Lithium batteries are what allow electric vehicles to travel several hundred miles on one charge. Their capacity for energy storage is well known, but so is their tendency to occasionally catch on fire—an occurrence known to battery researchers as “thermal runaway.” These fires occur most frequently when the batteries overheat or cycle rapidly. With more and more electric vehicles on the road each year, battery technology needs to adapt to reduce the likelihood of these dangerous and catastrophic fires.
Researchers from the University of Illinois at Chicago College of Engineering report that graphene—wonder material of the 21st century—may take the oxygen out of lithium battery fires. They report their findings in the journal Advanced Functional Materials.
The reasons lithium batteries catch fire include rapid cycling or charging and discharging, and high temperatures in the battery. These conditions can cause the cathode inside the battery—which in the case of most lithium batteries is a lithium-containing oxide, usually lithium cobalt oxide—to decompose and release oxygen. If the oxygen combines with other flammable products given off through decomposition of the electrolyte under high enough heat, spontaneous combustion can occur.
“We thought that if there was a way to prevent the oxygen from leaving the cathode and mixing with other flammable products in the battery, we could reduce the chances of a fire occurring,” said Reza Shahbazian-Yassar, associate professor of mechanical and industrial engineering in the UIC College of Engineering and corresponding author of the paper.
It turns out that a material Shahbazian-Yassar is very familiar with provided a perfect solution to this problem. That material is graphene—a super-thin layer of carbon atoms with unique properties. Shahbazian-Yassar and his colleagues previously had used graphene to help modulate lithium buildup on electrodes in lithium metal batteries.
Lithium cobalt oxide particles coated in graphene. Credit: Reza Shahbazian-Yassar.
Shahbazian-Yassar and his colleagues knew that graphene sheets are impermeable to oxygen atoms. Graphene is also strong, flexible and can be made to be electrically conductive. Shahbazian-Yassar and Soroosh Sharifi-Asl, a graduate student in mechanical and industrial engineering at UIC and lead author of the paper, thought that if they wrapped very small particles of the lithium cobalt oxide cathode of a lithium battery in graphene, it might prevent oxygen from escaping.
First, the researchers chemically altered the graphene to make it electrically conductive. Next, they wrapped the tiny particles of lithium cobalt oxide cathode electrode in the conductive graphene.
When they looked at the graphene-wrapped lithium cobalt oxide particles using electron microscopy, they saw that the release of oxygen under high heat was reduced significantly compared with unwrapped particles.
Next, they bound together the wrapped particles with a binding material to form a usable cathode, and incorporated it into a lithium metal battery. When they measured released oxygen during battery cycling, they saw almost no oxygen escaping from cathodes even at very high voltages. The lithium metal battery continued to perform well even after 200 cycles.
“The wrapped cathode battery lost only about 14% of its capacity after rapid cycling compared to a conventional lithium metal battery where performance was down about 45% under the same conditions,” Sharifi-Asl said.
“Graphene is the ideal material for blocking the release of oxygen into the electrolyte,” Shahbazian-Yassar said. “It is impermeable to oxygen, electrically conductive, flexible, and is strong enough to withstand conditions within the battery. It is only a few nanometers thick so there would be no extra mass added to the battery. Our research shows that its use in the cathode can reliably reduce the release of oxygen and could be one way that the risk for fire in these batteries—which power everything from our phones to our cars—could be significantly reduced.”
Rice University scientists are counting on films of carbon nanotubes to make high-powered, fast-charging lithium metal batteries a logical replacement for common lithium-ion batteries.
The Rice lab of chemist James Tour showed thin nanotube films effectively stop dendrites that grow naturally from unprotected lithium metal anodes in batteries. Over time, these tentacle-like dendrites can pierce the battery’s electrolyte core and reach the cathode, causing the battery to fail.
That problem has both dampened the use of lithium metal in commercial applications and encouraged researchers worldwide to solve it.
Rice University graduate student Gladys López-Silva holds a lithium metal anode with a film of carbon nanotubes. Once the film is attached, it becomes infiltrated by lithium ions and turns red. Photo by Jeff Fitlow
Lithium metal charges much faster and holds about 10 times more energy by volume than the lithium-ion electrodes found in just about every electronic device, including cellphones and electric cars.
“One of the ways to slow dendrites in lithium-ion batteries is to limit how fast they charge,” Tour said. “People don’t like that. They want to be able to charge their batteries quickly.”
The Rice team’s answer, detailed in Advanced Materials, is simple, inexpensive and highly effective at stopping dendrite growth, Tour said.
“What we’ve done turns out to be really easy,” he said. “You just coat a lithium metal foil with a multiwalled carbon nanotube film. The lithium dopes the nanotube film, which turns from black to red, and the film in turn diffuses the lithium ions.”
“Physical contact with lithium metal reduces the nanotube film, but balances it by adding lithium ions,” said Rice postdoctoral researcher Rodrigo Salvatierra, co-lead author of the paper with graduate student Gladys López-Silva. “The ions distribute themselves throughout the nanotube film.”
