Unveiling the environment-dependent mechanical properties of porous polypropylene separators

Porous polypropylene (PP) is commonly used as separator materials for lithium ion batteries (LIB). Its mechanical properties, especially critical for abuse tolerance and durability of LIB, are subject to change in different environments. To capture the mechanical responses of a porous PP separator, its microstructure was mapped into separate atomistic models of bulk crystalline phases and oriented amorphous nanofibers. These structures were relaxed and stretched in vacuum, water, and dimethyl carbonate (DMC) using molecular dynamics (MD). The simulation results revealed DMC molecules penetrated into the amorphous PP nanofiber, and reduced the local density and the Young's modulus. In contrast, water increased the Young's modulus of the amorphous PP nanofiber. Furthermore, neither water nor DMC had any impact on the Young's modulus of the crystalline phase. These results suggest that the DMC induced separator softening was attributed to the strong attraction of the less-polar DMC solvent with the amorphous fibrous PP nanofibers. [Display omitted] •The porous PP separator used in lithium ion battery reduces its modulus in electrolyte.•The softening effect is due to attraction between DMC and the amorphous PP nanofibers.•Atomic simulations revealed DMC solvent penetrates into the amorphous PP nanofibers.•In contrast, water will not penetrate into nor soften PP nanofiber.