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Lithium-ion battery cathodes are manufactured by coating slurries, liquid suspensions that typically include carbon black (CB), active material, and polymer binder. These slurries have a yield stress and complex rheology due to CB’s microstructural response to flow. While optimizing the formulation and processing of slurries is critical to manufacturing defect-free and high-performance cathodes, engineering the shear rheology of cathode slurries remains challenging. In this study, we conducted simultaneous rheo-electric measurements on 3 wt% CB suspensions in N-methyl-2-pyrrolidone containing various loadings of active material NMC811 and polyvinylidene difluoride. Accounting for the changes in the infinite shear viscosity, the yield stress, and the medium viscosity due to the presence of NMC and polymers, we defined the differential relative viscosity. This differential relative viscosity, Δ𝜂𝑟, is a measure of the distance from the infinite shear rate, where carbon black agglomerates are fully broken down. We find that Δ𝜂𝑟 collapses all flow curves regardless of formulation with an empirical relationship Δ𝜂𝑟=2.18𝑀𝑛𝑓−0.92, indicating a quantitative prediction of the flow curve of cathode slurries across a wide range of formulation space. We then used electrical conductivity to identify and quantify shear-induced structure memory, evidenced in the ratio of the shear conductivity over the post-shear quiescent conductivity. We find that similar to the changes in the yield stress, increasing NMC concentration increases memory retention, and in contrast, the addition of PVDF erases memory effects. Our findings here will provide valuable insight into engineering the formulation and processing conditions of lithium-ion battery cathodes.
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Rheo-electric measurements of carbon black suspensions containing polyvinylidene difluoride in N -methyl-2-pyrrolidone
Lithium-ion battery cathode slurries have a microstructure that depends sensitively on how they are processed due to carbon black's (CB) evolving structure when subjected coating flows. While polyvinylidene difluoride (PVDF), one of the main components of the cathode slurry, plays an important role in modifying the structure and rheology of CB, a quantitative understanding is lacking. In this work, we explore the role of PVDF in determining the structural evolution of Super C65 CB in N-methyl-2-pyrrolidinone (NMP) with rheo-electric measurements. We find that PVDF enhances the viscosity of NMP resulting in a more extensive structural erosion of CB agglomerates with increasing polymer concentration and molecular weight. We also show that the relative viscosity of all suspensions can be collapsed by the fluid Mason number (Mnf), which compares the hydrodynamic forces imposed by the medium to cohesive forces holding CB agglomerates together. Using simultaneous rheo-electric measurements, we find at high Mnf, the dielectric strength (Δε) scales with Mnf, and the power-law scaling can be quantitatively predicted by considering the self-similar break up of CB agglomerates. The collapse of the relative viscosity and scaling of Δε both suggest that PVDF increases the hydrodynamic force of the suspending medium without directly changing the CB agglomerate structure. These findings are valuable for optimizing the rheology of lithium ion battery cathode slurries. We also anticipate that these findings can be extended to understand the microstructure of similar systems under flow.
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- Award ID(s):
- 2047365
- PAR ID:
- 10409499
- Publisher / Repository:
- Society of Rheology
- Date Published:
- Journal Name:
- Journal of Rheology
- Volume:
- 67
- Issue:
- 3
- ISSN:
- 0148-6055
- Page Range / eLocation ID:
- p. 647-659
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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