The growing popularity of lithium-ion batteries in recent years has put a strain on the world’s supply of cobalt and nickel—two metals integral to current battery designs—and sent prices surging.
In a bid to develop alternative designs for lithium-based batteries with less reliance on those scarce metals, researchers at the Georgia Institute of Technology have developed a promising new cathode and electrolyte system that replaces expensive metals and traditional liquid electrolyte with lower cost transition metal fluorides and a solid polymer electrolyte.
“Electrodes made from transition metal fluorides have long shown stability problems and rapid failure, leading to significant skepticism about their ability to be used in next generation batteries,” said Gleb Yushin, a professor in Georgia Tech’s School of Materials Science and Engineering. “But we’ve shown that when used with a solid polymer electrolyte, the metal fluorides show remarkable stability—even at higher temperatures—which could eventually lead to safer, lighter and cheaper lithium-ion batteries.”
In a typical lithium-ion battery, energy is released during the transfer of lithium ions between two electrodes—an anode and a cathode, with a cathode typically comprising lithium and transition metals such as cobalt, nickel and manganese. The ions flow between the electrodes through a liquid electrolyte.
For the study, which was published Sept. 9 in the journal Nature Materials and sponsored by the Army Research Office, the research team fabricated a new type of cathode from iron fluoride active material and a solid polymer electrolyte nanocomposite. Iron fluorides have more than double the lithium capacity of traditional cobalt- or nickel-based cathodes. In addition, iron is 300 times cheaper than cobalt and 150 times cheaper than nickel.
To produce such a cathode, the researchers developed a process to infiltrate a solid polymer electrolyte into the prefabricated iron fluoride electrode. They then hot pressed the entire structure to increase density and reduce any voids.
Gleb Yushin, a professor in Georgia Tech’s School of Materials Science and Engineering and Kostiantyn Turcheniuk, research scientist in Yushin’s lab, inspect a battery using a new cathode design that replaces expensive metals and traditional liquid electrolyte with lower cost transition metal fluorides and a solid polymer electrolyte. Credit: Allison Carter
Two central features of the polymer-based electrolyte are its ability to flex and accommodate the swelling of the iron fluoride while cycling and its ability to form a very stable and flexible interphase with iron fluoride. Traditionally, that swelling and massive side reactions have been key problems with using iron fluoride in previous battery designs.
“Cathodes made from iron fluoride have enormous potential because of their high capacity, low material costs and very broad availability of iron,” Yushin said. “But the volume changes during cycling as well as parasitic side reactions with liquid electrolytes and other degradation issues have limited their use previously. Using a solid electrolyte with elastic properties solves many of these problems.”
The researchers then tested several variations of the new solid-state batteries to analyze their performance over more than 300 cycles of charging and discharging at elevated temperature of 122 degrees Fahrenheit, noting that they outperformed previous designs using metal fluoride even when these were kept cool at room temperatures.
The researchers found that the key to the enhanced battery performance was the solid polymer electrolyte. In previous attempts to use metal fluorides, it was believed that metallic ions migrated to the surface of the cathode and eventually dissolved into the liquid electrolyte, causing a capacity loss, particularly at elevated temperatures. In addition, metal fluorides catalyzed massive decomposition of liquid electrolytes when cells were operating above 100 degrees Fahrenheit. However, at the connection between the solid electrolyte and the cathode, such dissolving doesn’t take place and the solid electrolyte remains remarkably stable, preventing such degradations, the researchers wrote.
“The polymer electrolyte we used was very common, but many other solid electrolytes and other battery or electrode architectures—such as core-shell particle morphologies—should be able to similarly dramatically mitigate or even fully prevent parasitic side reactions and attain stable performance characteristics,” said Kostiantyn Turcheniuk, research scientist in Yushin’s lab and a co-author of the manuscript.
In the future, the researchers aim to develop new and improved solid electrolytes to enable fast charging and also to combine solid and liquid electrolytes in new designs that are fully compatible with conventional cell manufacturing technologies employed in large battery factories.
Tesla watchers know that Jeff Dahn and his team at Dalhousie University near Halifax, Nova Scotia, are world leaders in lithium-ion battery research. For years, Dahn worked exclusively for 3M, but when that arrangement ended, Tesla swooped in and signed a contract for Dahn to work for the Silicon Valley car/tech/energy company.
In addition, solid-state batteries are less like to catch fire or explode if they get too hot. That in turn means electric car manufacturers can make simpler, less costly cooling systems for their battery packs, driving down the cost of EVs. It also reassures the public their shiny new electric cars aren’t going to explode in the garage, as recently happened to the owner of a Hyundai Kona EV in Canada.
Research published by Dahn and his team in the journal Nature Energy on July 15 reveals they have created new lithium-ion pouch cells that may outperform solid-state technology battery. Here’s the abstract of that research report:
“Cells with lithium-metal anodes are viewed as the most viable future technology, with higher energy density than existing lithium-ion batteries. Many researchers believe that for lithium-metal cells, the typical liquid electrolyte used in lithium-ion batteries must be replaced with a solid-state electrolyte to maintain the flat, dendrite-free lithium morphologies necessary for long-term stable cycling.
“Here, we show that anode-free lithium-metal pouch cells with a dual-salt LiDFOB/LiBF4 liquid electrolyte have 80% capacity remaining after 90 charge–discharge cycles, which is the longest life demonstrated to date for cells with zero excess lithium. The liquid electrolyte enables smooth dendrite-free lithium morphology comprised of densely packed columns even after 50 charge — discharge cycles. NMR measurements reveal that the electrolyte salts responsible for the excellent lithium morphology are slowly consumed during cycling.”
Credit: Jeff Dahn, et al./Nature Energy
Those pesky dendrites are the bane of lithium-ion batteries. They are little projections like stalagmites in caves that can poke through the insulating layer inside individual cells, leading to short circuits and potential fires. Eliminating them would be a big step forward, particularly for use in electric vehicles.
Is Tesla on the verge of replacing the cylindrical cells in its battery packs with Jeff Dahn’s pouch cells? Not just yet. There is a lot of research and testing left to do before they becomes suitable for commercial production, but they may signal an important step forward for energy storage in the years ahead.
