The Preparation And Modification Of Separator For Lithium-ion Batteries

With the lithium-ion(Li-ion)batteries technology boosting,Li-ion batteries have considered as one of the most attractive energy storage devices,which are extending into field of electronic vehicles(EV)and electronic hybrid vehicles(HEV),due to high energy density,long-term cyclic lifespan and non-memory effect.Separator regarded as one of the most important components of Li-ion batteries has played a critical role in preventing the physical contact between positive and negative electrodes;the properties of separator are strongly affect to Li-ion batteries.Most conventional Li-ion battery separators are fabricated from polyolefin,predominantly polyethylene(PE)or polypropylene(PP),which have high mechanical strength and exhibit high electrochemical stability.However,electrolyte infiltration of these types of separator is difficult,owing to their low polarity and porosity;this leads to poor compatibility between the separators and electrolyte,as well as electrodes in the batteries,thereby resulting in poor electrochemical performances.Another problem is thermal induced safety hazard,caused by poor thermal-resistance of the separator.To solve these two main problems of hydrophilicity and thermal stability,in this paper,we developed a series of well-defined separators and modified separators by new method and novel materials.Furthermore,the nanostructure control principles,the effect of the hydrophilicity on lithium electrochemical kinetic,as well as relationship between safety problem and thermal resistance are deeply investigated.The specific results are shown as follows:Hydrophilic high-density polyethyl ene/methylcellulose(HDPE/MC)blend microporous membranes were prepared via the thermally induced phase separation process.The effect of MC on the HOPE membranes was investigated by examining the morphology and determining the phase diagram,hydrophilicity,crystallinity,and mechanical properties.In addition,the electrochemical properties were evaluated from button cells consisting of the pure HDPE membrane and HDPE/MC blended microporous membranes as separators saturated by the LiPF6 electrolyte.The obtained results indicated that the cloud point temperature and the surface morphology of the membranes was shifted to high values and changed from dense to porous,respectively,with increasing MC content.The blended membranes also had a higher electrical uptake than the pure HDPE membrane.Moreover,the maximum ionic conductivity(1.01×10-3 S/cm)and minimum activation energy(Ea)value(10.48 kJ/mol)were obtained at an MC content of 2 wt%.Furthermore,compared to those consisting of the pure HDPE membrane,button cells consisting of the HDPE/MC blend microporous membranes exhibited higher charge-discharge capacity and better discharge performance at various current densities.The low ionic conductivity of polypropylene(PP)separators has stymied their use as polymer electrolytes in high-power lithium-ion batteries.To improve the ionic conductivity,we coated PP separators with cellulose aerogel based on hydroxyethyl cellulose(HEC),via ice segregation induced self-assembly(ISISA).The coating was characterized via scanning electron microscopy(SEM),attenuated total reflection-infrared spectra(ATR-IR),thermogravimetric analysis(TGA),and differential scanning calorimetry(DSC).In addition,the electrochemical performances of the separator were evaluated by using a cell consisting of the coated separator,lithium foil as the counter and reference electrodes,and LiFePO4 as the cathode.The porous cellulose aerogel-coated separator exhibited superior dimensional stability,electrolyte uptake,and hence a higher ionic conductivity and better cycling performance,than its non-coated counterpart.Furthermore,the separator was coated without the use of toxic solvents,thereby rendering the preparation process cost effective and highly efficient.To develop a separator with remarkable thermal-resistance for high-safety lithium-ion(Li-ion)batteries,a graphene oxide(GO)-grafted hyper-branched polyether(GO-g-HBPE)macro-porous membrane without any polymer binder was designed and prepared using polystyrene(PS)nanoparticles as hard templates.GO(inorganic part)provides the membrane-formation ability for GO-g-HBPE separator and HBPE(polymer part)imparts excellent affinity with the liquid electrolyte(158%for liquid electrolyte uptake).The GO-g-HBPE membrane,serving as a separator for the batteries,exhibited robust thermal dimensional stability with no dimensional changes at 200 ? for 0.5 h.Moreover,it shows a better electrochemical performance(cycle performance and rate capability)than a commercialized PP separator,implying a promising potential for application in high-safety and high-power Li-ion batteries.Thermal runaway is a hazardous behavior of lithium-ion batteries under extreme conditions and is mainly cause for restraint of its commercial applications in development of high-power and high-rate lithium-ion batteries.In this paper,a new dual-functions coating layer fabricated from polystyrenepoly(butyl acrylate)copolymer encapsulated silica nanoparticles as "thermal shutdown switch" with a reasonable shutdown temperature o?80 ? is designed and coated on polypropylene separator.The shell polymer owing to its self-adhesion upon glass transition temperature(Tg)can retard off the Li+ conduction between the electrodes,thus prevents cell from thermal runaway,the core nanoparticles protect the separator from significant thermal shrinkage when the cell temperature keeps going up.Moreover,the coated separator has no negative affection on the normal electrochemical performance of the batteries,thereby implying that this coating layer provides a simple and effective approach to control the safety of commercial lithium-ion batteries by internal self-protecting.