System Bits: April 30

Future batteries could use a graphene sponge
Researchers at Sweden’s Chalmers University of Technology devised a porous, sponge-like aerogel, made of reduced-graphene oxide, to serve as a freestanding electrode in the battery cell. This utilization has the potential to advance lithium sulfur batteries, which are said to possess a theoretical energy density about five times greater than lithium-ion batteries.

This catholyte (a liquid combining a cathode with an electrolyte), built around a graphene-based “sponge,” would provide the basis for lithium sulfur batteries.

Taking a standard coin cell battery case, the researchers first insert a thin layer of the porous graphene aerogel.

“You take the aerogel, which is a long thin tube, and then you slice it – almost like a salami. You take that slice, and compress it, to fit into the battery,” says Carmen Cavallo of the Department of Physics at Chalmers, and lead researcher on the study. Then, a sulfur-rich solution – the catholyte – is added to the battery. The highly porous aerogel acts as the support, soaking up the solution like a sponge.

“The porous structure of the graphene aerogel is key. It soaks up a high amount of the catholyte, giving you high enough sulfur loading to make the catholyte concept worthwhile. This kind of semi-liquid catholyte is really essential here. It allows the sulfur to cycle back and forth without any losses. It is not lost through dissolution – because it is already dissolved into the catholyte solution,” says Carmen Cavallo.

“Furthermore, sulfur is cheap, highly abundant, and much more environmentally friendly. Lithium sulfur batteries also have the advantage of not needing to contain any environmentally harmful fluorine, as is commonly found in lithium ion batteries,” says Aleksandar Matic, Professor at Chalmers Department of Physics, who leads the research group behind the paper.

The problem with lithium sulfur batteries so far has been their instability, and consequent low cycle life. Current versions degenerate fast and have a limited life span with an impractically low number of cycles. But in testing of their new prototype, the Chalmers researchers demonstrated an 85% capacity retention after 350 cycles.

Image credit: Qian Cheng, Columbia Engineering

Meanwhile, at Columbia University in New York City, a Columbia Engineering team reports coming up with a new technique for safely prolonging the life of batteries by adding a nanocoating of boron nitride to a stable solid electrolyte of lithium metal batteries.

“We decided to focus on solid, ceramic electrolytes. They show great promise in improving both safety and energy density, as compared with conventional, flammable electrolytes in Li-ion batteries,” says Yuan Yang, assistant professor of materials science and engineering, who led the team. “We are particularly interested in rechargeable solid-state lithium batteries because they are promising candidates for next-generation energy storage.”

Most solid electrolytes are ceramic, and therefore non-flammable, eliminating safety concerns. In addition, solid ceramic electrolytes have a high mechanical strength that can actually suppress lithium dendrite growth, making lithium metal a coating option for battery anodes. However, most solid electrolytes are unstable against Li—they can be easily corroded by lithium metal and cannot be used in batteries.

“Lithium metal is indispensable for enhancing energy density and so it’s critical that we be able to use it as the anode for solid electrolytes,” says Qian Cheng, the paper’s lead author and a postdoctoral research scientist in the department of applied physics and applied mathematics who works in Yang’s group. “To adapt these unstable solid electrolytes for real-life applications, we needed to develop a chemically and mechanically stable interface to protect these solid electrolytes against the lithium anode. It is essential that the interface not only be highly electronically insulating, but also ionically conducting in order to transport lithium ions. Plus, this interface has to be super-thin to avoid lowering the energy density of batteries.”

To address these challenges, the team worked with colleagues at Brookhaven National Lab and the City University of New York. “While earlier studies used polymeric protection layers as thick as 200 µm, our BN protective film, at only 5~10 nm thick, is record-thin—at the limit of such protection layers—without lowering the energy density of batteries,” Cheng says. “It’s the perfect material to function as a barrier that prevents the invasion of lithium metal to solid electrolyte. Like a bullet-proof vest, we’ve developed a lithium-metal-proof ‘vest’ for unstable solid electrolytes and, with that innovation, achieved long-cycling lifetime lithium metal batteries.”

Developing light sources with superinjection technology
Lasers and light-emitting diodes are light sources made with semiconductor heterostructures, using a physical effect known as superinjection. Research into this technology was conducted in the 1960s, resulting in laser printers and advanced networking. For years, it was believed that superinjection was only possible in heterostructures, structures composed of two or more semiconductor materials. Moscow Institute of Physics and Technology researchers say the superinjection effect can be achieved with semiconductor homostructures, made of a single material.

The institute’s Igor Khramtsov and Dmitry Fedyanin made a discovery that drastically changes the perspective on how light-emitting devices can be designed. The physicists found that it is possible to achieve superinjection with just one material. What is more, most of the known semiconductors can be used.

“In the case of silicon and germanium, superinjection requires cryogenic temperatures, and this casts doubt on the utility of the effect. But in diamond or gallium nitride, strong superinjection can occur even at room temperature,” Dr. Fedyanin said. This means that the effect can be used to create mass market devices. According to the new paper, superinjection can produce electron concentrations in a diamond diode that are 10,000 times higher than those previously believed to be ultimately possible. As a result, diamond can serve as the basis for ultraviolet LEDs thousands of times brighter than what the most optimistic theoretical calculations predicted. “Surprisingly, the effect of superinjection in diamond is 50 to 100 times stronger than that used in most mass market semiconductor LEDs and lasers based on heterostructures,” Khramtsov pointed out.

The physicists emphasized that superinjection should be possible in a wide range of semiconductors, from conventional wide-bandgap semiconductors to novel two-dimensional materials. This opens up new prospects for designing highly efficient blue, violet, ultraviolet, and white LEDs, as well as light sources for optical wireless communication (Li-Fi), new types of lasers, transmitters for the quantum Internet, and optical devices for early disease diagnostics.

Using 5G tech to protect the environment
Fifth-generation cellular communications will offer faster mobile Internet connections and many other factors in mobile technology. “Such is the promise of 5G technology, at least for urban millennials who enjoy hacking their commute. But the most important impact of 5G will be on the health and safety of everyone who lives, works, and plays in a city,” writes Aileen Nowlan of the Environmental Defense Fund.

The areas where 5G tech will shine in the future include deliveries and returns, multi-modal transportation, pollution monitoring, and road safety, she writes.

“To install new 5G equipment, companies will need to access hundreds of thousands of light/electrical poles across the country — an immense opportunity to build out the digital infrastructure for data-ready cities at the same time. Where companies are building from scratch rather than retrofitting, it is incredibly low cost to add smart city infrastructure for safety, pollution monitoring, and a circular economy.

“While it will take some time for 5G networks to roll out, many wonders will soon be possible, including environmental innovations we can’t even imagine today,” Nowlan concludes.