Stanford Engineering Library Blog

Stanford's nanowire battery holds 10 times the charge of existing ones

BY DAN STOBER

Stanford researchers have found a way to use silicon nanowires to reinvent the rechargeable lithium-ion batteries that power laptops, iPods, video cameras, cell phones, and countless other devices.

The new version, developed through research led by Yi Cui, assistant professor of materials science and engineering, produces 10 times the amount of electricity of existing lithium-ion, known as Li-ion, batteries. A laptop that now runs on battery for two hours could operate for 20 hours, a boon to ocean-hopping business travelers.

"It's not a small improvement," Cui said. "It's a revolutionary development."

The breakthrough is described in a paper, "High-performance lithium battery anodes using silicon nanowires," published online Dec. 16 in Nature Nanotechnology, written by Cui, his graduate chemistry student Candace Chan and five others.

The greatly expanded storage capacity could make Li-ion batteries attractive to electric car manufacturers. Cui suggested that they could also be used in homes or offices to store electricity generated by rooftop solar panels.

"Given the mature infrastructure behind silicon, this new technology can be pushed to real life quickly," Cui said.

The electrical storage capacity of a Li-ion battery is limited by how much lithium can be held in the battery's anode, which is typically made of carbon. Silicon has a much higher capacity than carbon, but also has a drawback.

Silicon placed in a battery swells as it absorbs positively charged lithium atoms during charging, then shrinks during use (i.e., when playing your iPod) as the lithium is drawn out of the silicon. This expand/shrink cycle typically causes the silicon (often in the form of particles or a thin film) to pulverize, degrading the performance of the battery.

Cui's battery gets around this problem with nanotechnology. The lithium is stored in a forest of tiny silicon nanowires, each with a diameter one-thousandth the thickness of a sheet of paper. The nanowires inflate four times their normal size as they soak up lithium. But, unlike other silicon shapes, they do not fracture.

Research on silicon in batteries began three decades ago. Chan explained: "The people kind of gave up on it because the capacity wasn't high enough and the cycle life wasn't good enough. And it was just because of the shape they were using. It was just too big, and they couldn't undergo the volume changes."

Then, along came silicon nanowires. "We just kind of put them together," Chan said.

For their experiments, Chan grew the nanowires on a stainless steel substrate, providing an excellent electrical connection. "It was a fantastic moment when Candace told me it was working," Cui said.

Cui said that a patent application has been filed. He is considering formation of a company or an agreement with a battery manufacturer. Manufacturing the nanowire batteries would require "one or two different steps, but the process can certainly be scaled up," he added. "It's a well understood process."

Also contributing to the paper in Nature Nanotechnology were Halin Peng and Robert A. Huggins of Materials Science and Engineering at Stanford, Gao Liu of Lawrence Berkeley National Laboratory, and Kevin McIlwrath and Xiao Feng Zhang of the electron microscope division of Hitachi High Technologies in Pleasanton, Calif

From the Stanford Report, December 18, 2007

Two faculty members receive grants for innovative energy research

BY JOHN CANNON

Competition for federal funding is fierce, and the odds seem even slimmer for unconventional research, regardless of its potential. But two Stanford professors will receive awards to finance their work over the next year for just that kind of outside-of-the-box thinking.

"We recognize the difficulty faculty can have, particularly early in their careers, in gaining funding for high-risk, unproven projects," said Katherine "KT" Moortgat. She is a partner at Mohr Davidow Ventures, which is sponsoring the $75,000 grants. "The MDV award aims to enable new possibilities for these extraordinary faculty innovators."

Assistant Professor Yi Cui and Associate Professor Michael McGehee, both in materials science and engineering, submitted proposals detailing projects with, according to MDV, "potential to disrupt current thinking in their field or provoke new areas of research." The Menlo Park-based venture capital firm announced the four winners of its inaugural Innovators Award (the other two are from the University of California-Berkeley) on Nov. 15.

