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Batteries made from an electrically conductive mixture the consistency of molasses could help solve a critical piece of the decarbonization puzzle. An interdisciplinary team from MIT has found that an electrochemical technology called a semisolid flow battery can be a cost-competitive form of energy storage and backup for variable renewable energy (VRE) sources such as wind and solar. The group’s research is described in a paper published in Joule.

“The transition to clean energy requires energy storage systems of different durations for when the sun isn’t shining and the wind isn’t blowing,” says Emre Gençer, a research scientist with the MIT Energy Initiative (MITEI) and a member of the team. “Our work demonstrates that a semisolid flow battery could be a lifesaving as well as economical option when these VRE sources can’t generate power for a day or longer—in the case of natural disasters, for instance.”

The rechargeable zinc-manganese dioxide (Zn-MnO2) battery the researchers created beat out other long-duration energy storage contenders. “We performed a comprehensive, bottom-up analysis to understand how the battery’s composition affects performance and cost, looking at all the trade-offs,” says Thaneer Malai Narayanan SM ’18, Ph.D. ’21. “We showed that our system can be cheaper than others, and can be scaled up.”

Narayanan, who conducted this work at MIT as part of his doctorate in mechanical engineering, is the lead author of the paper. Additional authors include Gençer, Yunguang Zhu, a postdoc in the MIT Electrochemical Energy Lab; Gareth McKinley, the School of Engineering Professor of Teaching Innovation and professor of mechanical engineering at MIT; and Yang Shao-Horn, the JR East Professor of Engineering, a professor of mechanical engineering and of materials science and engineering, and a member of the Research Laboratory of Electronics (RLE), who directs the MIT Electrochemical Energy Lab.

Going with the flow

In 2016, Narayanan began his graduate studies, joining the Electrochemical Energy Lab, a hotbed of research and exploration of solutions to mitigate climate change, which is centered on innovative battery chemistry and decarbonizing fuels and chemicals. One exciting opportunity for the lab: developing low- and no-carbon backup energy systems suitable for grid-scale needs when VRE generation flags.
While the lab cast a wide net, investigating energy conversion and storage using solid oxide fuel cells, lithium-ion batteries, and metal-air batteries, among others, Narayanan took a particular interest in flow batteries. In these systems, two different chemical (electrolyte) solutions with either negative or positive ions are pumped from separate tanks, meeting across a membrane (called the stack). Here, the ion streams react, converting electrical energy to chemical energy—in effect, charging the battery. When there is demand for this stored energy, the solution gets pumped back to the stack to convert chemical energy into electrical energy again.

The duration of time that flow batteries can discharge, releasing the stored electricity, is determined by the volume of positively and negatively charged electrolyte solutions streaming through the stack. In theory, as long as these solutions keep flowing, reacting, and converting the chemical energy to electrical energy, the battery systems can provide electricity.

“For backup lasting more than a day, the architecture of flow batteries suggests they can be a cheap option,” says Narayanan. “You recharge the solution in the tanks from sun and wind power sources.” This renders the entire system carbon free.

But while the promise of flow battery technologies has beckoned for at least a decade, the uneven performance and expense of materials required for these battery systems has slowed their implementation. So, Narayanan set out on an ambitious journey: to design and build a flow battery that could back up VRE systems for a day or more, storing and discharging energy with the same or greater efficiency than backup rivals; and to determine, through rigorous cost analysis, whether such a system could prove economically viable as a long-duration energy option.

Multidisciplinary collaborators

To attack this multipronged challenge, Narayanan’s project brought together, in his words, “three giants, scientists all well-known in their fields”: Shao-Horn, who specializes in chemical physics and electrochemical science, and design of materials; Gençer, who creates detailed economic models of emergent energy systems at MITEI; and McKinley, an expert in rheology, the physics of flow. These three also served as his thesis advisors.

“I was excited to work in such an interdisciplinary team, which offered a unique opportunity to create a novel battery architecture by designing charge transfer and ion transport within flowable semi-solid electrodes, and to guide battery engineering using techno-economics of such flowable batteries,” says Shao-Horn.

While other flow battery systems in contention, such as the vanadium redox flow battery, offer the storage capacity and energy density to back up megawatt and larger power systems, they depend on expensive chemical ingredients that make them bad bets for long duration purposes. Narayanan was on the hunt for less-pricey chemical components that also feature rich energy potential.

