New Type Of Graphene Paves The Way For Sustainable Sodium Batteries

Artist impression of sodium ions (green) within layers of Janus Graphene. Image Credit: Marcus Folino and Yen Strandqvist/Chalmers University of Technology

Lithium-ion batteries are a cornerstone of our technology due to their energy storage capabilities. But lithium itself is not that great when one considers the environmental and monetary costs of its production. A better alternative would be sodium, which is abundant on Earth but making sodium batteries is not without its difficulties. However, one of those might have been sorted.

Researchers from Chalmers University of Technology have been able to build high-performance electrode materials for sodium batteries with a special type of graphene. The battery shows energy capacity very close to what can be found in standard lithium batteries. The breakthrough is published in Science Advances.

In lithium batteries, the anode is made of graphite, the material in a pencil lead, which is layer upon layer of graphene. Lithium ions are small enough to move in and out of graphene layers, but sodium is much chunkier so it can't use the same trick. And that’s where the Janus graphene comes in. This version of graphene can be organized in layers that allow sodium ions to flow.

“We have added a molecule spacer on one side of the graphene layer. When the layers are stacked together, the molecule creates larger space between graphene sheets and provides an interaction point, which leads to a significantly higher capacity,” lead author Dr. Jinhua Sun said in a statement.

The capacity of a sodium battery using standard graphite is 35 milliampere hours per gram, which is less than one-tenth of what you get from a standard lithium battery. The new approach with the Jannus graphene gets to 322 milliampere hours per gram. Still less than lithium, but much closer. And they are fully reversible and have high cycling stability.

“It was really exciting when we observed the sodium-ion intercalation with such high capacity. The research is still at an early stage, but the results are very promising. This shows that it’s possible to design graphene layers in an ordered structure that suits sodium ions, making it comparable to graphite,” added Professor Aleksandar Matic at the Department of Physics at Chalmers.

The name of this graphene comes from the Roman god, Janus. Graphene is made of carbon organized in a single one-atom-thick layer. Janus is known for having two faces so here it symbolizes the fact that this layer looks different on one side due to the molecule spacer. Janus is also the god of doors, and the team liked the idea that the material will open the door to new technology.

“Our Janus material is still far from industrial applications, but the new results show that we can engineer the ultrathin graphene sheets – and the tiny space in between them – for high-capacity energy storage. We are very happy to present a concept with cost-efficient, abundant and sustainable metals,” concluded Vincenzo Palermo, Affiliated Professor at the Department of Industrial and Materials Science at Chalmers.

The race is on to find killer applications for graphene

Illustration of graphene molecule.

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The future may be two-dimensional — at least, the future of materials.

Since 2004, when Russian physicists Andre Gejm and Kostya Novosëlov succeeded in isolating the first monoatomic layer of graphene, starting from the graphite tip of a simple pencil, the Pandora's box of 2D materials has been opened: an extended family of atomic lattices with sci-fi-like properties, which until then were considered inaccessible. Between daring experiments, research and technical obstacles, the race to the killer application for graphene and its siblings has thus begun, promising solutions in various fields that not only perform better than the current ones, but also more energy efficient and circular.

We had a chat about this with Camilla Coletti, enthusiastic coordinator of the research line "2D Materials Engineering" and of the Graphene Lab at the Italian Institute of Technology.

Giorgia Marino: What are two-dimensional materials? How can we imagine them?

Camilla Coletti: The concept isn’t self-explanatory. Two-dimensional materials, such as graphene, are literally materials that have a thickness of one atom. We can imagine a sheet with the thickness of an atom, so thin it appears to be invisible. This is precisely why we were so late in discovering its existence.

Graphene can be defined as the "mother" of all 2D materials and of allotropes of carbon: think of graphite, carbon nanotubes, fullerenes, all materials already studied during the 20th century. Graphite is nothing more than a superposition of layers of graphene, stacked one on top of the other; by curling graphene on itself, we obtain the nanotube; if we squeeze it into a ball, we get a fullerene.

Marino: But why did it take so long to discover graphene?

Coletti: First of all, because we lacked the methodologies to be able to see something so thin and transparent. In reality, the right technique existed, but what was perhaps an even more important obstacle was that this material was thought to be unstable from a thermodynamic point of view. This means that detaching and isolating it from graphite, for example, would make it curl on itself and therefore it wouldn’t exist as a sheet of atomic thickness.

The discovery of graphene, which [won] the Nobel Prize to Russian physicists Andrej Konstantinovič Gejm and Konstantin Sergeevič Novosëlov, is one of those stories that is almost unbelievable for the randomness and simplicity with which it happens…

It was a discovery born from the desire to go beyond what is believed possible. In the world of scientists, on Friday evenings, instead of going out for a drink, they enjoy doing "weird" experiments. So these two Russian scientists decided, one Friday night in Manchester, to try exfoliating the tip of a pencil with Scotch tape, over and over again, in a repetitive motion, to see if it was possible to isolate a single layer of graphene. They finally succeeded and won the Nobel Prize. People have often gossiped that the prize was awarded too hastily, for a childish experiment. But in reality, the two scientists have not only isolated graphene, they have studied its properties: This was what changed everything.

