There’s a renaissance underway in structural battery research, which aims to build energy storage into the very devices and vehicles they power.
ELON MUSK MADE a lot of promises during Tesla’s Battery Day last September. Soon, he said, the company would have a car that runs on batteries with pure silicon anodes to boost their performance and reduced cobalt in the cathodes to lower their price. Its battery pack will be integrated into the chassis so that it provides mechanical support in addition to energy, a design that Musk claimed will reduce the car’s weight by 10 percent and improve its mileage by even more. He hailed Tesla’s structural battery as a “revolution” in engineering—but for some battery researchers, Musk’s future looked a lot like the past.
“He’s essentially doing something that we did 10 years ago,” says Emile Greenhalgh, a materials scientist at Imperial College London and the engineering chair in emerging technologies at the Royal Academy. He’s one of the world’s leading experts on structural batteries, an approach to energy storage that erases the boundary between the battery and the object it powers. “What we’re doing is going beyond what Elon Musk has been talking about,” Greenhalgh says. “There are no embedded batteries. The material itself is the energy storage device.”
Today, batteries account for a substantial portion of the size and weight of most electronics. A smartphone is mostly a lithium-ion cell with some processors stuffed around it. Drones are limited in size by the batteries they can carry. And about a third of the weight of an electric vehicle is its battery pack. One way to address this issue is by building conventional batteries into the structure of the car itself, as Tesla plans to do. Rather than using the floor of the car to support the battery pack, the battery pack becomes the floor.
But for Greenhalgh and his collaborators, the more promising approach is to scrap the battery pack and use the vehicle’s body for energy storage instead. Unlike a conventional battery pack embedded in the chassis, these structural batteries are invisible. The electrical storage happens in the thin layers of composite materials that make up the car’s frame. In a sense, they’re weightless because the car is the battery. “It’s making the material do two things simultaneously,” says Greenhalgh. This new way of thinking about EV design can provide huge performance gains and improve safety because there won’t be thousands of energy-dense, flammable cells packed into the car.
A lithium-ion battery inside a phone or EV battery pack has four main components: the cathode, anode, electrolyte, and the separator. When a battery is discharged, lithium-ions flow through the electrolyte from the negative anode to the positive cathode, which are partitioned by a permeable separator to prevent a short circuit. In a conventional battery, these elements are either stacked like a wedding cake or wound around each other like a jelly roll to pack as much energy as possible into a small volume. But in a structural battery, they have to be reconfigured so the cell can be molded into irregular shapes and withstand physical stress. A structural battery doesn’t look like a cube or a cylinder; it looks like an airplane wing, car body, or phone case.
The first structural batteries developed by the US military in the mid-2000s used carbon fiber for the cell’s electrodes. Carbon fiber is a lightweight, ultrastrong material that is frequently used to form the bodies of aircraft and high-performance cars. It’s also great at storing lithium ions, which makes it a good substitute for other carbon-based materials like graphite that are used as anodes in typical Li-ion batteries. But in a structural battery, carbon fiber infused with reactive materials like iron phosphate is also used for the cathode because it needs to provide support. A thin sheet of woven glass separates the two electrodes, and these layers are suspended in an electrolyte like fruit in an electrochemical jello. The entire ensemble is only a few millionths of a meter thick and can be cut into any desired shape.
Leif Asp, a materials scientist at the Chalmers University of Technology in Sweden, has been at the forefront of structural battery research for the past decade. In 2010, Asp, Greenhalgh, and a team of European scientists collaborated on Storage, a project that aimed to build structural batteries and integrate them into a prototype hybrid Volvo. “At that time, I didn’t think it would have much impact on society, but as we moved along it struck me that this could be a very useful idea,” says Asp, who characterizes the conventional battery as a “structural parasite.” He says the main benefit of structural batteries is that they reduce the amount of energy an EV needs to drive the same distance—or it can increase its range. “We need to focus on energy efficiency,” says Asp. In a world where most electricity is still produced with fossil fuels, every electron counts in the fight against climate change.
