Scientists at Rice University (Houston) have created a rechargeable lithium metal battery with three times the capacity of commercial lithium-ion (Li-ion) batteries by solving the dendrite issue that has long stumped researchers.
The team at Rice designed a battery that stores lithium in a unique anode—a seamless hybrid of graphene and carbon nanotubes. First created at Rice in 2012, the material essentially is a 3D carbon surface that provides abundant area for lithium to inhabit. The anode itself approaches the theoretical maximum for storage of lithium metal while resisting the formation of damaging dendrites or “mossy” deposits.
Dendrites have bedeviled attempts to replace lithium-ion with advanced lithium metal batteries that last longer and charge faster, according to the researchers. Dendrites are lithium deposits that grow into the battery’s electrolyte. If they bridge the anode and cathode and create a short circuit, the battery may fail, catch fire or even explode.
Rice researchers led by chemist James Tour found that when the new batteries are charged, lithium metal evenly coats the highly conductive carbon hybrid in which nanotubes are covalently bonded to the graphene surface. As reported in the American Chemical Society journal ACS Nano, the hybrid replaces graphite anodes in common Li-ion batteries that trade capacity for safety.
“Lithium-ion batteries have changed the world, no doubt,” Tour said, “but they’re about as good as they’re going to get. Your cellphone’s battery won’t last any longer until new technology comes along.” Tour said the new anode’s nanotube forest, with its low density and high surface area, has plenty of space for lithium particles to slip in and out as the battery charges and discharges. The lithium is evenly distributed, spreading out the current carried by ions in the electrolyte and suppressing the growth of dendrites.
Though the prototype battery’s capacity is limited by the cathode, the anode material achieves a lithium storage capacity of 3351 milliamp hours per gram, close to the theoretical maximum and 10 times that of Li-ion batteries, Tour said. Because of the low density of the nanotube carpet, the ability of lithium to coat all the way down to the substrate ensures maximum use of the available volume, he said.
The Rice researchers had their “Aha!” moment in 2014, when co-lead author Abdul-Rahman Raji, a former graduate student in Tour’s lab and now a postdoctoral researcher at the University of Cambridge, began experimenting with lithium metal and the graphene-nanotube hybrid. “I reasoned that lithium metal must have plated on the electrode while analyzing results of experiments carried out to store lithium ions in the anode material combined with a lithium cobalt oxide cathode in a full cell,” Raji said. “We were excited because the voltage profile of the full cell was very flat. At that moment, we knew we had found something special.”
Within a week, Raji and co-lead author Rodrigo Villegas Salvatierra, a Rice postdoctoral researcher, deposited lithium metal into a standalone hybrid anode so they could have a closer look with a microscope. “We were stunned to find no dendrites grown, and the rest is history,” Raji said.
To test the anode, the Rice lab built full batteries with sulfur-based cathodes that retained 80% capacity after more than 500 charge-discharge cycles, approximately two years’ worth of use for a normal cellphone user, Tour said. Electron microscope images of the anodes after testing showed no sign of dendrites or the moss-like structures that have been observed on flat anodes. To the naked eye, anodes within the quarter-sized batteries were dark when empty of lithium metal and silver when full, the researchers reported.
“Many people doing battery research only make the anode, because to do the whole package is much harder,” Tour said. “We had to develop a commensurate cathode technology based upon sulfur to accommodate these ultrahigh-capacity lithium anodes in first-generation systems. We’re producing these full batteries, cathode plus anode, on a pilot scale, and they’re being tested.”
Co-authors of the paper are Rice postdoctoral researcher Nam Dong Kim, visiting researchers Xiujun Fan and Junwei Sha and graduate students Yilun Li and Gladys López-Silva. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice. The Air Force Office of Scientific Research Multidisciplinary University Research Initiative supported the research. An abstract of the paper is available at http://pubs.acs.org/doi/abs/10.1021/acsnano.7b02731.
Nanotech Key to Increasing Performance of Industrial Catalyst
Nanoscale stretching or compressing significantly boosts the performance of ceria, a material widely used in catalytic converters and clean-energy technologies. A tiny amount of squeezing or stretching can produce a big boost in catalytic performance, according to a study by Stanford University and SLAC National Accelerator Laboratory.
The discovery, published May 18 in the journal Nature Communications, focuses on an industrial catalyst known as cerium oxide, or ceria, a spongy material commonly used in catalytic converters, self-cleaning ovens and various green-energy applications, such as fuel cells and solar water splitters.
