Inorganic Modification Of Electrospun Poly(Vinylidene Fluoride) Separators And Their Application In Lithium-ion Batteries

Separator is a critical component in rechargeable lithium-ion batteries and it plays an important role in determining the interface structure and internal resistance. In addition, it directly affect the specific capacity, cycle performance, and safety of the lithium-ion batteries. Therefore, optimizing the performance of Separator is beneficial to improve the comprehensive property of the lithium-ion batteries. Currently, most commercial separators are polyole?n microporous membranes made from polyethylene(PE) and polypropylene(PP). These polyole?n separators have good mechanical strength and excellent chemical stability, but they also have the disadvantages such as low porosity, poor wettability, heat deformation and etc. Thus, in this paper, the poly(vinylidene fluoride)(PVDF) with good wettability for organic electrolyte was chosen as the matrix material, and electrospinning technique was chosen as processing method. Furthmore, different dimensions inorganic nanomaterials were added to modify the performances of PVDF nanofibrous membrane. The morphology, function group, crystallinity, and mechanical properties of membranes are investigated by scanning electron microscope(SEM), Fourier Transform infrared spectra(FT-IR), differential scanning calorimetry(DSC), and tensile test, respectively. The electrochemical performance and battery property of the composite membrane were tested in the form of a button battery. The main work is given as follows:First, the zero-dimensional material SiO2 is added to PVDF nanofibrous by in-situ composite. The porosity and electrolyte uptake of composite separator with 6wt% SiO2 is 87.48% and 421%. All the PVDF/SiO2 composite separators have high ionic conductivity at room temperature(? 2.5 mS?cm-1). The interface impedance between the PVDF/SiO2 composite separators and the lithium electrode reduce gradually with the increase of SiO2 content. The capacity of Battery used PVDF/SiO2-6% membrane as separator is higher and decay more slowly than the batteries used PP microporous membrane and PVDF membrane as separator. Battery used PVDF/SiO2-6% membrane separator has a high retention rate of capacity when the C rate increases.Secondly, the PVDF nanofibrous membrane is modified by the carboxylic carbon nanotubes(MWCNT, one-dimensional nanomaterial). All of the composite membranes have high porosity(86-89%) and electrolyte uptake(about five times of the PP microporous membrane). The result shows that incorporation of MWCNT increases the Tm of PVDF membrane, and reduces the melting enthalpy and crystallinity. The composite membrane with 0.5% of MWCNT shows a highest ionic conductivity at room temperature of 3.75 mS?cm-1. The results of electrochemical impedance spectra indicate that the interface impedance composite membrane with 0.1% of MWCNT is low(156 Ω), and the interface impedance of any other composite membrane is greater than 300 Ω. The electrochemical stability window of the PVDF/MWCNT membrane is higher than 5V, which can meet the requirements of lithium-ion batteries. Battery with PVDF/MWCNT-0.1% membrane has high capacity, stable cycle performance, and excellent C-rate performance.At last, two-dimension nanomaterial montmorillonite(MMT) is used to improve the performance of PVDF nanofibrous membrane. The porosity of the composite membranes is about 84%. The electrolyte uptake of the composite membranes is between 3 to 4 times of PP microporous membrane. The increase of interlayer spacing of MMT in compositions indicates that PVDF intercalates into the silicate layer of MMT. When the MMT content is 7%, the tensile strength and elongation-at-break of composite membrane reach the highest point, 3.32 Mpa and 65.1%. The composite membrane with 5% of MMT has the highest ionic conductivity(4.2 mS?cm-1) and the lowest interfacial resistance(97 Ω). Battery with PVDF/MMT-5% membrane has high capacity(147 m A?h?g-1), good cycle performance, and excellent C-rate performance.By comprehensive comparison, the mechanical property of PVDF nanofibrous membrane can be significantly improved by adding MMT. The capacity of Battery used PVDF/SiO2-6% membrane as separator at 0.2C is highest. Battery used PVDF/MWCNT-0.1% membrane separator has the highest retention rate of capacity at high rate.