Investigation On Processing Physics Of Polyethylene Separator With Synchrotron X-ray Scattering

Lithium ion batteries are widely used in 3C products,which have been the indispensable consumables.It is no doubt that extending the life expectancy of battery and enhancing the charge and discharge stability are the greatest expectations of consumers.As one of the most important component of lithium ion,the failure of separator can induce the direct contact of positive and negative electrodes,which could result in a short circuit.Therefore,the separator plays an important role in determining the safety,life expectancy,as well as the stability of the charge and discharge.While,without in-depth comprehension of the foundational physical problems in preparing separator,one cannot improve its performances efficiently.Though,many researches have been focused on the preparation of lithium ion battery separators,most of them are based on the polypropylene(PP)via dry process and polyethylene(PE)via wet process.While,relatively few systematical work has paid attention on PE separator via dry process.There still have many questions and noduses on polyethylene separator preparation.Aiming to answer physical questions regarding to flow induced crystallization,the effect of mechanical energy and thermal effect on the micropore nucleation and growth,the deformation modes in amorphous areas of oriented lamellar stacks,the deformation mechanism of lamellar stacks in temperature-strain space,etc.,high density polyethylene(HDPE)was chosen as research object.Combining high time-,space-resolved synchrotron radiation X-ray,scanning electron microscope(SEM),air permeability,etc.the multi-scale structural evolution during precursor film preparation and post stretching of microporous membrane were tracked,which accumulate basic data and ideas of product exploitation for polyethylene separator with high performance.The main results and conclusions are summarized as follows:1)The strength of flow field was changed by varied draw ratios and die temperatures during the preparation of HDPE precursor films,which were further subjected to a new stretching method(constrained stretching)to generate microporous membranes.Structures like long period,lateral size of lamellae,orientation parameter,fibril content of the precursor films were obtained via small/wide angle X-ray scattering(SAXS/WAXS),and morphologies of the microporous membranes were examined with scanning electron microscope(SEM).It was found that the content of fibrils and orientation parameter of lamellar stacks in the precursor films played counter effects on the capability of micropore formation.A critical window to balance the conflict was revealed with an optimal fibril content of about 6.4%,at which maximal porosity and most homogeneous porous structure were obtained.A schematic model was proposed to account for the role of the twisted lamellar stacks,fibrils and the oriented lamellar stacks during pore formation.This thesis established the relationship between the content of fibrils-microstructure of precursor films-ability to generate micropores.2)Stress relaxation after HDPE precursor film subjected to cold stretching was performed at low and elevated temperature with strain keeping constant,respectively,and corresponding structure evolution was tracked by in situ and ex situ SAXS.It was found that stress-induced density fluctuation during cold stretching could be completely recovered as soon as the stress was unloaded,which was called as micropore embryo.Mechanical energy release at low temperature could promote micropore nucleation after an induction period.However,the growth of micropore nuclei was inhomogeneous,developing as non-uniform micropores with poor interconnectivity during hot stretching.While during mechanical energy release at elevated temperature,micropore embryos could be converted into evenly distributed micropores together with the formation of fibrils,which could supply growth sites for opened-pores during hot stretching.Consequently,microporous membranes with homogeneously distributed micropores and good permeability could be obtained after the subsequent hot stretching.It was suggested that irreversible micropore nucleation and growth at low temperature were undesirable,and temperature elevation was favorable process for the preparation of microporous membrane with narrow micropore size distribution and good air permeability.3)The structural evolution of HDPE precursor film containing highly oriented lamellar stacks deformed under two orders of magnitude of the tensile rate was tacked via time-resolved SAXS.It was found that the deformation modes were different,which might be affected by the relaxation response of amorphous chains.When the tensile rate was slow,the structures(such as areas with small entanglement density or the tails of the chains)with short relaxation time could be stretched out,which would act as stress concentration sites.The stress concentration expanded to heterogeneous deformation with further stretching,which caused relatively weak of second order scattering.While under high tensile rate,the relaxation of all the amorphous chain could not occur during the stretching,therefore,the deformation was homogenous in amorphous areas,resulting the enhancement of second order scattering and the weakness of heterogeneous scattering(Icav).The deformation modes could be defined into three regions:region I(0.04 mm/s-0.4 mm/s)heterogeneous deformation in amorphous areas,and cavitation took place;region Ⅱ(0.4 mm/s-2.4 mm/s)coexistence of heterogeneous and homogenous deformation,the ability of the two deformation modes was the same;region Ⅲ(2.4 mm/s-4 mm/s)homogenous deformation mode,stretching induced density fluctuation occurred.This may provide a new idea for detecting the relaxation time spectrum in amorphous chains of highly oriented lamellar stacks.4)HDPE precursor film with highly oriented lamellar stacks is chosen for microporous membrane preparation via varied cold stretching conditions.The evolutions of lamellar stacks and micropore formation were tracked by combining in-situ and ex-situ X-ray scattering and scanning electron microscopy methods.It was found that when stretched at 25℃,cold stretching strain mainly affects separation degree of lamellar stacks under strain lower than 60%,while slippage of crystal takes place at higher strain.As for cold stretching temperature,stretching below 60 ℃ can promote separation of lamellar stacks,while stretching over 80 ℃is beneficial to pull out molecular chains from lamellae,resulting in fibrils.Moreover,it is the first time to establish that a linear relation between separation degree of lamellar stacks and air permeability of final membrane membranes with high performance.