During the three-year project, the Storage team successfully integrated commercial lithium-ion batteries into a plenum cover, a passive component that regulates air intake into the engine. It wasn’t the car’s main battery, but a smaller secondary pack that supplied electricity to the air-conditioning, stereo, and lights when the engine temporarily turned off at a stop light. This was the first proof of concept for a structural battery that was integrated into the body of a working car and was essentially a small-scale version of what Tesla is trying to achieve.
But sandwiching a bunch of conventional Li-ion cells into the body of a car isn’t as efficient as making the car’s body serve as its own battery. During the Storage collaboration, Asp and Greenhalgh also developed a structural supercapacitor that was used as a trunk lid. A supercapacitor is similar to a battery but stores energy as electrostatic charge, rather than a chemical reaction. The one made for the Volvo trunk consisted of two layers of carbon fiber infused with iron oxide and magnesium oxide, separated by an insulating layer. The whole stack was wrapped in laminate and molded into the shape of the trunk.
Supercapacitors don’t hold nearly as much energy as a battery, but they’re great at rapidly delivering small amounts of electric charge. Greenhalgh says that they’re also easier to work with and were a necessary stepping stone toward accomplishing the same thing with a battery. The Volvo was a proof of concept that structural energy storage was viable in an EV, and the success of the Storage project generated a lot of hype about structural batteries. But despite that enthusiasm, it took a few years to procure more funding from the European Commission to push the technology to the next level. “This is a very challenging technology and something that’s not going to be solved with a few million pounds thrown at it,” says Greenhalgh of the financing difficulties. “We got a lot more funding, and now it’s really starting to snowball.”
This summer, Asp, Greenhalgh, and a team of European researchers wrapped up a three-year research project called Sorcerer that had the goal of developing structural lithium-ion batteries for use in commercial aircraft. Aviation is arguably the killer app for structural energy storage. Commercial aircraft produce a lot of emissions, but electrifying passenger jets is a major challenge because they require so much energy. Jet fuel is terrible for the environment, but it’s about 30 times more energy-dense than state-of-the-art commercial lithium-ion cells. In a typical 150-passenger aircraft, that means you’d need about 1 ton of batteries per person. If you tried to electrify this jet with existing cells, the plane would never get off the ground.
Established aerospace companies like Airbus and startups like Zunum have been working on electrifying passenger aircraft for years. But even if they’re successful, packing a plane full of conventional cells has some major safety risks. A short circuit in a large battery pack could cause a disastrous fire or explosion. “The aerospace sector is very conservative, and they’re nervous about packing aircraft with these really high-powered batteries,” says Greenhalgh. Emerging battery chemistries, including solid electrolytes, could lower the risk, but meeting the massive energy requirements of a passenger jet is still a major challenge that could be solved with structural batteries.
As part of the Sorcerer project, Asp and his colleagues created structural batteries made from thin layers of carbon fiber that could conceivably be used to build parts of an airplane’s cabin or wings. The experimental batteries the Sorcerer team developed have significantly improved mechanical properties and energy densities compared to the batteries they produced during the Storage initiative a decade earlier. “Now we can make materials that have at least 20 to 30 percent of both energy storage capacity and the mechanical capacity of the systems we want to replace,” says Asp. “It’s a huge progression.”
But technical challenges are only half the battle when it comes to getting structural batteries out of the lab and into the real world. Both the automotive and aviation industries are heavily regulated, and manufacturers often run on thin margins. That means introducing new materials into cars and planes requires demonstrating their safety to regulators and their superior performance to manufacturers.