Through a series of bench experiments, the researchers came up with a novel electrode (electrical conductor) for the battery system: a mixture containing dispersed manganese dioxide (MnO2) particles, shot through with an electrically conductive additive, carbon black. This compound reacts with a conductive zinc solution or zinc plate at the stack, enabling efficient electrochemical energy conversion. The fluid properties of this battery are far removed from the watery solutions used by other flow batteries.

“It’s a semisolid—a slurry,” says Narayanan. “Like thick, black paint, or perhaps a soft-serve ice cream,” suggests McKinley. The carbon black adds the pigment and the electric punch. To arrive at the optimal electrochemical mix, the researchers tweaked their formula many times.

“These systems have to be able to flow under reasonable pressures, but also have a weak yield stress so that the active MnO2 particles don’t sink to the bottom of the flow tanks when the system isn’t being used, as well as not separate into a battery/oily clear fluid phase and a dense paste of carbon particles and MnO2,” says McKinley.

This series of experiments informed the technoeconomic analysis. By “connecting the dots between composition, performance, and cost,” says Narayanan, he and Gençer were able to make system-level cost and efficiency calculations for the Zn-MnO2 battery.

“Assessing the cost and performance of early technologies is very difficult, and this was an example of how to develop a standard method to help researchers at MIT and elsewhere,” says Gençer. “One message here is that when you include the cost analysis at the development stage of your experimental work, you get an important early understanding of your project’s cost implications.”

In their final round of studies, Gençer and Narayanan compared the Zn-MnO2 battery to a set of equivalent electrochemical battery and hydrogen backup systems, looking at the capital costs of running them at durations of eight, 24, and 72 hours. Their findings surprised them: For battery discharges longer than a day, their semisolid flow battery beat out lithium-ion batteries and vanadium redox flow batteries. This was true even when factoring in the heavy expense of pumping the MnO2 slurry from tank to stack. “I was skeptical, and not expecting this battery would be competitive, but once I did the cost calculation, it was plausible,” says Gençer.

But carbon-free battery backup is a very Goldilocks-like business: Different situations require different-duration solutions, whether an anticipated overnight loss of solar power, or a longer-term, climate-based disruption in the grid. “Lithium-ion is great for backup of eight hours and under, but the materials are too expensive for longer periods,” says Gençer. “Hydrogen is super expensive for very short durations, and good for very long durations, and we will need all of them.” This means it makes sense to continue working on the Zn-MnO2 system to see where it might fit in.

“The next step is to take our battery system and build it up,” says Narayanan, who is working now as a battery engineer. “Our research also points the way to other chemistries that could be developed under the semi-solid flow battery platform, so we could be seeing this kind of technology used for energy storage in our lifetimes.”

Ceramics are resilient to heat and extreme environments, but they are fragile and crack easily. Recently, in a study published in Science Advances, researchers at Texas A&M University have discovered a self-healing mechanism within a type of ceramics, called MAX phases.

They have shown that these engineered ceramics form natural faults or kink-bands during loading that can not only effectively stop cracks from growing, but can also close and heal them, thereby preventing catastrophic failure.

“What’s really exciting about MAX phases is that they readily form kink-bands under loading which can self-heal cracks even at room temperature, making them suitable for a variety of advanced structural applications,” said Ankit Srivastava, assistant professor in the Department of Materials Science and Engineering, and a corresponding author on the study. “So far, self-healing of cracks in ceramics has only been achieved at very high-temperatures by oxidation and that is why self-healing of cracks at room-temperature by kink-band formation is remarkable.”

This remarkable behavior of MAX phases can be traced back to their atomically layered structures.

“Imagine a plain loaf of bread, it is homogeneous, so if I slice it up, each slice will look the same—similar in idea to conventional ceramics,” said Miladin Radovic a professor in the Department of Materials Science and Engineering and also a corresponding author on the study. “But MAX phases are layered like a peanut butter sandwich with peanut butter between two slices of bread.”

The researchers then investigated if this unique layered structure of MAX phases make them any different from conventional ceramics. For their experiments, they used single crystal samples of chromium aluminum carbide MAX phase synthesized by Thierry Ouisse of Université Grenoble Alpes, France and a senior author on the study, and loaded them inside an electron microscope using an in-house designed test fixture.

When the researchers viewed the deforming sample in the electron microscope while applying loading, they observed that there were kink-band like defects that formed in the material, resembling those formed in natural rocks. More interestingly, they discovered that the material within kink-bands rotate during loading which not only form barrier against crack propagation but also eventually close and heal the cracks. As a consequence, the sample was no longer vulnerable to catastrophic failure.

