Polymeric binder design frameworks for high capacity Li-ion battery anodes
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Battery electrodes are complex mesoscale systems comprised of an active material, conductive agent, current collector, and polymeric binder. While the focus of research related to the design of robust, high-performance Li-ion batteries relates to the synthesis of active particles, the binder plays a crucial role in stability and ensures electrode integrity during volume changes that occur with cycling. Conventional polymeric binders such as poly(vinylidene difluoride) generally do not interact with active particle surfaces and fail to accommodate large changes in particle spacing during cycling. Recently, a poly[3-(potassium-4-butanoate)thiophene] (PPBT) binder component, coupled with a polyethylene glycol (PEG) surface coating for the active material was demonstrated to enhance both electron and ion transport in magnetite based anodes; and it was established that the PEG/PPBT approach aids in overall battery electrode performance. In this thesis, the PEG/PPBT system is first used as a model polymeric binder system for understanding cation effects in anode systems. Potassium showed the most stable electrochemical performance, which is attributed to cation size and proposed to be a result of higher ionic conductivity. Next, a series of water-soluble, carboxylated polymers with varying functional groups were investigated as alternative polymeric binders to aid in electron and ion transport in magnetite-based anodes. Conjugated polymers under investigation include PPBT, a poly(3,4-ethylenedioxythiophene) (PEDOT) derivative, and the potassium salt form of poly(acrylic acid) (PAA-K). The results of this investigation create a framework of desirable qualities necessary for polymeric binders by investigating how different functional groups aid or hinder overall electrochemical performance in the overall design of composite Li-ion battery anodes. Finally, the model PPBT polymer was functionalized onto the magnetite surface using two functionalization methods. Direct attachment of the polymer onto the magnetite surface using Fischer-esterification led to enhanced performance. Additional work focused on understanding polymeric binder and carbon additive interactions for composite electrode design. Altogether, these studies demonstrate frameworks for designing optimal polymeric binders to enhance performance of high-capacity anodes, often hindered by their large volume changes during cycling.