Carboxylated Poly(thiophene) Binders for High-Performance Magnetite Anodes: Impact of Cation Structure

While the focus of research related to the design of robust, high-performance Li-ion batteries relates primarily 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. Thus, attention is now turning toward the exploration of interfacial interactions between composite electrode constituents as a key element in ensuring electrode stability. 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. Herein, the PEG/PPBT system is used as a model polymeric binder for understanding cation effects in anode systems. As such, the potassium ion was replaced with sodium, lithium, hydrogen, and ammonium through ion exchange. The potassium salt exhibited the most stable electrochemical performance, which is attributed to the cation size and resultant electrode morphology that facilitates ion transport. The lithium analogue demonstrated an initial increase in capacity but was unable to maintain this performance over 100 cycles; while the sodium-based system exhibited initially lower capacity as a result of slow reaction kinetics, which increased as cycling progressed. The parent carboxylic acid polymer and its ammonium salt were notably inferior. The results exploring the effect of ion exchange creates a framework for understanding how cations associated directly with the polymer impact electrochemical performance and aid in the overall design of binders for composite Li-ion battery anodes.