Carbon-fiber composites that double as batteries

  • 13-Nov-2013 01:25 EST
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A researcher tests the fit of a prototype structural-power trunk lid for a Volvo S80.

The high costs and limited driving range that have so far hampered sales of electric vehicles (EVs) in the U.S. and elsewhere can be ascribed primarily to the priciest and heaviest component in the car, the battery. But what if electrical production and structural load-bearing functions could be combined?

If batteries could be integrated directly into car structures such as doors and body panels, EVs could weigh less and travel farther on a single charge, according to Emile Greenhalgh, a researcher at The Composites Center of Imperial College London (ICL). He led a three-and-a-half-year, $4.7-million collaborative project that was funded by the European Union to develop structural power storage technology for automobiles.

“We like to call it ‘massless energy,’” he said.

Volvo, one of eight industrial and research organizations that took part in the EU storage project, recently demonstrated in an S80 sedan a prototype underhood plenum panel—a structural member below the windshield wipers—that is made of carbon-fiber composites that contain lithium-ion batteries. The dual-function device, which was developed at Swerea SICOMP, a nonprofit institute in Piteå, Sweden, that specializes in polymer-fiber composites research, replaces the small conventional battery for the mild-hybrid Volvo’s stop-start system.

“The new plenum demonstrates that it can also replace both the rally bar, a strong structural piece that stabilizes the car in the front, and the start-stop battery,” Volvo said in a statement. “This saves more than 50% in weight and is powerful enough to supply energy to the car’s 12-V system.”

Completely substituting such in-built technology for an electric car’s existing battery components could reduce vehicle weight by more than 15%, Greenhalgh said. For a city car, halving the weight typically doubles the range. Practical structural power “also gets the batteries out of the engine compartment and where they can sit out in the airflow for cooling,” he noted.

Carbon-fiber batteries

The automotive structural battery prototype in the S80 is built using electrochemical cells composed of stacks of electrically active carbon fibers, said Leif Asp, head of research at Swerea SICOMP. The embedded cells, which were developed by a team of five professors and 10 PhD candidates, are made of rows of commercial PAN-based carbon-fiber anodes that are coated first with a 20-micron-thick (787-µin) coating of an electrically isolative but ion-permeable polymer electrolyte, followed by a similar thin layer of lithium oxide (1.5 atom% Li) that serves as the cathode, he said. Both the thin-film cathode coating and the activated carbon-fiber anodes are connected electrically in parallel to form a circuit, and all is bound together and encased by a thermoset epoxy matrix.

“In 2007 we had something of a breakthrough,” Asp recalled, “when we succeeded in showing that we could treat commercially available carbon fibers and fabrics and get reasonable electrical performance.”

The interdisciplinary team “had to find carbon fibers that were most suitable” and then learn to use ion-intercalation, polymer synthesis, and other methods to introduce the lithium charge carriers, enhance the fibers’ surface area, as well as improve multifunctional performance.

The SICOMP group estimates that the current design could reach specific energy (volume and mass) levels of at least 400 W·h/L and 180 W·h/kg. The structural batteries have good performance, Asp said, “because the electrodes are separated by a very short distance” that enhances ion conduction.

As an example, he continued, consider a car roof-top application in a plug-in hybrid that has a roof 1.5 m (4.9 ft) on a side and a battery with a capacity of 12 kW·h. Using a 400-W·h/L structural battery would require a device thickness of only about 1 cm (0.4 in).

Early consumer application

Asp reported that a pair of patents on the invention are still pending but should be assigned to Swerea soon. Although the technology is still in its early stages, the SICOMP team is optimistic that commercialization can start fairly rapidly.

“The concept works well enough that we don’t need the structural aspect,” he said. “We’re now aiming to commercialize applications in portable electronics products such as soft, protective laptop cases made of structural batteries by 2016. It could take 10 years to get automotive structural batteries to the market, but it should be considerably easier to make them for powering a mobile phone or computer.”

“We still have to demonstrate production scale-up as well as volume manufacturing of a finished product that must have a perfect thin-film coating to work correctly,” he noted, adding that no electrical shorting and better energy density are the near-term goals.

In-situ supercapacitors

The EU-supported storage project in addition yielded a prototype composite trunk lid that incorporates structural supercapacitors that can light a strip of LEDs. It was developed by ICL and partner researchers.

“The boot lid is a functioning electrically powered storage component and has the potential to replace the standard batteries seen in today’s cars,” Volvo said in the statement. “It is lighter than a standard boot lid, saving on both volume and weight.”

The original component weighed 13 kg (29 lb), but its replacement came in at only 5 kg (11 lb).

Greenhalgh explained that the structural supercapacitors contain “slabs” that comprise electrodes of activated carbon-fiber mats sandwiching an electrically insulating but ion-permeable, glass-fiber mat separator, all of which is infused with a structural resin/electrolyte.

This material was difficult to formulate because “there’s a trade-off between strength and conductivity,” he said. The group worked with Belgium-based partner Nanocyl, a maker of carbon nanotubes, to graft carbon nanotubes to the carbon fibers to boost their surface area.

“The issue here was to greatly increase reinforcement surface area without degrading mechanical properties,” the researcher said. The ICL group also exploited “a two-phase polymer system that spontaneously forms a bi-continuous nanostructure—one phase provides ionic conductivity, the other structural rigidity.”

They also overcame problems with the cure speed, short circuits that resulted from autoclave processing, and so forth to obtain “good electrical performance.” The resulting prototypes survived crashworthiness tests as well.

Greenhalgh noted that the need to find space for the slabs’ electrical contacts kept the prototype device from reaching its power output goals. “It was designed for six supercapacitor slabs, but we could fit in only four slabs,” he said.

“Probably the ‘eureka’ moment of our research came a little later when we developed a new, better-performing formulation for the structural resin/electrolyte,” he said.

The team employed an ultra-porous carbon aerogel precursor that provides increased surface area for charge transfer, helps stiffen the resin, and provides better mechanical properties. New devices that use the advance are in development now.

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