Designing Polyolefin Separators to Minimize the Impact of Local Compressive Stresses on Lithium Ion Battery Performance

Abstract

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 separator more 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.

A Review on Lithium-Ion Battery Separators towards Enhanced Safety Performances and Modelling Approaches

“…Lagadec et al [ 36 ] characterized how the microstructural properties (including porosity, tortuosity, and permeability) of the separators change as a function of compressive strain and predicted the influence of these changes on the Li-ion transport through the separator by mechanical simulations. They also concluded that a given compressive strain negatively impacts the microstructure of PE separator more than that of a PP separator, because PE has a lower Young’s modulus, smaller pore sizes, and a more isotropic structure.…”

The unusual delamination phenomenon of three kinds of lithium‐ion battery separators

“…The separator is placed between two electrodes to prevent direct contact between them while serving as the electrolyte reservoir to enable ionic transport 6–11 . Many researchers have found that the pore structure of battery separators affects battery performance 12–18 . Even separators with the same porosity and through‐plane tortuosity exhibit different performances 19,20 …”

Experimental Combined Numerical Approach for Evaluation of Battery Capacity Based on the Initial Applied Stress, the Real-Time Stress, Charging Open Circuit Voltage, and Discharging Open Circuit Voltage

“…So far, it is still a great challenge to establish an accurate model between mechanical properties of local components and the capacity because of the diversity and complexity of materials. For example, the pressure on the separator changes with the aging of the battery [18]. The porous size of the electrode, the pressure in the manufacturing process, and the electrolyte around it also affect its mechanical properties [19].…”