Over the next few years, the batteries that go into electric vehicles are going to get cheap enough that an EV should cost no more than an equivalent-sized vehicle with an internal combustion engine. But those EVs are still going to weigh more than their gas-powered counterparts—particularly if the market insists on longer and longer range estimates—with the battery pack contributing 20-25 percent of the total mass of the vehicle.
But there is a solution: turn some of the car’s structural components into batteries themselves. Do that, and your battery weight effectively vanishes because regardless of powertrain, every vehicle still needs structural components to hold it together. It’s an approach that groups around the world have been pursuing for some time now, and the idea was neatly explained by Volvo’s chief technology officer, Henrik Green, when Ars spoke with him in early March:
What we have learned… just to take an example: “How do you integrate the most efficiently a battery cell into a car?” Well, if you do it in a traditional way, you put the cell into the box, call it the module; you put a number of modules into a box, you call that the pack. You put the pack into a vehicle and then you have a standardized solution and you can scale it for 10 years and 10 manufacturing slots.
But in essence, that’s a quite inefficient solution in terms of weight and space, etc. So here you could really go deeper, and how would you directly integrate the cells into a body and get rid of these modules and packs and stuff in between? That is the challenge that we are working with in future generations, and that will change how you fundamentally build cars. You might have thought that time of changing that would have ended, but it has just been reborn.
Tesla is known to be working on designing new battery modules that also work as structural elements, but the California automaker is fashioning those structural modules out of traditional cylindrical cells. There’s a more elegant approach to the idea, though, and a group at Chalmers University of Technology in Sweden led by professor Leif Asp has just made a bit of a breakthrough in that regard, making each component of the battery out of materials that work structurally as well as electrically.
The structural battery combines a carbon-fiber anode and a lithium-iron phosphate-coated aluminum foil cathode, which are separated by a glass fiber separator in a structural battery electrolyte matrix material. The anode does triple duty, hosting the lithium ions, conducting electrons, and reinforcing everything at the same time. The electrolyte and cathode similarly support structural loads and do their jobs in moving ions.
The researchers tested a couple different types of glass fiber—both resulting in cells with a nominal voltage of 2.8 V—and achieved better results in terms of battery performance with thinner, plain weave. The cells using this construction had a specific capacity of 8.55 Ah/kg, an energy density of 23.6 Wh/kg (at 0.05 C), a specific power of 9.56 W/kg (at 3 C), and a thickness of 0.27 mm. To put at least one of those numbers in context, the 4680 cells that Tesla is moving to have an energy density of 380 Wh/kg. However, that energy density figure for the cylindrical cells does not include the mass of the structural matrix that surrounds them (when used as structural panels).
Speaking of structural loads, the greatest stiffness was also achieved with plain glass fiber weave, at 25.5 GPa. Again, to put that number into context, it’s roughly similar to glass fiber-reinforced plastic, whereas carbon fiber-reinforced plastic will be around 10 times greater, depending on whether it’s resin transfer molding or woven sheets pre-impregnated with resin (known as pre-preg).
Professor Asp’s group is now working to see if swapping the cathode’s aluminum foil for carbon fiber will increase both stiffness (which it should) and electrical performance. The group is also testing even thinner separators. He hopes to reach 75 Wh/kg and 75 GPa, which would result in a cell that is slightly stiffer than aluminum (GPa: 68) but obviously much lighter.
Building electric cars or even airplanes out of structural composite batteries is still a longer-term project, and even at their best, structural battery cells may never approach the performance of dedicated cells. But since they would also replace heavier metal structures, the resulting vehicle should be much lighter overall.
Meanwhile, Asp thinks other products could see the benefits sooner. “The next generation structural battery has fantastic potential. If you look at consumer technology, it could be quite possible within a few years to manufacture smartphones, laptops, or electric bicycles that weigh half as much as today and are much more compact,” Asp said.
Listing image by Marcus Folino