An illustration shows how lithium metal anodes developed at Rice University are protected from dendrite growth by a film of carbon nanotubes. Courtesy of the Tour Group
When the battery is in use, the film discharges stored ions and the underlying lithium anode refills it, maintaining the film’s ability to stop dendrite growth.
The researchers reported the full lithium metal cells retained 99.8 percent of their coulombic efficiency, the measure of how well electrons move within an electrochemical system.
Rice University scientists have discovered that a film of multiwalled carbon nanotubes quenches the growth of dendrites in lithium metal-based batteries. Courtesy of the Tour Group
Co-authors of the paper are Rice alumni Almaz Jalilov of the King Fahd University of Petroleum and Minerals, Saudi Arabia; Jongwon Yoon, a senior researcher at the Korea Basic Science Institute; and Gang Wu, an instructor, and Ah-Lim Tsai, a professor of hematology, both at the McGovern Medical School at the University of Texas Health Science Center at Houston.
Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice.
The research was supported by the Air Force Office of Scientific Research, the National Institutes of Health, the National Council of Science and Technology, Mexico; the National Council for Scientific and Technological Development, Ministry of Science, Technology and Innovation and Coordination for the Improvement of Higher Education Personnel, Brazil; and Celgard, LLC.
Rice University chemist James Tour, left, graduate student Gladys López-Silva and postdoctoral researcher Rodrigo Salvatierra use a film of carbon nanotubes to prevent dendrite growth in lithium metal batteries, which charge faster and hold more power than current lithium-ion batteries. Photo by Jeff Fitlow.
Nanoscale reinforcement with graphene girders boosts performance of silicon anodes, Warwick team discovers
When you want to make a structure stronger, put a girder across it. It’s a simple principle that every civil engineer knows well. But a team at Warwick Manufacturing Group has found that it applies just as well on very small scales as in megastructures. Melanie Loveridge and colleagues are studying methods for improving lithium-ion batteries, and have found that minute girders could provide an answer to a problem that has been plaguing the field.
Ever since their first introduction in the early 1990s, the anode of lithium batteries has been made of graphite. It has long been apparent that silicon would be a better material, as it can hold ten times more charge per gramme than carbon. But the mechanics of lithium ion batteries, where lithium ions are absorbed into the anode, create problems.
When silicon is lithiated, it expands. But it is an inelastic material, and repeated expansion and contraction — as happens during charge-discharge cycles — can lead to cracking and crumbling, which makes the capacity of the battery fade over time. Graphene has been tried as a reinforcing material for nanostructured silicon, but this has led to other problems.
Loveridge’s team is looking at a material known as FLG (few-layer graphene). As the name implies, this is composed of a few connected layers of single-atom-thick graphene sheets, which can be manipulated together.
In a paper in Nature Scientific Reports, the WMG team describes how FLG can improve the performance of anodes containing micron-sized particles of silicon. The team started with a mixture of 60 per cent micro-silicon, 16 per cent FLG, 14 per cent sodium/polyacrylic acid and 10 per cent carbon additives, and put these anodes through 100 charge-discharge cycles.
“The flakes of FLG were mixed throughout the anode and acted like a set of strong, but relatively elastic, girders. These flakes of FLG increased the resilience and tensile properties of the material greatly reducing the damage caused by the physical expansion of the silicon during lithiation. The graphene enhances the long range electrical conductivity of the anode and maintains a low resistance in a structurally stable composite,” Loveridge said.
Moreover, she added, the graphene girders keep the silicon particles apart. In their absence, the particles tend to ‘weld’ together, restricting lithium diffusion through the anode and reducing the surface area available for lithiation.
“The presence of FLG in the mixture tested by the WMG University of Warwick led researchers to hypothesise that this phenomenon is highly effective in mitigating electrochemical silicon fusion,” Loveridge stated.
The team is now working on scaling up their graphene girders discovery to produce pouch cells based on their reinforced anodes, as part of a two-year graphene flagship project along with Varta Micro-innovations, Cambridge University, CIC, Lithops and IIT (Italian Institute of Technology).
Extending the battery life of our tech is something that preoccupies manufacturers and consumers alike. With every new phone launch we’re treated to new features, such as increasingly high-res displays and better cameras, but it’s longer battery life we all want. For most of us, being able to use our phone for a full day still means charging it every night, or lugging your charger around all day and hunting for a power socket. And when the electric car revolution reaches full speed, fast-charging, long-life batteries are going to be essential.
Advances in battery life are being made all the time, even if we’re yet to see the full benefits in our day-to-day gadgets.
But what’s beyond that? Wireless power. And we don’t mean laying our phone on a charging pad – we’re talking about long-range wireless power. If this is cracked we could have all our devices at full juice all the time, no matter where we are.
The current tech
The batteries in your current phone, and in electric cars, are lithium-ion. These charge quickly, last for plenty of cycles and offer decent capacity. But devices are more juice-hungry than ever, and with cars in particular fast charging needs to become more effective, because batteries aren’t going away any time soon.
While wireless power could be a viable option in the future, in the short-to-medium term we need to enhance batteries so that individuals and energy providers can first transition from fossil fuels to green renewable power.
