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Monday, March 25th, 2019
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| 11:59a |
New approach could boost energy capacity of lithium batteries Researchers around the globe have been on a quest for batteries that pack a punch but are smaller and lighter than today’s versions, potentially enabling electric cars to travel further or portable electronics to run for longer without recharging. Now, researchers at MIT and in China say they’ve made a major advance in this area, with a new version of a key component for lithium batteries, the cathode.
The team describes their concept as a “hybrid” cathode, because it combines aspects of two different approaches that have been used before, one to increase the energy output per pound (gravimetric energy density), the other for the energy per liter (volumetric energy density). The synergistic combination, they say, produces a version that provides the benefits of both, and more.
The work is described today in the journal Nature Energy, in a paper by Ju Li, an MIT professor of nuclear science and engineering and of materials science and engineering; Weijiang Xue, an MIT postdoc; and 13 others.
Today’s lithium-ion batteries tend to use cathodes (one of the two electrodes in a battery) made of a transition metal oxide, but batteries with cathodes made of sulfur are considered a promising alternative to reduce weight. Today, the designers of lithium-sulfur batteries face a tradeoff.
The cathodes of such batteries are usually made in one of two ways, known as intercalation types or conversion types. Intercalation types, which use compounds such as lithium cobalt oxide, provide a high volumetric energy density — packing a lot of punch per volume because of their high densities. These cathodes can maintain their structure and dimensions while incorporating lithium atoms into their crystalline structure.
The other cathode approach, called the conversion type, uses sulfur that gets transformed structurally and is even temporarily dissolved in the electrolyte. “Theoretically, these [batteries] have very good gravimetric energy density,” Li says. “But the volumetric density is low,” partly because they tend to require a lot of extra materials, including an excess of electrolyte and carbon, used to provide conductivity.
In their new hybrid system, the researchers have managed to combine the two approaches into a new cathode that incorporates both a type of molybdenum sulfide called Chevrel-phase, and pure sulfur, which together appear to provide the best aspects of both. They used particles of the two materials and compressed them to make the solid cathode. “It is like the primer and TNT in an explosive, one fast-acting, and one with higher energy per weight,” Li says.
Among other advantages, the electrical conductivity of the combined material is relatively high, thus reducing the need for carbon and lowering the overall volume, Li says. Typical sulfur cathodes are made up of 20 to 30 percent carbon, he says, but the new version needs only 10 percent carbon.
The net effect of using the new material is substantial. Today’s commercial lithium-ion batteries can have energy densities of about 250 watt-hours per kilogram and 700 watt-hours per liter, whereas lithium-sulfur batteries top out at about 400 watt-hours per kilogram but only 400 watt-hours per liter. The new version, in its initial version that has not yet gone through an optimization process, can already reach more than 360 watt-hours per kilogram and 581 watt-hours per liter, Li says. It can beat both lithium-ion and lithium-sulfur batteries in terms of the combination of these energy densities.
With further work, he says, “we think we can get to 400 watt-hours per kilogram and 700 watt-hours per liter,” with that latter figure equaling that of lithium-ion. Already, the team has gone a step further than many laboratory experiments aimed at developing a large-scale battery prototype: Instead of testing small coin cells with capacities of only several milliamp-hours, they have produced a three-layer pouch cell (a standard subunit in batteries for products such as electric vehicles) with a capacity of more than 1,000 milliamp-hours. This is comparable to some commercial batteries, indicating that the new device does match its predicted characteristics.
So far, the new cell can’t quite live up to the longevity of lithium-ion batteries in terms of the number of charge-discharge cycles it can go through before losing too much power to be useful. But that limitation is “not the cathode’s problem”; it has to do with the overall cell design, and “we’re working on that,” Li says. Even in its present early form, he says, “this may be useful for some niche applications, like a drone with long range,” where both weight and volume matter more than longevity.
“I think this is a new arena for research,” Li says.
The work was supported by the Samsung Advanced institute of Technology, the National Key Technologies R&D Program of China, the National Science Foundation of China, and MIT’s Department of Materials Science and Engineering. The team also included professor Jing Kong and others at MIT, as well as researchers at the Chinese Academy of Sciences in Beijing, the Songshan Lake Materials Laboratory in Guangdong, China, the Samsung Advanced Institute of Technology America in Burlington, Massachusetts, and Tongji University in Shanghai. | | 12:55p |
Fine-tuning multiphysics problems “Stretching myself radically to learn a new kind of physics or code is exactly what I want to do,” says Miriam Kreher. “It’s how I solve problems and find new ones.”