Below is a video of Dahn when he won the prestigious National Sciences and Engineering Research Council of Canada award in 2017. Here is a fellow who knows what he is talking about. If he says pouch cells can outperform solid state cells, we should pay heed.
Dr. Stephen Campbell, Chief Technology Officer at Nano One Materials Corporation has announced the issuance of US Patent No. 10,374,232. In the race to commercialize lithium ion battery powered electric vehicles, this patent adds value to Nano One’s high energy cathode materials as it defines the unique physical form of the powdered materials and provides a proprietary means of improving durability, safety, handling and cost.
Dr. Campbell said “This patent is particularly significant as it defines the properties of our high energy NMC cathode powders, rather than the underlying process to make them. These powders have unique physical properties, related to size and nanostructure, that Nano One is exploiting for improved durability, handling, safety and cost. It complements our process patent portfolio and adds substantially to our strategy with recently announced automotive partners to develop a new generation of low cost and durable high energy cathodes.”
NMC cathodes are typically comprised of lithium, nickel, manganese and cobalt. There are global initiatives underway to increase nickel for more energy and reduce cobalt to mitigate supply chain risk. However, this shift to nickel-rich materials compromises stability and safety in the battery, and the air sensitive materials require special handling. Nano One’s unique powders are differentiated from these efforts and they enable an innovative approach to lowering cost and increasing the durability of NMC powders.
Utilizing proprietary manufacturing technologies, which are themselves protected by patents in the US, Canada, Taiwan, China, Japan and Korea, Nano One is able to carefully control the formation of lithium ion battery materials resulting in unique forms and improved electrical properties. The improved NMC materials themselves are now patent protected in the US and Korea.
“The granting of this patent is great news”, said Dr. Joseph Guy, Director of Nano One and Patent Agent. “Our NMC powders are different because of very fine particles and layered nanostructures. It gives Nano One a sustainable means of differentiating its NMC cathode powder for improved performance and cost in lithium ion batteries. This is an important cornerstone in the execution of Nano One’s business plan and provides valuable leverage going forward.”
April – 2019 – Rensselaer Polytechnic Institute – Material Science
Creating a lithium-ion battery that can charge in a matter of minutes but still operate at a high capacity is possible, according to research from Rensselaer Polytechnic Institute just published in Nature Communications. This development has the potential to improve battery performance for consumer electronics, solar grid storage, and electric vehicles.
A lithium-ion battery charges and discharges as lithium ions move between two electrodes, called an anode and a cathode. In a traditional lithium-ion battery, the anode is made of graphite, while the cathode is composed of lithium cobalt oxide.
These materials perform well together, which is why lithium-ion batteries have become increasingly popular, but researchers at Rensselaer believe the function can be enhanced further.
“The way to make batteries better is to improve the materials used for the electrodes,” said Nikhil Koratkar, professor of mechanical, aerospace, and nuclear engineering at Rensselaer, and corresponding author of the paper. “What we are trying to do is make lithium-ion technology even better in performance.”
Vanadium disulfide – a promising new monolayer material for Li-ion batteries
Koratkar’s extensive research into nanotechnology and energy storage has placed him among the most highly cited researchers in the world. In this most recent work, Koratkar and his team improved performance by substituting cobalt oxide with vanadium disulfide (VS2).
“It gives you higher energy density, because it’s light. And it gives you faster charging capability, because it’s highly conductive. From those points of view, we were attracted to this material,” said Koratkar, who is also a professor in the Department of Materials Science and Engineering.
Excitement surrounding the potential of VS2 has been growing in recent years, but until now, Koratkar said, researchers had been challenged by its instability–a characteristic that would lead to short battery life. The Rensselaer researchers not only established why that instability was happening, but also developed a way to combat it.
The team, which also included Vincent Meunier, head of the Department of Physics, Applied Physics, and Astronomy, and others, determined that lithium insertion caused an asymmetry in the spacing between vanadium atoms, known as Peierls distortion, which was responsible for the breakup of the VS2 flakes. They discovered that covering the flakes with a nanolayered coating of titanium disulfide (TiS2)–a material that does not Peierls distort–would stabilize the VS2 flakes and improve their performance within the battery.
“This was new. People hadn’t realized this was the underlying cause,” Koratkar said. “The TiS2 coating acts as a buffer layer. It holds the VS2 material together, providing mechanical support.”
Once that problem was solved, the team found that the VS2-TiS2 electrodes could operate at a high specific capacity, or store a lot of charge per unit mass. Koratkar said that vanadium and sulfur’s small size and weight allow them to deliver a high capacity and energy density. Their small size would also contribute to a compact battery.
When charging was done more quickly, Koratkar said, the capacity didn’t dip as significantly as it often does with other electrodes. The electrodes were able to maintain a reasonable capacity because, unlike cobalt oxide, the VS2-TiS2 material is electrically conductive.
Koratkar sees multiple applications for this discovery in improving car batteries, power for portable electronics, and solar energy storage where high capacity is important, but increased charging speed would also be attractive.
** Contributed from Nature Communications Open Source Article
The ever-increasing demands for advanced lithium-ion batteries have greatly stimulated the quest for robust electrodes with a high areal capacity. Producing thick electrodes from a high-performance active material would maximize this parameter. However, above a critical thickness, solution-processed films typically encounter electrical/mechanical problems, limiting the achievable areal capacity and rate performance as a result.
Herein, we show that two-dimensional titanium carbide or carbonitride nano sheets, known as MXenes, can be used as a conductive binder for silicon electrodes produced by a simple and scalable slurry-casting technique without the need of any other additives.
“The nano sheets form a continuous metallic network, enable fast charge transport and provide good mechanical reinforcement for the thick electrode (up to 450μm). Consequently, very high areal capacity anodes (up to 23.3 mAh cm-2) have been demonstrated.” Utilization of Li-ion chemistry to store the energy electro-chemically can address the ever-increasing demands from both portable electronics and hybrid electrical vehicles.
Such stringent challenges on the battery safety and lifetime issues require high-performance battery components, with most of the focus being on electrodes or electrolytes with novel nano-structures and chemistries.
