Swedish carbon fiber battery could revolutionize car design

Over the next several years, the batteries that go into electric vehicles will be cheap enough that an electric vehicle does not cost more than a vehicle of equivalent size with an internal combustion engine. But these EVs will still weigh more than their gasoline-powered counterparts – especially if the market insists on increasingly lengthy estimates – with the battery making up 20-25% of the vehicle’s total mass.

But there is a solution – turn some structural components of the car into batteries themselves. Do that, and the weight of your battery will be effectively taken off because no matter what 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 fully explained by Volvo’s CTO Henrik Green when Ars spoke to him in early March:

What we learned… just to take an example: “How do you most effectively integrate a battery cell into a car?” Well, if you do it the traditional way, you put the cell in the box, call it the module; you put a number of modules in a box, you call that the pack. You put the pack in a vehicle, then you have a standardized solution and you can upgrade it over 10 years and 10 manufacturing slots.

But in essence, it’s a pretty inefficient solution in terms of weight and space, etc. So here you can really go a step further, and how do you directly integrate cells into a body and get rid of these mods and packs and the like? This is the challenge that we are working with for future generations, and it will fundamentally change the way you build cars. You might have thought that this time of change would have ended, but it has just been reborn.

Tesla is known to work on the design of new battery modules that also function as structural elements, but the California automaker is fashioning these structural modules from traditional cylindrical cells. There is a more elegant approach to the idea, however, and a group from Chalmers University of Technology in Sweden led by Professor Leif Asp have just made a small breakthrough in this regard, making every component of the battery materials that function structurally. as well as electrically.

The structural battery combines a carbon fiber anode and an aluminum foil cathode coated with lithium iron phosphate, which are separated by a glass fiber separator in a structural battery electrolytic matrix material. The anode does a triple duty, harboring lithium ions, conducting electrons and strengthening everything at the same time. The electrolyte and the cathode similarly support structural charges and do their job in moving ions.

Researchers tested a few different types of fiberglass – both resulting in cells with a nominal voltage of 2.8V – and got better battery performance results with thinner plain weave. Cells using this construct 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 4,680 cells Tesla is moving have an energy density of 380 Wh / kg. However, this energy density figure for cylindrical cells does not include the mass of the structural matrix surrounding them (when used as structural panels).

Speaking of structural loads, the greatest stiffness was also achieved with single fiberglass weave, at 25.5 GPa. Again, to put this number in context, it’s roughly similar to glass-fiber reinforced plastic, while carbon-fiber reinforced plastic will be around 10 times larger, depending on whether it is. resin transfer molding or pre-impregnated woven sheets with pre-preg resin).

Professor Asp’s group is now working to see whether replacing the cathode’s aluminum foil with carbon fiber will increase both stiffness (which it should) and electrical performance. The group is also testing even thinner separators. It 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 from structural composite batteries is always a longer term project, and even at best, structural battery cells may never come close to the performance of dedicated cells. But since they would also replace heavier metal structures, the resulting vehicle is expected to be much lighter overall.

Meanwhile, Asp believes other products may see the benefits sooner. “The next-generation structural battery has fantastic potential. If you look at consumer technology, it might be quite possible in a few years to make smartphones, laptops or e-bikes that weigh half the weight of today. ‘hui and are much more compact,’ said Asp told me.

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