A second-year doctoral student in nuclear science and engineering, Kreher is finding just the kind of challenges she craves as a member of MIT’s Computational Reactor Physics Group (CRPG). Her task: helping to develop vastly improved software simulations of the complex interactions taking place inside nuclear reactors.
“Some people focus on how neutrons move, and others look at how water flowing around the core affects temperature,” she explains. “But in nuclear reactors, these physics phenomena of neutron transport and fluid flow affect each other through complex feedback, and we need to understand both at the same time.”
This tight coupling of physics phenomena has preoccupied nuclear engineering for some time. “Getting a more precise picture of these interactions would allow for finer-tuned operational margins in reactors,” notes Kreher, a Department of Energy Computational Science Graduate Fellow. More accurate simulations could help the current fleet of commercial reactors work at higher powers, and aid in designing the next generation of reactors.
However, modeling multiphysics problems is not simple. Even in steady-state, there are countless neutrons interacting with the fuel and coolant, depositing large amounts of energy that alters the temperature of everything inside the reactor. Add a time variable, or alter the position of the control rod, which determines the rate of fission reactions, and the modeling proves more difficult still. Current high-fidelity simulations are expensive, requiring weeks or longer to render. But quite recently, scientists have begun to gain ground on these problems.
“Computers have now become powerful enough to address these multiphysics problems, permitting stable simulations in a shorter amount of time,” says Kreher. “This could be the basis for much less expensive modeling.”
Under the supervision of CRPG faculty leads Kord Smith and Benoit Forget, Kreher is developing computational tools that will yield high fidelity simulations with representative temperature and density conditions inside a reactor core. To tackle the complex multiphysics problems she confronts as she goes about this task, Kreher is taking a sequence of tough math and computation classes so she can test new modeling approaches.
“I want to help develop computational methods that will permit other researchers to simplify or speed up their simulations, so eventually they won’t need to depend on the world’s fastest computers,” she says. “I am excited to be part of something that could set the groundwork for science of the future.”
Kreher found her way into nuclear engineering early. Brought up in Morocco and France, she arrived at a magnet high school for science and technology in Marseille. There, Kreher became engaged by a class combining physics and English that included a survey on energy.
“I thought nuclear made the most sense, since it produced energy and cleaned up the atmosphere, so I decided I should contribute to the field somehow,” she recalls. She pursued engineering at the University of Pittsburgh — the city where her father grew up — concentrating in nuclear engineering. She found mentors who offered her research opportunities, and received her first taste of coding. “I give Pitt a lot of credit for that; I got my first internship at Bettis Atomic Power Laboratory because I knew MATLAB.”
Kreher also plunged into policy work, joining the Nuclear Engineering Student Delegation in 2014 for a week-long visit to Washington, where she learned about the intersection of politics and technology.
“If I had been drawn to fields other than math and physics, I might have become an advocate or lobbyist, because public perception of nuclear power is really important,” she says. To this day, Kreher says she likes to pop in on her Washington representative to discuss energy issues when her schedule permits.
She applied to the MIT Summer Research Program in 2015 to sharpen her resume for graduate school, and landed a research spot with Benoit Forget’s group.
“I was just an intern, but all the students helped me, and I eventually contributed to the group’s research,” says Kreher. “It felt fun and dynamic, a natural fit, and I decided on MIT for graduate school.”
Her work on multiphysics problems evolved quickly during meetings with Forget and Smith, who became her advisors. Summer research at several Department of Energy laboratories provided her opportunities to acquire additional coding techniques.
With her anticipated graduation in 2022, Kreher has years more of meticulous computation before her. “I keep the larger research vision in view, and while I struggle with coding issues from time to time I always get a rush when I solve them, which makes it feel worthwhile to go on to the next problem.”
