Electrochemical Performance of Commercial Carbon Fibers Towards Usage As Electrodes in Structural Li-Ion Batteries
Authors: Hagberg Johan , Leijonmarck Simon , Lindbergh Göran 
Published: February 2020

There is a continuing and growing demand for new and improved battery technologies. Extensive research is dedicated to pushing the limits of Li-ion batteries in terms of safety, environmental aspects, cost, lifetime and performance. A lot of the focus lies on improving graphite and other types of carbon-based electrodes [1-2] and extensive research has been on going since the beginning of the 90’s. Another possible route is improving technologies on a system level by introducing multifunctional materials. One way to do this is with structural batteries, where a battery can replace a load-bearing component in an electric or a hybrid electric vehicle for instance, and still maintaining the battery function. This way the weight is reduced (as well as freeing up space) on a system level and the overall performance is increased. A trade off between mechanical and electrochemical properties is expected, however, to increase overall effectiveness the performance of the battery does not need to be on par with a state of the art Li-ion battery. This concept of utilizing multifunctional materials in battery applications for instance has previously been studied with various methods and results [3-5].

This work focuses on investigating the electrochemical properties of commercial PAN based carbon fibers as electrodes for a future structural application. The cycleability and lifetime of the battery are two important factors for this concept to be realized. The focus is on evaluating the coulombic efficiency of carbon fibers which previous studies [6-8] has shown to have a good capacity as well as mechanical properties. The capacity was determined to be around 80% of the theoretical capacity of commercial graphite electrodes when cycled at a C/10 rate (fully charged in ten hours). One of the most promising carbon fibers is IMS65, which is an intermediate modulus fiber with a high strength and stiffness. The combination of good mechanical and electrochemical properties makes this fiber (and other similar) a very promising candidate for a structural battery application, which is the background to the work currently being done.
The goal of this work is thus to thoroughly examine the electrochemical properties, in particular the coulombic efficiency, of commercially available carbon fibers to find a good candidate for usage in a structural battery application in terms of lifetime, power and capacity. The methodology utilizes a half-cell setup where the carbon fibers are used as working electrode and lithium foil as counter electrode. A glass-microfiber filter is used as a separator and the liquid electrolyte is 1.0 M LiPf6in EC/DEC (1:1 by weight, Selectilyte LP40) with and without VC (Vinylene Carbonate) added as an electrolyte additive. The cell is placed in a heat box with a controlled temperature and cycled in a high precision charger setup to accurately determine the coulombic efficiency and capacity of the carbon fibers.
In a final concept, a liquid electrolyte cannot be used if the battery is to have structural properties as well. Instead, utilizing a solid polymer electrolyte is proposed. The effect of the solid polymer electrolyte on the electrochemical performance will therefore also be presented. Other important parameters studied are the surface coating (normally a sizing of epoxy or polyurethane) and epoxy tabbing (for tensile testing) of the fibers and its effect on the electrochemical properties.
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