A new approach to the design of a liquid battery, using a passive, gravity-fed arrangement similar to an old-fashioned hourglass, could offer great advantages due to the system’s low cost and the simplicity of its design and operation, says a team of MIT researchers who have made a demonstration version of the new battery. Liquid flow batteries — in which the positive and negative electrodes are each in liquid form and separated by a membrane — are not a new concept, and some members of this research team unveiled an earlier concept three years ago. The basic technology can use a variety of chemical formulations, including the same chemical compounds found in today’s lithium-ion batteries. In this case, key components are not solid slabs that remain in place for the life of the battery, but rather tiny particles that can be carried along in a liquid slurry. Increasing storage capacity simply requires bigger tanks to hold the slurry. But all previous versions of liquid batteries have relied on complex systems of tanks, valves, and pumps, adding to the cost and providing multiple opportunities for possible leaks and failures. The new version, which substitutes a simple gravity feed for the pump system, eliminates that complexity. The rate of energy production can be adjusted simply by changing the angle of the device, thus speeding up or slowing down the rate of flow. The concept is described in a paper in the journal Energy and Environmental Science, co-authored by Kyocera Professor of Ceramics Yet-Ming Chiang, Pappalardo Professor of Mechanical Engineering Alexander Slocum, School of Engineering Professor of Teaching Innovation Gareth McKinley, and POSCO Professor of Materials Science and Engineering W. Craig Carter, as well as postdoc Xinwei Chen, graduate student Brandon Hopkins, and four others. Chiang describes the new approach as something like a “concept car” — a design that is not expected to go into production as it is but that demonstrates some new ideas that can ultimately lead to a real product. The original concept for flow batteries dates back to the 1970s, but the early versions used materials that had very low energy-density — that is, they had a low capacity for storing energy in proportion to their weight. A major new step in the development of flow batteries came with the introduction of high-energy-density versions a few years ago, including one developed by members of this MIT team, that used the same chemical compounds as conventional lithium-ion batteries. That version had many advantages but shared with other flow batteries the disadvantage of complexity in its plumbing systems. The new version replaces all that plumbing with a simple, gravity-fed system. In principle, it functions like an old hourglass or egg timer, with particles flowing through a narrow opening from one tank to another. The flow can then be reversed by turning the device over. In this case, the overall shape looks more like a rectangular window frame, with a narrow slot at the place where two sashes would meet in the middle. In the proof-of-concept version the team built, only one of the two sides of the battery is composed of flowing liquid, while the other side — a sheet of lithium — is in solid form. The team decided to try out the concept in a simpler form before making their ultimate goal, a version where both sides (the positive and negative electrodes) are liquid and flow side by side through an opening while separated by a membrane. Solid batteries and liquid batteries each have advantages, depending on their specific applications, Chiang says, but “the concept here shows that you don’t need to be confined by these two extremes. This is an example of hybrid devices that fall somewhere in the middle.” The new design should make possible simpler and more compact battery systems, which could be inexpensive and modular, allowing for gradual expansion of grid-connected storage systems to meet growing demand, Chiang says. Such storage systems will be critical for scaling up the use of intermittent power sources such as wind and solar. While a conventional, all-solid battery requires electrical connectors for each of the cells that make up a large battery system, in the flow battery only the small region at the center — the “neck” of the hourglass — requires these contacts, greatly simplifying the mechanical assembly of the system, Chiang says. The components are simple enough that they could be made through injection molding or even 3-D printing, he says. In addition, the basic concept of the flow battery makes it possible to choose independently the two main characteristics of a desired battery system: its power density (how much energy it can deliver at a given moment) and its energy density (how much total energy can be stored in the system). For the new liquid battery, the power density is determined by the size of the “stack,” the contacts where the battery particles flow through, while the energy density is determined by the size of its storage tanks. “In a conventional battery, the power and energy are highly interdependent,” Chiang says. The trickiest part of the design process, he says, was controlling the characteristics of the liquid slurry to control the flow rates. The thick liquids behave a bit like ketchup in a bottle — it’s hard to get it flowing in the first place, but then once it starts, the flow can be too sudden. Getting the flow just right required a long process of fine-tuning both the liquid mixture and the design of the mechanical structures. The rate of flow can be controlled by adjusting the angle of the device, Chiang says, and the team found that at a very shallow angle, close to horizontal, “the device would operate most efficiently, at a very steady but low flow rate.” The basic concept should work with many different chemical compositions for the different parts of the battery, he says, but “we chose to demonstrate it with one particular chemistry, one that we understood from previous work. We’re not proposing this particular chemistry as the end game.” Venkat Viswanathan, an assistant professor of mechanical engineering at Carnegie Mellon University, who was not involved in this work, says: “The authors have been able to build a bridge between the usually disparate fields of fluid mechanics and electrochemistry,” and in so doing developed a promising new approach to battery storage. “Pumping represents a large part of the cost for flow batteries,” he says, “and this new pumpless design could truly inspire a class of passively driven flow batteries.” The work was supported by the Joint Center for Energy Storage Research, funded by the U.S. Department of Energy. The team also included graduate students Ahmed Helal and Frank Fan, and postdocs Kyle Smith and Zheng Li.
