Lightning hitting car

Using power units as walls and struts, Leif Asp and his team are designing a brave new world of energy

Carlos David Martinez Arevalo

Sometimes, a car hood is just a car hood. But if researchers at Chalmers University of Technology have their way, that hood could double up as a battery.

Thing is, electric cars, planes and electronic gear have often been weighed down by their batteries – usually heavy units that eat up space and, in the case of vehicles, curtail range. For example, a quarter of the weight of the Tesla S is the battery. Imagine if this weight became functionally zero.

The Chalmers team appear to have brought us a little closer to that possibility, making power storage part of the structure. Their breakthrough in what is optimistically termed “massless storage” stores almost 10 times the energy that their predecessors did.

“The structural battery composite is a multifunctional material that can store electrical energy (chemically) and simultaneously carry mechanical loads,” Leif Asp, the main investigator, told Truly Curious.

According to Asp, a professor of material and computational mechanics at the Department of Industrial and Materials Science at Chalmers, “The aim is to provide a multifunctional material that can replace monofunctional materials and devices to reduce the mass.” He described how cars, aircraft, laptops and phones could all benefit from being lighter and less bulky.

Lead investigator Leif Asp with a bobbin of carbon fiber yarn. The electrodes in a structural lithium ion battery consist of such yarn arranged in a grid in a polymer. Every length of yarn consists of 24,000 individual carbon fibers. Pic Johan Bodell / Chalmers University of Technology

But even before we discuss structure and strength, we need to start with the battery.

A battery is a unit in which two or more chemicals that react with each other when electrons – the tiny mobile units of electricity are permitted to move between the chemicals via an external conductor, usually a wire in normal use.

Asp’s research is focused on efficiently designing carbon fiber composites for the transportation sector.
When asked about the objective of the investigation he told Truly Curious it was to replace materials with just one function with those that have many. This reduces the mass of products that need to be light for better energy efficiency, such as car or aircraft; those that need to be light for user acceptance, such as laptops or phones; or in cases that volume is a factor, such as in spacecraft.

That is true. Why add batteries if the battery can be incorporated into the structure?

Asp explains the problem: “It has proven difficult to make a structural battery with both high energy storage capability and high elastic modulus.” Elastic modulus gauges how resistant the measured object is to deformation under stress.

“We have overcome that, and with the technology we have developed we make truly multifunctional materials,” says Leif. “In two years we expect to have a material [with] ten times [better] multifunctional properties.”

A schematic of a laminated structural battery with carbon fiber electrodes and a structural electrolyte. Pic Quay2021 / Wikipedia CC BY-SA 4.0

Asp agrees these batteries cannot take on the old lead-acid batteries quite yet.

“The energy density is low on the cell level compared to traditional batteries,” Leif says. “However, given that a lot of the battery boxes and connectors become obsolete, [our] structural battery energy density is closing on that of the traditional battery.” He points out that his team’s batteries do not need that kind of energy density since the whole product is lighter because the batteries also form the structure.

“[We] can introduce the energy storage capability in other parts of the car, like the roof, interior panel or chairs.” Asp says.

The first work on Li-ion batteries dates back to the 1970s, and the first commercial version came out in 1991. What about structural batteries?

“This depends on industrial interest and investment,” says Asp. “We aim to have cells available with an energy density and modulus of 75 W-h/kg and 75 GPa by the end of 2022. These will all be lab cells with dimensions of maximum 100 * 50 mm2. For use in laptops, and similar devices which require low … power, I think these can be matured … to be used within a couple of years.” That is about five years, he estimates.
Things could take longer with cars, he admits.

“Upscaling on the manufacture [for cars] is more demanding,” Asp says. ”Larger cells, with higher power capability, need to be manufactured, the variability between cells’ performance must be reduced and the large battery systems (more cells than for traditional EVs are expected) must be managed. For car applications (beyond the 12 V system) I think we can expect up to ten years before they can be introduced.”

Car. plane with a battery body
The researchers propose products where a large part of the body consists of structural lithium ion batteries. Multi-functional carbon fiber can work as battery electrodes and load-bearing material consecutively. ​Illustration: Yen Strandqvist / Chalmers University of Technology

Asked why he decided to study the problem, Asp responds: ”I started this in 2007, inspired by work by Eric Wetzel and his team at the US Army Research Labs.”

