Preparation And Properties Of Dry Uniaxially Stretched Lithium-ion Battery Separator

A technology,which is used for preparing polypropylene battery seperator for lithium-ion battery via a dry-unidirectional stretching process,and the properties of the produced memberane,is studied in this thesis.The process mainly includes casting,annealing,superposition,stretching,separation,release of stress and cutting.Among these steps,casting,annealing and stretching have significant influence on the performance of battery separator.In the process,molten polypropylene flows out of die of extruder and crystallises under high-speed stress field to produce the prefab film.The film has a parallel lamellar structure and the arrangement is perpendicular to the extrusion direction.Then,the film is annealed to improve the crystallinity.After that,the lamella crystals are separated by precisely stretching and micropores forms in the film.DSC,SEM,pore size distribution,gas permeability,crystallinity,porosity,thickness,elastic recovery,heat shrinkage and transmittance were measured to study the behaves of produced films.The influences of key process parameters,such as draw ratio during casting process,temperature of cold rolls,temperature and time during annealing,temperature and draw ratio and run speed during stretching process,on the important properties of battery separator were studied.The optimized parameters for adjusting properties of diaphragm are achieved.Both of the contribution rate of drawing to micropores formation,and the calculation formula are described in this thesis.The difficulty of micropores formation can be characterized by the ratio of actual porosity to theoretical maximum porosity.The results show that the elastic recovery of casting film reaches 80%,the porosity of the streched membrane reaches 38%,gas permeability is less than 400 s,when the casting temperature of cold rolls is between 75 ? and 95 ? and annealing temperature is between 139 ? and 147 ?.That products meet the performance requirement for battery diaphragm.Annealed for more than 40 s,the crystallinity of cast film reaches about 50% and keeps stable.When the stretching speed is between 5.5 m/min and 8.5 m/min,the permeability of membrane decrease.Especially,once the speed exceeds 7.5 m/min,the permeability decreases sharply.After annealed with rollers for 52 s,the casting film reaches the same performance annealed with traditional oven for 6 h.By using the roller-annealing process,the production efficiency increases obviously and the power consumption decreases meanwhile.