Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Silicon-carbon electrodes snap, swell, don't pop

15.03.2012
Nanocomposite electrodes being charged with electricity reveal performance advantages that could lead to longer-lasting, cheaper vehicle batteries
A study that examines a new type of silicon-carbon nanocomposite electrode reveals details of how they function and how repeated use could wear them down. The study also provides clues to why this material performs better than silicon alone. With an electrical capacity five times higher than conventional lithium battery electrodes, silicon-carbon nanocomposite electrodes could lead to longer-lasting, cheaper rechargeable batteries for electric vehicles.

Published online in the journal Nano Letters last week, the study includes videos of the electrodes being charged at nanometer-scale resolution. Watching them in use can help researchers understand the strengths and weaknesses of the material.

"The electrodes expand as they get charged, and that shortens the lifespan of the battery," said lead researcher Chongmin Wang at the Department of Energy's Pacific Northwest National Laboratory. "We want to learn how to improve their lifespan, because silicon-carbon nanofiber electrodes have great potential for rechargeable batteries."

Plus & Minus

Silicon has both advantages and disadvantages for use as a battery material. It has a high capacity for energy storage, so it can take on a hefty charge. Silicon's problem, though, is that it swells up when charged, expanding up to 3 times its discharged size. If silicon electrodes are packed tightly into a battery, this expansion can cause the batteries to burst. Some researchers are exploring nano-sized electrodes that perform better in such tight confines.

A multi-institution group led by PNNL's Wang decided to test nano-sized electrodes consisting of carbon nanofibers coated with silicon. The carbon's high conductivity, which lets electricity flow, nicely complements silicon's high capacity, which stores it.

Researchers at DOE's Oak Ridge National Laboratory in Oak Ridge, Tenn., Applied Sciences Inc. in Cedarville, Ohio, and General Motors Global R&D Center in Warren, Mich. created carbon nanofibers with a thin layer of silicon wrapped around. They provided the electrodes to the team at PNNL to probe their behavior while functioning.

First, Wang and colleagues tested how much lithium the electrodes could hold and how long they lasted by putting them in a small testing battery called a half-cell. After 100 charge-discharge cycles, the electrodes still maintained a very good capacity of about 1000 milliAmp-hours per gram of material, five to 10 times the capacity of conventional electrodes in lithium ion batteries.

Although they performed well, the team suspected that the expansion and contraction of the silicon could be a problem for the battery's longevity, since stretching tends to wear things out. To determine how well the electrodes weather the repeated stretching, Wang popped a specially designed, tiny battery into a transmission electron microscope, which can view objects nanometers wide, in DOE's EMSL, the Environmental Molecular Sciences Laboratory on the PNNL campus.

They zoomed in on the tiny battery's electrode using a new microscrope that was funded by the Recovery Act. This microscope allowed the team to study the electrode in use, and they took images and video while the tiny battery was being charged and discharged.

Not Crystal Glass

Previous work has shown that charging causes lithium ions to flow into the silicon. In this study, the lithium ions flowed into the silicon layer along the length of the carbon nanofiber at a rate of about 130 nanometers per second. This is about 60 times faster than silicon alone, suggesting that the underlying carbon improves silicon's charging speed.

As expected, the silicon layer swelled up about 300 percent as the lithium entered. However, the combination of the carbon support and the silicon's unstructured quality allowed it to swell evenly. This compares favorably to silicon alone, which swells unevenly, causing imperfections.
In addition to swelling, lithium is known to cause other changes to the silicon. The combination of lithium and silicon initially form an unstructured, glassy layer. Then, when the lithium to silicon ratio hits 15 to 4, the glassy layer quickly crystallizes, as previous work by other researchers has shown.

Wang and colleagues examined the crystallization process in the microscope to better understand it. In the microscope video, they could see the crystallization advance as the lithium filled in the silicon and reached the 15 to 4 ratio.

They found that this crystallization is different from the classic way that many substances crystallize, which builds from a starting point. Rather, the lithium and silicon layer snapped into a crystal all at once when the ratio hit precisely 15 to 4. Computational analyses of this crystallization verified its snappy nature, a type of crystallization known as congruent phase transition.

But the crystallization wasn't permanent. Upon discharging, the team found that the crystal layer became glassy again, as the concentration of lithium dropped on its way out of the silicon.

To determine if repeated use left its mark on the electrode, the team charged and discharged the tiny battery 4 times. Comparing the same region of the electrode between the first and fourth charging, the team saw the surface become rough, similar to a road with potholes.

The surface changes were likely due to lithium ions leaving a bit of damage in their wake upon discharging, said Wang. "We can see the electrode's surface go from smooth to rough as we charge and discharge it. We think as it cycles, small defects occur, and the defects accumulate."

But the fact that the silicon layer is very thin makes it more durable than thicker silicon. In thick silicon, the holes that lithium ions leave behind can come together to form large cavities. "In the current design, because the silicon is so thin, you don't get bigger cavities, just like little gas bubbles in shallow water come up to the surface. If the water is deep, the bubbles come together and form bigger bubbles."

In future work, researchers hope to explore the thickness of the silicon layer and how well it bonds with the underlying carbon to optimize the performance and lifetime of the electrodes.

MORE VIDEO: Late in this video, reflections change when the lithium-silicon crystallizes in the left-hand screen and dots flicker in the X-ray diffraction in the right-hand screen.

Reference: Chong-Min Wang, Xiaolin Li, Zhiguo Wang, Wu Xu, Jun Liu, Fei Gao, Libor Kovarik, Ji-Guang Zhang, Jane Howe, David J. Burton, Zhongyi Liu, Xingcheng Xiao, Suntharampillai Thevuthasan, and Donald R. Baer, 2012. In situ TEM investigation of congruent phase transition and structural evolution of nanostructured silicon/carbon anode for lithium ion batteries, Nano Letters March 2, doi: 10.1021/nl204559u. (http://pubs.acs.org/doi/full/10.1021/nl204559u)

Mary Beckman | EurekAlert!
Further information:
http://www.pnnl.gov

More articles from Power and Electrical Engineering:

nachricht Stretchable biofuel cells extract energy from sweat to power wearable devices
22.08.2017 | University of California - San Diego

nachricht Laser sensor LAH-G1 - optical distance sensors with measurement value display
15.08.2017 | WayCon Positionsmesstechnik GmbH

All articles from Power and Electrical Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Fizzy soda water could be key to clean manufacture of flat wonder material: Graphene

Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.

As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...

Im Focus: Exotic quantum states made from light: Physicists create optical “wells” for a super-photon

Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.

Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...

Im Focus: Circular RNA linked to brain function

For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.

While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...

Im Focus: RAVAN CubeSat measures Earth's outgoing energy

An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.

The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...

Im Focus: Scientists shine new light on the “other high temperature superconductor”

A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.

Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Call for Papers – ICNFT 2018, 5th International Conference on New Forming Technology

16.08.2017 | Event News

Sustainability is the business model of tomorrow

04.08.2017 | Event News

Clash of Realities 2017: Registration now open. International Conference at TH Köln

26.07.2017 | Event News

 
Latest News

What the world's tiniest 'monster truck' reveals

23.08.2017 | Life Sciences

Treating arthritis with algae

23.08.2017 | Life Sciences

Witnessing turbulent motion in the atmosphere of a distant star

23.08.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>