Carbon fibers have already beeen demonstrated as high-capacity Li-ion battery anodes, opening the way for their use as structural electrodes—i.e., simultaneously carrying mechanical load and storing electrical energy.
Now, researchers led by Chalmers University of Technology, Sweden, have linked the carbon fiber (CF) microstructure to the lithium insertion mechanism and the resulting electrochemical capacity. In an open-access paper published in the journal Multifunctional Materials, the team reports that carbon fibers with improved multifunctional performance can be realized by tailoring the graphitic order and crystallite sites of the carbon fiber.
From the relationships found between the carbonaceous microstructure and the electrochemical performance we have confirmed the significant feature of having small crystals, with limited directional orientation, to reach high storage capacity for CF based electrodes. This is why the IM CFs with a lithiation mechanism reminiscent of disordered carbons outperform the HM CF with its larger crystallites highly oriented along the fibre direction. In contrast, the HM CF has a lithium intercalation mechanism close to that of graphite, but with impeded formation of staged structures due to the persisting presence of turbostratic disorder and crosslinking between crystallites.
Armed with the here derived understanding of the two competing lithiation mechanisms for HM and IM CFs, future structural and flexible electrodes/batteries can be designed—where stiffness is traded for electrochemical capacity and mechanical strength, and vice versa.—Fredi et al.
The researchers studied the microstructure of different types of commercially available carbon fibers. They discovered that carbon fibers with small and poorly oriented crystals have good electrochemical properties but a lower stiffness in relative terms. If you compare this with carbon fibers that have large, highly oriented crystals, they have greater stiffness, but the electrochemical properties are too low for use in structural batteries.
We now know how multifunctional carbon fibers should be manufactured to attain a high energy storage capacity, while also ensuring sufficient stiffness. A slight reduction in stiffness is not a problem for many applications such as cars. The market is currently dominated by expensive carbon fiber composites whose stiffness is tailored to aircraft use. There is therefore some potential here for carbon fiber manufacturers to extend their utilisation.—Leif Asp, Professor of Material and Computational Mechanics at Chalmers University of Technology
In the study the types of carbon fiber with good electrochemical properties had a slightly higher stiffness than steel, whereas the types whose electrochemical properties were poor are just over twice as rigid as steel.
The researchers are collaborating with both the automotive and aviation industries. Leif Asp explains that for the aviation industry, it may be necessary to increase the thickness of carbon fiber composites, to compensate for the reduced stiffness of structural batteries. This would, in turn, also increase their energy storage capacity.
The key is to optimize vehicles at system level – based on the weight, strength, stiffness and electrochemical properties. That is something of a new way of thinking for the automotive sector, which is more used to optimizing individual components. Structural batteries may perhaps not become as efficient as traditional batteries, but since they have a structural load-bearing capability, very large gains can be made at system level.
In addition, the lower energy density of structural batteries would make them safer than standard batteries, especially as they would also not contain any volatile substances.—Leif Asp
The research was funded by Vinnova, the Swedish Energy Agency, the Swedish Research Council and Alistore European Research Institute.
Giulia Fredi et al. (2018) “Graphitic microstructure and performance of carbon fibre Li-ion structural battery electrodes” Multifunct. Mater. doi: 10.1088/2399-7532/aab707