Processing - Structure - Performance Relationships: A Rheological Perspective

Abstract

Optimal performance of commercial battery electrodes is dependent on a homogeneous distribution of three components: 1) active material 2) conductive additive 3) polymer binder. In commercial batteries, the conductive additive and polymer binder are less than 10\% of the entire electrode, but homogeneous distributions of these components is non-trivial and greatly impacts electrode performance. Processing of battery electrodes is a multi-step process with a multitude of variables that could possibly impact homogeneity. Typically the study of processing parameters is associated with manufacturers. However, processing parameters are not universal or well transplanted from system to system and as a result academia has begun to investigate processing from a fundamental standpoint. Literature has demonstrated the complex relationship between processing and performance and how correlations can become convoluted if careful experimentation is not performed. During the first unit operation, an active material, conductive additive and polymer binder are dispersed in solvent creating a slurry. Battery slurries can create a wide variety of microstructures depending on the specific components, concentration, and component interactions. Rheological measurements have risen in prominence as tools to better understand the battery slurry's microstructure. The work presented here investigates the fundamentals of the mixing unit operation and its impact on the slurry microstructure and electrode performance. The first part of the work presented here investigates the particle-polymer interactions that develop in Lithium Nickel Manganese Cobalt Oxide (NMC), carbon black (CB) and polyvinyldiene diflouride (PVDF) cathode slurries and how they are impacted by component parameters and shear during mixing. The investigation determined that depletion interactions develop between the CB particles and PVDF, the sign of which determined whether CBs attraction to itself is increased or decreased. In the case of low molecular weight PVDF, the attraction between the CB particles increases due to the presence of PVDF. The increased attraction causing flocculation of the particles and at a critical volume fraction of particles a gravity withstanding network formed. Interestingly, the presence of NMC particles does not impact the gelation transition. Since the NMC particles do not participate in gelation the microstructure is directly related to the concentration of CB in the slurry. On the other hand, when high molecular weight PVDF is used gelation of CB is prevented by changing the sign of the interaction potential. Shear during mixing was shown to be sufficient enough to cause polymer scission of the PVDF chains decreasing molecular weight and causing gelation when high molecular weight PVDF was started with. Uncontrolled polymer scission was also shown as a potential reason for reproducibility issues during processing. Overall the first two chapters demonstrate how different slurry microstructures develop and its dependence on component size and mixing parameters. Once the basics of creating the initial slurry microstructure was determined, the role of other processing steps could be considered. The second half of the work presented here investigates how a processing step called "dry-mixing" impacts the electrode performance and techniques for investigating electrodes. Dry-mixing coats the conductive additive onto the surface of the active material prior to wet mixing with the polymer binder and solvent. By investigating dry-mixing at commercially relevant concentrations of CB, it was determined that dry-mixing improves performance by decreasing the resistance between active material particles. It is difficult to measure particle-particle resistances directly. Particle-particle resistances are incorporated into the Dualfoil(c) model for Li-ion batteries. Preliminary investigations into the use of this model are documented. Additionally this work discusses combining energy dispersive x-ray spectroscopy (SEM/EDS) and x-ray computational tomography (XCT) for mapping of the conductive additive and polymer binder in electrodes. Due to the elemental and density similarities of the conductive additive and polymer binder they are difficult to distinguish in both techniques. The use of a contrast agent was shown to improve both SEM/EDS and XCT results and allow for material specific connectivity to be calculated in XCT. The work presented here sets the frame work for fundamental investigations into the impact of processing on electrode performance. By using a consistent high performance electrode system conclusions about performance can be made in the absence of electrochemical limitations. The findings presented here demonstrate that complex interactions occur during mixing and that a variety of fluid phase microstructures can develop. In addition, the work presented here addresses the issue of reproducibility and demonstrates that short-range particle-particle resistances are the main limiting property in commercial battery electrodes. This work hopes to create a basis on which further understanding of the importance of processing can be based.Ph.D., Chemical Engineering -- Drexel University, 201