Search

Filling of the electrode and the separator with an electrolyte is a crucial step in the lithium ion battery manufacturing process. Incomplete filling negatively impacts electrochemical performance, cycle life, and safety of cells. Here, we apply concepts from the theory of partial wetting to explain the amount of gas entrapment that occurs during electrolyte infilling and show that this can explain the lower than expected effective transport coefficients that are measured experimentally. We consider a polyethylene separator as a model system. Quasi-static infilling simulations on 3D reconstructions of the separator structure indicate that there can be up to 30% gas entrapment upon infilling due to the geometry of the separator, which results in a reduction of effective transport by >40%. Considering the dynamics of the electrolyte (e.g., viscosity) and the infilling process explains why the residual gas phase is typically less (15%-20%) and why, for electrolytes that wet well, increasing viscosity leads to higher values of gas entrapment, which is observed experimentally as decreased effective electrolyte conductivity. This work highlights the importance of optimizing not only the physiochemical properties of the electrolyte and pore surfaces, but also the 3D structure of the pore space, providing insights how to do so. [ABSTRACT FROM AUTHOR]

In this paper, we study how polyolefin separators respond to compressive stress, which can occur during battery operation as active particles lithiate, expand, and push into and deform the separator. We use real microstructures of polyethylene and polypropylene separators acquired with focused ion beam scanning electron microscopic (FIB-SEM) tomography and simulate how these structures deform under compressive strain. After validating our mechanical simulations, we characterize how the microstructural properties of the separators change as a function of compressive strain and simulate the influence these changes in microstructure have on the lithium ion transport through the separator. We find that a given compressive strain negatively impacts the microstructure of polyethylene separatormore than that of a polypropylene separator. To understand the origins of the different response to compressive strains in polyethylene and polypropylene separators, we use a network-based analysis to assess the type of mechanical deformations occurring in the separator membrane and show that it is the combination of material properties and structure that are responsible for the greater stability of polypropylene. This work highlights how the structure of a separator plays an important role in its mechanical robustness and prevention of cell degradation. [ABSTRACT FROM AUTHOR]