LiFePO4-coated carbon fiber electrodes for structural batteries

Abstract: Lithium-ion batteries (LIBs) have a central role in products, from portable devices to large-scale energy storage for the electric grid and continue to undergo rapid development. The surge in electric vehicles has intensified the focus on technological advancements and new-generation technologies. Structural batteries have received considerable attention for their multifunctionality and lightweight properties. These batteries utilize carbon fibers to combine their mechanical strength with battery functionalities in a single structure, consequently reducing overall weight and increasing energy density. Similar to traditional LIBs, structural batteries comprise negative and positive electrodes, reinforced within a structural battery electrolyte (SBE). While extensive research has been conducted on carbon fibers as negative electrodes, there has been a relative scarcity in the development of positive electrodes that align with the structural battery concept.            This thesis explores coating methodologies on polyacrylonitrile (PAN)-based carbon fibers (CF) with positive electrode active material, specifically focusing on the utilization of lithium iron phosphate (LFP). Electron microscopy and electrochemical tests were conducted to evaluate the relation between structure with long-term and rate performances of these electrodes in half-cells.             Spray coating and siphon impregnation (later referred to as ‘powder impregnation’ in this thesis) techniques were employed to coat the carbon fibers, which serve as current collectors instead of conventional aluminum foil. The spray coating method utilized a standard electrode slurry based on an organic solvent, with efforts made to optimize parameters such as the height of the spray gun and plate temperature. The sprayed coating was quite thin, resulting in excellent rate capability. In the powder impregnation method, a water-based slurry was utilized with polyethylene glycol (PEG) as a binder. Efforts were made to obtain good fiber distribution within a homogeneous matrix of coating in the electrode. The parameters, including slurry viscosity, binder effect, electrode design, cell design, electrode preparation, and drying temperatures, were regulated for the best electrochemical performance and cell life. It was found that a binder is necessary for ensuring robust electrodes. Elevated drying temperatures are essential to eliminate moisture from the water-based process and components. Additionally, conductive carbon additives such as carbon black and graphene were incorporated, and their impact was assessed. A small amount of carbon additive (< 1 wt.%) improved performance at higher cycling rates.             The electrodes produced via powder impregnation were finally integrated into double-sided full cells versus uncoated PAN-derived CFs serving as negative electrodes in commercial liquid electrolyte or SBE, respectively. The LFP-coated CF electrodes exhibited good performance in full cells, indicating promising performance for the structural battery. The main limitation was observed in the power losses in the CF negative electrodes and in the ionic conductivity of the SBE. Overall, the thesis shows that the encapsulation of individual PAN-derived carbon fiber filaments using the applied coating methodologies was successful and that the use of carbon fibers as current collectors proved to be effective.