However, equally important is the development of electrode additives, which are required to main-tain the electrode’s conductive network and mechanical integrity. Traditionally, electrode additives are made of dual components based on a conductive agent (i.e. carbon black, CB) and a poly-meric binder.
While the former ensures the charge transport throughout the electrode, the latter mechanically holds the active materials and CB together during cycling. Although these traditional electrode additives have been widely applied in Li-ion battery technologies, they fail to perform well in high-capacity electrodes, especially those displaying large volume changes.
This is because the polymeric binder is not mechanically robust enough to withstand the stress induced during lithiation/deli-thiation, leading to severe disruption of the conducting networks. This results in rapid capacity fade and poor lifetime.
In summary, the efficient utilization of 2D MXene nanosheets as a new class of conductive binder for high volume-change Si electrodes is of fundamental importance to the electrochemical energy storage field.
The continuous network of MXene nanosheets not only provides sufficient electrical conductivity and free space for accommodating the volume change issue but also well resolves the mechanical instability of Si. Therefore, the combination of viscous MXene ink and high-capacity Si demonstrated here offers a powerful technique to construct advanced nanostructures with exceptional performance.
Of equal importance is that the formation of these high-mass-loading Si/MXene electrodes can be achieved by means of a commercially compatible, slurry-casting technique, which is highly scalable and low cost, allowing for large-area production of high-performance, Si-based electrodes for advanced batteries.
Considering that more than 30 MXenes are already reported, with more predicted to exist, there is certainly much room for further improving the electrochemical performance of such electrodes by tuning the electrical, mechanical and physicochemical properties of this exciting 2D MXene family.
Professor Valeria Nicolosi
Professor of Nanomaterials and Advanced Microscopy at Trinity College Dublin
How to build a better Battery through Nanotechnology
PALO ALTO, CALIFORNIA (Note to Readers: This original article was published in 2016 May. Recent updates, News Releases and a YouTube Video have been provided)
On a drizzly, gray morning in April, Yi Cui weaves his scarlet red Tesla in and out of Silicon Valley traffic. Cui, a materials scientist at Stanford University here, is headed to visit Amprius, a battery company he founded 8 years ago. Amprius Latest News Release(December 2018)
It’s no coincidence that he is driving a battery-powered car, and that he has leased rather than bought it. In a few years, he says, he plans to upgrade to a new model, with a crucial improvement: “Hopefully our batteries will be in it.”
Cui and Amprius are trying to take lithium–ion batteries—today’s best commercial technology—to the next level. They have plenty of company. Massive corporations such as Panasonic, Samsung, LG Chem, Apple, and Tesla are vying to make batteries smaller, lighter, and more powerful. But among these power players, Cui remains a pioneering force.
Unlike others who focus on tweaking the chemical composition of a battery’s electrodes or its charge-conducting electrolyte, Cui is marrying battery chemistry with nanotechnology. He is building intricately structured battery electrodes that can soak up and release charge-carrying ions in greater quantities, and faster, than standard electrodes can, without producing troublesome side reactions. “He’s taking the innovation of nanotechnology and using it to control chemistry,” says Wei Luo, a materials scientist and battery expert at the University of Maryland, College Park.
“I wanted to change the world, and also get rich, but mainly change the world.”
In a series of lab demonstrations, Cui has shown how his architectural approach to electrodes can domesticate a host of battery chemistries that have long tantalized researchers but remained problematic. Among them: lithium-ion batteries with electrodes of silicon instead of the standard graphite, batteries with an electrode made of bare lithium metal, and batteries relying on lithium-sulfur chemistry, which are potentially more powerful than any lithium-ion battery. The nanoscale architectures he is exploring include silicon nanowires that expand and contract as they absorb and shed lithium ions, and tiny egg like structures with carbon shells protecting lithium-rich silicon yolks.
(Article continues below Video)
Watch a YouTube Video on the latest Update from Professor Cui (November 2018). A very concise and informative Summary of the State of NextGen Batteries.
** Amprius already supplies phone batteries with silicon electrodes that store 10% more energy than the best conventional lithium-ion batteries on the market.
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Another prototype beats standard batteries by 40%, and even better ones are in the works. So far, the company does not make batteries for electric vehicles (EVs), but if the technologies Cui is exploring live up to their promise, the company could one day supply car batteries able to store up to 10 times more energy than today’s top performers. That could give modest-priced EVs the same range as gas-powered models—a revolutionary advance that could help nations power their vehicle fleets with electricity provided by solar and wind power, dramatically reducing carbon emissions.
Cui says that when he started in research, “I wanted to change the world, and also get rich, but mainly change the world.” His quest goes beyond batteries. His lab is exploring nanotech innovations that are spawning startup companies aiming to provide cheaper, more effective air and water purification systems. But so far Cui has made his clearest mark on batteries. Luo calls his approach “untraditional and surprising.” Jun Liu, a materials scientist at the Pacific Northwest National Laboratory in Richland, Washington, put it more directly: Cui’s nanotech contributions to battery technology are “tremendous.”
Making leaps in battery technology is surprisingly hard to do. Even as Silicon Valley’s primary innovation, the computer chip, has made exponential performance gains for decades, batteries have lagged. Today’s best lithium-ion cells hold about 700 watt-hours per liter. That’s about five times the energy density of nickel-cadmium batteries from the mid-1980s—not bad, but not breathtaking. In the past decade, the energy density of the best commercial batteries has doubled.
Battery users want more. The market for lithium-ion batteries alone is expected to top $30 billion a year by 2020, according to a pair of recent reports by market research firms Transparency Market Research and Taiyou Research. The rise in production of EVs by car companies that include Tesla, General Motors, and Nissan accounts for some of that surge.
But today’s EVs leave much to be desired. For a Tesla Model S, depending on the exact model, the 70- to 90-kilowatt-hour batteries alone weigh 600 kilograms and account for about $30,000 of the car’s price, which can exceed $100,000. Yet they can take the car only about 400 kilometers on a single charge, substantially less than the range of many conventional cars. Nissan’s Leaf is far cheaper, with a sticker price of about $29,000. But with a smaller battery pack, its range is only about one-third that of the Tesla.
