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Abstract
Lithium-ion batteries play a central role in the widespread adoption of clean renewable energy. While advances in the manufacturing of lithium-ion batteries lead to dramatic price reductions and improved performance, quantitative descriptions of the cathode processing-structure-property relationships are not well established. In particular, the cathode of lithium-ion batteries has a porous structure which determines the electronic and ionic conductivity, and therefore the performance. To manufacture cathodes, slurries consisting of suspended solid particles and polymer binder dissolved in an organic solvent are coated into a thin film which is subsequently dried and calendered. The thin film microstructure is far from equilibrium, meaning that the structure strongly depends on both the slurry formulation and its processing history.
Here in this thesis, rheo-electric measurements were first conducted to evaluate cathode slurries under mimicking flow conditions encountered during the coating process. The rheology of battery slurry suspensions, regardless of the polymer binder or the active material loadings, was rationalized and predicted using a dimensionless number. Electrical properties, such as dielectric strength and electronic conductivity, were also extracted from simultaneous electric measurements and connected to the microstructure achieved under different flow conditions. To characterize the cathode microstructure in the nanoscale, new data analysis methods were developed for contrast variation neutron scattering measurements, and important in situ structure parameters were obtained and correlated to the long-term performance and important electrode properties such as the tortuosity. Finally, the extensional rheology of polymer binder solutions was systematically examined and showed distinct behaviors that cannot be predicted from shear rheology alone. Results on slot-die coating further demonstrated that the theoretical coating window prediction did not consider the extensional stress that is prevalent in the coating process of battery slurries and many other complex fluids. Preliminary studies on a slot-die coater sample environment for neutron scattering were also presented and discussed.
This thesis presents novel findings that contribute to both applied engineering science, particularly in the manufacturing of lithium-ion battery cathodes, and fundamental disciplines such as rheology and neutron scattering. Additionally, the methodology and framework developed herein are broadly applicable to other liquid-based systems and may inform future materials design.
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