A binder plays a critical role in dispersion of coating liquids and the quality of coating. Poly(vinylidene fluoride) (PVDF) is widely used as a binder in cathode slurries; however, its role as a binder is still under debate. In this paper, we study the role of PVDF on the rheology of cathode battery slurries consisting of Li(Ni1/3Mn1/3Co1/3)O2 (NCM), carbon black (CB) and N-methyl-2-pyrrolidone (NMP). Rheology and microstructure of cathode slurries are systemically investigated with three model suspensions: CB/PVDF/NMP, NCM/PVDF/NMP and NCM/CB/PVDF/NMP. To highlight the role of PVDF in cathode slurries, we prepare the same model suspensions by replacing PVDF with PVP, and we compare the role of PVDF to PVP in the suspension rheology. We find that PVDF adsorbs neither onto NCM nor CB surface, which can be attributed to its poor affinity to NCM and CB. Rheological measurements suggest that PVDF mainly increases matrix viscosity in the suspension without affecting the microstructure formed by CB and NCM particles. In contrast to PVDF, PVP stabilizes the structure of CB and NCM in the model suspensions, as it is adsorbed on the CB surface. This study will provide a useful insight to fundamentally understand the rheology of cathode slurries.
The dynamics of model colloidal gels under a steady shear flow is studied by means of a Brownian dynamics (BD) simulation while applying orthogonal superposition rheometry, which superimposes a small amplitude oscillatory flow orthogonal to the main flow direction. Orthogonal dynamic frequency sweep (ODFS) curves are obtained at various magnitudes of the main flow, which shows shear thinning behavior of the colloidal gel. The viscoelastic spectra of the ODFS can be superimposed onto a master curve by the horizontal shift factor, which is equivalent to particle viscosity. That is, the shear rate controls a single master clock for all viscoelastic spectra of the ODFS in the form of a time–shear rate superposition, which bears an analogy with the time–temperature superposition of polymeric systems. In the low-frequency region of the master curve, both orthogonal moduli are well superimposed onto a single master curve, whereas the loss modulus deviates slightly from the master curve in the high-frequency region, which coincides with the experimental findings. We observe spatial and time-varying structural properties in both low- and high-frequency regions on the ODFS curves by decomposing the pair distribution function. It is verified that each flow condition shifted onto the same stress level on the master curve shows identical spatial orthogonal moduli at all radial distances despite the differences in the aggregate size and average particle connectivity.
The dispersion quality and the storage stability of the electrode slurry are very important industrial issues as they directly affect the productivity of the electrode process as well as the performance of the battery. To maintain the dispersion stability, the prepared electrode slurry is agitated in a storage tank before entering the subsequent coating process. However, our understanding on the dispersibility of electrode slurries during storage is not enough. In this study, we systematically investigate the changes in the dispersion state and the rheological properties of Ni-rich NMC-based cathode slurries under various agitating conditions during storage. Most of the conductive nanoparticles form large spherical agglomerates of several tens of micrometers under low-speed agitating conditions, resulting in a dramatic change in the rheological properties. We also report that the change in the dispersion state and the rheological properties during storage can be characterized by the hydrodynamic stress induced by the flow. The mechanism of change in the microstructure of the cathode slurry during storage can be understood by considering the relative affinity between the particles and the flow characteristics during agitation. This study clearly demonstrates how the cathode slurry should be cared during storage to prevent quality problems.
The nonlinear rheology of concentrated lithium-ion battery anode slurry was examined under large amplitude oscillatory shear and interpreted with a sequence of physical process (SPP) analysis. The complex interplay of three anode slurry components-graphite (Gr) as the active material, carbon black (CB) as the conductive additive, and carboxymethyl cellulose (CMC) as the binder-leads to a two-step yielding behavior, represented as the secondary plateau in dynamic strain and stress sweep tests. We demonstrate that the two-step yielding behavior is manifested as double deltoids in the SPP analysis, through the study of intra-cycle rheological transition under oscillatory shear flow. Slurries of Gr-CMC exhibit two-step yielding behavior; slurries of CB-CMC do not, suggesting that Gr and CMC are the primary causes of two-step yielding in an anode slurry. A sedimentation test on a dilute Gr-CMC solution yielded phase separation between graphite particles, with CMC adsorbed on their surface and graphite particles aggregated via hydrophobic attraction. This indicates two possible types of interactions in a concentrated slurry: a hydrophobic interaction between graphite particles and a physical interaction caused by CMC adsorbed on graphite particles. The first yielding step relates to the hydrophobic attraction between graphite particles, resulting in a network structure that is expected to be brittle and rupture at a small strain. The second yielding step is attributed to the interaction between concentrated CMC, which is corroborated by the overlap of the secondary deltoid of the anode slurry and the single deltoid of the concentrated CMC solution in the SPP analysis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.