Physical scaling for predicting shear viscosity and memory effects of lithium-ion battery cathode slurries

Abstract

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, Δη_r, is a measure of the distance from the infinite shear rate, where carbon black agglomerates are fully broken down. We find that Δη_r collapses all flow curves regardless of formulation with an empirical relationship Δη_r = 2.18Mn^−0.92_f, 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 under-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.