Tuning Conjugated Polymers for Binder Applications in High-Capacity Magnetite Anodes

Battery electrodes are complex mesoscale systems comprising an active material, conductive agent, current collector, and polymeric binder. Although significant research on composite electrode materials for Li-ion batteries focuses on the design, synthesis, and characterization of the active particles, the binder component has been shown to critically impact stability and ensure electrode integrity during volume changes induced upon cycling. Herein, we explore the ability of water-soluble, carboxylated conjugated polymer binders to aid in electron and ion transport in magnetite-based anodes. Specifically, poly[3-(potassium-4-butanoate)thiophene] (PPBT) and a potassium carboxylate functionalized 3,4-propylenedioxythiophene (ProDOT)-based copolymer (WS-PE₂) were investigated and evaluated against the control, potassium salt form of poly(acrylic acid) (PAA-K). When used in conjunction with a polyethylene glycol (PEG) surface coating for the magnetite active material, PPBT provided for overall improved electrode performance as a result of more favorable intermolecular interactions between the composite constituents. The ProDOT-based copolymer WS-PE₂ exhibited comparable cycling performance to PPBT, whereas PAA-K and PPBT were similar with respect to rate capability. This investigation compares and contrasts a series of carboxylated polymers to elucidate the roles of different functional groups and identify materials chemistry-based structural parameters that can be manipulated to assist overall electrochemical performance of composite Li-ion battery anodes.