Electrochemical Performances of Powder-Impregnated Carbon Fiber-Based Positive Structural Electrodes

Abstract

Carbon fibers have been in focus within the last decades in designing and constructing lightweight structures. They can be used in a large range of applications such as clothing, transportation gadget, sports materials, as well as in more unusual applications such as batteries. Their mechanical functionality in addition to good electrochemical and electrical features suggest that they can replace battery components such as the graphite anode and the current collector foils. A battery fully based on carbon fibers, often referred to as the structural battery introduced in 2004 by Wetzel et.al. [1], is a multifunctional battery that can carry the load and store the energy simultaneously. An advantage of this battery design is the less overall weight of the device due to eliminating dead mass by removing current collectors and additives from the total structure. Instead, introducing carbon fibers into lithium-ion batteries decreases the non-active mass as well as provides mechanical stability to the system. Carbon fibers show similar electrochemical behavior to graphite, and can therefore be used as negative active material thanks to their capability of reversible lithium intake capacity up to 350 mAh g-1. Furthermore, the fibers have an electrical conductivity of about 1000 S cm-1 and are therefore very suitableas current collectors [2]. Ultimately in structural batteries, also called laminated composite batteries, the carbon fiber-containing electrodes are separated by a structural battery electrolyte (SBE) consisting of a bulk phase (polymer/matrix) that encases the reinforcing between the laminas. The polymer matrix holds the carbon fibers together and transfers the loads to fibers, while carbon fibers carry the load. Figure 1a shows a cross-sectional schematic illustration of a structural battery. The lower lamina corresponds to the negative electrode where the SBE is reinforced with carbon fibers. In the upper lamina, SBE is reinforced with carbon fibers that are coated with a positive electrode material (e.g. LiFePO4 /LFP). A structural battery electrolyte (SBE) has been developed at KTH as a load-carrying polymer matrix that simultaneously conducts ions [3] and that can be combined with the carbon fiber negative electrode material. However, manufacturing the positive structural electrodes is a challenge, as the continuous carbon fibers need to be in close proximity with active materials that should also adhere well to the carbon fibers. In this context, achieving an even distribution of the positive electrode particles is expected to affect many of the structural battery performance metrics, and in particular the electrical, electrochemical and mechanical functionality [2].In this work, we produced positive structural electrodes based on PAN-based carbon fibers with powder impregnation technique in a water-based slurry composition. This technique is not only environmentally friendly but also allows good control of impregnation uniformity and active material/fiber ratio. By this method, continuous carbon fibers were impregnated (i.e., tightly mixed) with active material (LiFePO4) along with electrode components to obtain self-standing positive electrodes. The as-prepared samples show a good and uniform mixing with the active material through interlayers of carbon fibers in tow, leading to a lower internal resistance (SEM images in Figure1c.). The carbon fiber-based positive electrodes have been tested with different electrochemical methods to obtain their performances in half-cells (Figure1b.). The results show promising capacity retention and rate capability.