As a structural battery is charged and discharged, lithium ions are shuttling in and out of the carbon-fiber cathodes, which changes their shape and mechanical properties. It’s important for manufacturers and regulators to be able to predict precisely how these structural batteries will react when they’re being used and how that affects the performance of the vehicles they power. To that end, Greenhalgh and Asp are building mathematical models that will show exactly how the structure of vehicles built from these batteries changes during use. Asp says it will probably be more than a decade before structural batteries are deployed in vehicles because of their significant power demands and regulatory challenges. Before that happens, he predicts, they will become commonplace in consumer electronics.
Jie Xiao, the chief scientist and manager of the Batteries & Materials System group at Pacific Northwest National Laboratory, agrees. She thinks a particularly promising and often overlooked area of application is in microelectronics. These are devices that could comfortably fit on your fingertip and are particularly useful for medical implants. But first, there needs to be a way to power them.
“Structural batteries are extremely helpful for microelectronics, because the volume is very restricted,” says Xiao. While it is possible to scale down conventional batteries to the size of a grain of rice, these cells still take up valuable space in microelectronics. But structural batteries don’t take up more space than the device itself. At PNNL, Xiao and her colleagues have studied some of the fundamental issues with the design of microbatteries, like how to maintain alignment between electrodes when a structural battery is bent or twisted. “From a design point of view, it’s very important that your positive and negative electrodes face each other,” says Xiao. “So even if we can take advantage of void spaces, if those electrodes are unaligned they are not participating in the chemical reaction. So this limits the designs of irregular-shaped structural batteries.”
Xiao and her team have worked on several niche scientific applications for micro structural batteries, like injectable tracking tags for salmon and bats. But she says it’s still going to be a while before they find mainstream application with emerging technologies like electronic skin for prosthetics. In the meantime, however, structural batteries could be a boon for energy-hungry robots. In a laboratory on the Ann Arbor campus at the University of Michigan, chemist and chemical engineer Nicholas Kotov oversees a menagerie of small biomimetic robots he developed with his graduate students. “Organisms distribute energy storage throughout the body so that they serve double or triple functions,” says Kotov. “Fat is a great example. It has lots of energy storage. The question is: How do we replicate it?”
The team’s goal is to create machines that mimic animals, and so they require a power source that can integrate with their robotic skeletons, much like fat and muscle hem to ours. Some of their latest creations include robotic scorpions, spiders, ants and caterpillars that skitter around the floor. All of them are powered by a unique structural battery integrated with their moving parts. The battery sits on the back of the robot like a silver shell, and it both energizes and protects the robot’s mechanical guts. It’s taking a cue from nature to improve the unnatural.
Unlike the carbon-fiber and lithium-ion sheets being developed by Asp and Greenhalgh, Kotov and his students created a zinc-air structural battery for their automatons. This cell chemistry is able to store much more energy than conventional Li-ion cells. It consists of a zinc anode, a carbon cloth cathode, and a semi-rigid electrolyte made from polymer-based nanofibers that is nanoengineered to mimic cartilage. The energy carriers in this type of battery are hydroxide ions that are produced when oxygen from the air interacts with the zinc.
While structural batteries for vehicles are highly rigid, the cell developed by Kotov’s team is meant to be pliable to cope with the movements of the robots. They’re also incredibly energy-dense. As Kotov and his team detailed in a paper published earlier this year, their structural batteries have 72 times the energy capacity of a conventional lithium-ion cell of the same volume. For now, their batteries are being used to power robotic toys and small drones as a proof of concept. But Kotov says he expects they’ll be used in midsize robots as well as larger hobby drones in the not-so-distant future. “Drones and medium-size robots need to have new solutions for energy storage,” Kotov says. “I can guarantee you that structural batteries will be a part of that.”
The battery has always been an addendum, a limiting factor, and a parasite. Today it’s vanishing before our eyes, melting into the fabric of our electrified world. In the future, everything will be a battery, and stand-alone energy storage will seem as quaint as landline telephones and portable CD players. It’s a disappearing act worthy of a great magician: Now you see it—and soon you won’t.