For these reasons and more, many experts are increasingly interested in making electricity a local affair. This idea, useful for both cost savings and for backup power, moves the main source of electricity away from remote large-scale plants to smaller local ones. This approach is called distributed energy. Distributed energy devices can produce enough electricity to power a home, business, or small community, making them ideal backups for power when energy is cut from the main grid in the case of a power outage. They can be solar panels, small wind turbines, batteries, fuel cells or microturbines that connect directly to the home or local electrical grid, but their purpose is to meet the specific power needs of a local population. "Until maybe about 10 years ago, we never accommodated people's different energy needs—now we can," said George Crabtree, director of Argonne's Joint Center for Energy Storage Research center. "It may be the solar panel on my roof or the battery in my garage, but in the end, these different needs give rise to new technologies that give us a greater amount of choice, and ultimately, control." Large power plants have several major drawbacks. Firstly, they waste a lot of energy—about two-thirds —when converting fuel to electricity. Secondly, many of them sit idle for a good part of the year (on average about half the time) as they may only be needed to meet peak demand—usually the time of day when the majority of people come home in the evening. Thirdly, in shipping the power from the large stations to the consumer, we waste another 5-7% in transmission and distribution losses. On the other hand, distributed systems present smaller, more flexible options. And because they are located close to where the power is needed, less energy is lost in delivery. Distributed energy devices such as solar panels, fuel cells, microturbines, and batteries come in a variety of types and sizes, from as little as 1 kilowatt—enough to power ten 100-watt light bulbs—up to as much as 10,000 kilowatts, which might be enough to power a university campus or a neighborhood or community microgrid. Single homes and small businesses can benefit from rooftop solar panels on sunlit days or portable natural gas generators in the evening. Hospitals and small towns that use considerably more energy can use a mix of microturbines, generators, and industrial-scale batteries. However, tying these devices together in a way that allows them to communicate with electrical grid operators is a challenge. Each new device connected to the grid becomes another device that operators have to account for when balancing an area's energy demand. As information-sharing between devices gets more complex, and as more consumers install distributed systems in their homes and communities, the next challenge becomes how to share data with local utilities so they can accurately respond to real-time energy demand. Distributed energy resources are growing rapidly in some parts of the country. At the end of 2014, the U.S. had close to 650,000 solar-powered homes with a new a solar project installed every 2.5 minutes, and the grid has managed to handle it fine so far. "But if we're talking about moving from coordinating distributed energy for tens of millions of people, we can't even envision that amount of data," said Argonne energy systems analyst Guenter Conzelmann. "We are talking about a very different scale than the one we are used to, and we just don't have the systems in place to handle that right now." National grid operators are quite efficient at balancing energy supply and demand, because they can rely on hundreds of power plants to respond instantly each time a device is turned on or off. This type of balance becomes more challenging on smaller neighborhood scales. "Controlling things on a smaller scale can be more challenging because you have fewer resources," said Jianhui Wang, the head of Argonne's Advanced Power Grid Modeling section. "The solar panels, the wind or gas turbines, the electric cars—all of these things have to communicate with to each other to balance out the demand and supply within that community, just as an electrical grid operator has to balance out a region." Figuring out how distributed energy systems will communicate effectively within a grid the size of a neighborhood is the focus of a new project on Chicago's south side. Argonne and the Illinois Institute of Technology (IIT) are teaming up with electrical utility ComEd to build a microgrid in the Chicago neighborhood of Bronzeville. A microgrid is a small community that can make and use most or all of its own power using a mix of distributed energy generators and energy storage units. Argonne is working with ComEd, IIT, and a number of industry partners to develop advanced control system software to help this microgrid balance relatively few resources within a strictly defined area, while matching the energy demand of its community. Though we'll still rely on remotely-generated power, an increase in the number of distributed systems will decrease this reliance over time, Argonne experts say. "That transition is going to happen, but you need to tie in all of these resources into the grid to be able to manage it," said Argonne mechanical engineer Sreenath Gupta. "Thankfully, innovation is driving costs down, which is definitely going to help increase the adoption of these types of technologies."
