Microstructure Regulation,surface Inorganic Functionalization And Application Of Polyimide Nanofibers

Polyimide(PI),due to the combination of lightweight,high strength,high and low temperature resistance(long-term use temperature range from-269? to+300?),excellent dielectric properties,radiation resistance,chemical and dimensional stability,has been fabricated as film,coating,advanced composite material,fiber,foamed plastic,engineering plastic,adhesive and widely used in the field of chemical industry,electric and electronic,information,military,nuclear industry,aeronautics and astronautics.With the development of electrospinning,preparation of nanofiber has become efficient,convenient,and easily tailored.PI nanofibers have integrated the excellent properties of PI materials with the surface effect and size effect of nanofibers.Through further functionalization,PI nanofibers are expected to become a kind of high-performance and multi-functional nanofibers.In this dissertation,two kinds of PI nanofabric separators with the crosslinking structure were prepared by the alkaline-induced crosslinking method and thermally induced crosslinking method,respectively.By sol-impregnation method and reverse in situ hydrolysis method,PI nanofibers were composited with zirconia(ZrO2)and silica(SiO2),respectively,to prepare two kinds of PI nanofiber-based flexible ceramic separators.The thermal properties,mechanical properties and electrochemical properties of these four separators were characterized.PI nanofibers were functionalized with silver nanoparticles through direct ion exchange-in situ reduction method,and its enhancement effect as surface enhanced Raman scattering(SERS)substrate was characterized.In the alkali-induced crosslinking method,poly(amic acid)(PAA),as the precursor of insoluble and infusible PI,was used to prepare nanofiber membrane by electrospinning.Based on the mechanism of alkaline hydrolysis of PAA,the microstructure of PAA nanofabric was regulated via alkali-induced crosslinking and thermal imidization to prepare PI crosslinked nanofabric separator.The influence of the concentration of alkali liquor and induction time on the structure of PI nanofabric were investigated.The introduction of the crosslinking structure increases the tensile strength of the PI nanofiber membrane by 3.5 times and the initial thermal deformation temperature from 328? to 380?.The LiFePO4/Li half-cell assembled with this separator delivered a discharge capacity of 162.5 mA h g-1 at 0.1 C and maintained a high capacity of 124.5 mA h g-1 at 5C.The discharge capacity decreased from 146.2 mA h g-1 to 140.4 mA h g-1,and the capacity retention was 96%after 100 cycles at 1C.At 120? and 1C,the cell possessed excel cycling stability.The discharge capacity,rate performance,and cycling stability at both room and high temperature of the cell with PI crosslinked nanofabric separator are better than that with Celgard-2400,making it a good candidate as high-temperature Li-ion battery separator.In the thermally induced crosslinking method,soluble and fusible polyimide was used to prepare nanofiber membrane by electrospinning.Polyimide fabric separator with crosslinking structure was prepared by welding the joints among fibers through thermally induced crosslinking.The introduction of the crosslinking structure increases the tensile strength of the separator by 5 times.This separator shows better thermal dimensional stability than polyolefin separator.The LiFePO4/Li half-cell assembled with this separator delivered a discharge capacity of 154.5 mA h g-1 at 0.1C and maintained a capacity of 113 mA h g-1 at 5C.Its discharge capacity at high C rate is better than Celgard-2400 separator.More importantly,this separator exhibits thermal shutdown function at 260?,showing its potential application as the separator in safe Li-ion battery.Based on the good affinity between PAA and zirconium sol,the PI nanofibers were coated by the ZrO2 layer via sol-impregnation method to prepare the PI/ZrO2 composite nanofabric based flexible ceramic separator.The ZrO2 layer endowed PI nanofiber membrane with increased tensile strength from 8 MPa to 39 MPa,improved thermal dimensional stability and wettability(water contact angle was reduced from 1260 to 18°).The LiFePO4/Li half-cell assembled with this separator delivered a discharge capacity of 164.4 mA h g-1 at 0.1C and maintained a high capacity of 135.6 mA h g-1 at 5C.The discharge capacity decreased from 153.1 mA h g-1 to 150.4 mA h g-1,and the capacity retention was 98.4%after 100 cycles at 1C,showing its potential application as the separator in high-power Li-ion battery.PI/SiO2 composite nanofabric based flexible ceramic separator with special core(SiO2 nanoparticles doped PI)/shell(SiO2 coating)structure was prepared by the reverse in situ hydrolysis method.During the in situ hydrolysis,the SiO2 layer was fused at the joints of the nanofibers,and the crosslinking structure was introduced into the PI nanofiber membrane.Consequently,the tensile strength and modulus of the PI nanofiber membrane were increased by 5 times,and the thermal dimensional stability was further improved.Compared with pristine PI nanofiber membrane,the wettability and wetting rate of electrolyte were significantly improved.The excellent fire resistance of PI nanofiber membrane was given by SiO2 coating and SiO2 doping,and the interface properties between the PI nanofabric separator and electrodes were also improved.The LiFePO4/Li half-cell assembled with this separator delivered a discharge capacity of 166.3 mA h g-1 at 0.1 C and maintained a high capacity of 139.4 mA h g-1 at 5C.The capacity retained 98.4%after 100 cycles at 1C and room temperature.At 1C and 120?,the cell shows good cycling stability and good capacity retention of 99%.The above-mentioned excellent properties make PI/SiO2 composite nanofabric separator have good application potential in the field of high-power and high-safety Li-ion batteries.PI nanofibers coated by silver(Ag)nanoparticles were prepared by direct ion exchange-in situ reduction method,and factors influencing the preparation were investigated.With p-aminothiophenol(p-ATP)as the probe molecule,the enhancement effect of PI/Ag composite nanofiber membrane as a SERS substrate was characterized.The results show that the PI/Ag composite nanofiber membrane has excellent enhancement effect and good uniformity.When the concentration of p-ATP was as low as 10-14 M,there was still a significant signal on the substrate.It was proved that the PI/Ag composite nanofiber membrane was a simply prepared and excellent SERS substrate.