Preparation Of Inorganic Modified Seperator For Lithium Ion Battery By An Electrospinning Method

Nanofiber membrane prepared by an electrospinning method has great potential in polymer lithium ion battery due to its high porosity and ionic conductivity. As a common material for electrospun seperator, Poly (vinylidenefluoride)(PVdF) has good chemical and electrochemical stability, as well as excellent wettability for organic electrolyte. However, without strong bonding between fibers, electrospun PVDF membrane has poor mechanical strength which can’t meet the requirements for assembly of batteries, so its application has been limited to a great extent.In this paper, nanoparticles including SiO2and TiO2were added to modify PVDF spinning solution to improve the mechanical strength of electrospun PVDF seperator, taking advantage of the interaction between the polar groups on the surface of ceramic fillers and the fluorine atoms of PVDF molecular chain to enhance the bonding of fibers. Meanwhile, the presence of ceramic fillers can reduce the crystallinity of PVDF, which is benificial to the increase of electrolyte uptake and ionic conductivity of electrospun seperators.The morphology of seperators was characterized by Scanning Electron Microscopy (SEM), and the fiber diameter distribution was also calculated using an image analysis software. Differential Scanning Caborimetry (DSC) measurements were carried out to investigate seperator’s thermal properties such as melting point, melting enthalpy and crystallinity. The mechanical strength was determined by tensile tests. The ionic conductivity was determined by AC impedence method, and the decomposition potential was determined by Linear Sweeping Voltammetry (LSV).First, the effects of parameters including solvent, concentration of solution and electrospinning voltage on the morphology and mechanical properties of PVDF seperators were investigated. The optimization condition is a15wt%PVDF solution using DMAc/acetone (3:7, by volume) as solvent, electrospun under a voltage of25kV. PVDF seperator obtained under this condition has a typical fiber interconnected porous structure without beads and drops, and its tensile strength and elongation are0.7MPa and35.3%, its ionic conductivity at room temperature is3.9mS/cm.Second,3wt%oleophilic and hydrophilic SiO2were directly added into PVDF spinning solution to prepare PVDF/SiO2composite seperators. The tensile strength of compostite seperators containing oleophilic and hydrophilic SiO2were0.4MPa and1.4MPa, respectively, indicating that hydrophilic SiO2has better dispersibility in PVDF spinning solution. Ultrasonic treatments were taken to facilitate the dispersion of oleophilic SiO2in PVDF spinning solution, and the tensile strength of oleophilic composite seperator increased to1.4MPa after ultrasonic treatment of15minutes.Third, PVDF/SiO2composite separators were prepared by electrospinning of PVDF solution containing in-situ generated SiO2by the hydrolysis of tetraethyl silicate (TBSi). The tensile strength and breaking elongation of composite seperaor with6wt%SiO2were2.1MPa and108.8%. The polymer electrolytes based on it showed a high ionic conductivity at room temperature of4.4mS/cm, and cells using it showed higher discharge capacity and cycle performance than cells using polymer electrolytes based on PVDF.At last, PVDF/TiO2composite separators were prepared by electrospinning of PVDF solution containing in-situ generated TiO2by the hydrolysis of tetrabutyl titanate (TBTi). The tensile strength and breaking elongation of composite seperaor with5wt%TiO2were2.3MPa and121.7%, with increase rate of228.6%and244.8%compared to PVDF. The polymer electrolytes based on it showed a high ionic conductivity at room temperature of5.1mS/cm, and cells using it showed higher discharge capacity and cycle performance than cells using polymer electrolytes based on PVDF and PVDF/SiO2composite seperator.