For research breaks, she works up a sweat swing dancing with MIT’s Lindy Hop Society. “I have no background in dancing, but it makes me really happy, especially since I’m not one of those people who exercise,” she says. “It just gives me those natural endorphins from moving, plus it’s a social outlet for me.” And as MIT co-president of the student section of the American Nuclear Society, Kreher engages in the kind of outreach work on energy issues that remain important to her.
At the end of the doctoral road, a position at one of the national labs or perhaps a faculty position, beckons. In the meantime, MIT life is working out well. “Being here is very special, because I can problem solve with people, and they share things with me,” she says. “Culturally, socially, I’m very happy at MIT.”
She’s also a big fan of the 32-year-old MSRP, and of Institute efforts to make the science and engineering communities more inclusive. | | 3:48p |
New 3-D printing approach makes cell-scale lattice structures A new way of making scaffolding for biological cultures could make it possible to grow cells that are highly uniform in shape and size, and potentially with certain functions. The new approach uses an extremely fine-scale form of 3-D printing, using an electric field to draw fibers one-tenth the width of a human hair.
The system was developed by Filippos Tourlomousis, a postdoc at MIT’s Center for Bits and Atoms, and six others at MIT and the Stevens Institute of Technology in New Jersey. The work is being reported today in the journal Microsystems and Nanoengineering.
Many functions of a cell can be influenced by its microenvironment, so a scaffold that allows precise control over that environment may open new possibilities for culturing cells with particular characteristics, for research or eventually even medical use.
While ordinary 3-D printing produces filaments as fine as 150 microns (millionths of a meter), Tourlomousis says, it’s possible to get fibers down to widths of 10 microns by adding a strong electric field between the nozzle extruding the fiber and the stage on which the structure is being printed. The technique is called melt electrowriting.
“If you take cells and put them on a conventional 3-D-printed surface, it’s like a 2-D surface to them,” he explains, because the cells themselves are so much smaller. But in a mesh-like structure printed using the electrowriting method, the structure is at the same size scale as the cells themselves, and so their sizes and shapes and the way they form adhesions to the material can be controlled by adjusting the porous microarchitecture of the printed lattice structure.
“By being able to print down to that scale, you produce a real 3-D environment for the cells,” Tourlomousis says.
He and the team then used confocal microscopy to observe the cells grown in various configurations of fine fibers, some random, some precisely arranged in meshes of different dimensions. The large number of resulting images were then analyzed and classified using artificial intelligence methods, to correlate the cell types and their variability with the kinds of microenvironment, with different spacings and arrangements of fibers, in which they were grown.
Cells form proteins known as focal adhesions at the places where they attach themselves to the structure. “Focal adhesions are the way the cell communicates with the external environment,” Tourlomousis says. “These proteins have measurable features across the cell body allowing us to do metrology. We quantify these features and use them to model and classify quite precisely individual cell shapes.”
For a given mesh-like structure, he says, “we show that cells acquire shapes that are directly coupled with the substrate’s architecture and with the melt electrowritten substrates,” promoting a high degree of uniformity compared to nonwoven, randomly structured substrates. Such uniform cell populations could potentially be useful in biomedical research, he says: “It is widely known that cell shape governs cell function and this work suggests a shape-driven pathway for engineering and quantifying cell responses with great precision,” and with great reproducibility.
He says that in recent work, he and his team have shown that certain type of stem cells grown in such 3-D-printed meshes survived without losing their properties for much longer than those grown on a conventional two-dimensional substrate. Thus, there may be medical applications for such structures, perhaps as a way to grow large quantities of human cells with uniform properties that might be used for transplantation or to provide the material for building artificial organs, he says. The material being used for the printing is a polymer melt that has already been approved by the FDA.
The need for tighter control over cell function is a major roadblock for getting tissue engineering products to the clinic. Any steps to tighten specifications on the scaffold, and thereby also tighten the variance in cell phenotype, are much needed by this industry, Tourlomousis says.
The printing system might have other applications as well, Tourlomousis says. For example, it might be possible to print “metamaterials” — synthetic materials with layered or patterned structures that can produce exotic optical or electronic properties.
The team included Thrasyvoulos Karydis and Andreas Mershin at MIT, and Chao Jia, Hongjun Wang, Dilhan Kalyon, and Robert Chang at the Stevens Institute of Technology in Hoboken, New Jersey. The work was funded by the National Science Foundation. |
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