Structural composite batteries are a novel type of multifunctional devices, which have a great potential to remarkably reduce the mass of electric vehicles, and thus increase their energy efficiency. In these batteries, carbon fibres (Carbon fibres) play dual roles: reinforcements (as in CARBON FIBRE composites) and negative electrodes (as in batteries). However, the relationship between the microstructure and the electrochemical property of the Carbon fibres is not well understood. In this study, the microstructure of two Carbon fibres, M60J and IMS65, were studied by using scanning electron microscopy and transmission electron microscopy. Detailed microstructural features were revealed, and correlated to the electrochemical properties of the Carbon fibres. The more disordered microstructure, and rather large pores are the reasons for the better electrochemical properties of IMS65 compared to M60J

Asp, Leif

Liu, Fang

Rashidi, Masoud

Chalmers Research

Initial study of the microstructure of carbon fibres acting as negative electrodes in structural battery composites

Chalmers Publication Library

ECCM17 - 17th European Conference on Composite Materials     
Munich, Germany, 26-30th June 2016 1 
Fang Liu, Masoud Rashidi and Leif Asp 
 
 
 
INITIAL STUDY OF THE MICROSTRUCTURE OF CARBON FIBRES 
ACTING AS NEGATIVE ELECTRODES IN STRUCTURAL BATTERY 
COMPOSITES 
 
Fang Liu1, Masoud Rashidi2 and Leif E. Asp3 
 
1 Department of Physics, Chalmers University of Technology, 41296 Göteborg, Sweden 
Email: fang.liu@chalmers.se  
1 Department of Physics, Chalmers University of Technology, 41296 Göteborg, Sweden 
Email: masoud.rashidi@chalmers.se  
3 Department of Applied Mechanics, Chalmers University of Technology, 41296 Göteborg, Sweden  
Email: leif.asp@chalmers.se 
 
 
Keywords: structural batteries, carbon fibres, microstructure, SEM, TEM,  
  
 
Abstract 
Structural composite batteries are a novel type of multifunctional devices, which have a great potential 
to remarkably reduce the mass of electric vehicles, and thus increase their energy efficiency. In these 
batteries, carbon fibres (Carbon fibres) play dual roles: reinforcements (as in CARBON FIBRE 
composites) and negative electrodes (as in batteries). However, the relationship between the 
microstructure and the electrochemical property of the Carbon fibres is not well understood. In this 
study, the microstructure of two Carbon fibres, M60J and IMS65, were studied by using scanning 
electron microscopy and transmission electron microscopy. Detailed microstructural features were 
revealed, and correlated to the electrochemical properties of the Carbon fibres. The more disordered 
microstructure, and rather large pores are the reasons for the better electrochemical properties of 
IMS65 compared to M60J. 
 
 
1. Introduction 
 
Imagine the polymeric composite frame of a car simultaneously stores electrochemical energy. This 
can be realized by a multifunctional device called structural composite battery. Multifunctionality is 
more efficient in realizing substantial weight/volume savings, compared to the conventional approach 
by optimising the individual subsystems, i.e. in this case the load-bearing structure and the power 
storage component. A Swedish interdisciplinary team of scientists formed in 2009 is now one of the 
leading teams in the new research field of structural composite batteries. In 2012 we proposed a 
radically novel concept for structural battery [1]:  using the individual carbon fibres as battery 
electrodes. We have demonstrated such a battery with an energy density of 10 Wh/kg. By optimizing 
different components of the battery an energy density of 175 Wh/kg and a shear modulus of 1 GPa can 
be realized [2].  
 
The most crucial component for a structural composite battery is carbon fibres, which provide 
mechanical strengthening and simultaneously function as negative electrodes, thanks to their 
capability of intercalating lithium ion at very low electrode potential. Under the same condition, 
electrochemical capacity of different carbon fibres (i.e. the capacity of lithiation and delithiation) can 
vary up to nearly eight times [3]. The reason lies in the microstructure of carbon fibres. Breaking the 
microstructure of a carbon fibre down to the most basic level, it contains turbostratic (or disordered) 
graphite – graphene layers with relatively large distances (~ 0.3 nm) in between. Twisted graphite 
ECCM17 - 17th European Conference on Composite Materials     
Munich, Germany, 26-30th June 2016 2 
Fang Liu, Masoud Rashidi and Leif Asp 
 
domains form fibrils, which further align along the fibre axis and finally form carbon fibres with a 
diameter ~ 5 µm. In the fibres there are always some structural defects, such as pores. The porosity 
rate can sometimes reach up to 20%. How the microstructure of carbon fibres governs lithium 
intercalation remains largely unknown. 
 
