Preparation And Electrochemical Properties Of Carbon-Based Composite Anode Materials Derived From Polyimide

In recent years,with the increase in energy demand and the rapid depletion of fossil fuels,the exploration of sustainable energy sources and the development of corresponding electrochemical energy storage/conversion technologies have been continuously strengthened.These technologies include rechargeable batteries,supercapacitors and fuel cells for electric vehicles(EVs),portable electronic devices and stationary power plants.Among several rechargeable batteries,lithium-ion batteries(LIBs)have the advantages of relatively high energy density,power density,working voltage,long cycle life,low self-discharge rate and little environmental pollution,and have good development prosects.In addition,LIBs are flexible in design,suitable for a wide range of portable electronic devices,and can be manufactured in various sizes and shapes to effectively fill the available space.However,in recent years,with the rapid rise of new energy vehicles,the performance requirements of LIBs have been significantly improved.Therefore,it is very important to develop electrode materials with high specific capacity and good cycle stability.Organic carbonyl electrode materials,as high energy density electrode materials for environmentally friendly metal ion batteries without heavy metals,have the advantages of high theoretical capacity,adjustable redox center and flexible structural design,but their low electronic conductivity and poor battery rate performance limit their application.In this thesis,nitrogen-doped carbon materials with excellent electrochemical properties were prepared by doping and carbonization based on polyimide.The main contents of this thesis are as follows:(1)Polyamic acid(PAA)was used as a precursor,coating film,soaking in deionized water,and annealing at 700°C,800°C,900°C,and 1000°C under inert gas to obtain carbon with a three-dimensional three-dimensional porous structure material.Carbon material was used as the negative electrode of a lithium-ion battery,and the effect of annealing temperature on its electrochemical performance was studied.It is found that compared with other annealing temperatures,the initial discharge capacity of the porous carbon material annealed at 800?is 549 m Ah g^-1,and the discharge capacity of 192 m Ahg^-1 can still be retained after 1000cycles at 500 m Ag^-1,which shows the best electrochemical performance.The carbon material obtained after annealing at 800?is not completely graphitized,but the PAA still retains more nitrogen after being calcination by imidization,which increases the structural defects of the carbon material and improves the conductivity of the carbon material.On the other hand,the firm pore structure has a large specific surface area,which increases the reaction area of active sites,so that the electrochemical reaction can proceed in an orderly manner.(2)Carbon nanofibers with uniform thickness and Fe₃O₄ particles were prepared by electrospinning and carbonization of PAA and Fe(acac)₃ with different qualities.In the electrochemical test,Fe₃O₄ provides the main specific capacity in the cycle,but the volume of Fe₃O₄ will expand and crush during the electrochemical reaction,which will aggravate the irreversible reaction of the negative electrode.Carbon nanofibers will limit the volume expansion of Fe₃O₄ to a certain extent,but some Fe₃O₄ nanoparticles exposed on the surface of carbon nanofibers will still make the reaction irreversible.The test results show that compared with other doping amounts,when the mass ratio of PAA to Fe(acac)₃ is 1:3,the Fe₃O₄@CNF anode material has a high initial discharge capacity of 1290 m Ah g^-1 and a discharge capacity of 377 m Ah g^-1 after 500 cycles at 500 m A g^-1,which shows good electrochemical performance.(3)To solve the problem of volume expansion of Fe₃O₄ in carbon nanofibers,we put the nanofibers obtained by electrospinning into methanol solution containing dimethyl imidazole,so that Fe³⁺in ferric acetylacetonate coordinates with it to form crystals,and firmly fixes Fe³⁺in carbon nanofibers,thus ensuring the stability of the electrochemical reaction.Electrochemical tests show that the prepared Fe₃O₄@CNF material has an initial discharge capacity of 1482 m Ahg^-1and a capacity of 381 m Ahg-1 after 550 cycles at 1000 m A g^-1.Compared with Fe₃O₄@CNF,the capacity and cycle performance of Fe₃O₄@CNF are improved.