Effects of Carboxylated Polythiophenes in Fe3O4 Li-Ion Battery Anodes

Abstract

The high energy/power density of Li-ion batteries elicits intensive research efforts related to high capacity anode materials. The main obstacles which retard the practical employment of high-capacity electrochemically active particles including Si, Sn, metal oxide and their derivatives, stem from large volume changes associated with Li insertion/extraction and the resultant electrical contact loss, thereby leading to poor cycling performance. Efforts have been made to circumvent the breakdown of electron pathways through the introduction of electrical conducting functionalities to the surface of active materials, such as carbon coatings and conductive polymeric binders, or the fabrication of a stable battery anode with electrically inactive polymeric binders that enable the system to maintain its integrity. These approaches are reasonably effective, however, incorporation of a porous entity which helps ion transport, into a composite electrode is also essential for improved performance. In principle, electrochemical reactions that occur within the electrode, reveal the intrinsic energy capacity when a Li ion encounters an electron inside an active site. Electron and ion transport are both critical factors to determine the internal resistance of the electrodes, which in turn influences their electrochemical performance. Here we present how to improve both electronic and ionic transport in Li-ion battery electrodes using conjugated polymers. The approach includes poly[3-(potassium-4-butanoate)thiophene] (PPBT) — a water-soluble, carboxylate substituted polythiophene — as a binder component, and polyethylene glycol (PEG) as a surface coating on active material, namely, Fe3O4 nanoparticles. Additionally, carbon nanotubes (CNTs) are considered for use as the conducting networks to facilitate design of a light-weight, flexible web electrode. To enhance the electronic conduction in the electrodes, connection between active materials (PEG- Fe3O4) and conducting agents (or CNTs) through binding components is of importance. PEG coating and carboxylated polythiophenes play an important role in dispersing the Fe3O4 nanoparticles and conducting agents (or CNTs), respectively, in a water medium, which allow for well-developed and interconnected electrode structures. Furthermore, carboxylated polythiophenes (e.g. PPBT) can boost electronic conduction, based on their high conducting properties and through electrochemical doping during electrochemical testing. The presence of carboxylic moieties on the side chain of polythiophenes such as PPBT could facilitate the formation of stable electrodes via chemical interactions between PPBT moieties (COO-) and the Fe3O4 surface (–OH). The results will show that this methodical consideration of both ion and electron transport through introduction of a carboxylated PPBT component, can remarkably enhance the performance of Fe3O4 based high-capacity anodes.