In this paper we present the initial findings of fibre microstructure of two commercially available 
carbon fibres and compare these with their measured electrochemical properties. Here, only pristine 
fibres are studied using a high-resolution scanning electron microscope (SEM) and transmission 
electron microscopes (TEM). 
 
 
2. Materials and experiment 
 
Two commercially available polyacrylonitrile (PAN)-based carbon fibres were studied: M60J (Toray), 
and IMS65 (Toho Tenax). The strength and modulus are 1760 MPa and 330 GPa for M60J, and 
6000 MPa and 290 GPa for IMS65.  
 
A Leo Ultra 55 SEM with a field emission gun was used. The SEM was operated at a low voltage 
range 1–10 kV. An in-lens secondary electron detector was used to acquire SEM micrographs. In order 
to investigate the transverse cross-sections of the carbon fibres using SEM, the fibres were broken, 
while merged in liquid nitrogen, using a pair of tweezers. 
 
A Titan 80-300 TEM, operated at 300 kV, was used to analyse the axial cross-sections of carbon 
fibres. Both bright field and dark field images were acquired.  Convergent beam electron diffraction 
was performed to study ordering of the microstructure in carbon fibres. In order to prepare TEM 
specimens with a thickness of ~100 nm, the in-situ lift-out technique was employed using an FEI 
Versa 3D workstation, a combined focused ion beam microscope and SEM (FIB/SEM), equipped with 
an Omniprobe micromanipulator. To protect the very surface of the carbon fibre from ion damaging 
during the specimen preparation procedure, a layer of Pt was deposited with the aid of the gas 
injection system in the workstation. Figure 1 (a) shows a carbon fibre with a deposited Pt layer. After 
this step, a small piece was cut out using the FIB; then lifted out from its surrounding; transferred to a 
3 mm TEM grid using the micromanipulator; and finally ion polished down to 100 nm thick with the 
FIB. Thus, a TEM specimen was obtained. Figure 1 (b) shows the TEM specimen with axial cross-
section of the carbon fibre. A detailed description of this sample preparation process technique was 
reported elsewhere [4]. 
 
 
  
 
Figure 1. SEM micrographs showing the TEM specimen preparation procedures. (a) A carbon fibre 
with a Pt layer on top to protect the surface. (b) A TEM specimen with axial cross-section of the 
carbon fibre. 
ECCM17 - 17th European Conference on Composite Materials     
Munich, Germany, 26-30th June 2016 3 
Fang Liu, Masoud Rashidi and Leif Asp 
 
3. Results and discussion  
 
The fractured surfaces are very rough (Figure 2 (a) and (c)). The topography of the surface varies 
remarkably especially for IMS65, probably due to the poorly controlled fracture procedure. However, 
it reflects the difference in the microstructure. Both amorphous and crystalline areas were observed in 
the transverse cross-sections of M60J and IMS65. The amorphous areas appear very smooth, while the 
crystalline areas with small granules appear rough in the high resolution SEM micrographs (Figure 
2(b) and (d)). Dozens of cross-sections were investigated; the proportion of amorphous areas varies 
quite a lot even for the same grade of fibre. In the crystalline areas of M60J, the granules appear 
spherical and have a rather smooth surface, with a size around 100 nm. In the crystalline areas of 
IMS65, the granules are much smaller, under 20 nm. Therefore, it yields a very rough surface. Another 
striking difference between these two fibres is that in M60J almost no pores were observed using 
SEM, while in IMS65 the voids are ubiquitous, and the size of the pores is about 20 nm.  
 
 
  
  
 
Figure 2. SEM micrographs showing detailed microstructure of the fractured transvers cross-sections 
of M60J and IMS65. (a) Overview of the surface of M60J. (b) Detailed microstructure in M60J 
showing smooth areas and rough granular areas. The granules are spherical shape, the size is rather 
big, around 100 nm, and their surface is rather smooth. There are no visible voids. (c) Overview of the 
surface of IMS65. (d) Detailed microstructure in IMS65 showing the smooth areas, rough granular 
areas, and voids. The size of granules is much smaller, under 20 nm. The size of the voids is about 20 
nm. 
 
