The researchers at the Deakin Institute for Frontier Materials, Australia and Chalmers University of Technology, Sweden have developed a new type of carbon fiber able to function simultaneously as a strong structural material and an effective electrode for lithium-ion batteries. Usually, engineers have to choose between the strength of the material and its ability to store energy. However, the new partially carbonized carbon fibers have demonstrated a rare combination of properties: as the processing temperature rises, both their mechanical strength and ability to store electricity improve simultaneously. This paves the way for creation of the so-called structural batteries, in which structural components (such as an electric vehicle body or an airplane wing) are able to store energy themselves, reducing the size and the weight of the main battery.
The idea to use carbon fiber as an electrode is not new. Such materials have long been used in aerospace composites and sports equipment where strength and light weight are particularly important. In addition, carbon is capable of incorporating lithium ions into its structure, allowing it to function as a battery anode. But there has always been a trade-off here. Traditional high-strength fibers, optimized for mechanical loads, did not store energy well. Conversely, fibers with a looser and more disordered structure absorbed lithium well but proved too brittle for practical application.
The researchers decided to change the very approach to the material production. They took standard polyacrylonitrile fiber (the basis for most modern carbon fibers) and subjected it to partial carbonization. The material was heated to the temperatures ranging 800 to 1100 °C, but was never brought to complete carbonization as it is typically done in the production of industrial carbon fibers. This resulted in four types of materials – PC800, PC900, PC1000, and PC1100 – where the number denotes the maximum processing temperature. Then the fibers were examined with an electron microscopy, spectroscopy, X-ray diffraction analysis, mechanical testing, and tests in experimental batteries.
The results were unexpected. Typically, as the processing temperature increases, the carbon structure becomes more ordered with denser and stronger carbon regions forming within the material. However, at the same time, the number of defects and pores capable of trapping lithium ions decreases, causing the battery capacity to drop. In the new fibers with incomplete carbonization, this contradiction was virtually absent.
For example, the PC1100 material treated at 1100 °C was approximately 2.5 times stiffer than the PC800 fiber and conducted electricity much better. At the same time, its ability to store and release lithium was approximately 40% higher than that of the commercial T800 carbon fiber, which is currently considered the benchmark for structural batteries.
The researchers attribute this effect to a unique “golden mean” in the material structure. During partial carbonization, the fibers do not yet transform into fully ordered graphite which is poorly suited for lithium storage but they also do not remain a chaotic amorphous mass. A complex structure forming inside consists of small graphite regions surrounded by less ordered carbon with a large number of defects and nanopores. The ordered regions provide a good electrical conductivity and mechanical strength, while the defective areas and pores create additional sites for lithium ions storage.
What has become particularly important is that the researchers did not detect usual tradeoff between the mechanical and the electrochemical properties. Across the entire temperature range, increasing the degree of carbonization simultaneously improved both the strength and the battery performance. The same PC1100 fiber consistently withstood more than 125 charge and discharge cycles with virtually no energy loss, while remaining a very stiff and strong material.
At the same time, the PC800 fiber treated at the lowest temperature turned out to be so poorly conductive that it could not be used in a battery at all. This indicates existence of a minimum carbonization threshold below which the material loses its practical value.
Thus, the researchers have demonstrated that partial carbonization of carbon fibers opens up new possibilities for designing structural batteries: depending on the processing temperature, we can select the desired combination of strength and energy density for a specific application. The researchers also believe that such a material structure could prove promising not only for lithium-ion batteries but also for sodium-ion or potassium-ion batteries, which are considered to be a cheaper alternative.