“Ceria stores and releases oxygen as needed, like a sponge,” said study co-author Will Chueh, an assistant professor of materials science and engineering at Stanford and a faculty scientist at SLAC. “We discovered that stretching and compressing ceria by a few percent dramatically increases its oxygen storage capacity. This finding overturns conventional wisdom about oxide materials and could lead to better catalysts.”
Ceria has long been used in catalytic converters to help remove air pollutants from vehicle exhaust systems, the researchers noted. “In your car, ceria grabs oxygen from poisonous nitrogen oxide, creating harmless nitrogen gas,” said study lead author Chirranjeevi Balaji Gopal, a former postdoctoral researcher at Stanford. “Ceria then releases the stored oxygen and uses it to convert lethal carbon monoxide into benign carbon dioxide.”
Studies have shown that squeezing and stretching ceria causes nanoscale changes that affect its ability to store oxygen. “The oxygen storage capacity of ceria is critical to its effectiveness as a catalyst,” said study co-author Aleksandra Vojvodic, a former staff scientist at SLAC now at the University of Pennsylvania, who led the computational aspect of this work. “The theoretical expectation based on previous studies is that stretching ceria would increase its capacity to store oxygen, while compressing would lower its storage capacity.”
To test this prediction, the research team grew ultrathin films of ceria, each just a few nanometers thick, on top of substrates made of different materials. This process subjected the ceria to stress equal to 10,000 times the Earth’s atmosphere. This enormous stress caused the molecules of ceria to separate and squeeze together a distance of less than one nanometer.
Typically, materials like ceria relieve stress by forming defects in the film. But atomic-scale analysis revealed a surprise. “Using high-resolution transmission electron microscopy to resolve the position of individual atoms, we showed that the films remain stretched or compressed without forming such defects, allowing the stress to remain in full force,” said Robert Sinclair, a professor of materials science and engineering at Stanford.
To measure the impact of stress under real-world operating conditions, the researchers analyzed the ceria samples using the brilliant beams of X-ray light produced at Lawrence Berkeley National Laboratory’s Advanced Light Source.
The results were even more surprising. “We discovered that the strained films exhibited a fourfold increase in the oxygen storage capacity of ceria,” Gopal said. “It doesn’t matter if you stretch it or compress it. You get a remarkably similar increase.”
The high-stress technique used by the research team is readily achievable through nanoengineering, Chueh added. “This discovery has significant implications on how to nanoengineer oxide materials to improve catalytic efficiency for energy conversion and storage,” he said. “It’s important for developing solid oxide fuel cells and other green-energy technologies, including new ways to make clean fuels from carbon dioxide or water.”
Other Stanford co-authors of the study are Max Garcia-Melchor, now at Trinity College Dublin (Ireland), and graduate students Sang Chul Lee, Zixuan Guan, Yezhou Shi and Matteo Monti. Additional co-authors are Andrey Shavorskiy of Lund University (Sweden) and Hendrik Bluhm of Lawrence Berkeley National Laboratory. This work was supported by the US Department of Energy, the National Science Foundation and the Stanford Precourt Institute for Energy.
New Robotic Design Tool Speeds Creation of Custom Bots
A new interactive design tool developed by the Robotics Institute at Carnegie Mellon University (Pittsburgh) can help novices and experts build customized legged or wheeled robots with 3D-printed components and off-the-shelf actuators.
Using a easy drag-and-drop interface, robot builders can choose from a library of components and place them into the design. The tool suggests components compatible with each other, offers potential placements of actuators and can automatically generate structural components to connect those actuators. Once the design is complete, the tool provides a physical simulation environment to test the robot before fabricating it, enabling users to iteratively adjust the design to achieve a desired look or motion.
“The process of creating new robotic systems today is notoriously challenging, time-consuming and resource-intensive,” said Stelian Coros, assistant professor of robotics at the Robotics Institute at CMU. “In the not-so-distant future, however, robots will be part of the fabric of daily life and more people—not just roboticists—will want to customize robots.”
On May 26, robotics doctoral student Ruta Desai presented a report on the design tool that she developed with Coros and fellow graduate student Ye Yuan at the IEEE International Conference on Robotics and Automation (ICRA 2017) in Singapore. Coros’ team designed a number of robots with the tool and verified its feasibility by fabricating two of them—a wheeled robot with a manipulator arm that can hold a pen for drawing, and a four-legged “puppy” robot that can walk forward or sideways.
The National Science Foundation supported the research.
Tech Front is edited by Senior Editor Patrick Waurzyniak; pwaurzyniak@sme.org.
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