Study Of Polyarylether Separator For Lithium-ion Battery Prepared By Electrospinning Technique

A separator is one of the important components of the Lithium-ion battery. The properties of the separator decide not only the electrochemical performance, but also the safety of the battery. Currently, polyolefin microporous membranes have been used as major separators for Lithium-ion batteries. But rate capability of the battery assembled with this separator is not enough for high power application due to poor thermal stablility, low wettability and low porosity of the polyolefin microporous membrane. Novel soluble Poly(phthalazinone ether sulfone ketone) resins have excellent thermal resistance properties and good wettability in liquid electrolyte, therefore are used to prepare novle separators for Lithium-ion batteries by electrospinning technique in this paper. Moreover, in our experiment Poly(vinylidene fluoride) resins were used for preparation of PPESK/PVDF/PPESK sandwiched composite separators to further improve the thermal safety of electrospun PPESK separators. The influences of electrospinning process parameters on morphology, structure, mechanical performance and ionic conductivity of the electrospun PPESK separator were invesgated. And the effects of materials’characteristic and porous structure of the separators on electrolyte uptake, electrochemical performance and thermal safety were disscussed.In this paper, appropriate porous structure was obtained to improve mechanical performance and ionic conductivity of electrospun PPESK fabrics separators by selecting suitable electrospinning parameters during electrospinning process, including solution concentration, voltage and collecting velocity. Morphology and tensile strength of the separator were characterized by scanning electron microscopy (SEM) and tensile testing, respectively. The influences of electrospinning parameters on fiber diameter, orientation, pore size and mechanical behavior were disscussed. Porosity and electrolyte uptake of the separator were tested. AC impedance spectroscopy was used to characterize the ionic conductivity. The influence of micro-structure on electrolyte uptake and electrochemical performance was analyzed. The results indicate that electrospun fiber diameter and pore size of the separator are increased by increasing solution concentration and decreasing voltage and collecting velocity. Large pore size structure contributes to the improvement of permeability, porosity, electrolyte uptake and ionic conductivity of the electrospun separator. But too large pore size can induce electrolyte leakage and ionic conductivity decrease. Increasing collecting velocity appropriately can increase jet strengthing effect during electrospining process, which is benifical to improve the orientation of fibers and enhance the tensile strength of electrospun fabrics.PVDF resins have suitable melting temperature and excellent electrochemical property, therefore are used to prepare PPESK/PVDF/PPESK sandiwiched composite separators for futher improving thermal safety of PPESK separator in this paper. Deferential scanning calorimetry (DSC) was used to evaluate thermal properties of PPESK/PVDF/PPESK separator. AC impedance spectroscopy was used to characterize the ionic conductivity of the composite separator before and after thermal treatment. The influences of the component of composite separator on ionic conductivity, thermal dimensional stability and shut-down characteristic were analyzed. The results indicate that PPESK/PVDF/PPESK composite separator with4:3mass rate between PPESK and PVDF has higher ionic conductivity and better thermal dimensional stability than other mass rates. And shut-down characteristic is also observed for this PPESK/PVDF/PPESK composite separator.Linear sweep voltammetry (LSV), AC impedance spectroscopy and charge-discharge testing were used to characterize the electrochemical stability, analyze the ionic conductivity mechanism and evaluate the capacity, rate discharger properties and cycling properties of the separators, respectively. The results indicate that electrospun PPESK separator and PPESK/PVDF/PPESK sandiwiched separator display good electrochemical stability. The ionic conductivity of the both separators is higher than5.94×10-3S/cm at room temperature. The ionic conductive behavion of the two separators/electrolyte systems is in accord with Arrhenius equation. The cells assembled with these two separators exhibite high discharge capacity. The initial discharge capacity of the cells can reach to143mAh/g, which is increased by40%than that of the cells assembled with Celgard(?)2325. And the cells with the both kinds of separators can retain more than84.7%of initial discharge capacity after100times cycling charger-discharge process. Moreover, discharge rate capacity and charge-discharge effectivity of the cells are improved due to being assembled with PPESK separator or PPESK/PVDF/PPESK sandiwiched separator.