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?”