Improving batteries could make a major impact. Doubling a battery’s energy density would enable car companies to keep the driving range the same while halving the size and cost of the battery—or keep the battery size constant and double the car’s range. “The age of electric vehicles is coming,” Cui says. But in order for EVs to take over, “we have to do better.”
He recognized the need early in his career. After finishing his undergraduate degree in his native China in 1998, Cui moved first to Harvard University and then to the University of California (UC), Berkeley, to complete a Ph.D. and postdoc in labs that were pioneering the synthesis of nanosized materials. Those were the early days of nanotechnology, when researchers were struggling to get a firm handle on how to create just the materials they wanted, and the world of applications was only beginning to take shape.
While at UC Berkeley, Cui spent time with colleagues next door at the Lawrence Berkeley National Laboratory (LBNL). At the time, LBNL’s director was Steven Chu, who pushed the lab to invent renewable energy technologies that had the potential to combat climate change, among them better batteries for storing clean energy. (Chu later went on to serve as President Barack Obama’s secretary of energy from 2009 to 2013.)
“At the beginning, I wasn’t thinking about energy. I had never worked on batteries,” Cui says. But Chu and others impressed on him that nanotechnology could give batteries an edge.
As Chu says now, it offers “a new knob to turn, and an important one,” enabling researchers to control not only the chemical composition of materials on the smallest scales, but also the arrangement of atoms within them—and thus how chemical reactions involving them proceed.
After moving to Stanford, Cui quickly gravitated to the nexus between nanotechnology and the electrochemistry that makes batteries work—and accounts for their limitations. Take lithium-ion rechargeable batteries. In principle, these batteries are simple: They consist of two electrodes divided by a membrane “separator” and a liquid electrolyte that allows ions to glide back and forth between the electrodes.
When a battery is charging, lithium ions are released from the positive electrode, or cathode, which consists of a lithium alloy, commonly lithium cobalt oxide or lithium iron phosphate. They are drawn toward the negatively charged electrode, called the anode, which is usually made of graphite. There they snuggle in between the graphite’s sheets of carbon atoms. Voltage from an external power source drives the whole ionic mass migration, storing power.
When a device—say, a power tool or a car—is turned on and demands energy, the battery discharges: Lithium atoms in the graphite give up electrons, which travel through the external circuit to the cathode. Meanwhile, the lithium ions slip out of the graphite and zip through the electrolyte and the separator to the cathode, where they meet up with electrons that have made the journey through the circuit (see diagram below).
Nano to the rescue
Cui and colleagues have applied several nanotechnology-inspired solutions to keep silicon anodes from breaking down and to prevent battery-killing side reactions.
Graphite is today’s go-to anode material because it is highly conductive and thus readily passes collected electrons to the metal wires in a circuit. But graphite is only so-so at gathering lithium ions during charging. It takes six carbon atoms in graphite to hold on to a single lithium ion. That weak grip limits how much lithium the electrode can hold and thus how much power the battery can store.
Silicon has the potential to do far better. Each silicon atom can bind to four lithium ions. In principle, that means a silicon-based anode can store 10 times as much energy as one made from graphite. Electrochemists have struggled in vain for decades to tap that enormous capacity.
It’s easy enough to make anodes from chunks of silicon; the problem is that the anodes don’t last. As the battery is charged and lithium ions rush in to bind to silicon atoms, the anode material swells as much as 300%. Then, when the lithium ions rush out during the battery’s discharge cycle, the anode rapidly shrinks again. After only a few cycles of such torture, silicon electrodes fracture and eventually split into tiny, isolated grains. The anode—and the battery—crumbles and dies.
Cui thought he could solve the problem. His experience at Harvard and UC Berkeley had taught him that nanomaterials often behave differently from materials in bulk. For starters, they have a much higher percentage of their atoms at their surface relative to the number in their interior. And because surface atoms have fewer atomic neighbors locking them in place, they can move more easily in response to stresses and strains. Other types of atomic movement explain why thin sheets of aluminum foil or paper can bend without breaking more easily than chunks of aluminum metal or wood can.
In 2008, Cui thought that fashioning a silicon anode from nanosized silicon wires might alleviate the stress and strain that pulverize bulk silicon anodes. The strategy worked. In a paper in Nature Nanotechnology, Cui and colleagues showed that when lithium ions moved into and out of the silicon nanowires, the nanowires suffered little damage. Even after 10 repeated cycles of charging and discharging, the anode retained 75% of its theoretical energy storage capacity.
Unfortunately, silicon nanowires are much more difficult and expensive to fashion than bulk silicon. Cui and colleagues started devising ways to make cheaper silicon anodes. First, they found a way to craft lithium-ion battery anodes from spherical silicon nanoparticles. Though potentially cheaper, these faced a second problem: The shrinking and swelling of the nanoparticles as the lithium atoms moved in and out opened cracks in the glue that bound the nanoparticles together. The liquid electrolyte seeped between the particles, driving a chemical reaction that coated them in a non-conductive layer, known as a solid-electrolyte interphase (SEI), which eventually grew thick enough to disrupt the anode’s charge-collecting abilities. “It’s like scar tissue,” says Yuzhang Li, a graduate student in Cui’s lab.
A few years later, Cui and his colleagues hit on another nanotech solution. They created egg like nanoparticles, surrounding each of their tiny silicon nanoparticles—the yolk—with a highly conductive carbon shell through which lithium ions could readily pass. The shell gave silicon atoms in the yolk ample room to swell and shrink, while protecting them from the electrolyte—and the reactions that form an SEI layer. In a 2012 paper in Nano Letters, Cui’s team reported that after 1000 cycles of charging and discharging, their yolk-shell anode retained 74% of its capacity.
They did even better 2 years later. They assembled bunches of their yolk-shell nanoparticles into micrometer-scale collections resembling miniature pomegranates. Bunching the silicon spheres boosted the anode’s lithium storage capacity and reduced unwanted side reactions with the electrolyte. In a February 2014 issue of Nature Nanotechnology, the group reported that batteries based on the new material retained 97% of their original capacity after 1000 charge and discharge cycles.