Marino: What are these properties?

Coletti: Graphene is a structure in which carbon atoms are arranged in a honeycomb, hexagonal shape. There is nothing exotic about this: graphite is the same thing, except it’s built by many layers of graphene, one on top of the other. The possibility to have exotic properties with graphene had already been studied and predicted, but scientists thought that in nature it would be impossible to isolate a 2D material without it disappearing instantly. When they succeeded, the properties found are really exceptional. That's why people started to talk about graphene as the "wonder material."

First of all we can say it’s a playground for physicists. In this material, in fact, electrons, charge carriers, travel at a relativistic speed that is comparable to light speed. Many of the theoretical predictions made at the time of Einstein, which were not verifiable because there was no suitable material to verify them, have finally found an experimental platform in graphene.

Then, of course, people have started thinking about practical applications. These very fast electrons open possibilities to design very fast computers: the dream of graphene inside was born, which means the existence of computers with graphene processors instead of silicon. A thing that then turned out to be problematic, because graphene always leads, it has no "on" and "off." Therefore, in the 17 years since its discovery, the vague graphene computers have not been realized and probably will not be the application of choice of this material.

Marino: Going back to 2D material properties...

Coletti: The main property is the fact that its charge carriers travel at incredible speed, approaching the speed of light. Practically these electrons move almost without encountering resistance, and this feature also determines the property of super conduction.

Then there is the transparency: 97 percent of light in the visible spectrum passes through it. In addition, graphene is an incredibly flexible material but at the same time it’s also very resistant: as strong as diamond but as flexible as plastic. In short, we could do a lot of interesting things with it.

Marino: What are the critical points when it comes to working with these materials?

Coletti: As we were saying, we have the problem of "on and off." From the point of view of practical work, the first problem is, of course, the fact that the material is so thin that you need a trained eye to see it even under the microscope.

The other big problem has been trying to produce it on a large scale. It's one thing to have a flake, a sheet of paper with a diameter as thin as a hair: if you're in a research lab, you can have fun making a small device and see how well it works. But if we talk about applications, with such infinitesimal dimensions you can't do anything. So one of the first difficulties that scientists tried to overcome was to produce graphene in a scalable way while maintaining its properties: with the same crystalline quality, with all its regular hexagons, without missing an atom or finding something else instead of carbon. It wasn't easy at all, but a lot of progress was made early on, and several approaches allowed us to get high-quality graphene "grown" on various substrates.

Marino: Grown?

Coletti: I say "grow" because that's the method we use in our labs at IIT and it's one of the most classic ways to obtain it. You use a circular plate, which we call wafer, to deposit graphene on it. The type of substrate is chosen according to the final application and can be silicon, silicon carbide, sapphire, copper or any other material typically used in electronic applications. The wafer is placed in a large oven heated with methane at temperatures above 1000 degrees Centigrade and after a few minutes the reaction called "chemical vapor deposition" (CVD) takes place: in practice, the carbon from the methane is deposited on the substrate forming the graphene. At the end of the process, we cool the oven and take it out. Most of the time you can't see anything with the naked eye and you have to check with spectroscopy or microscope if you have obtained a perfect monolayer of graphene.

Marino: Are there other ways to get the graphene?

Coletti: Yes, there are; for example, manufacturing processes that start with graphite: by centrifuging it at ultrasonic speed in a solvent, "inks" are obtained, which can then be literally sprayed and mixed with other materials to strengthen their basic properties. They are ideal for low-tech applications, such as helmets and rackets, and so this system has explored a lot in recent years and has enabled the production of products that are already on the market.

The oven system, on the other hand, allows us to have very high quality material, pure and not in solution form. Once obtained, we manufacture devices, for example for data transmission, which prove to be very efficient, since graphene works well as both transmitter and receiver.

Marino: Why are these building blocks of photonics made from graphene so interesting?

Coletti: Covid, if there was any need, has shown us how much we need data transmission. Demand has become enormous and now we have reached a doubling of bandwidth every two years, but this must come at the same cost, energy consumption and footprint, i.e. the size of the devices. Demands that we can no longer keep up with existing technologies: in electronics we have arrived at the minimum size achievable with silicon devices and in photonics we have arrived at the maximum possible achievable with the materials we use. We need new materials.

