## Swedish researchers explore use of carbon fiber as active electrode in structural battery for electric vehicles

##### 05 June 2014
 Woven carbon fiber can act as an electrode for lithium ion batteries. (Photo: Peter Larsson) Click to enlarge.

Researchers in Sweden are exploring the use of carbon fiber as an active electrode in a multifunctional structural Li-ion battery in an electric car; i.e., electrical storage is incorporated into the body of the car. Carbon fiber material is a good candidate for structural electrodes since it has high specific tensile stiffness and ultimate tensile strength (UTS) as well as high lithium (Li)-intercalation capability.

Mats Johansson at Sweden’s KTH Royal Institute of Technology says the work is about improving the mechanical properties of batteries so that it not only stores energy but is part of the design. For example, he suggests, the hood of the car could be part of the battery. The concept of such a multifunctional structural vehicle battery has attracted a great deal of other research interest, including:

• A project by researchers from Imperial College London and their European partners, including Volvo Car Corporation, to develop a prototype multifunctional structural composite material composed of carbon fibers and a polymer resin which can store and discharge electrical energy and which is also strong and lightweight enough to be used for car parts.

In this €3.4-million (US$4.7-million) project, the scientists are planning to develop the composite material so that it can be used to replace the metal flooring in wheel well, which holds the spare wheel. Volvo is investigating the possibility of fitting this wheel well component into prototype cars for testing purposes. (Earlier post.) • Volvo Car Group has also created two multifunctional components for the testing and further development of the technology. These are a trunk lid and a plenum cover, tested within the Volvo S80. (Earlier post.) • In 2013, the US Advanced Research Projects Agency - Energy (ARPA-E) awarded a total of$8.75 million to four separate projects (led by Stanford University, UC San Diego, Arizona State University, and Penn State) to develop multifunctional structural batteries for vehicles as part of its RANGE program for transformative EV storage. (Earlier post.)

The potential for a multifunctional composite material which can simultaneously carry mechanical loads while storing and delivering electrical energy, said Dr. Emile Greenhalgh, project coordinator, at Imperial College London, was demonstrated by researchers at the US Army Research Lab in 2005.

In a paper presented at the Materials Research Society Symposium in 2005, South et al. provided three examples of multifunctional power-generating and energy-storing materials: structural lithium-ion batteries, structural proton exchange membrane (PEM) fuel cells, and structural capacitors. These systems were deliberately designed, the researchers wrote, so that material elements participating in power or energy processes are also carrying significant structural loads, a necessary condition for achieving mass savings through multifunctional design.

KTH. The Swedish project is run as a partnership between three professors at Sweden’s KTH Royal Institute of Technology: Göran Lindbergh, Chemical Engineering; Mats Johansson, Fibre and Polymer Technology; and Dan Zenkert, Aeronautical and Vehicle Engineering. The research is done in cooperation with Swerea SICOMP and Luleå Institute of Technology.

Eric Jacques, a researcher in vehicle and aerospace engineering at KTH (and whose doctoral thesis was on structural batteries), says carbon fiber can fill two functions in an electric car: as a lightweight composite reinforcement material for the car’s body, and as an active electrode in lithium ion batteries.

The objective of our research was to develop a structural battery consisting of multifunctional lightweight materials that simultaneously manage mechanical loads, and store electrical energy. This can result in a weight reduction for electric vehicles.

—Eric Jacques

In a 2013 paper in the Journal of the Electrochemical Society, Jacques and his colleagues reported that at moderate lithiation rates, 100 mA g−1, several grades of commercially-available polyacrylonitrile (PAN)-based carbon fibers displayed a reversible capacity close to or above 100 mAh g−1 after ten full cycles. The main factor affecting the measured capacity was the lithiation rate. Decreasing the current by a tenth yielded an increase of capacity of around 100% for all the tested grades. From the measurements performed in that study, they concluded that carbon fibers could be used as the active negative material and current collector in structural batteries.

In a paper published earlier this year in the journal Carbon, Jacques and his colleagues investigated the relationship between the amount of intercalated Li and the changes induced in the tensile stiffness and UTS of polyacrylonitrile-based CF tows. Among their findings:

• After a few electrochemical cycles the stiffness of the CFs was not degraded and independent of the measured capacity.

• A drop in the UTS of lithiated CFs was only partly recovered during delithiation and was larger at the highest measured capacities, but remained less than 40% at full charge.

• The reversibility of this drop with the C-rate and measured capacity supports the conclusions that the fibers are not damaged, that some Li is irreversibly trapped in the delithiated CFs and that reversible strains develop in the fiber. However, the drop in the strength does not vary linearly with the measured capacity and the drop in the ultimate tensile strain remains lower than the CF longitudinal expansion at full charge.

• These results suggest that the loss of strength might relate to the degree of lithiation of defectives areas which govern the tensile failure mode of the CFs.

The research project has demonstrated very good results, but we have some work to do before we can display finished batteries.

—Eric Jacques

Resources

• Eric Jacques, Maria H. Kjell, Dan Zenkert, Göran Lindbergh (2014) “The effect of lithium-intercalation on the mechanical properties of carbon fibers,” Carbon, Volume 68, Pages 725-733 doi: 10.1016/j.carbon.2013.11.056

• Maria H. Kjell, Eric Jacques, Dan Zenkert, Mårten Behm, and Göran Lindbergh (2013) “Electrochemical Characterization of Lithium Intercalation Processes of PAN-Based Carbon Fibers in a Microelectrode System,” J. Electrochem. Soc. 160(9): A1473-A1481 doi: 10.1149/2.053112jes

• Joseph T. South, Robert H. Carter, James F. Snyder, Corydon D. Hilton, Daniel J. O’Brien, and Eric D. Wetzel (2005) Multifunctional Power-Generating and Energy-Storing Structural Composites for US Army Applications (Mater. Res. Soc. Symp. Proc. Vol. 851)