Environmental Stress-Induced Fracture Behavior of Polypropylene Battery Separators: Insights into Creep Behavior and Mechanical Stability

Abstract

In the manufacturing and usage processes of lithium batteries, separators are subjected to a diverse array of environmental conditions. The stability of polypropylene (PP) porous separators in these environments plays a crucial role in ensuring the safety of lithium batteries. This study investigates the fracture behavior of PP battery separators in the face of environmental stressors such as electrolyte, temperature variations, and applied stress. Through a tensile creep experiment in various temperatures and solvents, we reveal a unique three-stage creep behavior under conditions that mimic actual battery use, including the transition from primary to secondary creep, followed by fracture. Our findings demonstrate that elevated temperatures or stress levels increase secondary creep strain, leading to greater overall fracture strain due to increased mobility of molecular chains and transformation of the crystalline region. Apparent activation energy (E a ) calculations indicate that PP separators exhibit higher E a in the electrolyte, pointing to a propensity for brittle fracturing at lower strains due to early failure of fibrous connections. By applying a three-component model, we capture the intricacies of long-term creep behavior, offering insights into the mechanical relaxation and response to creep. This research underscores the importance of the environmental stability of PP separators in lithium batteries, offering essential insights for the development of more stable separators.