Graphene is interesting in this field first of all because it has the ability to convert light energy into electrical signals and does so in an energy efficient way. It allows us to make devices that transmit and receive with low energy consumption and outstanding performance, wider bandwidth, very small size. The cost is also promising, because atomic-thick monolayers can be ported to existing photonic platforms without asking companies to change all their production lines.

Marino: It's an interesting prospect also in regard of the possibility of replacing critical raw materials such as the so-called rare-earth elements…

Coletti: Certainly a carbon-based material is a more sustainable solution, even from the point of view of e-waste pollution, which is becoming a big problem.

Marino: Since we are dealing with carbon, is it possible to hypothesize CO2 capture processes from which to produce graphene?

Coletti: Using CO2 instead of methane is actually one of the first solutions we thought of, but we had to stop because of technical limitations of our machinery. However, there is already research using different types of waste, organic products and vegetable oils to obtain carbon from which to produce graphene. The quality is not stellar yet, but certainly these processes will be refined in the near future.

Marino: Can we imagine a circular economy of graphene, then?

Coletti: Yes, the possibilities are there. It's all about being able to demonstrate not only that it can be done — because we already know this — but that it can be done while maintaining the high quality of the material obtained.

Above all, my dream and the dream of those who work on graphene is to find the so-called killer application, that is, the important application that can really change something in people's lives. Without detracting from low-tech products that are already on the market, such as rackets, bike tires or others, I would like to see graphene in an application capable of revolutionizing our lives for the better. We could have, as I said, widely used technology for faster and greener transmissions. If we imagine such a landscape, having the ability to produce high-quality graphene with circular processes would be ideal.

Marino: Speaking of green applications, there are also graphene devices to purify water currently being studied.

Coletti: Yes, graphene has shown to have possible interesting applications and good performance in this field as well. Being a very versatile material, depending on how it’s processed, it can become either highly impermeable or porous, and in this case it lends itself precisely to absorption.

Marino: We have talked a lot about graphene, but it’s not the only 2D material. After its discovery, a universe of new two-dimensional materials seems to have opened up: how many are there?

Coletti: There are so many of them. There are theoretical studies that speak of hundreds and hundreds of materials, but obviously in practice those that we can synthesize, isolate and study are way fewer. The ones that the most interest is focused on at the moment are graphene's white brother, boron nitride, and transition metal dichalcogenides (TMD). Boron nitride is made exactly like honeycomb graphene, but instead of having all carbon atoms, there is an alternating boron atom and nitrogen atom. Unlike graphene, it is an insulating material, so putting them together makes dreamy electronic devices.

Then there is the whole family of transition metal dichalcogenides (TMD) that are formed by a chalcogenic atom (sulfur or selenium for example) and a transition metal such as tungsten or molybdenum: so we have tungsten disulfide, molybdenum disulfide and so on. These materials are actually three-dimensional, but the layers are so weakly connected to each other that you can exfoliate them and get other 2D materials. The interesting thing is that as three-dimensional materials they have certain properties, but by isolating the single atom or the single layer of atoms, the properties are completely different. For example, tungsten sulfide is normally a semiconductor and does not emit light, but in 2D form it is the opposite and you can therefore make a lot of applications in electronics and optoelectronics.

Marino: What is still missing and what will be needed in the near future to get to tangible, marketable applications for 2D materials?

Coletti: Certainly from the point of view of investments, there has been a strong help from the European community with the Graphene Flagship project, of which we are part and of which many European countries and institutions are members. This project has made it possible to create synergies between companies and research institutes in order to move rapidly towards the development of practical applications.

What is missing is to be able to identify the best applications among all the possible ones and in that direction continuing to work in a synergistic way with companies and with those who could be the final users of the application. I would say that we are at a good point: if this virtuous circle is not interrupted now, we have good chances to see 2D materials in relevant applications in five or 10 years.

Fortunately, the interest from the industrial world is there. The problem now is to be able to demonstrate the reproducibility of 2D materials on a large scale: everything we can do on 3 centimeters, we need to do on 15 centimeters. To summarize, now we need reproducibility and market.

Toward a Graphene Laser

Artist’s rendering of nanopillars in strained graphene, which can give rise to large pseudo-magnetic fields in the material, changing its optical properties. [Image: Courtesy of D.-H. Kang]

Graphene—atomically thin, 2D sheets of carbon, whose discovery captured the 2010 Nobel Prize in physics—has some remarkable properties. It’s strong, yet super-light; it’s hard, yet flexible; and it boasts extremely high electron mobility and a brisk photoelectric response. Thus the material is finding its way into designs for a variety of next-gen optoelectronic devices, in components such as modulators, photodetectors, low-loss waveguides and more.

One possibility, however, has remained elusive. Because graphene is a zero-band-gap material, it’s been hard to find a route to graphene lasers.

