Coupled effect of strain rate and solvent on dynamic mechanical behaviors of separators in lithium ion batteries

Mechanical properties of separators have a great impact on the electrochemical performance of lithium ion battery, such as capacity, charge/discharge behavior, charging cycles and among others. In the present study, two typical widely commercialized separators, Celgard 2400 and Celgard 2340, are the investigation objects with single layer and three-layer structures respectively. Firstly, to investigate material anisotropy and strain rate effects, tensile tests conducted on MTS and Instron at various strain rates from 0.01 to 50 s^-1 are carried out with samples prepared at three different directions: transverse direction (TD, 0°), machine direction (MD, 90°) and 45°. The failure strain decreases while failure stress increases with the strain rate for materials in all three directions. Material anisotropy is observed within the porous structure with nanofiber reinforced in the polymer matrix. Secondly, the environmental solvent effect is also examined: dimethyl carbonate (DMC) solution has a negative effect on the mechanical property of the separator while water may exert a positive effect. Furthermore, DMA Q800 is employed to study the viscoelasticity of separators at various temperatures and frequencies. A simple viscoelastic model based on Kelvin-Voigt model is proposed for the two types of separators. This research may serve as a solid step towards the comprehensive understanding of LIB separator and shed light on the future research of unveiling physical relation between mechanical properties and electrochemical performance of LIB.