| Electrospinning technique is a large-scale preparation method of nanofiber along with simple process,higher efficiency,easy control of dimension and extensive adaption to raw materials,which has realized industrialized production and application in some areas.Currently,electrospinning technique which enables mass production of nanofiber has been classified as(multi-needle)needle and needleless typs.Compared with(multi-needle)needle electrospinning technique,needleless technique has great advantages such as higher output,no blocking and easy cleaning.However,current industrialized needleless electrospinning apparatus for producing nanofiber require higher spinning voltage,severely decreasing the energy utilization rate.In addition,the diameter of the resultant nanofiber is uneven and the uniformity of nanofiber membrane is poor.Fractal structure has self-similarity and Von Koch curve is the most simple fractal structure geometric figure which also has self-similarity.The number of triangle tip in Von Koch curve could increase with the increase of fractal level,therefore,using the self-similarity of the Von Koch curve,the electric field uniformity of the spinning electrode based on the Von Koch curve could be improved:Using the Von Koch curve mult-tip feature,the electric field intensity and productivity could be improved,meanwhile which can save energy.As a consequence,a needleless electrospinning with tips based on fractal theory was designed and developed in the research,a linear fractal structure electrospinning system experiment platform was created and the effect of the parameters including solution concentration,voltage and receive distance on fiber morphology was discussed.The high electric field intensity and uniformity electric field distribution was beneficial to product thinner and uniform thickness nanofiber,which can produce electrospun membrane with high porosity and uniform thickness compared with thicker naofiber and nonuniform larger thickness electrospun membrane.Lithium ion battery separator should have high porosity and uniform thickness,hence the electrospun membrane acting as lithium ion battery separator prepared by fractal structure electrospinning system was beneficial to the development of the porosity,electrolyte uptake,electrochemical property and cycling performance.Currently,polyolefin separator is the mainly lithium ion battery separator in the market,which possesses excellent mechanical property and chemical stability.However,the lower melting point and poor thermal stability may cause safety problem easily.In addition,the inferior wettability with liquid electrolyte is another disadvantage of the polyolefin separator.Therefore,in this paper a Al2O3@PI/PE/Al2O3@PI composite membrane was prepared based on fractal electrospinning apparatus through thermal calderning process with PE membrane,which has thermal shut down function.Although the Al2O3@PI/PE/Al2O3@PI composite membrane possesses good porosity and liquid electrolyte uptake,wettability,good electrochemical stability and cycling performance,but the weak mechanical property limited its application.For improving the mechanical property of the Al2O3@Pl/PE/Al2O3@PI composite membrane,the modified commercial PE membrane was employed to use in the composite membrane.Finally,the SiO2@PI/m-PE/SiO2@PI composite possesses thermal shut down function at low temperature,high porosity and better electrolyte uptake,superior wettability with liquid electrolyte,good electrochemical stability and cycling performance,meanwhile with excellent mechanical property.The specific content of this thesis research are as follows:Firstly,COMSOL finite element software was employed to simulate and research the electric field intensity and distribution of electrospinning system with first-level,second-level and third-level fractal structure.The second-level fractal spinneret was optimized based on the principle of relatively larger electric field intensity and uniform field distribution.Then second level circular type of Fractal spinneret and spiral type of Fractal spinneret were further simulated and analyzed,the results showed that the circular Fractal spinneret had better field intensity and distribution compared to spiral Fractal spinneret in the nonlinear Fractal electrospinning process.Secondly,experiments of electrospinning systems with linear second level fractal structures spinneret were made in order to verify the results of simulation researches.The feasibility of electrospinning system based on fractal theory in terms of large-scale electrospinning technique was further demonstrated.The mechanism of needleless elecreospinning technology was researched by discussing the influence of electrospinning solution concentration,voltage,and receiving distance on the electrospinning nanofiber.Finally,the parameters of the preparation of the PVDF nanofibers are as follow:concentration 10.41%,voltage 23.00 kV,receiving distance 22cm,and the average nanofiber diameter is 351nm,CV value is 15%.For further improved the electric field intensity and reduce the end effect,the spacing and height of the spinneret are researched.Relative location of the spinneret was employed to improve the electric field stability during the continuous production.At last the optimal model is as below:the spacing and the height difference are 40mm and 15mm relatively,the 2#and 4#spinnerets clockwise 36°Thirdly,the Al2O3@PI electrospinning nanofiber membrane was prepared by second level circular fractal structure spinneret.Then the Al2O3@PI/PE/Al2O3@PI three-layer composite membrane was prepared by Al2O3@PI electrospun membrane and PE membrane.The Al2O3@PI/PE/Al2O3@PI three-layer composite membrane except had high porosity,better electrolyte uptake and electrochemical property,also possesses the property of shutdown function at low temperature.Finally,to further to improve the mechanical property of the lithium battery separator,the SiO2@PI/m-PE/SiO2@PI composite membrane was prepared by modified PE membrane and SiO2@PI electrospun membrane fabricated by second level circular fractal structure spinneret.At last,the SiO2@PI/m-PE/SiO2@PI composite membrane as lithium ion battery separator except the good property and thermal shutdown function at low temperature,the mechanical property of the SiO2@PI/m-PE/SiO2@PI composite membrane was as high as commercial PP/PE/PP membrane.Specific results are as follows:porosity 78%,liquid electrolyte uptake 575%,contact angle 11°,strength 177.6 MPa,ion conductivity 0.94 mS/cm,interfacial impedance 72 Ω,electrochemical stability window 5.3V,first time charge-discharge capacity 166.1 mAh/g and 162.4 mAh/g.After 100 times charge-discharge,the discharge capacity was 135.6 mAh/g,and capacity retention was 83.5%。... |
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