recycling of the carbon that gets released when carbon-rich grains are destroyed by supernova explosions.

Stellar Nursery
A stellar nursery could be home to carbon nanotube factories (Credit: NASA, http://apod.nasa.gov/apod/ap021102.html)

The structure of the carbon nanotubes produced by Nuth and colleagues was determined by materials scientist Yuki Kimura, of Tohoku University, Japan, using transmission electron microscopy. He observed particles on which the original smooth graphite gradually morphed into an unstructured region and finally to an area rich in tangled hair-like masses. A closer look with an even more powerful microscope showed that these tendrils were in fact cup-stacked carbon nanotubes, resembling a stack of disposable drinking cups with the bottoms removed. If further testing indicates that the new method is suitable for materials-science applications, it could supplement, or even replace, the familiar way of making nanotubes, explains Kimura.

Researchers might also now evaluate whether graphite whiskers absorb light. A positive result would lend credence to the proposition that the presence of these molecules in space affects the observations of some supernovae.

LINKS

Astrophys J Lett, 2010, 710, L98-L101

http://dx.doi.org/10.1088/2041-8205/710/1/L98