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Scanning electron microscope photo of hollow carbon nanofiber-encapsulated sulfur tubes, at the heart of a new battery design. Credit: Wesley Guangyuan Zheng; Stanford

Stanford University associate professor of materials science and engineering, Yi Cui, knows his way inside and out of a carbon nanotube, and he’s using his knowledge of that terrain to design new electrodes for lithium-ion batteries and ultracapacitors. Two papers published in recent weeks in Nano Letters describe the details.

The first paper (published Sept. 14, subscription required) describes an approach to cathode design for Li-Ion batteries. Sulfur is an attractive cathode material because of its high storage capacity at relatively low voltage. It is also inexpensive, abundant and nontoxic. According to a Stanford press release, batteries with sulfur cathodes can store four to five times as much energy as existing Li-ion batteries.

Previous cathode designs coated sulfur onto porous carbon structures. However, they fail quickly during the charge-recharge cycle because intermediate lithium polysulfide compounds are in contact with the electrolyte solution and dissolve into it. Cui’s graduate student, Wesley Guangyuan Zheng, describes the problem: “[W]e don’t want a large surface area contacting the sulfur and the electrolyte, and on the other hand we want a large surface area for electrical and ionic conductivities.”

The Cui team separated the sulfur from the electrolyte by simply moving it inside the cathode.

Adapting a commercially available water filtration process, they coated the interior of CNTs with sulfur. The new design prevents polysulfides from leaking into the electrolyte solution, while still allowing easy transport of Li ion through the CNT wall. Tests showed a high specific capacity after 150 charge-discharge cycles. In the same paper, they reported improved coulumbic efficiency gained by adding LiNO₃ to the electrolyte.

Cui’s second paper (published Sept. 26) also investigates electrode efficiency, in this case MnO₂, which is a promising material for supercapacitors (also called ultracapacitors) because of its high theoretical specific capacity, low cost and nontoxicity. Although it is blessed with a high charge storage capacity, it has low electrical and ionic conductivity, so getting the charge in or out quickly is a barrier.

To improve the conductivity of the electrode at its surface, two conductive coatings were investigated: carbon nanotubes and a conductive polymer. Coatings were applied by dipping a graphene-MnO₂ nanostructured composite electrode into a solution of the coatings.

Both coatings increased electrode conductivity, and therefore capacitance. The specific capacitance of the CNT-coated electrode increased by 25 percent and that of the polymer-coated electrode increased by 45 percent. The paper also reports that the coated electrodes, which the authors describe as ternary composites, delivered superior cycling performance, retaining over 95 percent of their capacitance after more than 3000 cycles.

In a Technology Review story, it was pointed out that the energy density of the electrode has yet to be reported.

We first reported on Cui’s simple approaches to using CNTs to make supercapacitors about two years ago.

Coincidentally, Penn State just announced that is has received $5 million from DOE to develop a battery that can provide 600 watt-hours per hour. Included will be research on developing a “nanocomposite sulfur cathode and lithium-rich composite anode material.”

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Rice University researchers, Robert Vajtai, Enrique Barrera and Yao Zhao created a conductive cable from iodine-doped nanotubes capable of carrying household current. Credit: Jeff Fitlow/Rice University

Showing how something works is more effective than telling how it works. With the assistance of a fluorescent lightbulb, Rice University researchers demonstrated successful substitution of standard copper wiring with a carbon nanotube cable.

Using double-walled CNTs spun into a cable several centimeters long, a recent Rice PhD, Yao Zhao, constructed a rig that allowed him to run electricity through a CNT cable to a fluorescent lightbulb. The lightbulb was left “on” for several days without interruption and without any sign of degradation in the CNT cable. Zhao is in Enrique Barrera’s research group. CTT recently interviewed Barrara as part of the MSE football series.

The cable was constructed of billions of double-wall CNTs and fabricated by collaborators at Tsinghua University in China. The cables were doped with iodine to increase their conductivity, and Zhao found they could be tied together without losing conductivity.

In a Rice press release, Barrera says the cables have the potential to be just as effective as metal wiring, at about 1/6 the weight. He also said that the chemical processes used to make lab-scale cables will become part of a larger process that starts with raw materials and produces a steady stream of nanocable. The next step for the team is “to make longer, thicker cables that carry higher current while keeping the wire ligtweight.”

The work was published in the Nature journal, Scientific Reports.

Meanwhile, NIST has been studying the reliability of CNTs for electronic devices with the goal of developing measurement and techniques to test fabrication quality and reliability.

Recession and clumping, in gold electrodes after NIST researchers applied 1.7 volts of electricity to the carbon nanotube wiring for an hour. NIST reliability tests may help determine whether nanotubes can replace copper wiring in next-generation electronics.Credit: M. Strus; NIST

Possibly relevant to the Rice work, NIST researchers have been studying failure in CNT networks, where electrons physically jump from one CNT to another, and found that failure seemed to happen between nanotubes, which is the point of greatest resistance. In a press release, NIST postdoctoral researcher, Mark Strus said that by monitoring the initial starting resistance and stages of degradation, it was possible to predict whether the resistance would degrade gradually or sporadically. Gradual degradation is preferred because it allows for operational limits to be set for devices. NIST has developed some electrical stress tests “that link initial resistance to degradation rate, predictability of failure and total device lifetime. The test can be used to screen for proper fabrication and reliability of nanotube networks.”

