Don’t bother with solid-state batteries. Also, there’s all this talk about silicon anodes. Even the much-heralded aluminum-air battery, which cannot be recharged at home, is unlikely to cause a stir in the electric vehicle industry.
However, structural batteries may be able to help.
You might be wondering what a structural battery is. That’s a great question, and thankfully for me, who doesn’t want to pretend to have engineering knowledge I don’t have, the solution is very straightforward. Electric vehicles are now powered by batteries that are installed in the vehicle.
Oh, we’ve devised creative methods for concealing their mass, such as embedding all those lithium-ion cells in the chassis’s floor, resulting in the “skateboard” platform that has become synonymous with EV design. They are, however, still separate from the vehicle. To put it another way, it’s an add-on.
By structuring the entire chassis out of battery cells, a structural battery challenges the status quo. Not only would the load floor contain the battery in an apparently implausible future, but so would parts of the body — the A-pillars, the roof, and even, as one research house has demonstrated, the air filter plenum — not only be filled with batteries, but actually composed of battery cells. The car, in the words of Marshall McLuhan, is the battery.
What makes you think this is such a big deal?
They’re also hefty. Hefty in the sense of “broad load.” The standard method for calculating a battery’s energy density these days is that each kilogramme of lithium-ion can provide roughly 250 watt-hours. 250 Wh/kg, or 250 Wh/kg if you’re an engineer.
If you do the math, a 100-kilowatt-hour battery, like the one Tesla uses in its Model S, means you’re carrying approximately 400 kg of battery cells. And that’s only in the best, most efficient cases. For us laypeople, it’s probably more accurate to estimate that a 100 kWh battery weighs roughly 1,000 pounds. Half a tonne, to be precise.
Consider something like the new Hummer SUT, which is said to have up to 213 kWh of battery capacity. Even if The General has discovered some sort of efficiency breakthrough, a top-of-the-line Hummer will still be carrying around a tonne of batteries.
Yes, it will drive further, but the increase in range will not be proportional to the doubling of battery capacity, and its truck will need to have more powerful — and hence less efficient — motors just to match the performance of the lighter, shorter-range alternative. Every automotive engineer will tell you that weight is the enemy, whether they’re seeking speed or fuel economy.
This is when structural batteries enter the picture. Much of the added weight is eliminated by building the car out of batteries rather than adding them to an existing construction. Increasing a car’s range would come with very low weight penalty up to a point — notably, when everything structural has been transformed to a battery cell.
As you might imagine — because I’m sure you’re thinking, “What a brilliant idea!” — such a brilliant solution is not without its drawbacks. The first is mastering the ability to construct batteries from materials that can not only serve as the anodes and cathodes required by any basic battery, but also as a sufficiently strong — and light! — structure capable of supporting two tonnes of automobile and the passengers it will transport, hopefully safely.
Carbon fibre and aluminium are the two main elements of the most powerful structural battery to date, which was developed by Chalmers University of Technology with help from KTH Royal Institute of Technology, two of Sweden’s most prestigious engineering institutes. Carbon fibre serves as the negative electrode, while lithium-iron-phosphate-coated aluminium foil serves as the positive electrode. Carbon fibre also carries electrons, obviating the requirement for heavy silver and copper.
A fibreglass matrix separates the cathode and anode and also includes the electrolyte, so it not only transports lithium ions between electrodes but also distributes structural loads between the two. Each of these battery cells has a nominal voltage of 2.8 volts and, like all contemporary electric car batteries, would be connected to create the 400V and even 800V now seen in most EVs.
Even these high-tech cells, while a significant step forward, weren’t quite ready for prime time. Their energy density per kilogram was a meagre 25 watt-hours, and their structural stiffness was only 25 gigapascals (GPa), hardly more robust than way-too-weak-for-a-car-frame fibreglass. The latest versions, which were funded by the Swedish National Space Agency, have replaced the aluminum foil electrode with even more carbon fibre, which the researchers claim has both rigidity and energy density. These latest carbon/carbon cells are expected to generate up to 75 watt-hours per kilogram and have a Young’s modulus of 75 GPa.
Although its energy density still falls short of that of traditional lithium-ion batteries, its structural stiffness has improved to the point where it is now superior to aluminium. To put it another way, an electric car chassis/battery composed of these cells may be as structurally strong as one made of aluminium while weighing far less.
The first use of these high-tech batteries will almost assuredly be consumer electronics. “It should 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,” says Leif Asp, a professor at Chalmers. But, as the leader of the project notes, “we are really only limited by our imaginations here.”
The modern electric vehicle’s foundation and weakest link are batteries. Even the most optimistic estimates only anticipate current energy density doubling. We’ll have to do more than just add batteries to cars if we want the great ranges we’ve all been promised — after all, isn’t it every week that someone claims that 1,000 kilometers per charge is just around the corner? — we’ll have to create automobiles out of batteries.
Editor’s note: This article originally appeared on Driving.