“What’s really exciting is that this kinking or self-healing mechanism can occur over and over closing the newly formed cracks, thus delaying the failure of the material,” said Hemant Rathod, a doctoral student in the Department of Materials Science and Engineering and the lead author on the study.

The current discovery that materials resilient to heat and extreme environments such as MAX phases, also self-heal cracks which may form during service can advance a host of next-generation technologies, for instance, efficient jet engines, hypersonic flights, and safer nuclear reactors. The researchers also noted in the current study that the kink-band induced self-healing of cracks is most likely not unique to MAX phases and can be extended to other materials with similar atomically layered structures.

“This study demonstrates the serendipity of the scientific process,” says Siddiq Qidwai, a program director in National Science Foundation’s Directorate for Engineering. “We have had self-healing soft materials and polymer composites, and now, remarkably, ceramics.”

Researchers from Switzerland are tapping into an unexpected energy source right under our feet: wooden floorings. Their nanogenerator, presented September 1 in the journal Matter, enables wood to generate energy from our footfalls. They also improved the wood used in the their nanogenerator with a combination of a silicone coating and embedded nanocrystals, resulting in a device that was 80 times more efficient—enough to power LED lightbulbs and small electronics.

The team began by transforming wood into a nanogenerator by sandwiching two pieces of functionalized wood between electrodes. Like a shirt-clinging sock fresh out of the dryer, the wood pieces become electrically charged through periodic contacts and separations when stepped on, a phenomenon called the triboelectric effect. The electrons can transfer from one object to another, generating electricity. However, there’s one problem with making a nanogenerator out of wood.

“Wood is basically triboneutral,” says senior author Guido Panzarasa, group leader in the professorship of Wood Materials Science located at Eidgenössische Technische Hochschule (ETH) Zürich and Swiss Federal Laboratories for Materials Science and Technology (Empa) Dübendorf. “It means that wood has no real tendency to acquire or to lose electrons.” This limits the material’s ability to generate electricity, “so the challenge is making wood that is able to attract and lose electrons,” Panzarasa explains.

To boost wood’s triboelectric properties, the scientists coated one piece of the wood with polydimethylsiloxane (PDMS), a silicone that gains electrons upon contact, while functionalizing the other piece of wood with in-situ-grown nanocrystals called zeolitic imidazolate framework-8 (ZIF-8). ZIF-8, a hybrid network of metal ions and organic molecules, has a higher tendency to lose electrons. They also tested different types of wood to determine whether certain species or the direction in which wood is cut could influence its triboelectric properties by serving as a better scaffold for the coating.

The researchers found that a triboelectric nanogenerator made with radially cut spruce, a common wood for construction in Europe, performed the best. Together, the treatments boosted the triboelectric nanogenerator’s performance: it generated 80 times more electricity than natural wood. The device’s electricity output was also stable under steady forces for up to 1,500 cycles.

The researchers found that a wood floor prototype with a surface area slightly smaller than a piece of paper can produce enough energy to drive household LED lamps and small electronic devices such as calculators. They successfully lit up a lightbulb with the prototype when a human adult walked upon it, turning footsteps into electricity.

“Our focus was to demonstrate the possibility of modifying wood with relatively environmentally friendly procedures to make it triboelectric,” says Panzarasa. “Spruce is cheap and available and has favorable mechanical properties. The functionalization approach is quite simple, and it can be scalable on an industrial level. It’s only a matter of engineering.”

Besides being efficient, sustainable, and scalable, the newly developed nanogenerator also preserves the features that make the wood useful for interior design, including its mechanical robustness and warm colors. The researchers say that these features might help promote the use of wood nanogenerators as green energy sources in smart buildings. They also say that wood construction could help mitigate climate change by sequestering CO2 from the environment throughout the material’s lifespan.

The next step for Panzarasa and his team is to further optimize the nanogenerator with chemical coatings that are more eco-friendly and easier to implement. “Even though we initially focused on basic research, eventually, the research that we do should lead to applications in the real world,” says Panzarasa. “The ultimate goal is to understand the potentialities of wood beyond those already known and to enable wood with new properties for future sustainable smart buildings.”

Fitbits, car windows, oil refineries—the roll call of devices and technologies that rely on ultra-small sensors already numbers in the hundreds of billions.