Scientists at the US Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a novel electrolyte for use in solid-state lithium batteries. This electrolyte overcomes many of the problems that plague other solid electrolytes while also showing signs of being compatible with next-generation cathodes. Berkeley Lab battery scientist Nitash Balsara, working with collaborator Joseph DeSimone at the University of North Carolina at Chapel Hill, came up with a highly conductive hybrid electrolyte that combines the two primary types of solid electrolyte – polymer and glass. Their discovery is detailed in a paper published in Proceedings of the National Academy of Sciences (PNAS), co-authored by Berkeley Lab researchers Irune Villaluenga, Kevin Wujcik, Wei Tong, and Didier Devaux, and Dominica Wong of the University of North Carolina. Villaluenga, a postdoctoral fellow at Berkeley Lab, played a key role in designing and realizing the solid electrolyte; Balsara and DeSimone are the senior authors. "The electrolyte is compliant, which means it can readily deform to maintain contact with the electrode as the battery is cycled, and also has unprecedented room temperature conductivity for a solid electrolyte," said Balsara. The electrolyte carries electrical charge between the battery's cathode and anode and in most commercial batteries is liquid. Researchers are striving to develop a battery with all solid components, as it would likely perform better, last longer and be safer. The two main candidates as solid electrolytes – polymer and glass or ceramic – each come with their own set of issues. Polymer electrolytes don't conduct well at room temperature and need to be heated up. Ceramic electrolytes, on the other hand, do conduct well at room temperature but require a great deal of pressure to maintain contact with the electrodes. "It needs something like one ton over every square centimeter, so you need a big truck sitting on the battery as it cycles," Balsara said. The new material they developed, a glass-polymer hybrid, was made by taking particles of glass and attaching perfluoropolyether chains to their surface, adding salt, and then making a film out of these components. By tuning the polymer-to-glass ratio, the scientists were able to come up with a compliant electrolyte with high conductivity at room temperature and excellent electrochemical stability. Although the conductivity is not as good as that of a liquid electrolyte, being about 10 to 15 times lower, "it's probably good enough for some applications," Balsara said. "We don't necessarily need to match a liquid electrolyte because nearly all of the current in the hybrid electrolyte is carried by the lithium ion. In conventional lithium electrolytes, only 20–30% of the current is carried by the lithium ion. Nevertheless, it is likely that playing around with different glass compounds, particle size, and length and concentration of the polymer chains will result in improved conductivity." The researchers also demonstrated that their hybrid electrolyte should work with two of the most promising next-generation cathode materials being developed: sulfur and high-voltage materials such as lithium nickel manganese cobalt oxide. "People would like to use 5-volt cathodes, but electrolytes that are stable against those 5-volt cathodes are not readily available," Balsara said. "We have demonstrated this electrolyte is stable at 5 volts, though we have not incorporated the hybrid electrolyte in the cathode yet." Further experiments demonstrated that the hybrid electrolyte can be well suited to work with a sulfur cathode, which operates at a relatively low voltage but has the advantages of high capacity and low cost. A major failure mode in lithium-sulfur cells with conventional liquid electrolytes is the dissolution into the electrolyte of intermediate compounds formed as sulfur in the cathode is converted to lithium sulfide. However, the intermediates were found to be insoluble in the glass-polymer electrolyte. "Although much work remains to be done, we believe that our work opens a previously unidentified route for developing hybrid solid electrolytes that will address the current challenges of lithium batteries," the researchers wrote in the PNAS article. Funding for the research at Berkeley Lab was provided by DOE's Office of Science through the Joint Center for Energy Storage Research, a DOE Energy Innovation Hub. Part of the work was done at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory and at the Advanced Light Source at Berkeley Lab, both DOE Office of Science User Facilities. Balsara was one of the co-founders of battery startup Seeo, founded in 2007 to develop a solid block copolymer electrolyte. Balsara and DeSimone have also co-founded a startup company called Blue Current, which aims to commercialize a perfluoropolyether-based nonflammable electrolyte they developed together. This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
News Article | March 31, 2016
Ever worry that your cell phone will fade when you need it most? Or that the same thing will happen when driving your electric car? Lightweight lithium-sulfur batteries could be the answer, holding two times the energy of those on store shelves, but they often fade and won't hold a charge for long. Through the Joint Center for Energy Storage Research (JCESR), scientists at DOE's Pacific Northwest National Laboratory identified one of the reasons behind this problem.
