Taking materials to extremes
Ju Li models and develops nanomaterials for demanding energy applications.
Lithium-air batteries are the Holy Grail in the worldwide quest for better batteries, because they can store energy at very high densities, at least in theory. On the downside, they suffer from high losses in energy conversion and other drawbacks. Ju Li’s MIT research group, however, has designed a novel sealed lithium-oxygen nanotechnology chemistry that overcomes many of these problems.
That’s just one of many nanotech advances being developed by Li, who is the Battelle Energy Alliance Professor of Nuclear Science and Engineering as well as Professor of Materials Science and Engineering.
Li’s group works on nanomaterials for energy applications, performing theoretical modeling and synthesizing and characterizing the materials. “We model materials at atomistic and electronic structure scales, and we’ve been developing state-of the-art in situ capabilities to understand the performance of these materials under extreme environments,” he says.
Prototypes from Li's lab range from composites for nuclear reactors that resist high radiation levels to advanced battery components to energy storage for “smart dust” sensors. As he advances nanotech science and engineering in the lab, Li also is establishing industrial partnerships with consumer electronics firms, electric utilities, automobile makers, oil and gas companies, semiconductor manufacturers and nanotechnology companies.
Described in a July 2016 Nature Energy paper, the sealed lithium-oxygen battery created by Li and his colleagues was inspired by rocket fuels, which are made up of hydrogen fuel and solid oxidants, he says.
Like lithium-air batteries, the lithium-oxygen design discharges electrical power produced by an electrochemical reaction between lithium and oxygen. Unlike the case in lithium-air chemistry, however, the oxygen is not drawn from surrounding air but is held in three forms of solid lithium oxides.
These oxides are packaged in nanoscale particles of glass that are co-dispersed with cobalt oxide, which stabilizes the otherwise unstable lithium superoxide form and catalyzes their conversion, he explains. The technique stores energy by cycling through the three forms of lithium oxides, avoiding the enormous changes in density that oxygen undergoes as a gas in lithium-air batteries.
“Unlike conventional lithium-air batteries, which need pumps and membranes, we have a fully sealed battery,” says Li. “We have also reduced the energy loss by a factor of four, and prolonged the life of the cathode.”
In general, nanomaterials offer advantages for delivering electrical energy at high rates, Li says. But they also bring several well-known problems. One issue is the difficulty in packing in the materials densely enough. (The first-generation lithium-solid oxygen cathodes already have achieved greater energy density by volume than conventional metal-based cathodes, he says.) Additionally, some nanomaterials are not stable, and some have unwanted side reactions on their large surface areas that deplete electrolytes.
Safety is a big concern with all batteries, but the sealed lithium-solid oxygen battery can automatically avoid overcharging to minimize risks, he says.
Other battery work in Li’s lab focuses on devices that are really, really small.
In 2010, shortly before he joined the MIT faculty, Li and co-workers made a battery of a single nanowire, “which was just a few hundred nanometers in size, the smallest battery in the world,” he says. Today, his group seeks to construct batteries suitably sized for the ultra-small devices called “magic dust.”
Smart dust devices, a longstanding vision in nanotech, are “autonomous units that are smart and can communicate with each other,” Li explains. “These devices combine sensing, computing and energy harvesting and storage together in a single package. Nowadays we can easily make tens of thousands of transistors that can function in a micron-scale package, but what is really needed is energy storage.”
Magic dust eventually might support many sensing applications, Li suggests, such as tracking parts during manufacture, monitoring nuclear materials through their life cycles, or analyzing the structural integrity of structures such as bridges.
Resistance to radiation is a leading goal for materials for nuclear reactors, since those materials tend to fail unpredictably. “Sustaining high levels of radiation, they start to have voids, their volume changes, they lose their ductility and their ability to sustain impact and tensile load,” he says.
His lab is developing radiation-resistant nanomaterials, including materials with carbon nanotubes, graphene or oxide nanowires dispersed uniformly in the material, which provide venues for radiation defects to recombine and heal the radiation damage. “These materials manifest very good thermomechanical properties, they are creep resistant, they have better room temperature strength, and they retain their tensile ductility, but most importantly, they survive high radiation doses,” Li says.
Another major research theme is creating high-temperature nanomaterials. One example is a thermal super-insulator that remains intact up to 1400 degrees C and retains good structural properties. “This is a crystalline zirconate ceramic foam that is very stable both chemically and thermally, with thermal conductivity lower than that of air over a wide temperature range,” he says. “It’s porous, very light, and it can survive large compressive stress as well.”
This ceramic foam could act as a thermal barrier coating for turbine engines, improving their energy efficiency. Li also sees potential applications in high-temperature processing such as chemical fuel production, and in creating lab-on-a-chip devices that require high temperatures.
Readying such nanomaterials for manufacturing raises huge barriers. “Generally, to understand materials you only need milligrams of them, but to move to manufacturing, we often need to transition to kilogram or even ton-scale production,” Li says. “For example, we are striving to make batteries that can power a cell phone, and this sort of prototyping process is very important.”
Such prototyping is also quite a challenge in a university setting, he points out. His lab collaborates with industry to scale up nanomaterial fabrication and testing, and connections through the Industrial Liaison Program have proved extremely helpful.
Li also co-directs the MIT Energy Initiative’s new Low-Carbon Energy Center for Materials in Energy and Extreme Environments with Bilge Yildiz, Associate Professor of Nuclear Science and Engineering and of Materials Science and Engineering.
“Materials are pervasive in all aspects of energy generation, storage and transmission,” Li points out. “At our center, we are keen to connect MIT faculty with both traditional fossil-energy companies and renewable energy companies, in all aspects of research that involve both reducing environmental impact and improving the bottom line.”
“We envision having MIT faculty going into the plants to talk with not just management but technical persons and operators in the field to understand the limiting issues and how companies articulate their problems,” he adds. “Faculty then can bring the problems back to MIT with a much stronger and more intimate connection with industry’s technical challenges.”