Power/Performance Bits: July 3

Graphene foam; superatoms; higher-capacity cathodes.


Graphene foam devices
Scientists at Rice University developed a method for building conductive, three-dimensional objects out of graphene foam, which they say could offer new possibilities for energy storage and flexible electronic sensor applications.

The same lab initially created laser-induced graphene, or LIG, in 2014. The process involves heating inexpensive polyimide plastic sheets with a laser. The laser burns halfway through the plastic and turns the top into interconnected flakes of 2D carbon that remain attached to the bottom half. LIG can be made in macroscale patterns at room temperature.

The new method is based on laminated object manufacturing, in which layers of a material are assembled and then cut to shape. In this case, the bottom LIG layer remains attached to its polyimide base. A second layer is coated with ethylene glycol and placed facedown on the first. Its polyimide top is then burned into graphene; the process is repeated until the block is complete.

The ethylene glycol binder is evaporated away on a hot plate, and any remaining polyimide can be removed in a furnace. That leaves a pristine, spongy carbon block, said Duy Xuan Luong, a Rice graduate student. The researchers stacked up to five layers of foam and then used a custom-built fiber lasing system on a modified 3D printer to mill the block into complex shapes.

Rice graduate student Duy Xuan Luong suspends a three-dimensional block of laser-induced graphene atop two willows. The lab uses an industrial laser to transform inexpensive polyimide plastic into graphene foam at room temperature, and then binds the sheets to produce lightweight, conductive 3D graphene. (Source: Rice University)

To demonstrate the possibilities of the graphene blocks, the team assembled proof-of-concept lithium-ion capacitors that used 3D LIG as both anodes and cathodes. The anode’s gravimetric capacity of 354 milliamp hours per gram neared the theoretical limit of graphite, while the cathode’s capacity exceeded the average capacity of other carbon materials. Full test cells retained about 70 percent of their capacity after 970 charge-discharge cycles.

“This is excellent performance in these new-generation lithium-ion capacitors, which capture the best properties of lithium-ion batteries and capacitor hybrids,” said James Tour, a chemist at Rice.

The researchers also infused a block of 3D LIG with liquid polydimethylsiloxane through its 20- to 30-nanometer pores. This created a stronger, still-flexible, conductive material without changing the original foam’s shape. From this material, they made a flexible sensor that accurately recorded the pulse from the wrist of a volunteer. The team says further calibration of the device would let them extract blood pressure from the pulse waveform.

Creating superatoms
Researchers at Virginia Commonwealth University discovered a new strategy for creating superatoms — combinations of atoms that can mimic the properties of more than one group of elements of the periodic table.

Forming superatoms that can supply or accept multiple electrons while maintaining structural stability is a key requirement for creating better batteries or semiconductors, said Shiv Khanna, Commonwealth Professor and chair of the Department of Physics. The ability of superatoms to effectively move charges while staying intact is attributed to how they mimic the properties of multiple groups of elements.

Currently, alkali atoms, which form the first column of the periodic table, are optimal for donating electrons. These naturally occurring atoms require a low amount of energy to donate an electron. However, donating more than one electron requires a prohibitively high amount of energy.

The process developed by the team allows clusters of atoms to donate or receive multiple electrons using low levels of energy.

While such superatoms already have been made, there has not been a guiding theory for doing so effectively. The team theorized that organic ligands, molecules that bind metal atoms to protect and stabilize them, can improve the exchange of electrons without compromising energy levels.

In the work, the researchers used groups of aluminum clusters mixed with boron, carbon, silicon and phosphorous, paired with organic ligands. Using computational analysis, they demonstrated the cluster would use even less energy to donate an electron than francium, the strongest naturally occurring alkali electron donor.

“We could use ligands to take any cluster of atoms and turn it into either a donor or acceptor of electrons. We could form electron donors that are stronger than any element found on the periodic table,” said Khanna. “The possibility of having these building blocks that can accept multiple charges or donate multiple charges would eventually have wide-ranging applications in electronics.”

Higher-capacity cathodes
Scientists at the University of Maryland, the U.S. Department of Energy’s Brookhaven National Laboratory, and the U.S. Army Research Lab developed a new cathode material that could triple the energy density of lithium-ion battery electrodes.

The new cathode material is a modified and engineered form of iron trifluoride (FeF3), composed of iron and fluorine, both cost-effective and environmentally benign elements. Researchers have been interested in using chemical compounds like FeF3 in lithium-ion batteries because they offer inherently higher capacities than traditional cathode materials.

“Compared to the large capacity of the commercial graphite anodes used in lithium-ion batteries, the capacity of the cathodes is far more limited. Cathode materials are always the bottleneck for further improving the energy density of lithium-ion batteries,” said Xiulin Fan, a scientist at UMD.

“The materials normally used in lithium-ion batteries are based on intercalation chemistry,” said Enyuan Hu, a chemist at Brookhaven. “This type of chemical reaction is very efficient; however, it only transfers a single electron, so the cathode capacity is limited. Some compounds like FeF3 are capable of transferring multiple electrons through a more complex reaction mechanism, called a conversion reaction.”

Despite FeF3’s potential to increase cathode capacity, the compound has not historically worked well in lithium-ion batteries due to three complications with its conversion reaction: poor energy efficiency (hysteresis), a slow reaction rate, and side reactions that can cause poor cycling life. To overcome these challenges, the scientists added cobalt and oxygen atoms to FeF3 nanorods through chemical substitution. This allowed the scientists to manipulate the reaction pathway and make it more reversible.

Substituting the cathode material with oxygen and cobalt prevents lithium from breaking chemical bonds and preserves the material’s structure. (Source: Brookhaven National Laboratory)

To investigate the reaction pathway and assess the functionality of the material, the team took advantage of transmission electron microscopy as well as the National Synchrotron Light Source II’s X-ray Powder Diffraction (XPD) beamline.

“We also performed advanced computational approaches based on density functional theory to decipher the reaction mechanism at an atomic scale,” said Xiao Ji, a scientist at UMD. “This approach revealed that chemical substitution shifted the reaction to a highly reversible state by reducing the particle size of iron and stabilizing the rocksalt phase.”

The team says this research strategy could be applied to other high energy conversion materials, and future studies may use the approach to improve other battery systems.

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