| Aromatic heterocyclic polymer is a kind of polymer with outstanding heat resistance.Due to the excellent mechanical properties,high and low temperature resistance,chemical solvent resistance and good dielectric properties of the aromatic heterocyclic polymer,therefore,it is widely used in the fields of aerospace,chemical machinery,microelectronics,battery,military industry,and and other fields by the form of fiber,membrane,adhesives,composite material,nanofiber,coating and plastic.Polybenzimidazole?PBI?and polyimide?PI?are typical examples of aromatic heterocyclic polymers,showing excellent comprehensive performance.The aromatic heterocyclic polymer fibers have integrated the material and fiber characteristics,it is expected to achieve diversified applications by the functionalization of fibers and integration of advantages of different materials.In this paper,the aromatic heterocyclic polymer fibers about PBI and PI are studied.The first part of work is carried out around the complex spinning process and poor strength of high-performance PBI fiber.The novel PBI integrated spinning process is studied,and the ultrahigh strength PBI fiber is prepared.In the research process of PBI fiber,it is found that the characteristics of PBI meet the performance requirements of lithium-ion battery separator,so it is expected to be introduced into the research of separator to prepare high-performance composite separator.In the second part,the poor strength,loose structure and safety of PI nanofibers as lithium-ion battery separator are systematically studied.The main work includes the following four research systems.The self-bonding and micro-crosslinking technology is used to regulate polyamide acid?PAA?nanofibers to prepare high-strength PI nanofibers membrane,which improves the strength and structure stability of nanofibers membrane;Meanwhile,diversified applications of PBI is developed,and the application value of PBI in lithium-ion battery separator is proved;Therefore,in order to further optimize the performance of the PI nanofibers membrane,high-performance PBI material is introduced into the PI nanofibers to prepare the high-performance PI nanofibers composite membrane with ultrahigh strength,solving the problems of poor strength and loose structure of membrane;Based on the research of high-performance PI nanofiber composite membrane,PI nanofiber membrane of soluble and fusible system is used,and the advantages of the two nanofibers are integrated by in-situ welding technology.Thus,the high-safety PI nanofibers composite membrane with the shutdown function is prepared,which insures the safely running of battery.The related work are as follows:?1?To solve the problem of complicated spinning process and poor strength of PBI fiber,we have studied the spinning technology of PBI liquid crystal polymer solution,developed a new integrated spinning process for the first time,and prepared ultrahigh strength PBI fiber.PBI polymer liquid crystal solutions with different solid contents are synthesized by microwave reactor.The PBI liquid crystal solution as the spinning solution need to remove the bubbles,and the integrated spinning is carried out directly at high temperature.Studies have shown that a large number of hydrogen bonds in the PBI molecules will increase the viscosity of the polymer solution with the increase of solid content,making the polymerization reaction difficult to sustain and reducing the degree of polymerization.Therefore,the inherent viscosity of PBI fibers tends to decrease.The inherent viscosity of PBI fiber with 5%solid content reaches 2.25 dL/g,and the strength of the fiber is up to 0.91 GPa.Meanwhile,the thermal stability of PBI fiber in nitrogen reaches 600?,and the excellent flame-retardant property also are showed.Thus,the preparation of the high performance PBI fiber proves the feasibility of spinning technology.?2?To solve the problem of poor strength and loose structure of PI nanofiber membrane,we have regulated the micromorphology of PAA nanofibers membrane by self-bonding and micro-crosslinking technology and prepared high strength PI nanofiber membrane with porous-layer-coated morphology.The self-bonding and micro-crosslinking technology make the strength of PI nanofiber membrane as high as 2 8MPa.By adjusting the composition of the spray solution,the prepared PI nanofibers membrane shows outstanding wettability,and the nanofiber membrane exhibits excellent thermal stability and flame retardant self-extinguishing.The battery using the fabricated nanofibers membrane shows a higher discharge capacity.The discharge capacity of the battery is 157.7mA h g-1 at 0.1 C,and the discharge capacity of the battery is as high as 129.9mA h g-1 at 5 C.At the same time,the battery maintains excellent cyclic stability at the high temperature of 120?.Thus,the prepared PI nanofiber membrane can be used as a novel heat-resisting separator.?3?To verify the application value of high-performance PBI polymer in lithium-ion battery separator and confirm its excellent wetting,heat resistance and flame retardancy,we prepared the PBI microporous membrane with excellent comprehensive performance by using the small molecule imidazole monomer as the pore-making agent for the first time.By regulating the dosage of pore-forming agent,PBI microporous membrane is prepared by pore-forming technique.The contact angle of PBI microporous membrane toward water and electrolyte is far less than the contact angle of Celgard,confirming the excellent wettability of PBI microporous membrane.The size of PBI microporous membrane has not any change even after 300?,and the membrane shows obvious flame retardant.The battery using the PBI microporous membrane shows excellent discharge capacity from 0.1 C to 5 C.In addition,the capacity retention rate is almost 100%after 100 cycles.Therefore,we have verified that PBI has great application value as a new separator material.?4?To further optimize the performance of the PI nanofiber membrane,we have introduced PBI into the PI nanofibers by the form of adhesive coating.The high-performance PI@PBI nanofiber composite membrane with the ultrahigh strength is prepared by self-bonding and self-compression technology.Due to the existence of self-bonding technology,PI nanofiber is closely bonded and coated by PBI,and self-compression technology makes PI fiber membrane more compact.Thus,the strength of composite nanofibers membrane is up to 59 MPa,and the introduction of PBI further improves the wettability and flame retardancy of the separator.The discharge capacity of battery using the composite nanofibers membrane are about 158.6 mA h g-1 at 0.1 C and 130.2 mA h g-1 at 5 C.The rate capability is obvious better than that of the Celgard membrane.In addition,the battery shows excellent cyclic stability at 25? and 120?.Therefore,the prepared high-performance composite nanofibers membrane has laid a research foundation for the preparation of high-power lithium ion batteries.?5?To prepare the high security PI nanofibers composite membrane,the soluble and fusible PI nanofiber membrane is used as shut-down layer and the high-performance PI@PBI membrane is used as the structural supports,integrating the comprehensive performance of the different nanofibers membrane by the in-situ welding technology.Thus,we have prepared a high-safety PI@PBI/PEI/PI@PBI composite membrane with shutdown function and sandwich structure.Due to the existence of ultrahigh strength membrane as support structure and PEI nanofiber membrane with crosslinked structure,the prepared composite membrane shows good strength of 46.5 MPa,excellent wettability,super flame retardancy.The shutdown function at 235? can improve the safety of the battery.The battery assemblied by composite nanofibers membrane shows excellent discharge capacity,especially at 5C(132.4 mA h g-1),and cyclic performance is stable.Thus,the prepared composite nanofibers membrane can be used as advanced lithium battery separator with high power,high performance and high safety... |
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