Cui has big ideas he hopes will improve rechargeable lithium ion batteries like those found in cell phones, laptops and other portable electronic devices. But at the root of his big idea is something very small: tiny silicon filaments called nanowires that efficiently transmit charge.

"We are talking about an energy density two to three times higher than current technology, or even higher," Cui said. "Basically, we want to change the fundamental mechanism of how the battery works."

Silicon nanowires have a much higher capacity for energy than traditional carbon, at least in theory, Cui said, so they should allow a battery to hold more energy.

"The current technology is great, but we want to move it one big step further," he said. "If you charge up your laptop battery, you can use it for four hours. What if you want to use if for 24 hours? Say you're taking an international flight, and you aren't able to recharge it."

Because nanowires are so small—with a diameter 1,000 times smaller than the thickness of a sheet of paper—and there are many of them present, they provide greater total surface area to contact the chemicals in a battery than would a larger, single electrode. This increase in the area of the electrode transmitting charge should result in greater power.

McGehee is attacking the energy problem from another angle. For the past seven years, he has been studying ways to make solar cells from organic materials—that is, polymers.

"The general advantage of organic electronics is you can make large area devices at low cost," said McGehee. "Solar cells are an application where you need to cover a very large area at low cost."

A laptop, for example, uses about 100 watts of power, and to collect that from the sun would require a solar cell a square meter in area—about the size of a four-person dinner table. Imagine, then, how large an area would be required to begin to contribute to the country's energy needs as a whole. Clearly, there needs to be a more economical method of producing solar cells.

McGehee is interested in technology similar to a roll-to-roll coating machine—like the ones used to print newspapers—to roll out solar cells and increase the efficiency of production.

"We're hoping to cut the costs by a factor of five to 10," McGehee said.

Several of his students plan to start up companies after graduating, so they hope to benefit from collaboration with the venture capital firm. In turn, MDV will have access to emerging technologies and potential areas for investment.

And for McGehee and Cui, of course, it's important to have the support to try something new.

"At this point, the federal government would probably not fund this project because it's not clear we're going to be able to get it to work," McGehee said of his specific plans for the grant. "But it's nice to be able to try it because this new idea will be really exciting if it does work."

John Cannon is a science-writing intern at the Stanford News Service.

From the Stanford Report, December 27, 2007

Stanford researchers develop a quantum "light switch"

BY RACHEL TOMPA

Infinitely secure cryptography that renders any computer unhackable. Computers that can solve the structure of a complicated protein at the drop of a hat. Programs to decrypt complicated enemy secrets. Optical data connections up to 100 times faster than current technology allows.

Photons and atoms hold the power to make these innovations reality; scientists just have to figure out how to unlock their potential. Now, researchers at Stanford and the University of California-Santa Barbara have developed a quantum "light switch" that could have implications for the future of certain kinds of computing.

A team of scientists led by Jelena Vuckovic, assistant professor of electrical engineering, has succeeded in directly probing a solid quantum system with light. This finding could be a milestone on the road to building a functional "quantum computer," a machine where information is coded in individual particles that flip between different states instead of in transistors switching on and off. The finding could lead to better quantum cryptography and faster optical data connections. Their study was published in the Dec. 6 issue of Nature.

"This effect has been previously demonstrated only in complicated atomic physics systems," Vuckovic wrote in an e-mail, "but ours is the first demonstration in solid state."

Previous demonstrations of the technique on atoms suspended in a gaseous state used machines that would dwarf an office desk. Vuckovic's team used solid material on a chip smaller than a thumbnail.

Scientists have been dreaming of a quantum computer for over 25 years. In such a machine, bits of information would be encoded in systems that walk to the beat of quantum mechanics—the field of physics that describes the quirky behavior of tiny atomic and subatomic particles.

