New approach could boost energy capacity of lithium batteries

Molecular diagram shows the structure of molybdenum sulfide, one of the materials used to create the new kind of cathode for lithium-sulfur batteries.

Researchers around the globe have been on a quest for batteries that pack a punch but are smaller and lighter than today's versions, potentially enabling electric cars to travel further or portable electronics to run for longer without recharging. Now, researchers at MIT and in China say they've made a major advance in this area, with a new version of a key component for lithium batteries, the cathode.

The team describes their concept as a "hybrid" cathode, because it combines aspects of two different approaches that have been used before, one to increase the energy output per pound (gravimetric energy density), the other for the energy per liter (volumetric energy density). The synergistic combination, they say, produces a version that provides the benefits of both, and more.

The work is described today in the journal Nature Energy, in a paper by Ju Li, an MIT professor of nuclear science and engineering and of materials science and engineering; Weijiang Xue, an MIT postdoc; and 13 others.

Today's lithium-ion batteries tend to use cathodes (one of the two electrodes in a battery) made of a transition metal oxide, but batteries with cathodes made of sulfur are considered a promising alternative to reduce weight. Today, the designers of lithium-sulfur batteries face a tradeoff.

The cathodes of such batteries are usually made in one of two ways, known as intercalation types or conversion types. Intercalation types, which use compounds such as lithium cobalt oxide, provide a high volumetric energy density -- packing a lot of punch per volume because of their high densities. These cathodes can maintain their structure and dimensions while incorporating lithium atoms into their crystalline structure.

The other cathode approach, called the conversion type, uses sulfur that gets transformed structurally and is even temporarily dissolved in the electrolyte. "Theoretically, these [batteries] have very good gravimetric energy density," Li says. "But the volumetric density is low," partly because they tend to require a lot of extra materials, including an excess of electrolyte and carbon, used to provide conductivity.

In their new hybrid system, the researchers have managed to combine the two approaches into a new cathode that incorporates both a type of molybdenum sulfide called Chevrel-phase, and pure sulfur, which together appear to provide the best aspects of both. They used particles of the two materials and compressed them to make the solid cathode. "It is like the primer and TNT in an explosive, one fast-acting, and one with higher energy per weight," Li says.

Among other advantages, the electrical conductivity of the combined material is relatively high, thus reducing the need for carbon and lowering the overall volume, Li says. Typical sulfur cathodes are made up of 20 to 30 percent carbon, he says, but the new version needs only 10 percent carbon.

The net effect of using the new material is substantial. Today's commercial lithium-ion batteries can have energy densities of about 250 watt-hours per kilogram and 700 watt-hours per liter, whereas lithium-sulfur batteries top out at about 400 watt-hours per kilogram but only 400 watt-hours per liter. The new version, in its initial version that has not yet gone through an optimization process, can already reach more than 360 watt-hours per kilogram and 581 watt-hours per liter, Li says. It can beat both lithium-ion and lithium-sulfur batteries in terms of the combination of these energy densities.

With further work, he says, "we think we can get to 400 watt-hours per kilogram and 700 watt-hours per liter," with that latter figure equaling that of lithium-ion. Already, the team has gone a step further than many laboratory experiments aimed at developing a large-scale battery prototype: Instead of testing small coin cells with capacities of only several milliamp-hours, they have produced a three-layer pouch cell (a standard subunit in batteries for products such as electric vehicles) with a capacity of more than 1,000 milliamp-hours. This is comparable to some commercial batteries, indicating that the new device does match its predicted characteristics.

So far, the new cell can't quite live up to the longevity of lithium-ion batteries in terms of the number of charge-discharge cycles it can go through before losing too much power to be useful. But that limitation is "not the cathode's problem"; it has to do with the overall cell design, and "we're working on that," Li says. Even in its present early form, he says, "this may be useful for some niche applications, like a drone with long range," where both weight and volume matter more than longevity.

"I think this is a new arena for research," Li says.