Home > Press > Cleaning up hybrid battery electrodes improves capacity and lifespan: New way of building supercapacitor-battery electrodes eliminates interference from inactive components Abstract: Hybrid batteries that charge faster than conventional ones could have significantly better electrical capacity and long-term stability when prepared with a gentle-sounding way of making electrodes. Called ion soft-landing, the high-precision technique resulted in electrodes that could store a third more energy and had twice the lifespan compared to those prepared by a conventional method, the researchers report today in Nature Communications. Straightforward to set up, the method could eventually lead to cheaper, more powerful, longer-lasting rechargeable batteries. "This is the first time anyone has been able to put together a functioning battery using ion soft-landing," said chemist and Laboratory Fellow Julia Laskin of the Department of Energy's Pacific Northwest National Laboratory. The advantages come from soft-landing's ability to build an electrode surface very specifically with only the most desirable molecules out of a complex mixture of raw components. "It will help us unravel important scientific questions about this energy storage technology, a hybrid between common lithium rechargeable batteries and supercapacitors that have very high energy density," said lead author, PNNL chemist Venkateshkumar Prabhakaran. A different kind of hybrid Although lithium ion rechargeable batteries are the go-to technology for small electronic devices, they release their energy slowly, which is why hybrid electric vehicles use gasoline for accelerating, and take a long time to recharge, which makes electric vehicles slower to "fill" than their gas-powered cousins. One possible solution is a hybrid battery that crosses a lithium battery's ability to hold a lot of charge for its size with a fast-charging supercapacitor. PNNL chemists wanted to know if they could make superior hybrid battery materials with a technology -- called ion soft-landing -- that intricately controls the raw components during preparation. To find out, Laskin and colleagues created hybrid electrodes by spraying a chemical known as POM, or polyoxometalate, onto supercapacitor electrodes made of microscopically small carbon tubes. Off-the-shelf POM has both positively and negatively charged parts called ions, but only the negative ions are needed in hybrid electrodes. Limited by its design, the conventional preparation technique sprays both the positive and negative ions onto the carbon nanotubes. Ion soft-landing, however, separates the charged parts and only sets down the negative ions on the electrode surface. The question that Laskin and team had was, do positive ions interfere with the performance of hybrid electrodes? To find out, the team made centimeter-sized square hybrid batteries out of POM-carbon nanotube electrodes that sandwiched a specially developed ionic liquid membrane between them. "We had to design a membrane that separated the electrodes and also served as the battery's electrolyte, which allows conduction of ions," said Prabhakaran. "Most people know electrolytes as the liquid sloshing around within a car battery. Ours was a solid gel." They tested this mini-hybrid battery for how much energy it could hold and how many cycles of charging and discharging it could handle before petering out. They compared soft-landing with conventionally made hybrid batteries, which were made with a technique called electrospray deposition. They used an off-the-shelf POM containing positively charged sodium ions. Cheers for the POMs The team found that the POM hybrid electrodes made with soft-landing had superior energy storage capacity. They could hold a third more energy than the carbon nanotube supercapacitors by themselves, which were included as a minimum performance benchmark. And soft-landing hybrids held about 27 percent more energy than conventionally made electrospray deposited electrodes. To make sure the team was using the optimal amount of POM, they made hybrid electrodes using different amounts and tested which one resulted in the highest capacity. Soft-landing produced the highest capacity overall using the lowest amount of POM. This indicated the electrodes used the active material extremely efficiently. In comparison, conventional, sodium-based POM electrodes required twice as much POM material to reach their highest capacity. The conventionally-made devices used more POM, but the team couldn't count them out yet. They might in fact have a