Preparation Of PVDF And Its Composite Fiber Membranes For Separator In Lithium Ion Battery By Electrospinning

The lithium ion battery has become the main power supply in all kinds ofelectronic products for its no memory effect, high energy density, high workingvoltage, long use life and many other advantages. The growing popularity of thenew energy vehicles provides a wider space for the development of lithium ionbatteries. But at the same time it also raises higher requests to lithium ionbatteries, and separator becomes the bottleneck of the restriction for itsdevelopment. Large current charge and discharge require separator with highionic conductivity, electrolyte absorption rate and excellent performance, butseparator preparated by the traditional method has low porosity, electrolyteabsorption rate, so the study of the new generation of sepatator becomes of thecenter in the lithium ion battery materialsIn this paper the poly （vinylidene fluoride）（PVDF） and its organic,inorganic composite fiber membrane were made by electrospinning. Itspreparation process, physical properties and electrochemical properties werestudied.The single PVDF fiber membranes were prepared using electrospinningmethod, and their preparation technique was optimized. SEM, XRD, DSC, ACimpedance, linear sweep voltammetry and battery charging discharging and so onwere used to test the fiber membrane morphology, structure, degree ofcrystallinity, ionic conductivity, and battery charging discharging properties. Theresults show that20%PVDF fiber membrane ion conductivity is1.78×10^-3S/cm,electrochemical stability window was4.7V, it could meet the requirements of thecharging and discharging performance of the lithium ion battery, and had thehigh working voltage and discharge stability, but with the instability coulombefficiency.In view of the disadvantages of the single PVDF fiber membrane,PVDF/PMMA composite fiber membrane were prepared and tested through allkinds of representation and test method. finally20%PVDF/PMMA （6/4） fibremembrane showed better performance, having a porosity rate of71%, electrolyteabsorption rate of350%, ionic conductivity of5.90×10^-3S/cm, electrochemicalstability window of5.5V, and discharge specific capacity up to124.5mAh/g（PP/PE/PP separator was only100mAh/g）. Moreover the coulomb efficiency of20%PVDF/PMMA （6/4） fibre membrane was also very stable, and showed goodcycle performance. The heat shrinkage rate of20%PVDF/PMMA （6/4） fibremembrane coated by silica sol waslower than5%, and it had good dimensionalstability. Electrospinning was used to prepare the PVDF-B₂O₃fiber membranetentatively. The other kinds of composite fiber membranes, such asPVDF-HFP/PVDF-B₂O₃/PVDF-HFP and PVDF（acetone/DMF）/PVDF（acetone）were prepared by the alternate and coaxial electrospinning technology. The resultshowed that PVDF-B₂O₃nanofiber membrane had good coulomb efficiencystability about100%, but its capacity attenuated quickly; The three layercomposite membrane with sandwich structure can still show good cell cycleperformance under the10C rate. And its first discharge specific capacity couldreach about127mAh/g under the0.5C rate.