Research On The Preparation And Property Of New Separator Materials For Lithium Ion Battery

Lithium ion battery has been widely used as its merits, such as, high energy density, no memory effect,long cycle life, and friendly to environment. Separator, as one of the key materials of lithium ion battery,affects the battery performance, including energy density, power density, cycle life and safety greatly.Polyolefin microporous membrane has become one of the main commercialization separator because of itsgood strength, resistance to electrolyte corrosion, non-toxic, low price, and its closing performance at hightemperature.Polyethylene microporous membrane was usually prepared by wet process (thermally-induce phaseseparation, TIPS), which could form nearly round nano-pores. The consistency of microporous has a greateffect on the battery performance. Too large pore will speed up the self-discharge. And non-uniform poremay give rise to non-uniform current density. All of these may reduce the voltage consistency of the battery.Therefore, in order to improve the microporous structure, it is quite essential to add nucleating agent to thepolymer/diluent system.Polyethylene microporous membrane may shutdown when the temperature closing to its melting point,which will prevent further temperature rising, make the temperature lower than the melting point of lithiumor organic electrolytes to ensure the safety. But there is great shrinkage at the same time the pore is closing.Both shrinking and melting of the separator could result in contact of the electrodes so that direct chemicalreactions between the cathode material and anode material occur. And the generated heat causes thermalrunaway. So how to reduce the shutdown temperature and contractibility rate, increase the meltingtemperature, has become the key problem to improve the security of batteries.In addition, with the inherent hydrophobic surface and lower surface energy, the non-polarpolyethylene separators show poor ability of electrolytes uptake and maintaining. The hydrophilicmodification to the membranes surfaces is an effective way to improve the application performance ofmembranes.Considering the above analysis, research content was determined as follows:1、The research situation of the lithium ion battery separator was reviewed. And from the existing problems we put forward the research content.2、Polyethylene microporous membranes were prepared via thermally induced phase separation. Theratio of polyethylene and diulent were optimized. At the optimized conditions, nucleating agent PVP wasused to controll microporous structure. The morphology structure, porosity, electrolyte uptake, water flux,crystallinity, ionic conductivity and cycling performance of the separator were measured using scanningelectron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD),electrochemical workstations and battery test system. Results show that the nucleating agent PVP improvedthe porous structure successfully. When adding2.0%PVP, porosity and electrolyte uptake increased6.32%and40.87%, respectively.3、Because single layer polyethylene separator may soften and shrink seriously at110℃or highertemperature, polyethylene/polypropylene microporous membrane via thermally induced phase separationwas prepared. Effects of the blend ratio of polyethylene and polypropylene on the membrane structure,porosity, electrolyte uptake, crystallinity, thermal shrinkage, ionic conductivity and electrical performancewere explored systematically. Results show that, blending polypropylene with polyethylene not only canimprove the shrinkage obviously, but also enhance the performance of the separator further. When theconcentration of the polymer is30%, HDPE and iPP ratio is90:10, the porosity is49.97%, electrolyteuptake is175.0%, and the ionic conductivity rearches6.11×10-4S·cm-1, the longitudinal and transversecontractibility rate decreased1%~1.6%compared to pure PE membrane.4、To solve the poor wettability, electrolyte uptake ability and big shrinkage at high temperature,ceramic compositing method was used. The PE separator membranes were coated with polyvinylenefluoride (PVDF) and nano aluminum oxide (nano-Al2O3) to form porous coating by means of phaseinversion. Effects of solution concentration and phase converting conditions on the wettability, electrolyteuptake ability and thermal stability were explored systematically. And the electrical performance was alsoinvestigated by AC impedance and linear scanning current-voltage methods. Results show that, when thecoating solution concentration is15%and the nano-Al2O3content is30wt%, the porosity reaches37.21%.Compared to the membrane without Al2O3coating, the electrolyte uptake increased211.5%, and thewatercontactangle decreased41.3°, indicating the significant improvement of the wettability and electrolyteuptake ability of composite separator.