Also from NIST, a study of CNT interconnects between gold electrodes found that the CNTs carried very high current densities for awhile, but degraded under constant current. By about 40 hours, the edges of the metal electrodes receded and clumped, leading to device failure. Further NIST research is focusing on the intersections between CNT and metals, as well as between different CNTs. In th press release Strus said, “The common link is that we really need to study the interfaces.”

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A key tenet of the Materials Genome Initiative for Global Competitiveness (pdf) is using computation to reduce the time for materials development by 75%, from 20 years to 5 years. A recent press release from Fujitsu Laboratories in Japan gives an early clue about the feasibility of this approach.

Fujitsu is interested in developing materials for novel nanodevices to replace silicon large scale integration devices. The drive to shrink electronic devices is starting to run up against the physical limits of the material to be miniaturized.

Turning to computational methods, Fujitsu used a first-principles method to calculate the electrical properties of a 1,000-atom device based on carbon nanotubes and graphene electrodes. In the press release the company says the significance of this breakthrough is that “The new technology opens the door to the design of exceptionally high-speed, energy-efficient nanodevices that break totally new ground with their development.”

First-principles computation is based on the quantum mechanics of a material’s electrons and atoms, thus experimental data or empirical parameters are not needed. It is useful for simulating the properties of materials like carbon where small differences in atomic arrangement results in large property differences. Consider, for example, how different the electrical properties of charcoal, graphite and diamond are.

Electrical properties were calculated using software developed by the Japan Advanced Institute for Science and Technology and the computational power of a supercomputer at the Information Technology Center at Nagoya University. First-principles calculations are iterative and tend to need a lot of computing time and memory. Each iteration updates input values and the computation continues until the output values converge. It took about three days to calculate the electrical properties of a 1,000-atom nanodevice using about one-third of the supercomputer’s capacity.

In the press release, Fujitsu explained that they worked with JAIST to tweak the software somewhat, and that they also used a “hybrid parallel processing technique.” As a result, Fujitsu was able to include the modeling of several times more atoms than it had previously be able to do.

The nanodevice modeled is a carbon nanotube with graphene electrodes. Lithium atoms occupied the inner edges of the graphene electrodes and several hydrogen atoms bridge the atomic layer between the electrodes and the nanotube. This is a very simple system, atomically, compared to most commercial engineered materials, which often have complex compositions or atomic structures. However, the company said its success in this instance “significantly paves the way to designing novel nanodevices.”

Because the Materials Genome Initiative is aimed at elevating the United States’ national competitiveness, there is some irony of discussing the efforts of a Japanese enterprise. However, this example illustrates the type of technology—also available in the US—that the MGI intends on leveraging.

And, while Fujitsu’s work shows great promise for designing a type of nanodevice, it also demonstrates that this route to materials design requires sizeable computational investment (hardware and software). Even at the speed attained by Fujitsu, the span of potential materials compositions, crystalline structures, properties and applications make it clear that a lot of computational capacity and agility will be needed in the US.

The Fujitsu work was published in the Aug. 11 online edition of Applied Physics Express.

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Four-point conductivity measurement of the new transparent conducting film developed by Cor Koning (left) and Paul van der Schoot (right). Credit: Bart van Overbeeke, TU/e.

According to a news release, a research team at Eindhoven University of Technology (TU/e) believes it might have found a surrogate for indium tin oxide in some applications, based on exploiting the particle size and shape interplay in a colloid combination of carbon nanotubes, electrically conductive latex and polystyrene. They say that the material may provide an alternative to ITO, which might be needed if indium reserves continue to decline.

The trick, apparently, is in manipulating how the spherical latex particles and rod-like nanotubes interact. They say the interplay in the polymer matrix “introduces competing length scales that can strongly influence the formation of the system-spanning networks that are needed to produce electrically conductive composites. Interplay between the different species in the dispersions leads to synergetic or antagonistic [electrical] percolation, depending on the ease of charge transport between the various conductive components.”

The team, whose work is published in Nature Nanotechnology (doi:10.1038/nnano.2011.40), says initial attempts at using CNT-reinforced polymeric composites have led to the creation of a transparent and conductive film that mimics ITO properties. It doesn’t duplicate ITO perfectly: Films made with the current batch of composites have a conductivity much lower than ITO. However, the levels of conductivity they have achieved, for now, are good enough for some antistatic applications and shielding against electromagnetic radiation, even for flexible displays.

Moreover, the group believes it can quickly eliminate the conductivity shortfall by tinkering with the composition and increasing the amount of metallic CNTs.

The group says it started with CNTs disbursed in water. Then they added conductive polymer latex and polystyrene beads, the latter acting as a binder. Application of heat then fused the beads to form a film. Lastly, freeze-drying removes the remaining water. What was left behind was a conducting network of nanotubes and beads from the conducting latex.

Returning to the issue of increasing conductivity in the material, it seems to be mainly a problem of cost. One of the coleaders of the TU/e team, Cor Koning, blames the low conductivity on their use of less expensive “standard” CNTs, which are a mixture of metallic conducting and semiconducting tubes. “But as soon as you start to use 100 percent metallic tubes, the conductivity increases greatly. The production technology for 100 percent metallic tubes has just been developed, and we expect the price to fall rapidly,” says Koning.

Paul van der Schoot is the other coleader of the team. He describes the material as being more environmentally friendly then ITO because it avoids the use of tin.

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