Thixotropy modeling of battery slurries and computational analysis of slurry transportation through pipe systems
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The rheology of battery slurries has a significant effect on the battery manufacturing process. Therefore, it is important to understand and model the unique rheology of battery slurries, such as shear-thinning, yield stress, or thixotropic behaviors. In this study, we focused on thixotropy, which is a time-dependent decrease of the viscosity induced by flow, reversible when the flow is decreased or arrested. A number of thixotropy models have been proposed up to date, but most of the previous works have been focused on crude oils, suspensions, polymer solutions, etc. They all exhibit different behaviors from our target fluid, battery slurry, which is a suspension with complex particulate interactions. Here, we aimed to find the suitable model that can best express the thixotropic behaviors of the battery slurries. Among the existing thixotropy models, we adopted a fluidity model which employs fluidity, the reciprocal of viscosity, as a parameter referring to the microscopic state of the fluid. We modified the model to fit the measurement data of the anode slurry, which was prepared using a binder, active material, and conductive additive. A Series of rheological measurements including creep, steady-state shear flow, construction, and destruction experiment were carried out on a rheometer to fit the model parameters. Using the developed model, we analyzed the transportation of the slurry through pipe systems with numerical solutions obtained with the finite element method. In addition to mass and momentum conservation equations, another scalar equation that models the evolution of fluidity was integrated into a variational formulation to handle thixotropy. The spatial domain was discretized on a finite element space using the variational formulation, and the trapezoidal rule based on finite difference interrupts was used for time integration. Various pipe geometries, such as 4:1 contraction or 90$\degree$ bendings, were considered, and selected geometries were tested to analyze the effect of pipe geometry on the time-dependent behaviors of the slurry. Time scales of the flow and the evolution of microstructure were compared to highlight the importance of thixotropy on the pipe flows.