Certain problems that scientists want to answer, such as predicting the way a complicated protein will fold, which might aid drug discovery, or factoring large integers into prime numbers to decrypt encoded messages, are extremely difficult to do with classical computers. In 2005, a 200-digit number was decomposed into prime numbers using multiple computers running for 18 months—scientists estimated that it would have taken one relatively speedy computer over 50 years to do the same task. A single powerful quantum computer, if it existed, could have done it in minutes.

One of the difficulties in actually creating a quantum computer comes from the fact that no one particle can do it all, said Dirk Englund, doctoral student in applied physics and one of the lead authors of the study. Photons are great for carrying information, and they are easy to move around, but they can't interact with each other. Conversely, atoms can interact, but can't easily communicate information. Scientists hope to get around this problem by using both, through something called a quantum network that would connect a series of atoms with a photonic channel. "In this approach, you're trying to exploit the best parts of both the atom and the photon," Englund said. "Communicate with the photon, interact with the atom."

But the problem of how to transfer the information between a single atom and a single photon still remains. If you just lob a photon at an atom, chances are it will miss, Englund said. So to give the photon a fighting chance at finding the atom, the scientists built a cavity of mirrors. The photon shoots into the cavity from a finely tuned laser beam and, like a pinball in a pinball machine, it ricochets around and around until it finally hits its target.

In this case, the target is an artificial atom termed a "quantum dot"—a microscopic blob of semiconductor material—nestled in a cavity inside another semiconductor. The blob confines charged particles to a tiny volume, much like an atom confines electrons in the tiny boundaries of its shell. Because of this confinement, the quantum dot behaves much as an artificial atom, including the ability to occupy different energy states that could represent the binary "ones" and "zeros" of digital information. If you think of the quantum dot like a spinning top, Englund said, "you'd call a spinning top that's upright a 'one' and a spinning top that pointed down a 'zero.'"

When the quantum dot is inside the semiconductor cavity, the cavity can be switched from transparent to opaque when the laser beam shines on it—meaning the team of researchers has succeeded in making a light switch out of just one photon and one quantum dot. The team includes study co-authors Andrei Faraon and Ilya Fushman, doctoral students in applied physics.

Previous groups had probed the quantum dot/cavity pair using indirect methods, but nobody had ever directly accessed the quantum system with photons before, Englund said. A research team from the California Institute of Technology published a study in the Dec. 6 issue of Nature that also demonstrates direct probing of a quantum system with photons, using a different system and technique.

The tiny chips used by Vuckovic's group have the advantage that they could easily be manufactured using technologies similar to those for computer chip manufacturing, Englund said.

While it will probably be a while before Vuckovic's system challenges the transistor as a new computational unit of information, it has that potential, Englund said. The next important step is to make some changes to the quantum dot to demonstrate that information can actually be transferred from the photonic channel to the dot—that is, to show that a piece of information from the photon could be relayed by changing the dot's energy state or spin direction.

Quantum dots might pave the road to the computer of the future, but that doesn't mean quantum computers will stock the shelves of your local electronics store, Englund said. Quantum information devices are most sought after because of their special applications to certain problems, such as unbreakable encryption systems and simulations of intricate molecular structures.

"In the next 20 years you might well see a quantum computer in a scientific research setting or defense," Englund said, "but you won't see Dell making one."

The paper's other authors are Nick Stoltz and Pierre Petroff of the University of California-Santa Barbara.

Funding for the study was provided by the Office of Naval Research, the Okawa Foundation, the U.S. Department of Defense, the U.S. Army, the U.S. Disruptive Technology Office, the Center for Integrated Systems at Stanford and the National Science Foundation.

Rachel Tompa is a science-writing intern at the Stanford News Service.

From the Stanford Report, December 7, 2007

Join us for Join us for Engineering 311a this quarter!
What will I do with my engineering degree? Should I get a Ph.D?
Should I stick with my Ph.D? How will I balance career and
family? Should I go into academia or industry or the public sector?

Come hear how several successful executives, researchers, and
professors have grappled with these questions through the course of
their careers.

What:
The Mechanical Engineering Graduate Women's group will be hosting a
Winter quarter seminar series ENGR 311a - Women's Perspectives.
The theme this year is "Discovering your inner geek."

