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.