Study On Fabrication And Performance Of Poly(Phthalazinone Ether Sulfone Ketone)Fibrous Membrane For Lithium-ion Battery Separator

Electrospun fibrous membranes have been widely used in the research of novel lithium-ion battery separators,due to the advantages of large specific surface area,high porosity and wide range of material sources.The electrospun fibrous membranes can positively overcome the shortcomings of low porosity,weak electrolye wettability and insufficient thermal stability for traditional polyolefin separators.However,the mechanical property of electrospun fibrous membranes mainly depends on the weak bonding strength between fibers.The mechanical properties of electrospun fibrous membranes are quite low compared with the commercial polyolefin separators,and it is difficult to meet the tensile strength requirements during the winding and battery assembly process.Therefore,it is necessary to improve the mechanical strength of fibrous separators by using appropriate structural design and post-processing research without the loss of good electrochemical performance.Lithium-ion battery separators are mainly used to isolate the positive and negative electrodes of the battery,and conduct lithium-ions simultaneously.Generally.the separators are required to have enough thermal stability to prevent the direct contact with the positive and negative electrodes of the lithium-ion battery.while suffered abnormal high temperature.The direct contact with electrodes may lead to a short circuit of the battery,resulting in a serious fire or explosion disaster.From the perspective of thermal safety for lithium-ion battery,the thermal stability temperature of the separators should be as high as possible.It can maintain the normal charge/discharge cycle of the battery at a relatively high temperature,but not interrupt the exothermic reaction inside the battery.Uncontrolled temperature rise will eventually lead to a severe thermal runaway.Therefore.to further improve the thermal safety of lithium-ion battery,the separators are expected to quickly cut off the circuit when the battery is abnormally overheated.An ideal lithium-ion battery separator should have both high thermal stability and sensitive thermal shutdown performance.At the same time,the buffer temperature between the thermal shutdown temperature and the melting temperature of the separator should be as large as possible.In this dissertation,the heat-resistant and soluble poly(phthalazinone ether sulfone ketone)(PPESK)is chosen to fabricate the PPESK fibrous membrane as the lithium-ion battery separator by electrospinning method.The mechanical property of PPESK electrospun membrane are improved by the increase of fiber orientation,coaxial electrospinning and hot-pressing treatment.Besides.the fibrous separators with excellent thermal shutdown property are fabricated through coaxial electrospinning and multilayer composite methods.The modification of mechanical property and thermal shutdown property greatly improves the safety performance of lithium-ion batteries.The main research contents are as follows:(1).Randomly oriented PPESK fiber membranes and oriented PPESK fiber membranes were prepared using electrospinning technology,and the mechanical properties of PPESK membranes were effectively improved by hot-pressing treatment.After hot-pressed at 320?,the tensile strength of the PPESK membranes increases from 1.7 MPa to 19.8 MPa.But,the porosity and electrolyte uptake of the hot-pressed PPESK membranes are still as high as 78%and 720%,respectively,which are much higher than the commercial PP separators.The electrochemical impedance spectrum shows that the hot-pressed PPESK membranes exhibit higher ionic conductivity(2.38 mS/cm)and lower interface resistance(170 ?),compared with the commercial PP separators.The charge/discharge results of cells using hot-pressed PPESK fiber membranes as separators at different rates show excellent cycle performance and rate capability.In order to further improve the mechanical properties of PPESK fiber membranes.oriented PPESK fiber membranes are prepared by increasing the rotating speed of the collecting drum.Correspondingly,the tensile strength of oriented PPESK membranes along the fiber orientation direction is improved effectively.Then the oriented PPESK membranes are hot-pressed at 320? with two pieces of them overlaid vertically.The hot-pressed oriented PPESK composite membranes show a high tensile strength of 22.8 MPa in both longitudinal and horizontal directions.Compared with PP separators,the hot-pressed PPESK membranes also exhibit excellent thermal stability and superior electrochemical performance.(2)Coaxial PPESK/PVDF membranes are successfully prepared by electrospinning,and followed by hot-pressing near the melting temperature of PVDF shell fiber.After hot-pressed at 160?,the tensile strength of the coaxial PPESK/PVDF membranes increase significantly from 3.5 MPa to 21.0 MPa.The coaxial PPESK/PVDF membranes exhibit excellent thermal stability even at 200?.The porosity of the coaxial PPESK/PVDF membrane decreases after the hot-pressing,but due to the 3D network structure,the hot-pressed PPESK/PVDF membrane still have a high porosity of 80%and an electrolyte uptake of 805%.The results of the electrochemical impedance spectrum show that the hot-pressed PPESK/PVDF membrane has higher ionic conductivity(1.78 mS/cm)and lower interface resistance(182 ?)than that of the PP separator.The coin cells assembled with the hot-pressed PPESK/PVDF membranes also show good cycle performance and excellent rate capability.In addition,an ideal coaxial PPESK/PVDF fiber membrane with superior thermal stability and sensitive thermal shutdown function was prepared by adjusti,ng the flow rate of the core-layer PPESK solution and the shell-layer PVDF solution.The electrochemical impedance spectrum shows that the ionic conductivity of TS-PPESK/PVDF membrane decreases sharply from 1.35 mS/cm to 0.02 mS/cm after treated at 170? for 3 min.which indicates a timely thermal shutdown performance.The coin cells assembled with TS-PPESK/PVDF fiber membrane also show good charge/discharge performance and rate capability.(3)To further improve the thermal safety performance of lithium-ion batteries,the three-layer PPESK/PEO/PPESK membranes with timely thermal shutdown property and excellent thermal stability are prepared by electrospinning combined with hot-pressing method.Compared with the commercial PP/PE/PP separators,the PPESK/PEO/PPESK membranes exhibit higher porosity(74%),electrolyte absorption rate(755%)and thermally stable temperature(200?).The electrochemical impedance spectrum shows that the ionic conductivity of PPESK/PEO/PPESK membrane decreases sharply from 2.18 mS/cm at room temperature to 0.01 mS/cm after treated at 80? for 3 min,which indicates the sensitive thermal shutdown performance at 80?.Therefore,the PPESK/PEO/PPESK composite membrane is endowed with both lower thermal shutdown temperature and higher melting temperature,which can significantly enhance the thermal safety performance of lithium-ion batteries.