The seminars will explore various life and career paths open to
women in engineering. Our hope is to have the speakers share their
personal stories and also remark on how graduate school influenced
their career and life paths. The speaker schedule is listed below
as well as on our web site: http://www.stanford.edu/group/mewomen/
seminars/SeminarW08.html
Sign up on Axess under Engineering 311a. All are welcome, even if
you do not formally register for the seminar.

When:
The seminars are on Thursdays from 4:15 - 5:05pm, between January
10th and March 13th. Cookies and tea will be served at 4pm.

Where:
Building 550, Room 550A on Stanford's campus.

Why:
Be a part of the community of graduate women in the School of
Engineering. Explore your career options with your peers and those
who are well established in a variety of career paths.

Speaker Schedule:
January 10th
Sheri Sheppard, Professor of Mechanical Engineering, Stanford
University (and faculty advisor to the ME Women's Group)
"Where we are and where we want to be"

January 17th
Ellen Spertus, Research Scientist, Google and Associate Professor
of Computer Science at Mills College (on leave)
"From male identified misogynist to sexiest geek alive: My journey
as a woman in computer science"

January 24th
Deborah Gordon, Associate Director, Preventative Defense Project,
Stanford University, former Mayor of Woodside, CA
"Why not?"

January 31st
Trae Vassallo, Parter, Kleiner Perkins Caufield and Byers
"What's really going on in greentech"

February 7th
Alissa Fitzgerald, Founder and Managing Member, A.M. Fitzgerald &
Associates
"How to empower your inner geek: Essential skills that you won't
learn in engineering classes"

February 14th
Norman Fortenberry, Director, Center for the Advancement of
Scholarship on Engineering Education, National Academy of Engineering
"Confessions of a homemade social engineer"

February 21st
Emily Ma, Project Manager and Design Engineer, IDEO
"The conscious geek"
February 28th

Maria Klawe, President, Harvey Mudd College
"Embracing geekdom in art, math, and exercise"

March 6th
Marjolein van der Meulen, Associate Professor of Mechanical
Engineering, Cornell University
"How tenure frees you to follow your passions"

March 13th
Lily Sanchez, Process Validation Engineer, Cepheid
"'This will be the worst mistake of your life!' - not so much"

Findings on potential for harvesting wind power off California’s coast presented

By John Cannon

In many ways, wind energy seems an ideal energy source. Fields of mighty turbines spinning in rhythm could harness carbonless power and shuttle it off to homes and industries. But questions remain about the feasibility of wind parks: How much will they cost? Can this unpredictable energy source be relied upon to contribute appreciably to the country's power needs?

A team of Stanford researchers set out to find answers in a recent study of the California coast and will present their research during a Dec. 13 poster session at this year's meeting of the American Geophysical Union in San Francisco. The poster is titled "California Offshore Wind Energy Potential."

Michael Dvorak, a Stanford doctoral student in civil and environmental engineering, joined Mark Jacobson, professor of civil and environmental engineering, and Cristina Archer, consulting assistant professor of civil and environmental engineering, in evaluating the potential for harvesting wind energy offshore in California.

"This is basically the first study that's a detailed look at places where we could develop offshore wind energy in California," Dvorak said. "Some of the studies have looked at the wind speeds offshore, but they hadn't looked at the [water] depth and wind speeds at this high of resolution."

Deeper water means higher costs for building wind turbines. Not only would it require more materials to reach the bottom and anchor the structures, but, as the water depth increases, so does the power of the waves constantly slamming into the turbine supports, Dvorak said.

Furthermore, most engineering research worldwide has been focused on building turbines in shallow water, like that of the North Sea in Europe, where all of the existing offshore wind parks are. Consequently, most available technology is geared toward building turbines in water less than 20 meters deep. Though wind speeds are usually higher further offshore, the study concluded it would likely be more economical to build in shallower water.

