Microstructure Prediction And Optimization Of UHMWPE Separator Preparation Process

Lithium-ion battery,which possesses advantages in energy density,long life,and environmental-friendly,is one of the best power storage and supply choices.The lithium-ion battery is widely used in defense,E-vehicles,and 3C products.Lithium-ion batteries are composed by five parts: a positive electrode,a negative electrode,a separator,electrolyte,and a battery shell.The separator is a polymer film with a microporous structure and is called as the “third electrode” in the lithium-ion battery.The separator is used to prevent the positive and negative electrode contact and ensure that lithium ions transportation.Polyethylene-based separators are mainly manufactured by wet processing.The wet processing,which includes six steps: mixing,extrusion,cooling,biaxial stretching,extraction,and heat treatment,could generate a uniform pore distributed structure and even thickness.In the mixing process,the diluent is well dispersed in the polyethylene,and the location of the diluent determines the location of pores in the separator.Then,the phase separation behavior during cooling brings the porous structure,and the stretching process determines the final pore structure.Therefore,the research in this dissertation focuses on the mixing,cooling,and stretching processes during the preparation of Ultra High Molecular Weight Polyethylene(UHMWPE)separators.In this paper,the dissipative particle dynamics(DPD)method is used to study the wet processing of the UHMWPE separator.The interaction parameters and dissipative force parameters in the DPD method are determined based on the experimental data.The mesoscopic simulations of shearing,cooling,and stretching processes are conducted.A multi-scale CFD(Computational fluid dynamics)-DPD method is established in this paper,and the UHMWPE structure evolution during the actual preparation is obtained.In the shear process,the size and direction of polyethylene molecules can be affected by the shear rate.Shear rates also affect the porosity and pore size.The increased volume percentage of short PE chains will reduce the degree of polyethylene dispersion.The porosity and pore size also decreases with the increased percentage of short chains.In this paper,the DPD and CFD methods are integrated with the shear rate,and the evolution of the mesoscale structure in the model is obtained to analyze the effects of single-cycle and multi-cycle step shear rates.DPD simulations are then carried out to study the cooling steps and temperatures effects in the cooling process.It is found that during the cooling process,the porosity and pore size gradually increases while the number of pores gradually decreases.The temperature-time/step correlation in the casting model,which is obtained from the CFD simulation,was utilized in the DPD simulation.When the initial temperature is higher,the porosity and pore size change in a small range,and the number of pores increases gradually.When the temperature drops fast,the porosity and pore size increase,and the number of pores decreases significantly.In the stretching process simulation,uniaxially stretching is achieved by applying different stretching ratios and stretching rates.The porosity is increased by uniaxial stretching,but the pore size is not affected.Under different stretching rates,the higher the stretching rate being in the higher porosity and the larger pore size.Meanwhile,asynchronous stretching rendered the separator with higher porosity,while the separator prepared by synchronous stretching would possess smaller pores.