Hubble watches spun-up asteroid coming apart

This Hubble Space Telescope image reveals the gradual self-destruction of an asteroid, whose ejected dusty material has formed two long, thin, comet-like tails. The longer tail stretches more than 500,000 miles (800,000 kilometers) and is roughly 3,000 miles (4,800 kilometers) wide. The shorter tail is about a quarter as long. The streamers will eventually disperse into space.

A small asteroid has been caught in the process of spinning so fast it's throwing off material, according to new data from NASA's Hubble Space Telescope and other observatories.

Images from Hubble show two narrow, comet-like tails of dusty debris streaming from the asteroid (6478) Gault. Each tail represents an episode in which the asteroid gently shed its material -- key evidence that Gault is beginning to come apart.

Discovered in 1988, the 2.5-mile-wide (4-kilometer-wide) asteroid has been observed repeatedly, but the debris tails are the first evidence of disintegration. Gault is located 214 million miles (344 million kilometers) from the Sun. Of the roughly 800,000 known asteroids between Mars and Jupiter, astronomers estimate that this type of event in the asteroid belt is rare, occurring roughly once a year.

Watching an asteroid become unglued gives astronomers the opportunity to study the makeup of these space rocks without sending a spacecraft to sample them.

"We didn't have to go to Gault," explained Olivier Hainaut of the European Southern Observatory in Germany, a member of the Gault observing team. "We just had to look at the image of the streamers, and we can see all of the dust grains well-sorted by size. All the large grains (about the size of sand particles) are close to the object and the smallest grains (about the size of flour grains) are the farthest away because they are being pushed fastest by pressure from sunlight."

Gault is only the second asteroid whose disintegration has been strongly linked to a process known as a YORP effect. (YORP stands for "Yarkovsky-O'Keefe-Radzievskii-Paddack," the names of four scientists who contributed to the concept.) When sunlight heats an asteroid, infrared radiation escaping from its warmed surface carries off angular momentum as well as heat. This process creates a tiny torque that can cause the asteroid to continually spin faster. When the resulting centrifugal force starts to overcome gravity, the asteroid's surface becomes unstable, and landslides may send dust and rubble drifting into space at a couple miles per hour, or the speed of a strolling human. The researchers estimate that Gault could have been slowly spinning up for more than 100 million years.

Piecing together Gault's recent activity is an astronomical forensics investigation involving telescopes and astronomers around the world. All-sky surveys, ground-based telescopes, and space-based facilities like the Hubble Space Telescope pooled their efforts to make this discovery possible.

The initial clue was the fortuitous detection of the first debris tail, observed on Jan. 5, 2019, by the NASA-funded Asteroid Terrestrial-Impact Last Alert System (ATLAS) telescope in Hawaii. The tail also turned up in archival data from December 2018 from ATLAS and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) telescopes in Hawaii. In mid-January, a second shorter tail was spied by the Canada-France-Hawaii Telescope in Hawaii and the Isaac Newton Telescope in Spain, as well as by other observers. An analysis of both tails suggests the two dust events occurred around Oct. 28 and Dec. 30, 2018.

Follow-up observations with the William Herschel Telescope and ESA's (European Space Agency) Optical Ground Station in La Palma and Tenerife, Spain, and the Himalayan Chandra Telescope in India measured a two-hour rotation period for the object, close to the critical speed at which a loose "rubble-pile" asteroid begins to break up.

"Gault is the best 'smoking gun' example of a fast rotator right at the two-hour limit," said team member Jan Kleyna of the University of Hawaii in Honolulu.

An analysis of the asteroid's surrounding environment by Hubble revealed no signs of more widely distributed debris, which rules out the possibility of a collision with another asteroid causing the outbursts.

The asteroid's narrow streamers suggest that the dust was released in short bursts, lasting anywhere from a few hours to a few days. These sudden events puffed away enough debris to make a "dirt ball" approximately 500 feet (150 meters) across if compacted together. The tails will begin fading away in a few months as the dust disperses into interplanetary space.