To assess wind speeds, the team employed computer models like those used by meteorologists to predict weather patterns. The researchers looked at wind speeds in 2005 and 2006 at locations along California's coast to estimate how much power could be generated annually.

Findings indicated that two of the three study areas are less than ideal for harvesting wind energy. Water depths of greater than 50 meters in the San Francisco Bay Area would require floating platforms, similar to those used for oil and gas exploration, but not yet developed for use in wind technology. In most of Southern California, the winds die down during the summer and thus would not generate a steady amount of power throughout the year.

The third study area the researchers looked at was a specific area in Northern California off Cape Mendocino. They found that a wind park at this site would supplant about 5 percent of California's electricity coming from carbon-emitting sources, Dvorak said. When combined with offshore wind energy at several other sites, it may be possible to produce between at least a quarter—and potentially all—of California's electricity.

Unfortunately, most transmission lines available to deliver power are in the southern part of the state, where winds are not as strong. But Pacific Gas and Electric Co. is looking into ocean wave-energy projects in Northern California, which also would require new transmission lines.

"There's a chance the wind and wave-energy projects could dovetail together and lower the transmission costs for both projects," Dvorak said.

A recent study authored by Archer and Jacobson and published in the November Journal of Applied Meteorology and Climatology examined ways to link wind farms to further exploit economies of scale and thereby reduce the cost of wind energy. Interconnecting multiple parks can offset the intermittent nature of wind and make it a more dependable source of energy, the authors said. And, like the wave-energy project, it would be cheaper to have an integrated set of transmission lines instead of separate connectors to each wind park.

Offshore wind farms have made headlines lately, as some residents of Cape Cod have argued that a potential Cape Wind project there would spoil their pristine view. A survey conducted earlier this year by Opinion Research Corp. found that, despite a vocal minority, 84 percent of all Massachusetts residents and 58 percent who live on or near Cape Cod support the Cape Wind project, Dvorak said.

"The proposed Cape Cod wind project, if it was built, would be the largest offshore wind park in the world," Dvorak said, noting smaller projects in Europe have been met with more support. Projects in Denmark, for example, began with one or two offshore turbines, he added. The proposed Cape Cod wind park calls for the construction of 130 turbines in Nantucket Sound.

In informal conversations with people who live near Cape Mendocino, Dvorak said most people seemed willing to sacrifice their view to have an environmentally friendly source of power.

Still, he added, "You would want to do a pretty extensive survey of the local population and the environment to see how they would be affected."

Another limiting factor is the development of new technology. Under provisions of the Merchant Marine Act of 1920, the construction of ships and offshore equipment—both of which are needed to build the wind turbines—must be done in the United States, even though there are experienced crews and ships outfitted for this sort of work in Europe.

"You can't actually farm it out to a foreign vessel," Dvorak said. "So the first offshore wind project of this type is going to incur a lot of extra cost."

It would take seven to eight years before a wind park like the one in Northern California could start producing electricity, Dvorak said, given the required environmental considerations.

John Cannon is a science-writing intern at the Stanford News Service.
from the Stanford Report January 9, 2008

In continuing coverage from Thursday's briefing, in a front-page story, the New York Times (1/11, A1, Sengupta) reports that this week, when "Tata Motors unveiled the world's cheapest car, the $2,500 Nano," carmakers "from across the world [went] to New Delhi to peddle their wares to a bubbling Indian car market." While "incomes rise, car loans proliferate, and the auto industry churns out low-cost cars to nudge them off their motorcycles," Indians "bought 1.5 million cars last year," and, "[b]y some estimates, India is expected to soar past China this year as the fastest-growing car market." Not surprisingly, "Indian environmentalists have assailed the car craze, particularly because of the country's relatively relaxed emissions standards and the proliferation of diesel-powered cars."