Earlier this year, Cui and colleagues reported a solution that outdoes even their complex pomegranate assemblies. They simply hammered large silicon particles down to the micrometer scale and then wrapped them in thin carbon sheets made from graphene. The hammered particles wound up larger than the silicon spheres in the pomegranates—so big that they fractured after a few charging cycles. But the graphene wrapping prevented the electrolyte compounds from reaching the silicon. It was also flexible enough to maintain contact with the fractured particles and thus carry their charges to the metal wires. What’s more, the team reported in Nature Energy, the larger silicon particles packed more mass—and thus more power—into a given volume, and they were far cheaper and easier to make than the pomegranates. “He has really taken this work in the right direction,” Jun Liu says.
Powered by such ideas, Amprius has raised more than $100 million to commercialize lithium-ion batteries with silicon anodes. The company is already manufacturing cellphone batteries in China and has sold more than 1 million of them, says Song Han, the company’s chief technology officer. The batteries, based on simple silicon nanoparticles that are cheap to make, are only 10% better than today’s lithium-ion cells. But at Amprius’s headquarters, Han showed off nanowire-silicon prototypes that are 40% better. And those, he says, still represent only the beginning of how good silicon anodes will eventually become.
Now, Cui is looking beyond silicon. One focus is to make anodes out of pure lithium metal, which has long been viewed as the ultimate anode material, as it has the potential to store even more energy than silicon and is much lighter.
But there have been major problems here, too. For starters, an SEI layer normally forms around the lithium metal electrode. That’s actually good news in this case: Lithium ions can penetrate the layer, so the SEI acts as a protective film around the lithium anode. But as the battery cycles, the metal swells and shrinks just as silicon particles do, and the pulsing can break the SEI layer. Lithium ions can then pile up in the crack, causing a metal spike, known as a dendrite, to sprout from the electrode. “Those dendrites can pierce the battery separator and short-circuit the battery and cause it to catch fire,” says Yayuan Liu, another graduate student in Cui’s group.
Conventional approaches haven’t solved the problem. But nanotechnology might. In one approach to preventing dendrite formation, Cui’s team stabilizes the SEI layer by coating the anode with a layer of interconnected nanocarbon spheres. In another, they’ve created a new type of yolk-shell particle, made of gold nanoparticles inside much larger carbon shells. When the nanocapsules are fashioned into an anode, the gold attracts lithium ions; the shells give the lithium room to shrink and swell without cracking the SEI layer, so dendrites don’t form.
Improving anodes is only half the battle in making better batteries. Cui’s team has taken a similar nano inspired approach to improving cathode materials as well, in particular sulfur. Like silicon on the anode side, sulfur has long been seen as a tantalizing option for the cathode. Each sulfur atom can hold a pair of lithiums, making it possible in principle to boost energy storage several fold over conventional cathodes. Perhaps equally important, sulfur is dirt cheap. But it, too, has problems. Sulfur is a relatively modest electrical conductor, and it reacts with common electrolytes to form chemicals that can kill the batteries after a few cycles of charging and discharging. Sulfur cathodes also tend to hoard charges instead of giving them up during discharge.
Seeking a nanosolution, Cui’s team encased sulfur particles inside highly conductive titanium dioxide shells, boosting battery capacity fivefold over conventional designs and preventing sulfur byproducts from poisoning the cell. The researchers have also made sulfur-based versions of their pomegranates, and they have trapped sulfur inside long, thin nanofibers. These and other innovations have not only boosted battery capacity, but also raised a measure known as the coulombic efficiency—how well the battery releases its charges—from 86% to 99%. “Now, we have high capacity on both sides of the electrode,” Cui says.
Down the road, Cui says, he intends to put both of his key innovations together. By coupling silicon anodes with sulfur cathodes, he hopes to make cheap, high-capacity batteries that could change the way the world powers its devices. “We think if we can make it work, it will make a big impact,” Cui says.
It just might help him change the world, and get rich on the side.
Bio – Professor Yi Cui
Professor of Materials Science and Engineering, of Photon Science, Senior Fellow at the Precourt Institute for Energy and Prof, by courtesy, of Chemistry PhD, Harvard University (2002)
Cui studies nanoscale phenomena and their applications broadly defined. Research Interests: Nanocrystal and nanowire synthesis and self-assembly, electron transfer and transport in nanomaterials and at the nano interface, nanoscale electronic and photonic devices, batteries, solar cells, microbial fuel cells, water filters and chemical and biological sensors.
Specific energy and specific power of rechargeable batteries. Specific energy is the capacity a battery can hold in watt-hours per kilogram (Wh/kg); specific power is the battery’s ability to deliver power in watts per kilogram (W/kg). (Source: Battery University)
” … a research team from Sichuan University in China and Clarkson University in the U.S. have discovered a key design rule for Li metal batteries: If you want to suppress dendrites, you have to use a defect-free host. More generally, carbon defects catalyze dendrite growth in metal anodes … “
Rechargeable lithium-ion (Li-ion) batteries are the dominant technology not only for portable electronics but it also is becoming the battery of choice for electric-vehicle and electric-grid energy-storage applications.
In a Li-ion battery, the cathode (positive electrode) is a lithium metal oxide while the anode (negative electrode) is graphite. But researchers are looking for ways to replace graphite with lithium metal as the anode to boost the battery’s energy density.
Lithium metal-based batteries such as Li–sulfur and Li–air batteries have received considerable attention because the packing density of lithium atoms is the highest in its metallic form and Li metal can store 10 times more energy than graphite.
However, there are safety and performance concerns for these types of batteries that arise from the formation of dendrites on the metal electrodes; an issue that has been known and investigated since the 1960s.
These dendrites form when metal ions accumulate on the surface of the battery’s electrodes as the electrode degrades during the charging process. Dendrites are often responsible for the highly publicized violent battery failures reported in the news.
When these branch-like filament deposits elongate until they penetrate the barrier between the two halves of the battery, they can cause electrical shorts, overheating and fires. They also cause significant cycling efficiency losses.
To avoid dendrites, researchers are experimenting with new battery electrolyte chemistries, new separator technologies, and new physical hosts for the lithium metal.
Carbon hosts, in particular, are very promising, since they may be added to the anode with little additional cost and minimal modification of the manufacturing process and they are becoming an important way to stabilize Li metal anodes.
