Performance of Carbon Fibers with Various Coatings in Composite Lithium-Ion Structural Batteries

Abstract

Traditional Lithium-ion batteries (LIBs) are dominating the battery market where the energy density is very important. However, there is still potential for improvements. Normally, around 60% of the total cell is the active material whereas the rest is the weight of current collectors, casing, additives, etc [1].The structural battery concept was first introduced in 2004 by Wetzel et.al [2]. It is a multifunctional battery that can carry the load while storing the energy and therefore reduce the overall weight of a mobile electric device. Carbon fibers (CFs) are convenient for structural batteries due to their good electrical, electrochemical and mechanical properties [1]. It has been shown that carbon fibers can reversibly insert lithium ions with a capacity up to 350 mAh g-1 which is comparable to graphite (372 mAh g-1) [1]. Moreover, the electrical conductivity of the carbon fibers can reach around 1000 S cm-1 which allows the carbon fibers to be used without current collectors [1]. Removing current collectors and additives from the total structure and introducing carbon fibers into LIBs decreases the non-active mass as well as providing mechanical stability to the system. In a composite material, a bulk phase (polymer/matrix) encases the reinforcing phase which is carbon fiber in the structural battery. Polymer matrix holds the carbon fibers together and transfers the loads to fibers, while carbon fibers carry the load. A structural battery electrolyte (SBE) was developed at KTH as a load carrying polymer matrix that simultaneously conduct ions [3]. In Figure 1, the upper lamina corresponds to the negative electrode where the SBE is reinforced with carbon fibers. In the lower lamina SBE is reinforced with carbon fibers that are coated with a positive electrode material (e.g. LiFePO4). Carbon fibers in the negative lamina and the coated carbon fibers in the positive lamina are the active materials separated by a thin separator. The positive electrode is a challenge as carbon fibers need a coating with active material that adhere well to the carbon fibers. Obtaining an evenly distributed coating of positive electrode particles effects the mechanical performance of the structural battery. An excessive amount of active material coating will lower the fiber volume fraction in a structural battery and can decrease its mechanical properties whereas it would enhance the energy density. In this work, we present different coating techniques in order to make a structural positive electrode in a laminated structural battery. Accordingly, electrophoretic deposition (EPD), direct-ink writing (DIW), slurry casting and vacuum assisted slurry infusion methods are investigated individually. As it is important to have evenly coated single fibers within a tow, an even current distribution within the EPD cell is of high importance. A specific EPD cell is designed for this aim and it is used to coat carbon fibers electrochemically and uniformly. This new cell design gives the flexibility to obtain the electrochemical parameters (distance, coating thickness, current distribution, etc.) to encase the fibers in a tow homogeneously and hence, several micrometers of coating thickness can be obtained. Direct-ink writing on the carbon fibers is first shown here as a coating method. A specific ink is formulated to coat the fibers with the positive electrode material and used in a modified ink-printer with a specifically designed nozzle. Rheology of the ink and the coating speed is controlled in accordance with the coating quality. Slurry casting as a traditional method in LIBs is also investigated on fibers via doctor blade coating. The same ink is used in vacuum-assisted slurry infusion. The carbon fibers with various coatings are analyzed using surface morphology (SEM), electrochemical and mechanical tests in order to investigate the performance of the coated fibers in a laminated battery. Figure 1. Schematic illustration of laminated structural battery References Hagberg. Carbon Fibres for Multifunctional Lithium-Ion Batteries, Doctoral Thesis. KTH Royal Institute of Technology, Stockholm, Sweden, 2018.Wetzel. Multifunctional Composites for Future Energy Storage in Aerospace Structures. Communications, and Structure. AMPITAC Q. 8 (2004), 91-95. Ihrner, W. Johannisson, F. Sieland, D. Zenkert and M. Johansson. Structural lithium ion battery electrolytes: Via reaction induced phase-separation. Journal of Material Chemistry A 5.48 (2017), 25652-25659. Figure 1