Numerical Analysis Of Mechanical Properties And Thermal Stability Of Porous Polyethylene Film

The polyolefin film with nano-meter holes not only has good mechanical properties, but also has good ion penetration. Polyethylene and polypropylene porous membrane with high porosity, low resistance and high tensile strength, good acid and alkali resistance, good elasticity, have been widely used in lithium ion battery as separator. In lithium-based cells, the essential function of battery separator is to separate positive and negative electrodes and enable Li ionic transporting through the separator. The separator performance will directly influence on the battery capacity, safety and recycling. Most researchers investigated the mechanical properties of the separator and the mechanism of evolution of porous structure by experimental. However, the numerical simulation of mechanical behavior of nanoporous films is not enough. This thesis will simulate the mechanical properties and thermal stability of polyethylene film by molecular dynamics. The formation mechanism of nano-meter holes, structure characteristics, mechanical properties, thermal stability, and the penetration rate of lithium ion were investigated, then the relationship between the micro structure and macro properties were established. The main content includes the following three parts:Firstly, a simplified coarse-grained model of polyethylene (PE) separator was built using the method of self-avoiding random walk, then the final semi-crystalline PE model was obtained after the crystallization simulation. Based on the model the mechanical properties and formation mechanism of nanoporous structure were analyzed by MD. The results suggested that pure polyethylene film has good mechanical properties, pore-formed property and explain the evolution process of film hole generated on the micro scale.Secondly, the mechanical properties and thermal stability of the 2.8 w/w% SiO2 nanoparticle reinforced PE porous film were investigated by MD. The final semi-crystalline SiO2/PE model was obtained after the cooling crystallization simulation. Based on the model the mechanical properties and thermal stability were analyzed. The results suggested that the addition of 2.8 w/w% SiO2 nanoparticles presents increases of 26.6% in yielding strength, 19.9% in elastic modulus, and 11℃ in melting temperature respectively.At last, a molecular model was built to calculate lithium ion penetration rate (LIPR) of the separator, based on which the effect of the shape and size of holes on LIPR was investigated, and the porosity is 8.2%. The results showed that elliptical holes’LIPR 48.3% is higher than circular holes’ LIPR 39.5% with the same porosity and the greater the porosity, the greater the LIPR.