Louis Shaffer of power management solutions firm Eaton tells TechRadar: “We constantly hear about battery breakthroughs but still have the same lithium-ion batteries in our phones. Innovation takes time. It took over 30 years for li-ion batteries to enter the mainstream, from their invention in the 1980s to featuring in iPhones.”
Another factor in slowing this progress is highlighted by Chris Slattery, product manager at smart lighting manufacturer Tridonic. “The interesting point with mobile phones is that one of the major factors for upgrading your phone is the degradation of the current phone’s battery life,“ he says.
“Increasing the life of these batteries removes a major reason for upgrading to the latest smartphone when the feature set itself doesn’t change that greatly.”
Ultracapacitors are seen by many as the future of energy storage, as they store energy in an electric field, rather than in a chemical reaction as a battery does, meaning they can survive hundreds of thousands more charge and discharge cycles than a battery can.
Taavi Madiburk is CEO of Skeleton Technologies, a global leader in ultracapacitor-based storage solutions. He says: “The future, we believe, lies not in replacing lithium-ion, but coupling this technology with ultracapacitors in a hybrid approach.
“In doing so, it is possible to benefit from both the high energy density of batteries, and the high power density and output of ultracapacitors.
“Ultracapacitors can be re-charged in a matter of 2-3 seconds, providing one million deep charge/discharge cycles. Also, with ultracapacitors protecting batteries from high power surges, the lifetime of the battery pack is increased by 50% and the range by 10%.
Skeleton is already working to improve power grids to cater for the growing number of electric cars. It sees current large-scale electrical grids being replaced in certain areas by smaller, less centralized grids called microgrids, and, Madiburk adds, “We’re currently working on with ultracapacitors as a piece of that puzzle.”
Solid state batteries
One of the major advances in battery tech right now sticks with good old lithium.
Solid-state lithium batteries dispense with the electrolyte liquid that transfers charged particles, making them safer than current batteries yet still able to operate at super-capacitor levels, meaning that charging and discharging can happen faster.
This is great for car batteries, as it means more power can be utilized by the car for quick pull-away speed, but fast charging will mean drivers need to spend less time at charging stations.
One example of this, from , is a battery that can be fully charged from empty in just seven minutes.
Another promising area is , which have been placed in a car to deliver a whopping 1,100 miles on a single charge. Then there are , which – while still lithium-ion – manage to offer three times better performance than lithium-ion while being cheaper to make, non-toxic and environmentally friendly.
Whisper it, but one of the big hopes for improved batteries for a while now has been graphene. The Grabat battery from charges 33 times faster than lithium-ion units, and can deliver high power too, making it ideal for cars.
One way to go without batteries is to make gadgets super-low power consuming. A phone has been built that doesn’t even require a battery, so low are its power needs – and it was achieved using components that are available to anyone.
Engineers at the University of Washington designed the phone, which is able to pull power from the environment, with radio signals and light harvested by an antenna and tiny solar cell.
The result is enough power to run the 3.5 microwatt-consuming phone. You’re limited to making calls only, but the idea having a tiny credit card-sized backup phone in your wallet will appeal to everyone from constantly on-the-move workers who need to stay in touch, to hikers.
Other breakthroughs have also been based on drawing ambient power from the world around us. , so that simply talking into your phone generates power to charge it.
MIT scientists, meanwhile, have shown off a way to harvest power from water dew in the air; they’ve only been able to create a potential one microwatt so far, but combine these methods, throw in a bit more evolution and we could be looking at a battery-free future.
Over the air power
The dream of transmitting power over the air has existed since the days of the legendary inventor and electrical engineer Nikolas Tesla, but it’s only recently started to become a reality. One company that claims to have mastered the technology, taking it beyond the close-range Qi wireless charging now found in many smartphones, is uBeam.
The uBeam system was cracked by 25-year-old astrobiology grad Meredith Perry, who has since received over $28 million in funding.
This system uses microwaves to transmit energy several metres across a room to power devices. Perry has shown it off charging phones, but says it could be applied to TVs, computers and even cars.
It uses a lot of power, costs a lot to manufacture and offers a pretty slow charging rate; but there are no wires to be seen, and this way of delivering power could hail a future without batteries.
If it could be made efficient on a large scale, in a similar way to mobile phone networks, all our devices could draw power from such a system. Imagine phones and electric cars that never need charging.
But is this future as close as uBeam would have its investors and us believe? Probably not.
This is where things get really interesting – harnessing the power of human beings. Not like in The Matrix, where we’re reduced to a glorified battery, but through friction generated by movement.
Scientists have shown off the tech in action, powering 12 LED bulbs. That’s not going to change the way you use your gadgets right now, but it’s a step in the right direction.
The technology uses a 50nm thin gold film sitting under silicone rubber nanopillars which create maximum surface area with the skin. The result is lots of friction, and all the user has to do is strap the unit on, making it ideal for wearables.
And the Bill Gates Foundation has even developed a process that harvests enough power from our urine to charge a phone, dubbed the Microbial Fuel Cell; that’s pretty much the definition of sustainable power.
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