longer lifespan than the soft-landing produced electrodes. To test that, the team charged and discharged the hybrids 1,000 times and measured how long they lasted. As they did in the previous tests, the soft-landing-based devices performed the best, losing only a few percent capacity after 1000 cycles. The naked supercapacitors came in second, and the sodium-based, conventionally made devices lost about double the capacity of the soft-landed devices. This suggests that the soft-landing method has the potential to double the lifespan of these types of hybrid batteries. Looking good The team was surprised that it took so little of the POM material to make such a big difference to the carbon nanotube supercapacitors. By weight, the amount of POM was just one-fifth of a percent of the amount of carbon nanotube material. "The fact that the capacitance reaches a maximum with so little POM, and then drops off with more, is remarkable," said Laskin. "We didn't expect such a small amount of POM to be making such a large contribution to the capacitance." They decided to examine the structure of the electrodes using powerful microscopes in EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility at PNNL. They compared soft-landing with the conventionally made, sodium-POM electrodes. Soft-landing created small discrete clusters of POM dotting the carbon nanotubes, but the conventional method resulted in larger clumps of POM clusters swamping out the nanotubes, aggregates up to ten times the size of those made by soft-landing. This result suggested to the researchers that removing the positive ions from the POM starting material allowed the negative ions to disperse evenly over the surface. As long as the positive ions such as sodium remained, the POM and sodium appear to reform the crystalline material and aggregate on the surface. This prevented much of the POM from doing its job in the battery, thereby reducing capacity. When the team zoomed out a little and viewed the nanotubes from above, the conventionally made electrodes were covered in large aggregates of POM. The soft-landed electrodes, however, were remarkably indistinguishable from the naked carbon nanotube supercapacitors. In future research, the team wants to explore how to get the carbon materials to accept more POM, which might increase capacity and lifespan even further. ### This work was supported by the Department of Energy Office of Science and the Joint Center for Energy Storage Research, a Department of Energy Innovation Hub. Reference: Venkateshkumar Prabhakaran, B. Layla Mehdi, Jeffrey J. Ditto, Mark H. Engelhard, Bingbing Wang, K. Don D. Gunaratne, David C. Johnson, Nigel D. Browning, Grant E. Johnson and Julia Laskin. Rational Design of Efficient Electrode-Electrolyte Interfaces for Solid-State Energy Storage Using Ion Soft-Landing , Nature Communications April 21, 2016, DOI:10.1038/NCOMMS11399. About Pacific Northwest National Laboratory The Joint Center for Energy Storage Research (JCESR), a DOE Energy Innovation Hub, is a major partnership that integrates researchers from many disciplines to overcome critical scientific and technical barriers and create new breakthrough energy storage technology. Led by the U.S. Department of Energy's Argonne National Laboratory, partners include national leaders in science and engineering from academia, the private sector, and national laboratories. Their combined expertise spans the full range of the technology-development pipeline from basic research to prototype development to product engineering to market delivery. EMSL, the Environmental Molecular Sciences Laboratory, is a national scientific user facility sponsored by the Department of Energy's Office of Science. Located at Pacific Northwest National Laboratory in Richland, Wash., EMSL offers an open, collaborative environment for scientific discovery to researchers around the world. Its integrated computational and experimental resources enable researchers to realize important scientific insights and create new technologies. Follow EMSL on Facebook, LinkedIn and Twitter. Interdisciplinary teams at Pacific Northwest National Laboratory address many of America's most pressing issues in energy, the environment and national security through advances in basic and applied science. Founded in 1965, PNNL employs 4,400 staff and has an annual budget of nearly $1 billion. It is managed by Battelle for the U.S. Department of Energy's Office of Science. As the single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. 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