Researchers in Singapore have now taken an intriguing step toward that goal, by tweaking the strains in the material’s ultrathin sheets (Nat. Commun., doi: 10.1038/s41467-021-25304-0). Specifically, the team has shown that engineering a periodic structure of nanoscale pillars into a graphene sheet can produce localized tensile strains that give rise to extremely strong pseudo-magnetic fields. Those fields, in turn, can affect electron transport, open up energy gaps and significantly modify the material’s optical transitions. The result, the authors argue, could presage “a new class of graphene-based optoelectronic devices,” including lasers.

Faking magnetic fields

It’s been known for some time that subjecting graphene to an external magnetic field can create energy gaps in the 2D sheets, by affecting charge-carrier motion and relaxation times. The problem has been that pulling off the feat requires rather intense fields—on the order of those produced by laboratory-scale superconducting magnets. That’s hardly a practical approach for creating integrated electronic and photonic devices on the chip scale.

One alternative explored in the past decade has been creating a pseudo-magnetic field within the graphene itself, through judicious strain engineering. A variety of studies have shown that straining graphene flakes at the nanoscale can generate strong localized gauge fields, effectively giving rise to enormous pseudo-magnetic fields—as beefy as 800 T in one recent study. Those fields, in turn, should in principle allow development of so-called Landau quantization (discrete energy levels tied to electron motion in a magnetic field) in the graphene, thereby building an energy-gap structure in an otherwise zero-band-gap material.

Nanopillar array

Top: Nonuniform tensile strain at the edges of graphene nanopillars induces pseudomagnetic fields of opposite signs. Bottom: SEM image of nanopillar array (scale bars: 2 µm in main image, 1 µm in inset). [Image: D.-H. Kang et al., Nat. Commun., doi: 10.1038/s41467-021-25304-0 (2021); CC-BY 4.0]

What’s been missing from the picture has been experimental testing of how these giant pseudo-magnetic fields actually affect graphene’s optical properties. To take a step in that direction, Dong-Ho Kang, a postdoctoral fellow on the research team of Dongkuk Nam at Nanyang Technological University, Singapore, and colleagues drilled down into the dynamics of “hot” charge carriers—electrons and holes—in strained graphene.

To create a platform for the experiments, the team started out with a rectilinear array of nanopillars, chemically etched into an SiO₂/Si substrate and then topped with a 20-nm-thick Al₂O₃ layer. A graphene layer was then wet-transferred onto the top of the nanopillar array, and muscled into conforming to the array topography via capillary forces. Graphene’s exceptional mechanical properties allowed the accumulation of large, localized tensile strains—and, thus, the potential of strong pseudo-magnetic fields—at the nanopillar boundaries.

The team then used scanning electron microscopy and Raman spectroscopy to characterize the strain distribution in detail. Numerical modeling using the measured strain values suggested that the nanopillar-deformed graphene should host pseudo-magnetic fields as high as 100 T near the points of strongest deformation.

Measuring carrier relaxation

Finally, to see how such deformation-induced fields might affect the graphene’s optical properties, the Singapore researchers dug down into the strained graphene’s charge-carrier dynamics. Specifically, they fired femtosecond pump pulses into the material to excite charge carriers, followed by probe pulses at varying delay times to suss out how long it took the carriers to relax down to their original energy level.

The team found that the strained graphene sported carrier-relaxation times more than an order of magnitude longer than in unstrained graphene. Further experiments and numerical modeling suggested that the electron behavior was consistent with the formation of pseudo-Landau levels—and, thus, the creation of an energy-gap structure—in the graphene.

Graphene lasers ahead?

Pump–probe experiments (top) demonstrated a significant deceleration of the relaxation time of charge carriers in strained versus unstrained graphene, consistent with the development of Landau quantization in the material. The behavior could, according to the authors, enable the creation of pseudo-Landau-level lasers and other new optoelectronic devices in graphene. [Image: D.-H. Kang et al., Nat. Commun., doi: 10.1038/s41467-021-25304-0 (2021); CC-BY 4.0]

Lead author Dong-Ho Kang told OPN that under this system, the pseudo-magnetic field “can be easily tuned by varying the size of the nanopillars.” That, he maintains, makes it possible to have “an infinite number of unique graphene devices with different band gaps.”

More intriguing still, Kang says, is the possibility of leveraging these techniques to create graphene-based lasers. In a theoretical study published early this year in Optics Express, a number of authors on the new study argued that Landau quantization due to pseudo-magnetic fields in strained graphene could make the material “an excellent gain medium” that would support the building of chip-scale graphene lasers.

Thus the team’s demonstration and analysis of the optical properties in the strained 2D material should, Kang maintains, help with the project of “realizing the world’s strongest, thinnest lasers.” Such graphene lasers, he says, would “complete the last missing link toward the realization of all-graphene electronic-photonic integrated circuits … which is anticipated to revolutionize the way computer chips work.”