Based on observations by the Canada-France-Hawaii Telescope, the astronomers estimate that the longer tail stretches over half a million miles (800,000 kilometers) and is roughly 3,000 miles (4,800 kilometers) wide. The shorter tail is about a quarter as long.

Only a couple of dozen active asteroids have been found so far. Astronomers may now have the capability to detect many more of them because of the enhanced survey capabilities of observatories such as Pan-STARRS and ATLAS, which scan the entire sky. "Asteroids such as Gault cannot escape detection anymore," Hainaut said. "That means that all these asteroids that start misbehaving get caught."

The researchers hope to monitor Gault for more dust events.

Deep groundwater may generate surface streams on Mars

Recurrent Slope Linae on the Palikir Crater walls on Mars

In mid-2018, researchers supported by the Italian Space Agency detected the presence of a deep-water lake on Mars under its south polar ice caps. Now, researchers at the USC Arid Climate and Water Research Center (AWARE) have published a study that suggests deep groundwater could still be active on Mars and could originate surface streams in some near-equatorial areas on Mars.

The researchers at USC have determined that groundwater likely exists in a broader geographical area than just the poles of Mars and that there is an active system, as deep as 750 meters, from which groundwater comes to the surface through cracks in the specific craters they analyzed.

Heggy, who is a member of the Mars Express Sounding radar experiment MARSIS probing Mars subsurface, and co-author Abotalib Z. Abotalib, a postdoctoral research associate at USC, studied the characteristics of Mars Recurrent Slope Linea, which are akin to dried, short streams of water that appear on some crater walls on Mars.

Scientists previously thought these features were affiliated with surface water flow or close subsurface water flow, says Heggy.

"We suggest that this may not be true. We propose an alternative hypothesis that they originate from a deep pressurized groundwater source which comes to the surface moving upward along ground cracks," Heggy says.

"The experience we gained from our research in desert hydrology was the cornerstone in reaching this conclusion. We have seen the same mechanisms in the North African Sahara and in the Arabian Peninsula, and it helped us explore the same mechanism on Mars," said Abotalib Z. Abotalib, the paper's first author.

The two scientists concluded that fractures within some of Mars' craters, enabled water springs to rise up to the surface as a result of pressure deep below. These springs leaked onto the surface, generating the sharp and distinct linear features found on the walls of these craters. The scientists also provide an explanation on how these water features fluctuate with seasonality on Mars.

The study, to be published on March 28, 2018, in Nature Geoscience, suggests that groundwater might be deeper than previously thought in areas where such streams are observed on Mars. The findings suggest that the exposed part of these ground fractures associated with these springs as the primary location candidates to explore Mars' habitability. Their work suggests that new probing methods should be developed to study these fractures.

Method:

Previous research to explore groundwater on Mars relied on interpreting the returned electromagnetic echoes sent from the radar-probing experiments from orbit onboard Mars Express and Mars Reconnaissance Orbiter. These experiments measured the reflection of the waves from both the surface and the subsurface whenever penetration was possible. However, this earlier method did not yet provide evidence of groundwater occurrence beyond the 2018 South Pole detection.

The authors of this current Nature Geoscience study used hi-resolution optical images and modeling to study the walls of large impact craters on Mars. The goal was to correlate the presence of fractures with the sources of streams that generate short water flows.

Heggy and Abotalib, who have long studied subsurface aquifers and groundwater flow movement on Earth and in desert environments, found similarities between the groundwater moving mechanisms in the Sahara and on Mars.

"Groundwater is strong evidence for the past similarity between Mars and Earth -- it suggest they have a similar evolution, to some extent," says Heggy.

He says this deep source of groundwater is the most convincing evidence of similarities between the two planets -- it suggest both may have had wet periods long enough to create such an active groundwater system.

For Heggy, an advocate for water science and water science education in arid areas, this particular study is not about colonization. But he says these rare and puzzling water flows on Mars are of big interest to the science community.

