Multifunctional carbon fibres enable massless energy storage
Despite progress in energy storage technology, batteries still make up a significant part of the weight for devices such as laptops and even cars. Rather than focussing solely on the battery technology to tackle lightweight demands, Leif Asp at Chalmers University of Technology alongside a broad team of researchers in Sweden, Italy and France report in Multifunctional Materials that exploiting the electrochemical properties of carbon fibres could drop device masses by as much as 50%.
The mechanical properties of carbon fibres have been well understood for several decades, with a lot of seminal work dating back to the 1980s. While more recent, the promising electrochemical properties of carbon fibres have also been known since the late 2000s. However, no-one had looked into how to make carbon fibres that were stiff and strong while simultaneously demonstrating high-performance electrochemical properties, a gap in the knowledge that may be attributable to the nature of the research environment around carbon fibres. “When it comes to carbon fibres, knowledge is kept in the companies,” says Asp. “Few research groups are working on it.”
Asp and colleagues compared the microstructure and electrochemical performance for two types of commercial carbon fibre that have middling mechanical properties and one of the sector’s hardest hitters in terms of structural strength. Knowing that electrochemical properties improve for more amorphous microstructures with smaller more loosely oriented crystals, whereas mechanical properties improve with greater crystalline order, they were expecting a trade-off. What surprised Asp was that the compromise was far less than expected.
“The intermediate strength carbon fibres were much less organised than I expected,” says Asp as he describes some of the observations for the two middling carbon fibre types that remain competitive commercial options for applications that do not require extreme mechanical strength. “That these fibres still had such high mechanical properties means I might expect to be able to go to carbon fibres with even smaller crystals and I might still get good mechanical properties.”
Order isn’t everything
Carbon fibres are very sensitive to production conditions. The three types of carbon fibres at the focus of the current research – intermediate fibres T800 and IMS65, and the high modulus fibre M60J – were all produced by pyrolysis of polyacrylonitrile (PAN) but while the production temperature for T800 and IMS65 were similar, that for M60J was almost double. As a result the stiffness of the T800 and IMS65 is around 290 GPa and close to that of steel – still more than adequate for many applications – whereas that of M60J is almost double. The structural differences are immediately apparent from high-resolution transmission microscopy images, which are much more mottled for T800 and IMS65, whereas M60J shows a very finely stratified structure, with turbostratic defects where stratified layers run into each other.
Raman spectroscopy data, as well as axial swelling measurements taken as the carbon fibres were cycled through lithiation and delithiation suggest differences in the lithiation and resulting structural strains. M60J behaves akin to graphite, bar the absence of certain features attributed to turbostratic defects, while T800 and IMS65 behave like amorphous structures with small disoriented crystal sizes. The highly amorphous structure of T800 and IMS65 allows double the electrochemical capacity, while preserving good mechanical properties. The results raise the question “how far can you go” in terms of amorphous microstructures before the compromise in mechanical properties becomes too great.