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.

SWNT Anchored with Carboxylated Polythiophene "Links" on High-Capacity Li-Ion Battery Anode Materials.

Conjugated polymers possessing polar functionalities were shown to effectively anchor single-walled carbon nanotubes (SWNTs) to the surface of high-capacity anode materials and enable the formation of electrical networks. Specifically, poly[3-(potassium-4-butanoate) thiophene] (PPBT) served as a bridge between SWNT networks and various anode materials, including monodispersed Fe3O4 spheres (sFe3O4) and silicon nanoparticles (Si NPs). The PPBT π-conjugated backbone and carboxylate (COO-) substituted alkyl side chains, respectively, attracted the SWNT π-electron surface and chemically interacted with active material surface hydroxyl (-OH) species to form a carboxylate bond. Beneficially, this architecture effectively captured cracked/pulverized particles that typically form as a result of repeated active material volume changes that occur during charging and discharging. Thus, changes in electrode thickness were suppressed substantially, stable SEI layers were formed, electrode resistance was reduced, and enhanced electrode kinetics was observed. Together, these factors led to excellent electrochemical performance.

Carbon Nanotube Web with Carboxylated Polythiophene "Assist" for High-Performance Battery Electrodes.

A carbon nanotube (CNT) web electrode comprising magnetite spheres and few-walled carbon nanotubes (FWNTs) linked by the carboxylated conjugated polymer, poly[3-(potassium-4-butanoate) thiophene] (PPBT), was designed to demonstrate benefits derived from the rational consideration of electron/ion transport coupled with the surface chemistry of the electrode materials components. To maximize transport properties, the approach introduces monodispersed spherical Fe3O4 (sFe3O4) for uniform Li+ diffusion and a FWNT web electrode frame that affords characteristics of long-ranged electronic pathways and porous networks. The sFe3O4 particles were used as a model high-capacity energy active material, owing to their well-defined chemistry with surface hydroxyl (-OH) functionalities that provide for facile detection of molecular interactions. PPBT, having a π-conjugated backbone and alkyl side chains substituted with carboxylate moieties, interacted with the FWNT π-electron-rich and hydroxylated sFe3O4 surfaces, which enabled the formation of effective electrical bridges between the respective components, contributing to efficient electron transport and electrode stability. To further induce interactions between PPBT and the metal hydroxide surface, polyethylene glycol was coated onto the sFe3O4 particles, allowing for facile materials dispersion and connectivity. Additionally, the introduction of carbon particles into the web electrode minimized sFe3O4 aggregation and afforded more porous FWNT networks. As a consequence, the design of composite electrodes with rigorous consideration of specific molecular interactions induced by the surface chemistries favorably influenced electrochemical kinetics and electrode resistance, which afforded high-performance electrodes for battery applications.

Toward Uniformly Dispersed Battery Electrode Composite Materials: Characteristics and Performance.

Battery electrodes are complex mesoscale systems comprised of electroactive components, conductive additives, and binders. In this report, methods for processing electrodes with dispersion of the components are described. To investigate the degree of material dispersion, a spin-coating technique was adopted to provide a thin, uniform layer that enabled observation of the morphology. Distinct differences in the distribution profile of the electrode components arising from individual materials physical affinities were readily identified. Hansen solubility parameter (HSP) analysis revealed pertinent surface interactions associated with materials dispersivity. Further studies demonstrated that HSPs can provide an effective strategy to identify surface modification approaches for improved dispersions of battery electrode materials. Specifically, introduction of surfactantlike functionality such as oleic acid (OA) capping and P3HT-conjugated polymer wrapping on the surface of nanomaterials significantly enhanced material dispersity over the composite electrode. The approach to the surface treatment on the basis of HSP study can facilitate design of composite electrodes with uniformly dispersed morphology and may contribute to enhancing their electrical and electrochemical behaviors. The conductivity of the composites and their electrochemical performance was also characterized. The study illustrates the importance of considering electronic conductivity, electron transfer, and ion transport in the design of environments incorporating active nanomaterials.