Study Of Electrospun Polyvinylidene Fluoride In-situ Composite Membranes

Polyvinylidene fluoride(PVDF) has such outstanding properties as chemical corrosion resistance, thermostability, strong electric breakdown intensity, good mechanical behavior and electrochemical stability, and it is widely used in petrochemical industry, etl-semko and fluoro-carbon coatings. Because of its good chemical stability and electrical insulating property, modified PVDF polyporous membranes has made extensive application in lithium secondary battery separators, which have become the largest market of PVDF production.In this paper, PVDF/SiO₂ in-situ composite nanofibers with different concentrations of in situ generated silica were prepared by electrospinning with acetone and N,N-Dimethylacetamide (DMAC) as the solvents of polyvinylidene fluoride (PVDF) and in-situ composite technology as the method to prapare spinning solution. The effect of different concentrations of PVDF and ratios of acetone and DMAC on electrospun nanofibers'surface morphology was observed. Meanwhile, PVDF/SiO₂ composite nanofibers with 6% directly added silica were also prepared by electrospinning. Scanning electron microscopy (SEM) and atomic force microscope (AFM) were used to investigate the nanofibers'microstructure, while NDJ-79 rotary-type viscosity meter and QBZY full automatic surface tensiometer were employed to study the rheological properties of the composite solution for electrospinning. The chemical composition of the membranes was analyzed by FTIR, and the internal structure was characterized by TEM. DSC and TG were also used to analyze the thermal property of the composite membranes. Besides, related equipment were used to test the pore, single modulus and mechanical property. Finally, the charge-discharge performance was tested with the help of CT-3008W-5V1mA-S4 high accuracy battery performance test system. The electrochemical performance of the membranes was studied.Experimental results showed that the best concentration of PVDF was 10%, and the optimum solvent mass ratio was 6:4. The electrospinning process and fiber morphology were stable when the concentration of in situ silica was 0%-9%. FTIR and TEM confirmed that SiO₂ existed in in-situ composite membrane, and in the form of atom pellet dispersing homogeneously inside the fiber. SEM images showed that the addition of in situ silica improved the surface roughness of fibers leading to the improvement of specific area, which meets the requirements of battery membrane for liquid absorption performance. Pore test showed that the pore diameter and pore ratio become smaller with the increase of in situ silica concentration. Thermal property testing showed PVDF membrane need more heat to melt after the addition of inorganic silica, and the decomposition temperature of membrane is raised, which finally improved the safety performance. Mechanical property testing showed the addition of in situ silica had less effect on its tensile strength but with evident increase in breaking elongation. Especially when 6% in situ silica was added, the breaking elongation was 271.62%, which was much higher than that of 6% directly added SiO₂(45.45%). That could be explained that the addition of in situ silica can easily make PVDF nanofibers with good strength and flexibility. The force-distance curves showed the modulus of fiber surface increased by adding inorganic silica. Electrochemical performance test results indicated the charge-discharge performance of PVDF/SiO₂ composite membrane prepared by the method of in-situ composite was better, but the cycle index of charge-discharge was not so ideal. The charging voltage of PVDF/SiO₂ composite membrane by directly adding and PVDF membrane could not boost normally during the first cycle, so the normal charge-discharge performance could not be tested according to the setting parameters, the charge-discharge property of the two membranes still need to be improved. In general, the works did in this paper proved that in situ silica not only increased the basic performance but also improved the electrochemistry of the composite membrane.