I have written previously about the concept of structural batteries, such as this recent post on a concrete battery. The basic idea is a battery made out of material that is strong enough that it can bare a load. Essentially we’re asking the material to do two things at once – be a structural material and be a battery. I am generally wary of such approaches to technology as you often wind up with something that is bad at two things, rather than simply optimizing each function.

In medicine, for example, I generally don’t like combo medications – a single pill with two drugs meant to take together. I would rather mix and match the best options for each function. But sometimes there is such a convenience in the combination that it’s worth it. As with any technology, we have to consider the overall tradeoffs.

With structural batteries there is one huge gain – the weight and/or volume savings of having a material do double duty. The potential here is too great to ignore. For the concrete battery the advantage is about volume, not weight. The idea is to have the foundation of a building serve as individual or even grid power storage. For a structural battery that will save weight, we need a material is light and strong. One potential material is carbon fiber, which may be getting close to characteristics with practical applications.

Material scientists have created in the lab a carbon fiber battery material that could serve as a structural battery. Carbon fiber is a good substrate because it is light and strong, and also can be easily shaped as needed. Many modern jets are largely made out of carbon fiber for this reason. Of course you compromise the strength when you introduce materials needed for the energy storage, and these researchers have been working on achieving an optimal compromise. Their latest product has an elastic modulus that exceeds 76 GPa. For comparison, aluminum, which is also used for aircraft, has an elastic modulus of 70 GPa. Optimized carbon fiber has an elastic modulus of 200-500 GPa. Elastic modulus is one type of strength, specifically the resistance to non-permanent deformity. Being stronger than aluminum means it is in the range that is suitable for making lots of things, from laptops to airplanes.

How is the material as a battery? The basic features are good – it is stable, can last over 1000 cycles, and can be charged and discharged fast enough. But of course the key feature is energy density – their current version has an energy density of 30 Wh kg. For comparison, a typical Li Ion battery in an electric vehicle today has an energy density of 200-300 Wh kg. High end Amprius silicon anode Li ion batteries are up to 400-500 Wh kg.

So, as I said, the carbon fiber structural battery is essentially bad at two things. It is not as strong as regular carbon fiber, and it is not as good a battery as Li ion batteries. But is the combination worth it? If we run some numbers I think the answer right now is – probably not. What I don’t know is the cost to mass produce this material, which so far is just a laboratory proof of concept. All bets are off if the material is super expensive. But let’s assume cost is reasonable and focus on the weight and energy storage.

If, for example, we look at a typical electric vehicle how would the availability of this material be useful? It’s hard to say exactly because I would need to see the specs on a vehicle engineered to incorporate this material, but let’s do some rough estimates. A Tesla, for example, has a chassis made of steel and titanium, with a body that is almost entirely aluminum. So we can replace all the aluminum in such a vehicle with structural carbon fiber, which is stronger and lighter. Depending on the vehicle of course, we’re talking about 100kg of carbon fiber for the body of a car. The battery weighs about 500 kg. The carbon fiber battery has one tenth the specific energy as a Tesla Li ion battery, so 100 kg of carbon fiber battery would hold as much energy at 10 kg of battery. This would allow a reduction in the battery weight from 500 kg to 490 kg. That hardly seems worth it, for what is very likely to be a more expensive material than either the current battery or aluminum.

Of course you could beef up the frame, which could have the double advantage of making it stronger and longer lasting. Let’s say you have a triple thick 300 kg carbon fiber frame – that still only saves you 30 kg of battery. My guess is that we would need to get that energy density up to 100 Wh kg or more before the benefits start to become worth it.

The calculus changes, however, when we talk about electric aircraft. Here there is a huge range of models but just to throw out some typical figures – we could be talking about a craft that weighs 1,500 kg and battery that weight 2,000 kg. If the body of the craft were made out of structural carbon fiber, that could knock 150 kg off the weight of the battery, which for an aircraft is significant. For a commercial aircraft it might even be worth the higher cost of the plane, given the lower operating costs.

What about at the low end of the spectrum – say a laptop or smart phone? A laptop might be the sweet spot for this  type of material. If the case were made of a carbon fiber battery that could allow for a thinner and lighter laptop or it could extend the battery life of the laptop, both of which would be desirable. These are already expensive, and adding a bit to the overall cost to improve performance is likely something consumers will pay for. But of course the details matter.

Given all this – is the carbon fiber structural battery ready for commercial use? I think it’s marginal. It’s plausible for commercial electric aircraft and maybe laptops, depending on the ultimate manufacturing cost, and assuming no hidden gotchas in terms of material properties. We may be at the very low end of viability. Any improvement from this point, especially in energy density, makes it much more viable. Widespread adoption, such as in EVs, probably won’t come until we get to 100 Wh kg or more.