Design And Synthesis Of Functional Group-containing Polyimide And Its Application As Lithium-ion Battery Separator

Compared with other rechargeable batteries,lithium-ion batteries with high energy density,long cycle life,low price and environmental friendliness play a vital role in the fields of digital electronic products.In recent years,with the rapid developing of transportation,aerospace and stationary energy storage fields,lithium-ion batteries have become increasingly indispensable in the above fields.However,the lower energy and power densities are the limitations in the developing of lithium-ion batteries.Separator,as one of the indispensable parts of lithium-ion batteries,has great impacts on the electrochemical,cycle life and safety performance of the batteries.Polyolefin,owning to high mechanical properties and chemical stability,has often been used as separator.However,the separator exhibits limited capability in retaining electrolyte during cycle process was attributed to the smaller porosity and worse affinity to liquid electrolyte.Furthermore,polyolefin separators with lower thermal stability that prone to cause internal short circuit within electrodes and even explosion at higher temperatures.To solve the aforementioned problems,researchers have proposed to high requirements placed on the separator material.Compare with polyolefin separator,polyimide(PI)nanofiber membrane has becoming an ideal separator material of lithium-ion batteries because of the higher porosity,electrolyte wettability and excellent thermal stability.In this work,we have successfully obtained PI nanofiber membrane with carboxyl group on the surface by molecular structure design and alkaline solution hydrolysis.And the process without adding additional weight and thickness of nanofiber membrane.The method of modifying carboxyl groups on PI nanofiber membrane improves its polarity,which provides greater wettability with polar liquid electrolyte.In addition,the adjustable pore structure of the PI nanofiber membrane which effective increasing the mechanical properties and the operability during the process of battery assembly.According to the designable molecular structure of PI,the polycondensation was reacted under dianhydride and contained carboxyl structure diamine to obtain polyimide which modified with carboxyl groups.After inducing with the ammonia water,the carboxyl group on the nanofiber surface were successfully improved and achieved the functionalization on the surface of PI nanofiber membrane.In addition,the cross-linking structure was formed to improve the mechanical strength,wettability,glass transition temperature and thermal stability of the PI-COOH nanofiber membrane.Compared with the Celgard separator,PI-COOH nanofiber membrane has higher ion conductivity(3.05 mS·cm-1)as lithium ion battery separator.The specific discharge capacity could reach 124.1 mAh.g-1 at 5C rate and higher cycle stability(capacity retention of 99.3%after 100 cycle at 1C).In addition,a novel method of using alkaline solution hydrolysis to make ring-opening reaction of imide ring on the surface was developed to obtain carboxyl groups modified PI nanofiber membrane.The mechanical properties of PI nanofiber membrane were also raised from 12.9 MPa to 31.2 MPa due to the cross-linking structure.Wettability and thermal stability of PI nanofiber membrane were also obviously improved.Compared with the Celgard separator,the etching PI nanofiber membrane has higher ion conductivity(3.13 mS·cm-1),discharge specific capacity(121.1 mAh.g-1 at 5C rate)and cycle stability(capacity retention is above of 97%after 100 cycles at 1C).