"Understanding how groundwater has formed on Mars, where it is today and how it is moving helps us constrain ambiguities on the evolution of climatic conditions on Mars for the last three billion years and how these conditions formed this groundwater system. It helps us to understand the similarities to our own planet and if we are going through the same climate evolution and the same path that Mars is going. Understanding Mars' evolution is crucial for understanding our own Earth's long-term evolution and groundwater is a key element in this process. "

The new study suggests that the groundwater that is the source of these water flows could be at depths starting at 750 meters deep. "Such depth requires us to consider more deep-probing techniques to look for the source of this groundwater versus looking for shallow sources of water, " says Heggy.

The work is funded under NASA Planetary Geology and Geophysics Program.

Quantum optical cooling of nanoparticles

A tightly focused laser field traps a nanoparticle between two highly reflecting mirrors, i.e. an optical cavity. Preferential scattering along this optical resonator allows to induce cooling of the nanoparticle motion in all three directions.

One important requirement to see quantum effects is to remove all thermal energy from the particle motion, i.e. to cool it as close as possible to absolute zero temperature. Researchers  are now one step closer to reaching this goal by demonstrating a new method for cooling levitated nanoparticles.

Tightly focused laser beams can act as optical "tweezers" to trap and manipulate tiny objects, from glass particles to living cells. The development of this method has earned Arthur Ashkin the last years Nobel prize in physics. While most experiments thus far have been carried out in air or liquid, there is an increasing interest for using optical tweezers to trap objects in ultra-high vacuum: such isolated particles not only exhibit unprecedented sensing performance, but can also be used to study fundamental processes of nanoscopic heat engines, or quantum phenomena involving large masses.

A key element in these research efforts is to obtain full control over the particle motion, ideally in a regime where the laws of quantum physics dominate its behavior. Previous attempts to achieve this, have either modulated the optical tweezer itself, or immersed the particle into additional light fields between highly reflecting mirror configurations, i.e. optical cavities. However, laser noise and large required laser intensities have posed a substantial limit to these methods. "Our new cooling scheme is directly borrowed from the atomic physics community, where similar challenges for quantum control exist," says Uros Delic, lead author of the recent study published in Physical Review Letters by researchers at the University of Vienna, the Austrian Academy of Sciences and the Massachusetts Institute of Technology (MIT), which was headed by Markus Aspelmeyer. The idea goes back to early works from Innsbruck physicist Helmut Ritsch and from US physicists Vladan Vuletic and Steve Chu, who realized that it is sufficient to use the light that is scattered directly from the optical tweezer itself if the particle is kept inside an initially empty optical cavity.

A nanoparticle in an optical tweezer scatters a tiny part of the tweezer light in nearly all directions. If the particle is positioned inside an optical cavity a part of the scattered light can be stored between its mirrors. As a result, photons are preferentially scattered into the optical cavity. However, this is only possible for light of specific colors, or said differently, specific photon energies. If we use tweezer light of a color that corresponds to a slightly smaller photon energy than required, the nanoparticles will "sacrifice" some of their kinetic energy to allow photon scattering into the optical cavity. This loss of kinetic energy effectively cools its motion. The method has been demonstrated for atoms before by Vladan Vuletic, a coauthor of this work. This is, however, the first time it has been applied to nanoparticles and used to cool in all three directions of motion.

"Our cooling method is much more powerful than all the previously demonstrated schemes. Without the constraints imposed by laser noise and laser power quantum behavior of levitated nanoparticles should be around the corner," says Delic.

Discovery of life-extension pathway in worms demonstrates new way to study aging

Caenorhabditis elegans 

An enzyme-blocking molecule can extend the lifespan of Caenorhabditis elegans roundworms by as much as 45 percent, largely by modulating a cannabinoid biological pathway, according to a study from scientists at Scripps Research.

The scientists, whose work is published on March 25 in Nature Chemical Biology, also showed that the lifespan-extending cannabinoid pathway in C. elegans is related in unexpected ways to cannabinoid pathways found in humans and other mammals.