However, there are seemingly contradictory findings reported in hundreds of prior publications on the subject: The hosts, which are predominantly made from various nanostructured carbons such as graphene, are in some cases very effective in eliminating dendrites. In other cases, they don’t work at all, or actually make the dendrite problem worse.
Up to now, design of such host systems has been largely Edisonian: researchers use a trial-and-error approach to find an architecture/structure that works better than the rest.
These findings address the major scientific problem of explaining how the structure and chemistry of the carbon per se affects dendrite growth.
Left half: Defect-free graphene protecting lithium metal anode from the electrolyte. Right half: Defective graphene catalyzing dendrite growth. (Source: Mitlin Research Group, Clarkson University)
“We discovered a critical and unexpected relationship between the host (graphene) chemical/structural defectiveness and its ability to suppress dendrites,” Prof. David Mitlin, who led the work, explains to Nanowerk. “To do this, we created what may be the world’s most pure and ordered graphene and compared it to a standard graphene based on reduced graphene oxide. Using such opposite materials, provided unique and fundamental insight into the way lithium dendrites form and grow.”
“The key finding, which will rationally guide future lithium battery design efforts, is that the carbon defects are themselves catalytic for dendrite growth,” Prof Wei Liu from Sichuan University’s Institute of New Energy and Low Carbon Technology, and the paper’s first author, points out. “Much of the ‘damage’ to the anode ultimately responsible for the dendrites occurs even before the battery is fully charged for the first time. Defects in the carbon host corrode the electrolyte at low voltages, leading to early dendrite formation.”
The team hypothesized that it was the host structure/chemistry that mattered, but needed to create ideal model systems to test the hypothesis.
Prof. Wei Liu’s unique Flow Assisted Sonication (FAS) approach allowed them to create nearly defect-free graphene. Literally, such oxygen-free and structural defect-free graphene has never been synthesized prior by a wet chemistry method.
This graphene is 1-3 atomic layers thick and with only about one and a half percent oxygen. This is much purer than the typical eight percent or more oxygen found in most graphene materials.
“It served as a perfect test bed to explore our hypothesis,” says Liu. “Without such a pristine structure, it would not have been possible to obtain the conclusive answers to the dendrite growth problem.”
He emphasizes that this in itself is a transformative accomplishment for the carbon and energy communities, since prior only vapor deposition could obtain such ideal defect-free structures.
The team then compared their defect-free graphene to a standard highly defective Hummers graphene baseline found in literature.
“A direct one-to-one comparison allowed us to obtain unique insight into the role of carbon defects on Li dendrite growth,” says Mitlin. “A critical new finding is that solid electrolyte interphase (SEI) formation occurring BEFORE metal plating actually dictates dendrites. The fate of the Li metal anode is in effect sealed once the carbon host forms SEI at the initial charge!”
Going forward, the researchers plan to commercialize defect-free graphene host materials for next- generation lithium batteries. They also plan to further understand the complex relationship between carbon defects and metal dendrites by examining carbons with tuned structure/chemistry for lithium, sodium and potassium batteries.
Finding and improving renewable energy sources is becoming increasingly important. One strategy to generate energy is breaking water molecules (H2O) apart in an electrochemical reaction known as electrolysis.
This process allows us to convert energy from the sun or other renewable sources into chemical energy. However, electrochemically splitting water molecules requires an overpotential—an excess voltage that has to be applied in addition to the theoretical voltage (1.23V vs. reversible hydrogen electrode or RHE) so that the necessary reactions can occur.
Electrocatalysts are materials that, because of their electrical and morphological features, facilitate electrochemical processes. Researchers have been searching for electrocatalysts that can aid in the electrolysis of water, and some of the best catalysts are noble-metal oxides, which are rare and costly. Nickel-based hydroxide (Ni(OH)2) compounds are, fortunately, a better alternative.
A team of scientists, including Profs. Hyunsik Im and Hyungsang Kim from Dongguk University, intercalated polyoxovanadate (POV) nanoclusters into Ni(OH)2 arranged in ordered layers and found that doing this improves its conducting and morphological properties, which in turn enhances its catalytic activity.
File Photo: Dongguk University
They employed a promising method called chemical solution growth (CSG), wherein a highly saturated solution is prepared, and the desired material structure naturally forms as the solutes precipitate in a predictable and controlled fashion, creating a layer-by-layer structure with POV nanoclusters intercalated between the Ni(OH)2 layers.
The team demonstrated that the resulting house-of-cards-like structure greatly reduced the overpotential needed for the electrolysis of water. They attributed this to the morphological features of this material; the POV nanoclusters increase the spacing between the Ni(OH)2 layers and induce the formation of micropores, which increases the surface area of the final material and the number of catalytic sites where water molecules can be split. “Our results demonstrate the advantages of the CSG method for optimizing the pore structure of the resulting material,” explains Prof. Im.
Facilitating the electrolysis of water using novel catalysts is a step toward achieving a greener future. What’s more, the CSG method could be useful in many other fields. “The facile CSG deposition of nanohybrid materials may be useful for applications such as the production of Li-ion batteries and biosensors,” states Prof. Kim. Only time will tell what new uses CSG will find.
More information: Jayavant L. Gunjakar et al, Two-Dimensional Layered Hydroxide Nanoporous Nanohybrids Pillared with Zero-Dimensional Polyoxovanadate Nanoclusters for Enhanced Water Oxidation Catalysis, Small (2018). DOI: 10.1002/smll.201703481
Your knees and your smartphone battery have some surprisingly similar needs, a University of Michigan professor has discovered, and that new insight has led to a ‘structural battery’ prototype that incorporates a cartilage-like material to make the batteries highly durable and easy to shape. Credit: Evan Doughtry
Your knees and your smartphone battery have some surprisingly similar needs, a University of Michigan professor has discovered, and that new insight has led to a “structural battery” prototype that incorporates a cartilage-like material to make the batteries highly durable and easy to shape.
The idea behind structural batteries is to store energy in structural components — the wing of a drone or the bumper of an electric vehicle, for example. They’ve been a long-term goal for researchers and industry because they could reduce weight and extend range. But structural batteries have so far been heavy, short-lived or unsafe.