Sensors measure changing conditions and report the data electronically—say, to heat up a bedroom or cool down a nuclear reactor. Sensors with microcircuits are ubiquitous in today’s networks. Some estimates project ten trillion sensors will be in use by 2030.

Ultra-small sensors are powered by their counterparts: ultra-small batteries. As the pace of miniaturization and its applications accelerates, so does the need for ever-smaller sensor batteries that can last a lifetime.

Kristofer Pister, professor of electrical engineering and computer science, leads research to embed microbatteries directly into sensor circuitry, providing a built-in power source instead of an external one. The technology could shrink circuit boards to make the sensor, microprocessor and battery all one unit that can speak to the internet.

The Bakar Fellows Program supports his on-going work to fabricate and print microbatteries within the sensor circuitry itself. Much of the work is also being done by his post-doc, Dr. Anju Toor, who has developed a battery design and fabrication process to print high-capacity batteries. Pister describes the technology and its potential.

Q: Besides size, are there other features that make this battery-inside a sensor concept attractive to industry?

A: The number one requirement for batteries is connectivity. You have to get the data back to where it needs to go. And you need to do it reliably. The microbatteries we are developing pack a lot of power into a smaller package. They’re also lighter than current batteries and cheaper to produce. Manufacturing them uses the same fabrication and circuit printing technology already used to make trillions of chips.

Q. How are they made?

A: The circuitry is stencil printed into cavities on the silicon wafer. You can picture it as etching micro-trenches into the wafer. It’s the same principle as making screen-printed t-shirts. You have a pattern with holes in some areas and not in others. We deposit electrode slurries in the patterned cavities.

Q: How small are the batteries?

A: The batteries will be about the thickness of three sheets of paper.

Q: It seems that you’re creating something inside the box by thinking outside the box. Why do you think your idea of fabricating batteries directly in the sensor circuits had not been devised before?

A. Many people have made ultra-small batteries because they have seen this need coming. I really don’t know why the idea of embedding the batteries has not been thought of before. It has seemed obvious to me for about ten years.

I founded a company called Dust Networks in 2002. We produced ultra-low power wireless networking chips—we call them smart dust—that could function for a decade on the power of a AA battery. They are now deployed worldwide, from oil refineries to parking meters.

As with all microsensors on the market, I had the battery as separate component, and it was all assembled. But it became clear that to miniaturize the sensors further, the right thing to do is to print the batteries on the wafer.

Q. What part does your Spark award play in this development?

A: The Bakar Fellows Program support is about bringing the technology to the point that it can be commercialized. In our collaborations with industry up to now, we have been using micro-batteries off the shelf to power sensors. Built-in power sources would be very useful to support applications from the largest oil refineries to medical devices in clinics and in the home, and for personal health monitoring.

They would work very well for wearables, where comfort and appearance are important. As just one example, they are a natural for applications like smart bandages to monitor wound healing by assessing the healing process of the wound environment. We think this batteries-in-a chip design will be part of devices in the full range of commercial spaces.

Penske Truck Leasing, as part of its ongoing goal to expand the electrification of its fleet, is piloting software in conjunction with artificial intelligence energy storage provider Stem, Inc., to help drive off-peak electricity savings and minimize utility peak demand charges.

The companies have worked together since April to pilot Athena smart energy storage software and operate an advanced battery storage system.

Penske says Stem’s Athena software has driven a 40% decrease in its site peak energy consumption since starting the pilot program. The project included a 350 kilowatt/800 kilowatt hour battery system at Penske’s heavy-duty truck charging positions in Ontario, Calif.

Athena predicts when the charging site’s electricity demand will spike and uses the supplemental stored energy from on-site batteries to drive off-peak electricity savings and minimize peak demand charges. Athena can track battery performance in real time, optimize battery storage, and automatically store and release energy, among other abilities.

Stem collaborated with Penske and its other suppliers for the project, including optimizing energy tariffs, utilizing energy storage to secure funds from California Air Quality Resources and securing incentives from California’s Self Generation Incentive Program.

“The growth in demand for electric vehicles and fleet charging solutions will require smart energy storage to help optimize energy usage, support the grid, and help companies go electric,” says Stem CEO John Carrington.

Penske has been active in fleet electrification. In July the company announced a partnership with Roush CleanTech and Proterra to develop an all-electric Ford F-650. In late 2020 Penske began providing a battery-electric eM2 from the Freightliner Innovation Fleet for California-based delivery company Tecmo Logistics.

