Preparation And Characterization Of Electrospun PVDF-based Membrane As Lithium Battery Separator

Lithium-ion battery is widely used in many areas, including military affairs, wireless communication, livelihoods, etc.,due to its high energy density, high working voltage, non-memory effect, etc.. Separator is a critical component in lithium-ion battery, which can prevent physical contact of cathode and anode. Meanwhile, it also provides a migration channel for ions. The properties of separator play an important part in the comprehensive performance of lithium-ion battery by determining the interfacial structure, bulk resistance and so on.Nanofiber membrane produced by eletrospinning technique is one of the ideal materials for lithium-ion battery, attributed to its small pore size, high porosity and electrolyte uptake. In this paper, nanofibrous PVDF (Poly(vinylidene fluoride)) membrane were prepared by electrospinning. To successfully fabricate high performance lithium-ion battery separator, three methods were introduced to improve properties of PVDF nanofibrous web, which were thermal treatment, blending electrospinning and laminating. The main work was as follows:Firstly, pure PVDF membranes were fabricated by electrospinning technique, whose fiber morphology was studied by SEM (scanning electron microscopy). The influential factors of PVDF fiber morphology and diameter distribution in electrospinning were systematically discussed. The results showed that the optimum electrospinning parameters of PVDF was as follows:electrospinning solvent was a mixed solvent (DMAc(N,N-Dimethylacetamide)/acetone,7/3by volume), electrospinning solutions concentration was12wt.%, applied voltage was16KV, the distance between the needle tip and collector was20cm, and feed rate was0.4ml/h. The electrospun PVDF fiber diameter distribution was about100-500nm.Secondly, thermal treatment was introduced to improve the mechanical property and dimensional stability of the electrospun PVDF membrane. Characterizations, including SEM, DSC (differential scanning calorimeter) and WAXD (wide angle X-ray diffraction) were used to investigate morphology, crystalline structures, porosity, electrolyte uptake (EU) and ionic conductivity of the electrospun PVDF membranes before and after thermal treatment, respectively. The results showed that thermal treatment could notably increase the tensile property of the electrospun PVDF membrane, was enhanced by192%, from3.25MPa to9.50MPa, while the ionic conductivities were still higher than10-3S/cm.Thirdly, PVDF/PMMA (poly(methyl methacrylate)) composite membranes were fabricated by electrospinning with different mass ratio of polymer solution. The electrospun composite membranes were investigated using SEM, FTIR (Fourier transform infrared), porosity, EU, ionic conductivity, interfacial stability and other measurements. The results showed that when the mass ratios of PMMA lower than30wt.%, the electrospinning could be processed smoothly. With the increase of PMMA concentration, fiber diameter distribution became wider and surface of nanofibers were rougher. By with the increase of PMMA membrane, the crystallinity of PVDF/PMMA electrospun composite membranes decreased, while EU increased. The mechanical properties of PVDF/PMMA composite membrane were better than that of pure PVDF membrane, especially with the weight ratio of PVDF/PMMA became85:15, tensile strength of composite membrane reached the highest point,10.79Mpa. The electrospyun composite membranes exhibited high ionic conductivity, good electrochemical stability, whose highest ionic conductivity reached2.72×10-3S/cm at room temperature. The charge/discharge measurements were carried out with Li/LiFePO4cells by using PVDF/PMMA (85/15) as separator. The results showed the first discharge capacity was157.9mAh/g at0.2C-rate and after50cycles, the loss of capacity was less than10%, exhibiting a good cycle performance.Finally, PMMA/PVDF/PMMA laminating membranes were prepared based on combination of electrospinning and laminating method. The morphology of pure PMMA electrospun membrane was studied by SEM. The results showed that4wt.%was the most suitable concerntration for PMMA electrospinning. The physical properties, electrochemical properties, and battery performance of the battery used PMMA/PVDF/PMMA membrane as separator were tested. The results showed that PMMA/PVDF/PMMA membrane exhibited an extremely high EU (497%), superb ionic conductivity (3.63×10-3S/cm), low interface insistence (45Ω), wide electrochemical stability window (5.3V), which met the requirements of lithium ion battery excessively. Additionally, battery used PMMA/PVDF/PMMA membrane as separator showed the battery used PMMA/PVDF/PMMA membrane as separator showed a good charge and discharge performance, with the first discharge capacity of147.7mAh/g. And after20cycles, the capacity lost less than4.2%.