In a study published in ACS Nano, the researchers describe how they made a damage-resistant rechargeable zinc battery with a cartilage-like solid electrolyte. They showed that the batteries can replace the top casings of several commercial drones. The prototype cells can run for more than 100 cycles at 90 percent capacity, and withstand hard impacts and even stabbing without losing voltage or starting a fire.
“A battery that is also a structural component has to be light, strong, safe and have high capacity. Unfortunately, these requirements are often mutually exclusive,” said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering, who led the research.
Harnessing the properties of cartilage
To sidestep these trade-offs, the researchers used zinc — a legitimate structural material — and branched nanofibersthat resemble the collagen fibers of cartilage.
“Nature does not have zinc batteries, but it had to solve a similar problem,” Kotov said. “Cartilage turned out to be a perfect prototype for an ion-transporting material in batteries. It has amazing mechanics, and it serves us for a very long time compared to how thin it is. The same qualities are needed from solid electrolytes separating cathodes and anodes in batteries.”
In our bodies, cartilage combines mechanical strength and durability with the ability to let water, nutrients and other materials move through it. These qualities are nearly identical to those of a good solid electrolyte, which has to resist damage from dendrites while also letting ions flow from one electrode to the other.
Dendrites are tendrils of metal that pierce the separator between the electrodes and create a fast lane for electrons, shorting the circuit and potentially causing a fire. Zinc has previously been overlooked for rechargeable batteries because it tends to short out after just a few charge/discharge cycles.
Not only can the membranes made by Kotov’s team ferry zinc ions between the electrodes, they can also stop zinc’s piercing dendrites. Like cartilage, the membranes are composed of ultrastrong nanofibers interwoven with a softer ion-friendly material.
In the batteries, aramid nanofibers — the stuff in bulletproof vests — stand in for collagen, with polyethylene oxide (a chain-like, carbon-based molecule) and a zinc salt replacing soft components of cartilage.
Demonstrating safety and utility
To make working cells, the team paired the zinc electrodes with manganese oxide — the combination found in standard alkaline batteries. But in the rechargeable batteries, the cartilage-like membrane replaces the standard separator and alkaline electrolyte. As secondary batteries on drones, the zinc cells can extend the flight time by 5 to 25 percent — depending on the battery size, mass of the drone and flight conditions.
Safety is critical to structural batteries, so the team deliberately damaged their cells by stabbing them with a knife. In spite of multiple “wounds,” the battery continued to discharge close to its design voltage. This is possible because there is no liquid to leak out.
For now, the zinc batteries are best as secondary power sources because they can’t charge and discharge as quickly as their lithium ion brethren. But Kotov’s team intends to explore whether there is a better partner electrode that could improve the speed and longevity of zinc rechargeable batteries.
The research was supported by the Air Force Office of Scientific Research and National Science Foundation. Kotov teaches in the Department of Chemical Engineering. He is also a professor of materials science and engineering, and macromolecular science and engineering.
Drive for innovation: Electric vehicles are a major target for R&D on novel battery materials. (Image courtesy: imec)
31 Oct 2018
Note to Readers: This article first appeared in the 2018 Physics World Focus on Energy Technologies Engineering a sustainable, electrified future means developing battery materials with properties that surpass those found in current technologies.
The batteries we depend on for our mobile phones and computers are based on a technology that is more than a quarter-century old. Rechargeable lithium-ion (Li-ion) batteries were first introduced in 1991, and their appearance heralded a revolution in consumer electronics. From then on, we could pack enough energy in a small volume to start engineering a whole panoply of portable electronic devices – devices that have given us much more flexibility and comfort in our lives and jobs.
In recent years, Li-ion batteries have also become a staple solution in efforts to solve the interlinked conundrums of climate change and renewable energy. Increasingly, they are being used to power electric vehicles and as the principal components of home-based devices that store energy generated from renewable sources, helping to balance an increasingly diverse and smart electrical grid. The technology has improved too: over the past two and a half decades, battery experts have succeeded in making Li-ion batteries 5–10% more efficient each year, just by further optimizing the existing architecture.
Ultimately, though, getting from where we are now to a truly carbon-free economy will require better-performing batteries than today’s (or even tomorrow’s) Li-ion technology can deliver. In electric vehicles, for example, a key consideration is for batteries to be as small and lightweight as possible.
Achieving that goal calls for energy densities that are much higher than the 300 Wh/kg and 800 Wh/L which are seen as the practical limits for today’s Li-ion technology.
Another issue holding back the adoption of electric vehicles is cost, which is currently still around 300–200 $/kWh, although that is widely projected to go below 100 $/kWh by 2025 or even earlier. The time required to recharge a battery pack – still in the range of a few hours – will also have to come down, and as batteries move into economically critical applications such as grid storage and grid balancing, very long lifetimes (a decade or more) will become a key consideration too.
There is still some room left to improve existing Li-ion technology, but not enough to meet future requirements. Instead, the process of battery innovation needs a step change: materials-science breakthroughs, new electrode chemistries and architectures that have much higher energy densities, new electrolytes that can deliver the necessary high conductivity – all in a battery that remains safe and is long-lasting as well as economical and sustainable to produce.
To appreciate why this is such a challenge, it helps to understand the basic architecture of existing batteries. Rechargeable Li-ion batteries are made up of one or more cells, each of which is a small chemical factory essentially consisting of two electrodes with an electrolyte in between. When the electrodes are connected (for example with a wire via a lamp), an electrochemical process begins. In the anode, electrons and lithium ions are separated, and the electrons buzz through the wire and light up the lamp. Meanwhile, the positively-charged lithium ions move through the electrolyte to the cathode. There, electrons and Li-ions combine again, but in a lower energy state than before.
The beauty of rechargeable batteries is that these processes can be reversed, returning lithium ions to the anode and restoring the energy states and the original difference in electrical potential between the electrodes. Lithium ions are well suited for this task. Lithium is not only the lightest metal in the periodic table, but also the most reactive and will most easily part with its electrons. It has been chosen as the basis for rechargeable batteries precisely because it can do the most work with the least mass and the fewest chemical complications. More specifically, in batteries using lithium, it is possible to make the electric potential difference between anodes and cathodes higher than is possible with other materials.
