Preparation And Performance Of Modified PP And PVA-SiO₂ Separators

IVLithium-ion batteries(LIBs),with the merits of high energy density,low self-discharge,high working voltage,etc.,have attracted great attention in electrochemical energy storage technology and become one of the effective ways to alleviate energy shortage today.Separator,as a critical component in LIBs,plays an important role in physically separating the anode and the cathode and providing a path for free transport of lithium-ions.Currently,the commercial separators for LIBs are mainly polyolefin-based separators,such as polypropylene(PP)and polyethylene(PE)separators.However,these separators have poor electrolyte wettability,low electrolyte-absorbing as well as insufficient thermal resistance,raising negative affect on electrochemical and safety performances in some extent.Consequently,preparing high-performance separators,combining good electrolyte wettability,high electrolyte-absorbing with sufficient thermal resistance,has vital significance to the development of LIBs.In this paper,on the one hand,we modified PP separator by introducing Al2O3,SiO2 ceramic particles and prepared Al2O3/PP,SiO2/PP composite separators.On the other hand,we prepared PVA-SiO2 nanofiber membrane via electrospinning technique.At the same time,the physical properties and electrochemical performances of as-prepared separators were investigated by a series of evaluations.Main research content is as follows:(1)Al2O3/PP composite separators were prepared through a modification process with Al2O3 as ceramic particle,polyvinyl alcohol(PVA)as binder.The morphology,wettability,thermal resistance and electrochemical of bare pp separator and coated separator were characterized by X-ray diffraction,scanning electron microscope(SEM),differential scanning calorimetry(DSC),electrochemical test system.It was found that the electrolyte uptake of the coated separator increased from 98% to 203%,and the thermal shrinkage decreased from46.1% to 24.4% at 170 °C for 0.5 h.Moreover,compared to bare PP separator,the coated separator always exhibited higher discharge capability in different C-rates.The capacity retention of coated separator reached 90.34% when cycled for 100 cycles at 0.5C.(2)SiO2/PP composite separators were prepared by sol-gel and dip-coating methods.The effects of TEOS dosage on the morphology,wettability and thermal stability of composite separators were investigated using Fourier-transform infrared spectroscopy(FTIR),SEM,TG and contact angle measurements.According to the results,when TEOS dosage was 7.5 wt%,the composite separator had a porous structure,and gave a porosity of 50.5%,an electrolyte uptake of 201.2% as well as a thermal shrinkage of less than 10% after a heat storage at170 °C for 0.5 h,showing a nice comprehensive performance.The electrochemical test show that the cell assembled with composite separator containing 7.5 wt% TEOS gave an electrochemical stability window of 5.1 V,an ionic conductivity of 1.26 m S cm-1,and possessed great C-rate and cycling performances at the same time.(3)SiO2-PVA nanofiber membrane was prepared via sol-gel and electrospinning techniques.The morphology,wettability and thermal stability of all samples were characterized by FTIR,SEM,DSC and contact angle.Besides,the electrochemical performances of cells assembled with nanofiber membranes were evaluated by electrochemical workstation and electrochemical test system.Experimental results demonstrated that the PVA-SiO2 membranes possessed a unique three-dimensional interconnected porous structure and displayed a porosity of 73%,an electrolyte uptake of405%,and non-obvious thermal shrinkage after a heat storage at 170 °C for 0.5 h.Cells assembled with SiO2-PVA nanofiber membranes gave a sufficient electrochemical stability window,a higher ionic conductivity and nice C-rate and cycling performances.After cycled100 times at 0.5C,the cell possessed a discharge capacity of 129.5 m Ah g-1,coulombic efficiency of 99.8%,which were higher than those of PP separator under the same conditions.What mentioned above indicated that the SiO2-PVA nanofiber membrane could act as a separator in LIBs.