The AP (1/11) continues, "The potential impact of Tata's Nano has given environmentalists nightmares, with visions of the tiny cars clogging India's already-choked roads and collectively spewing millions of tons of carbon dioxide into the air." However, according to industry analysts, "For millions of people in the developing world, Tata Motors' new" subcompact sedan "may yield a transportation revolution as big as Henry Ford's Model T." John Casesa of the Casesa Shapiro Group called the car "a potentially gigantic development if it delivers what has been promised."

The U.K.'s Financial Times (1/11, Johnson) adds that Ratan Tata, chairman of Tata Motors, said that the company "made no claim to have made the most eco-friendly car in the world." However, "he pointed to the car's fuel-efficiency, noting it would achieve more than 20km a liter with its 0.62-liter engine and meet Indian and European emissions standards." Tarun Das, described by the Times as "chief mentor of the Confederation of Indian Industry," said that "he sees the Nano as a godsend for the Indian economy." Many Indian industrialists assert that "[s]uch entrepreneurial verve...will boost the confidence of Indian industrialists long overshadowed by China's manufacturing prowess."

"The Nano has sparked a race among global automakers to come up with vehicles at rock-bottom prices to appeal to the new lucrative segment of consumers in India and other emerging markets," noted the AFP (1/10, MacRae). "The budget car's nearest rival, the Maruti 800 from Japanese-owned Maruti Suzuki, sells for $4,800."

snipped from ASEE First Bell, January 11, 2008

Stanford Scientific Magazine Public Lecture Series Presents

IPCC, Kyoto, and the Next Steps to Meet the Challenge of Climate Change

* Who writes those IPCC reports?
* Why has addressing climate change been so difficult?
* What needs to happen for a meaningful response?

Join IPCC coauthor and prominent climate change scholar Chris Field for a lecture and audience Q&A.

(When) Wednesday, January 23 at 7:30pm
(Where) Cubberley Auditorium, Stanford University
(How much) Free and open to the public

The strong historical link between carbon emissions and economic activity highlights the dual economic and ecological nature of the looming challenge. Our challenge is cutting emissions while powering a global economic engine likely to grow more than 10-fold in the 21st century. Many low-emitting technologies are available today, but moving them from the laboratory scale to global energy system is the grand challenge of our era.

Prof. Chris Field is the founding director of the Carnegie Institution's Department of Global Ecology and Director of Jasper Ridge Biological Preserve. He was the coordinating lead author on the 2007 IPCC chapter on impacts of climate change on North America. A member of the US National Academy of Sciences, Field has appeared recently on BBC's "The World Today," CBC's "Quirks and Quarks," and NPR's "Science Friday."

HOW I WRITE

Professor Terry Root
Senior Fellow at the Woods Institute for the Environment and Professor of Biological Sciences

Tuesday, February 5, 2008
7PM
The Hume Writing Center,
Basement of Margaret Jacks Hall (460-020)

Terry L. Root, a Senior Fellow at the Woods Institute for the Environment and Professor, by courtesy, of Biological Sciences, primarily works on large-scale ecological questions with a focus on impacts of global warming. She actively works at making scientific information accessible to decision makers and the public (e.g., being a Lead Author for IPCC Third and Fourth Assessment Reports). She is the author of numerous scientific articles for such journals Science, Climatic Change, Biodiversity and Conservation, and Nature. She has written numerous chapters in books, as well as being the author of Atlas of Wintering North American Birds: An Analysis of Christmas Bird Count Data and co-author of Wildlife Responses to Climate Change: North American Case Studies. In 1999 she was chosen as an Aldo Leopold Leadership Fellow, in 1992 as a Pew Scholar in Conservation and the Environment, and in 1990 as a Presidential Young Investigator Award from the National Science Foundation.

Join Hilton Obenzinger, Associate Director of the Hume Writing Center, Honors and Advanced Writing, in a conversation on the techniques, quirks, and joys of advanced writers producing work in all fields and genres.

Sponsored by the Hume Writing Center, Undergraduate Advising and Research, and Stanford Continuing Studies

Look for previous How I Write Conversations on Stanford on i-Tunes