To date, therefore, the main challenge for battery scientists has been to find chemical compositions of electrodes and electrolyte that will let the lithium ions do their magic in the best possible way: electrodes that can pack in as many lithium ions as possible while setting up as high an electrical potential difference as possible; and an electrolyte that lets lithium ions flow as quickly as possible back and forth between the anode and cathode.
Seeking a solid electrolyte
The electrolyte in most batteries is a liquid. This allows the electrolyte not only to fill the space between the electrodes but also to soak them, completely filling all voids and spaces and providing as much contact as possible between the electrodes and the electrolyte. To complete the picture, a porous membrane is added between the electrodes. This inhibits electrical contact between the electrodes and prevents finger like outgrowths of lithium from touching and short-circuiting the battery.
For all the advantages of liquid electrolytes, though, scientists have long sought to develop solid alternatives. A solid electrolyte material would eliminate several issues at the same time. Most importantly, it would replace the membrane, allowing the electrodes to be placed much closer together without touching, thereby, making the battery more compact and boosting its energy density. A solid electrolyte would also make batteries stronger, potentially meaning that the amount of protective and structural casing could be cut without compromising on safety.
Unfortunately, the solid electrolytes proposed so far have generally fallen short in one way or another. In particular, they lack the necessary conductivity (expressed in milli-Siemens per centimetre, or mS/cm). Unsurprisingly, ions tend not to move as freely through a solid as they do through a liquid. That reduces both the speed at which a battery can charge and, conversely, the quantity of power it can release in a given time.
Scientists at imec – one of Europe’s premier nanotechnology R&D centres, and a partner in the EnergyVille consortium for sustainable energy and intelligent energy systems research – recently came up with a potential solution. The new material is a nanoporous oxide mix filled with ionic compounds and other additives, with the pores giving it a surface area of about 500 m2/mL – “comparable to an Olympic swimming pool folded into a shot glass,” says Philippe Vereecken, imec’s head of battery research. Because ions move faster along the pores’ surface than in the middle of a lithium salt electrolyte, he explains, this large surface area amplifies the ionic conductivity of the nanoengineered solid. The result is a material with a conductivity of 10 mS/cm at room temperature – equivalent to today’s liquid electrolytes.
Using this new electrolyte material, imec’s engineers have built a cell prototype using standard available electrodes: LFP (LiFePO4) for the cathode and LTO (Li4Ti5O12) for the anode. While charging, the new cell reached 80% of its capacity in one hour, which is already comparable to a similar cell made with a liquid electrolyte. Vereecken adds that the team hopes for even better results with future devices. “Computations show that the new material might even be engineered to sustain conductivities of up to 100 mS/cm,” he says.
Meanwhile, back at the electrode
Electrodes are conventionally made from sintered and compressed powders. Combining these with a solid electrolyte would normally entail mixing the electrode as a powder with the electrolyte also in powder form, and then compressing the result for a maximum contact. But even then, there will always remain pores and voids that are not filled and the contact surface will be much smaller than is possible with a liquid electrolyte that fully soaks the electrode.
Lithium-sulphur is a promising material that could store more energy than today’s technology allows
Imec’s new nano-composite material avoids this problem because it is actually applied as a liquid, via wet chemical coating, and only afterwards converted into a solid. That way it can impregnate dense powder electrodes, filling all cavities and making maximum contact just as a liquid electrolyte would. Another benefit is that even as a solid, the material remains somewhat elastic, which is essential as some electrodes expand and contract during battery charging and discharging. A final advantage is that because the solid material can be applied via a wet precursor, it is compatible with current Li-ion battery fabrication processes – something that Vereecken says is “quite important for the battery manufacturers” because otherwise more “disruptive” fabrication processes would have to be put in place.
To arrive at the energy densities required to give electric vehicles a long driving range, though, still more changes are needed. One possibility is to make the particles in the electrode powders smaller, so that they can be packed more densely. This would produce a larger contact surface with the electrolyte per volume, improving the energy density and charging rate of the cell. There is a catch, though: while a larger contact surface results in more ions being created and changing sides within the battery, it also gives more way for unwanted reactions that will degrade the battery’s materials and shorten its lifetime. “To improve the stability,” says Vereecken, “imec’s experts work on a solution where they coat all particles with an ultrathin buffer layer.” The challenge, he says, is to make these layers both chemically inert and highly conductive.
Introducing new materials
By combining solid electrolytes with thicker electrodes made from smaller particles, it may be possible to produce batteries with energy densities that exceed the current maximum of around 800 Wh/L. These batteries could also charge in 30 minutes or less. But to extend the energy density even further, to 1000 Wh/L and beyond, a worldwide effort is on to look for new and better electrode materials. Anodes, for example, are currently made from carbon in the form of graphite. That carbon could be replaced by silicon, which can hold up to ten times as many lithium ions per gram of electrode. The drawback is that when the battery is charged, a silicon anode will expand to more than three times its normal size as it fills with lithium ions. This may break up the electrode, and possibly even the battery casing.
A better alternative may be to replace carbon with pure lithium metal. A lithium anode will also store up to ten times as much lithium ions per gram of electrode as graphite, but without the swelling seen in silicon anodes. Lithium anodes were, in fact, used in the early days of Li-ion batteries, but as the metal is very reactive, especially in combination with liquid electrolytes, the idea was dropped in favour of more stable alternatives. Vereecken, however, believes that progress in solid electrolytes means it is “high time to revisit lithium metal as a material for the anode”, especially since it is possible to add protective functional coatings to nanoparticles.
Disruptive innovations are on the horizon for cathodes as well. Lithium-sulphur, for example, is a promising material that could store more energy than today’s technology allows. Indeed, the “ideal” lithium battery might well feature a lithium-air (lithium peroxide) cathode in combination with a pure lithium anode. But whereas the material composition of these batteries sounds simple, the path to realizing them will not be so easy, and there is still some way to go before any of these developments will be integrated into commercial batteries. Once that happens, though, huge payoffs are possible. The most obvious would be electrical cars that drive farther and charge faster, but better lithium batteries could also be the breakthrough needed to make renewable power ubiquitous – and thus finally let us off the fossil-fuel hook.
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