Carbon Fiber Based Positive Electrodes in Laminated Structural Li-Ion Batteries

Abstract

The structural battery concept was first introduced in 2004 by Wetzel et.al [1]. In general, approximately 60% of a total cell is the active material and the rest is dead mass such as casing, current collectors, additives, etc. The desire to develop safe, environmentally friendly, and more competitive electric vehicles give rise to a new type of multi-functional lightweight composite materials, also termed as structural battery composites. A structural battery is a multifunctional battery that can carry the load while storing the energy and therefore reduce the overall weight of a mobile electric device. The major component of a multifunctional battery is the carbon fibers as they are lightweight materials, have good electrical, electrochemical, and mechanical properties. Carbon fibers were shown that they can reversibly intake lithium ions with a capacity of up to 350 mAh g-1 that is similar to graphite (372 mAh g-1) [2]. Having around 1000 S cm-1 electrical conductivity, carbon fibers can be used without additional current collectors. Removing current collectors and additives from the total structure and introducing carbon fibers into Lithium-ion batteries decreases the non-active mass as well as providing mechanical stability to the system. Structural batteries also called laminated composite batteries consist of a negative and positive electrode where structural battery electrolyte (SBE) sits in between the laminas. 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 conducts ions [3]. A schematic illustration of the laminated structural battery is shown in Figure 1a. 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). The positive electrode is a challenge, as carbon fibers need a coating with an active material that adheres well to the carbon fibers. Obtaining an evenly distributed coating of positive electrode particles affects the mechanical performance of the structural battery [2].In this work, we present different coating techniques to make a structural positive electrode in a laminated structural battery. Accordingly, electrophoretic deposition (EPD) and spray coating methods are investigated individually. As it is important to have evenly coated single fibers within a tow, uniform 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. Spray coating is a versatile technique that is also applicable to the carbon fibers. Within this technique, an electrode slurry ink is prepared for the spray gun, and fibers are coated layer by layer to have as homogenous coating as possible. The coated carbon fibers are tested electrochemically and mechanically to investigate their performance in a battery cell (Figure 1.b.).Morphological analyses were also conducted using scanning electron microscopy (SEM) as illustrated in Figure 1c.