"This study reveals a new life-extension pathway, but more broadly, it introduces a powerful method for applying chemical probes to lab animals such as worms to discover biology that may be relevant to humans," says study senior author Benjamin Cravatt, PhD, Professor and Gilula Chair of Chemical Biology at Scripps Research.

Cravatt is known for his development of advanced "chemical proteomics" methods for studying enzymes and the biological pathways they regulate. In the new study his team deployed these methods to investigate aging in C. elegans roundworms. The tiny worms normally live for just a few weeks -- compared to two or three years for lab mice -- making them, in principle, more practical for lifespan studies.

Lifespan studies using C. elegans worms typically involve the deletion or silencing of a particular gene in the embryonic stage of life to see if that extends the average lifespan of affected animals. The Cravatt team's approach, by contrast, was to use small-molecule compounds to disrupt enzyme-related pathways in adult worms, in the hope that this would uncover pathways that regulate lifespan.

"The beauty of this approach is that any lifespan-extending compounds we identify can be useful tools to study whether the same mechanisms and targets also modulate aging in mammals," says study co-author Michael Petrascheck, PhD, associate professor in the Department of Molecular Medicine at Scripps Research.

The team used a library of about 100 such compounds, all known to inhibit enzymes called serine hydrolases in mammals. "Metabolic processes are very important in determining the rate of aging and lifespan, and serine hydrolases are major metabolic enzymes, so we thought there was a good chance we'd find an important aging-related enzyme this way," says study first author Alice Chen, a graduate student in the Cravatt lab.

After finding ways to get the compounds through the tough outer skin of the worms, Chen tested them on worms that were 1 day into adulthood, and found that some of the compounds extended average worm lifespan by at least 15 percent. One, a carbamate compound called JZL184, extended worm lifespan by 45 percent at the optimal dose. More than half the worms treated with JZL184 were still alive and apparently healthy at 30 days, a time when virtually all untreated worms were dead of old age.

JZL184 was originally developed by the Cravatt lab as an inhibitor of the mammalian enzyme monoacylglycerol lipase (MAGL), whose normal job includes the breakdown of a molecule called 2-AG. The latter is an important neurotransmitter and is known as an endogenous cannabinoid ("endocannabinoid") because it activates one of the receptors hit by the main psychoactive component in cannabis.

Curiously however, a corresponding MAGL enzyme does not exist in C. elegans worms, so JZL184's target in these animals was a mystery. Chen soon found, though, that one of the main target enzymes for JZL184 in worms was fatty acid amide hydrolase 4 (FAAH-4). Although FAAH-4 and MAGL are not related in terms of their amino-acid sequences or 3-D folds, further experiments revealed, surprisingly, that FAAH-4 in worms does what MAGL does in humans and other mammals: it breaks down 2-AG.

2-AG has been linked to aging in mammals; one recent study found evidence that its levels fall in the brains of aging mice, likely due to greater MAGL activity. The results suggest, then, that studying the FAAH-4/2-AG pathway in worms could one day yield lifespan-extending strategies for humans.

"It seems at least plausible at this point that both worms and mammals have a cannabinoid-related signaling pathway that affects longevity and possibly aging-related disorders," Cravatt says.

The study demonstrates more generally how libraries of small-molecule compounds and associated proteomics techniques can be used to reveal biological pathways that evolutionarily distant lab animals such as worms have in common with humans.

"In principle, with this approach one can quickly find a compound that has a desired biological effect and also find the target through which it works, all in a live and relatively complex model organism," Cravatt says.

The authors of the study, "Pharmacological convergence reveals a lipid pathway that regulates C. elegans lifespan," were Alice Chen, Kenneth Lum, Daisuke Ogasawara, Armand Cognetta, Alan To, William Parsons, Gabriel Simon, Michael Petrascheck, Liron Bar-Peled, and Benjamin Cravatt, all of Scripps Research; and Pablo Lara-Gonzalez and Arshad Desai of the University of California-San Diego.

The research was supported by the National Institutes of Health (DA033760, CA215249), the Damon Runyon Cancer Research Foundation, and Abide Therapeutics, Inc.