That first real effort at massless storage, involved the US Army working with a team at Drexel University, using carbon fiber, which is really strong, as the negative terminal (called the anode); and a coating of primarily lithium iron phosphate on the positive terminal (called the cathode). But to carry the current around they relied on a gel, which did not give the structure sufficient tensile elasticity, the ease at which it can deform. Mathematically speaking, that is stress divided by strain.

That team got energy density – or energy per unit volume – of 35 W-h/kg, which is about a tenth of a Tesla battery, and a tensile elasticity of -3 GPa (compared to 70 GPa for aluminum). This was too low to be considered suitable either for a battery or structural material.

Watt-hour per kilogram is a unit to measure energy density in batteries. Typically, lithium-ion batteries have an energy density of nearly 150 W-h/Kg and the most advanced ones can produce up to 250 W-h/Kg. The work at Chalmers predicts there will soon be a structural battery with 75 Wh/Kg and a modulus higher than 75 GPa.

Energy storage methodEnergy density (W-h/l)Specific energy (W-h/kg)
Compressed air1734
Supercapacitor3520
Lead acid battery4020
Nickel metal hydride9090
Lithium iron phosphate220110
Flywheel210120
Lithium-ion440175
Zinc-air1600470

With a specific energy of 75 W-h/kg Asp’s structural battery is still behind the widely used lithium-ion battery.

As the experiments continued, various teams got either a sturdy structure or an effective battery, never both in the same product. Using fiber ensured stiffness and strength. It even carried electricity. But it had no chemical function. That job is done well by conductive liquids, but then those cannot take a load.

In an earlier paper, Asp had described how the structural strength reduced as more lithium went into the carbon fibers.

“A eureka moment came when we pursued an idea to develop a micro-version of the structural battery,” Asp says. In 2010, the team worked with the KTH Royal Institute of Technology in Stockholm to coat the first single carbon fiber structural battery with charged particles, the thickness of the coating depending on the voltage.

“After that, I knew we will be able to make structural batteries, he says. According to Asp, the development of electrolytes for structural batteries at KTH have helped scale up the cells and allowed for laminated structural batteries.

Structural battery composites cannot store as much energy as lithium-ion batteries, but have several characteristics that make them highly attractive for use in vehicles and other applications. When the battery becomes part of the load-bearing structure, the mass of the battery essentially ‘disappears.’ ​Illustration: Yen Strandqvist

Asp assembled a strong team, including such researchers as Dan Zenkert (mechanics), Göran Lingbergh (electrochemistry), Mats Johansson (polymer synthesis), all from KTH; and Fang Liu (materials characterization), Kenneth Runesson (computational mechanics), and Patrik Johansson (materials physics), from Chalmers to address this highly interdisciplinary research.

The result was a battery with negative carbon electrode separated by glass fibers from a positive aluminum electrode coated with a lithium iron phosphate and a polymer electrolyte.

Although this is a big step in the field of structural batteries Asp says: “We still need to overcome the relatively high internal resistance in the structural battery. It is not a frustration but the most important issue for us to solve at the moment.”

Structural battery composite fabrication, showing the steps: battery component manufacture, pouch‐cell manufacture, and curing of the special electrolyte. Pic Advanced Energy and Sustainability Research (2021). CC BY 4.0

He denied having ever considered dropping the work out of frustration. On the contrary, he says “We have worked consistently on this… It has brought joy in my work and I have considered dropping other stuff to only focus on this. Not once have I considered to stop these activities.”

Asked if he had other interests besides research, he says, “Science is, to a large extent, a hobby. It is quite similar to a love for sports, with practice needed to be successful.” But he does enjoy time spent with his family, taking long walks with his dog, and traveling and meeting people – though all that is on hiatus for now. Asp also likes to head off to his summer cottage – to read and fish. So no, not just science.

* The headline is a reference to Walt Whitman’s poem, “I sing the body electric

Carlos Martinez Arevalo is a mathematics master’s student at IMCA-UNI, Peru. His main interests are probability theory and statistics in applied science. He works as a quantitative researcher/scientific programmer

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