Additionally, Penske produced the State of Sustainable Fleets 2020, an industry report on the advancement and accelerating the use of sustainable vehicles in the transportation industry.

Use of renewable fuel and energy options for fleets remains an important advance in the field. The Environmental Protection Agency as part of its SmartWay program estimates shipments of goods in the United States will grow by 45% by 2040. During this time the EPA says emissions from that growth will outpace emissions from all other sources, including passenger transportation.

Many companies have announced plans to continue to grow electric fleets as a response. FedEx, for example, says it will replace all its pickup and delivery fleet with battery-operated vehicles by 2040. Amazon has pledged to add 100,000 electric vehicles as part of a $1 billion sustainability bond.

In a move that could one day free the world’s countertops from their snarl of charging cords, researchers at the University of Michigan and University of Tokyo have developed a system to safely deliver electricity over the air, potentially turning entire buildings into wireless charging zones.

Detailed in a new study published in Nature Electronics, the technology can deliver 50 watts of power using magnetic fields.

Study author Alanson Sample, U-M professor of computer science and engineering, says that in addition to untethering phones and laptops, the technology could also power implanted medical devices and open new possibilities for mobile robotics in homes and manufacturing facilities. The team is also working on implementing the system in spaces that are smaller than room-size, for example a toolbox that charges tools placed inside it.

“This really ups the power of the ubiquitous computing world—you could put a computer in anything without ever having to worry about charging or plugging in,” Sample said. “There are a lot of clinical applications as well; today’s heart implants, for example, require a wire that runs from the pump through the body to an external power supply. This could eliminate that, reducing the risk of infection and improving patients’ quality of life.”

The team, led by researchers at the University of Tokyo, demonstrated the technology in a purpose-built aluminum test room measuring approximately 10 feet by 10 feet. They wirelessly powered lamps, fans and cell phones that could draw current from anywhere in the room regardless of the placement of people and furniture.

The system is a major improvement over previous attempts at wireless charging systems, which used potentially harmful microwave radiation or required devices to be placed on dedicated charging pads, the researchers say. Instead, it uses a conductive surface on room walls and a conductive pole to generate magnetic fields.

Devices harness the magnetic field with wire coils, which can be integrated into electronics like cell phones. The researchers say the system could easily be scaled up to larger structures like factories or warehouses while still meeting existing safety guidelines for exposure to electromagnetic fields.

“Something like this would be easiest to implement in new construction, but I think retrofits will be possible as well,” said Takuya Sasatani, a researcher at the University of Tokyo and the corresponding author on the study. “Some commercial buildings, for example, already have metal support poles, and it should be possible to spray a conductive surface onto walls, perhaps similar to how textured ceilings are done.”

A key to making the system work, Sample said, was building a resonant structure that could deliver a room-size magnetic field while confining harmful electric fields, which can heat biological tissues.

The team’s solution used devices called lumped capacitors. Placed in wall cavities, they generate a magnetic field that resonates through the room, while trapping electric fields inside the capacitors themselves. This overcomes a limitation of previous wireless power systems, which are limited to either delivering large amounts of power over a few millimeters or very small amounts of power over long distances.

A second hurdle was how to generate a magnetic field that reaches every corner of the room—magnetic fields tend to travel in circular patterns, creating dead spots in a square room. In addition, receivers need to align with the field in a specific way to draw power.

“Drawing power over the air with a coil is a lot like catching butterflies with a net,” Sample said. “The trick is to have as many butterflies as possible swirling around the room in as many directions as possible. That way, you’ll catch butterflies no matter where your net is or which way it’s pointed.”

To make that happen, the system generates two separate, 3D magnetic fields. One travels in a circle around the room’s central pole, while the other swirls in the corners, traveling between adjacent walls. This approach eliminates dead spots, enabling devices to draw power from anywhere in the space.

Tests with anatomical dummies showed that the system could deliver at least 50 watts of power to any location in the room without exceeding FCC guidelines for electromagnetic energy exposure. Sample said it’s likely, however, that it will be possible to deliver higher levels of power with further refinement of the system.

The researchers note that implementation of the system in commercial or residential settings is likely years away. They’re currently working to test the system in a building on U-M’s campus. They’ll implement it as both a retrofit and new construction in a series of rooms that use standard construction techniques, with a completion date set for this fall.

The team also includes Yoshihiro Kawahara, professor of electrical engineering and information systems at the University of Tokyo. The research was supported by the Japan Science and Technology Agency and the Japan Society for the Promotion of Science.