| The current rapid economic and social development and the sharp increase in energy demand are accompanied by important challenges such as the depletion of fossil energy and the deteriorating ecological environment.Lithium-ion batteries(LIBs)have become prominent energy storage devices for non-fossil energy routes due to their outstanding advantages such as high energy density,long cycle life,wide operating temperature range,and clean low-carbon energy,and more than 60%market share of the battery industry in transportation,smart grid and other fields.However,commercial LIBs usually use inorganic compounds such as metal compounds and graphite as electrode materials.The materials come from limited ore sources.They have high extraction energy consumption,heavy pollution,difficult recovery,and high cost.They also have low theoretical capacity or actual capacity close to the upper limit.And the inherent brittle mechanical characteristics are difficult to meet the rapid development of flexible electronic needs.Many challenges faced by inorganic electrode materials require urgent development of new green electrode materials to build high-performance LIBs.Polyimide(PI)has the characteristics of carbonyl electrochemical activity,fast reaction kinetics,high thermal stability,rich raw materials,designable structure,high mechanical strength,easy synthesis and flexible processing,and has become the most promising organic electrode material.In this paper,a series of PI electrode materials are prepared from monomer molecule selection,chain structure design,topology structure adjustment,aggregation state and micro-morphology control,and synthesis method optimization.And through cyclic voltammetry,constant current charge and discharge and electrochemical impedance and other electrochemical testing techniques,to study the relationship between its intrinsic structure,microstructure and lithium storage performance,lithium storage mechanism and regulation law.Seeking new ideas and new strategies to solve the challenges faced by the current PI electrode with fewer carbonyl active sites,limited electrochemical utilization,and low specific capacity.Its innovative work includes the following three aspects:(1)A new strategy of polymerization method design and regulation of PI microstructure and its electrochemical performance was established.Using the same naphthalene-1,4,5,8-tetracarboxylic acid(NTCDA)and 1,3,5-Tris(4-aminophenyl)benzene(TAPB)as reactive monomers,they were prepared by hydrothermal polymerization,conventional reflux method and solventhermal polymerization.The PIs with different shapes are named as h PI,c PI and s PI,respectively.The results show that c PI(10-30 nm)and s PI(50-100 nm)are in the form of nanoparticles,while h PI is a micrometer tubular structure with average outer and inner diameters of 184 and 104 nm,respectively,and has more excellent electrochemical performance.Taking h PI as the cathode material of LIBsl,at 1.5-3.5 V and25 m A g-1,the discharge capacities of h PI,c PI and s PI are 74.4,45.8 and 23.6 m Ah g-1,respectively.When increased to 1000 m A g-1,the initial capacity retention rate was 75.9,43.7and 53.0%,respectively,h PI showed higher specific capacity and rate performance;After 100cycles at current density of 25 m A g-1,the capacity retention rates of h PI,s PI,and c PI were90.2,72.2 and 72.9%,respectively.And after charging and discharging 15,000 times at current density of 1.5 A g-1,the capacity of h PI remains above 70%,and h PI has higher cycle stability.When h PI is used as the anode material,at 0.01-3.0 V and 100 m A g-1,the discharge capacity of the first cycle is up to 1662 m Ah g-1,and the reversible capacity is 630 m Ah g-1.The h PI can be used as the cathode and anode materials of LIBs at the same time,making it very promising in the field of all-organic soft-pack flexible batteries.(2)A general and simple green hydrothermal polymerization method was developed to prepare PI with different monomer sources,and the relationship and influence law between the macromolecular chain structure,micro-morphology and lithium storage performance were studied.Using NTCDA and three-type different triamine monomers,e.g.TAPB,tris(4-aminophenyl)amine(TAPA)and tris(2-aminoethyl)amine(TAEA)for hydrothermal polycondensation,and the obtained corresponding PIs are marked as PI-PB,PI-PA and PI-EA,respectively.It was found that PI-PA is a micron-level thick layer(?1.0?m)structure,PI-PB is a hollow tubular structure with a micron length and a thickness of about 40 nm,and PI-EA is a fibrous network structure with a nanometer diameter.Used as a cathode material,at current density of 25 m A g-1,the capacities of PI-PB,PI-PA and PI-EA are 80.5,28.5 and 118m Ah g-1,respectively,and the corresponding carbonyl utilization rates are 70,20 and 72.5%,respectively.When increased to 1000 m A g-1,the capacity retention rates of PI-PB,PI-PA,and PI-EA are 70.2,47.7 and 54.2%,respectively;After 100 cycles of 25 m A g-1,the capacities of PI-PB,PI-PA and PI-EA are 75.4,26.5 and 105.9 m Ah g-1,respectively,and the corresponding retention rates are 90,79.6 and 93%,respectively.Used as an anode material,at current density of 100 m A g-1,the capacities of PI-PB,PI-PA and PI-EA are 489.4,307.5and 491.1 m Ah g-1,respectively;When increased to 5.0 A g-1,the capacity retention rates of PI-PB,PI-PA and PI-EA are 15,43.9 and 27.8%,respectively.PI-EA has higher capacity and cycle performance are attributed to the high specific surface area conferred by the three-dimensional nanofibrous network,while PI-PB's superior rate performance is due to the one-dimensional tubular structure and strong?-conjugation effect.(3)A mixed solvothermal method for preparing covalent organic framework(COF)PI was developed,and the internal relationship and influence law of triamine monomer structure,COF microstructure and lithium storage performance were studied.In the mixed solvent of N-methylpyrrolidone,trimethylbenzene and isoquinoline,NTCDA anhydride reacts with TAPB,TAPA and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine(TAPT)to produce crystalline PI-COF-PB,PI-COF-PA and PI-COF-PT.It was found that PI-COF-PB and PI-COF-PA had a granular shape,while PI-COF-PT had a three-dimensional nanofiber network structure.When used as a cathode material,at 1.5-3.5 V and 25 m A g-1,the first-cycle discharge capacities of PI-COF-PB,PI-COF-PA and PI-COF-PT are 70.6,56.9 and 78.6 m Ah g-1,respectively.When increased to 1000 m A g-1,the capacity retention rate was 54.7,49.2 and 46.7%,respectively.After 100 cycles of 25 m A g-1,the capacity retention rates of PI-COF-PB,PI-COF-PA and PI-COF-PT were 85.6,94.4 and 83.9%,respectively.When used as a anode material,at0.01-3.0 V and 100 m A g-1,the first-cycle discharge capacities of PI-COF-PB,PI-COF-PA,and PI-COF-PT are 1503.7,590.3 and 995.9 m Ah g-1,respectively.And as the charge-discharge cycle proceeds to 200 cycles,the capacity of the three PI-COF anode electrodes continues to rise,due to the increase in electrochemical activation and active sites for lithium insertion. |
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