 
TEM was used to determine the degree of microstructural ordering in the carbon fibres in much detail. 
Figure 3 (a) shows a dark field TEM micrograph of the axial cross-section of M60J, using the (002) 
ECCM17 - 17th European Conference on Composite Materials     
Munich, Germany, 26-30th June 2016 4 
Fang Liu, Masoud Rashidi and Leif Asp 
 
diffraction spot. The inset is a typical convergent beam electron diffraction pattern taken from M60J. 
The size of the electron beam is barely a few nanometres, which means that these patterns reflects the 
local crystallographic information from a volume a few nanometres in width and 100 nm in depth. The 
(002) diffraction patterns are sharp, small, and round disks. Hence the grapheme layers (with the (002) 
index) are well aligned in this material. In addition, there are many elongated needle-shaped pores, 
appearing dark in contrast. Their sizes are a few nanometres wide and more than 10 nm long. 
Although the dark field micrographs of IMS65 appears similar to those of M60J, convergent beam 
electron diffraction patterns reveal significant difference (Figure 3 (b) inset). Instead of round disks, 
the (002) diffraction patterns appear as sections of an arc, which means that in the small volume of a 
few nanometres in width and 100 nm in depth, the orientation of the grapheme layers varies. The high 
resolution TEM micrograph of IMS65 also showed highly disordered structure (Figure 3 (b)).  
 
 
  
 
Figure 3. TEM micrographs showing the microstructure of the carbon fibres along the axial direction. 
(a) A dark field micrograph of M60J, using the (002) diffraction. Note that there are many pores with 
dark contract, and a few of them are highlighted by arrows. The pores are elongated (> 10 nm in 
length, and a few nanometres in width) and aligned along the axial direction. (b) HRTEM micrograph 
of IMS65. The structure is highly disordered. The inset is a typical convergent beam diffraction 
pattern. Note that the (002) patterns are sections of an arc. 
 
 
IMS65 have demonstrated promising electrochemical properties for applications in structural 
composite batteries [3,5]. The initial and cycling capacity of lithiation and delithiation of IMS65 is 
better than many other grades of carbon fibres, approaching 370 mAh g-1. This value is comparable 
with the maximum value for perfect graphitic carbons 372 mAh g-1. In addition, IMS65 have many 
other advantageous electrochemical properties such as mass transport, exchange current density, and 
electronic conductivity [5]. Interestingly, amorphous carbon with small dimensional graphite 
crystallites exhibits a much higher capacity than perfect graphite carbon – up to 900 mAh/g and more 
[6]. The present study show that indeed IMS65 have a more disordered microstructure, which explains 
their better electrochemical properties. In addition, the pores may also play important roles in 
accommodating Li ions. However, detailed electrochemical processes and mechanisms, for instance 
how lithium ions intercalate into the carbon fibres, needs further investigation.   
 
 
4.  Conclusions 
 
The detailed microstructure of M60J and IMS65 was studied using SEM and TEM. M60J has a much 
ECCM17 - 17th European Conference on Composite Materials     
Munich, Germany, 26-30th June 2016 5 
Fang Liu, Masoud Rashidi and Leif Asp 
 
more ordered microstructure with many small needle-shaped pores. IMS65 has a much more 
disordered microstructure, with many relatively large pores. The highly disordered microstructure, and 
rather large pores may explain the better electrochemical properties of IMS65 compared to M60J.  
 
 
Acknowledgements 
 
Funding from the strategic innovation programme LIGHTer, SRA1, provided by VINNOVA and 
Chalmers AoA Energy is gratefully acknowledged. 
 
 
References 
 
[1]   L. E. Asp,  et al., PCT Intl Appl No. PCT/EP2013/068024. 2012. 
[2]   L. E. Asp, and E. S. Greenhalgh, Comp Sci Technol, Vol. 101, 2014, pp. 41-61. 
[3]   M. H. Kjell, E. Jacques, D. Zenkert, M. Behm, G. Lindbergh, PAN-Based Carbon fibre Negative 
Electrodes for Structural Lithium-Ion Batteries, Journal of the Electrochemical Society, Vol. 
158(12), 1455-60, 2011. 
[4]  F. Liu,  H. Götlind, J.-E. Svensson, L.-G. Johansson, M. Halvarsson, TEM Investigation of the       
Microstructure of the Scale Formed on a FeCrAlRE alloy at 900oC – the Effect of Y-rich RE 
Particles, Oxidation of metals, Vol. 74, 11-32, 2010. 
[5]  M. H. Kjell, T. G. Zavalis,  M. Behm, G. Lindbergh, Electrochemical Characterization of Lithium 
Intercalation Processes of PAN-Based Carbon Fibres in a Microelectrode System Batteries and 
Energy Storage, Journal of the Electrochemical Society, Vol. 160(9), 1473-81, 2013.  
[6]  Lijun Fu et al. Negative Electrode materials based on carbon. in: Wu Yuping (Ed.), Lithium Ion 
Batteries, Fundamentals and Applications, CRC Press, 161-224 2015. 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 


Initial study of the microstructure of carbon fibres acting as negative electrodes in structural battery composites

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