Study On Salen-based Polymers And Their Composites For Lithium-ion Storage

As the commercial energy storage equipment,lithium-ion batteries(LIBs)have been widely employed as power sources in mobile devices,state grid and hybrid electrical vehicles because of their high energy density,long-term cycling life and environmental friendliness.Their electrochemical performances depend on the advanced anode materials,including inorganic and organic materials,which are one of the significant components of LIBs.Compared with inorganic materials,organic electrode materials have been widely utilized as anode for LIBs due to their environmential friendliness,sustainability and various structures.Among the numerous organic materials,the heteroatoms(N,O and S)in organic polymers exhibit outstanding electrochemical activity.Salen-based polymer(SP)is a class of two-dimensional or three-dimensional porous framework materials with rich heteroatoms.Moreover,the conjugated system in the organic polymers could accelerate the charge transfer to improve the electrochemical performances.When used as electrode materials,they have potential applications.However,SP as electrode materials of LIBs have been rarely studied and the storage mechanism is not clear.In this work,we focused on the design of SP and their composites as anode materials for LIBs,where the lithium electrochemical behavirors,storage mechanisms,and their diffusion and pseudocapacitance contributions have been studied.This main work contents and results of research are as following:1.Salen-polymers(SP-1 and SP-2)with abundant heteroatoms(N,O)and conjugated structure were constructured via Schiff-base reaction with different precursors.When employed as anodes for LIBs,SP-1 delivers a reversible specific capacity of 946.2 m A h g^-1at a current density of 2 A g^-1after 3500 cycles,while SP-2delivers a reversible capacity of 155.7 m A h g^-1at a current density of 2 A g^-1after4000 cycles.The high reversible capacity of SP-1 is ascribed to the activated benzene rings in organic skeleton,which is involved in an electrochemical process with 14electrons.Compared to SP-1,SP-2 exhibits a robust rate performance due to its large conjugated skeleton that could accelecrate the electron transfer.Based on the mechanism study,the results indicate that C=N,C-O and benzene rings in organic skeletons are responsible for the excellent electrochemical performances or SP-1 and SP-2.2.Reduced graphene oxide(RGO)/SP(SPRG)with nanosheet structure was constructed via in-situ polymerization reaction.The structure and electrochemical performances of SPRG were regulated by different mass of RGO,where SPRG-0.6,SPRG-0.8 and SPRG-1.0 were fabricated from the reactants with different RGO ratios of 60%,80%and 100%,respectively.When used as anodes for LIBs,SPRG-0.6,SPRG-0.8 and SPRG-1.0 exhibit high reversible capacities of 433,684.5 and 359 m A h g^-1at a current density of 0.05 A g^-1after 100 cycles,respectively,indicating their high reversible specific capacity.Moreover,SPRG-0.6,SPRG-0.8 and SPRG-1.0deliver high reversible specific capacities of 710.2,814.9 and 554.9 m A h g^-1at 3 A g^-1after 2400 cycles,respectively,indicating the excellent cycling stability of SPRG.3.RGO/SP/Co₃O₄(SPCRG)was constructed via one-pot approach.The structure and electrochemical performances of SPCRG were regulated by different mass of RGO,where SPCRG-0.1,SPCRG-0.3 and SPCRG-0.5 were fabricated from the reactants with different RGO ratios of 10%,30%and 50%,respectively.When utilized as anode for LIBs,SPCRG-0.1,SPCRG-0.3 and SPCRG-0.5 deliver high reversible specific capacities of 680.1,832.4 and 586.9 m A h g^-1at 0.05 A g^-1after 90cycles,respectively,indicating the outstanding reversible specific capacities of SPCRG.Moreover,SPCRG-0.1,SPCRG-0.3 and SPCRG-0.5 exhibit robust rate performances and stable long-term cycling performances with high specific capacities of 378.4,664 and 547 m A h g^-1at 3 A g^-1after 1100 cycles,suggesting the excellent cycling stability of SPCRG.Based on the mechanism study,C=N,C-O and benzene rings and Co₃O₄in SPCRG could be responsible for the excellent electrochemical performances.4.Fe-MIL-88B-NH₂/SP/Co₃O₄(FMNSPC)was prepared via a pot method.During the synthesized processes of FMNSPC,the stable C=N was obtained via Schiff-base reaction between–NH₂in Fe-MIL-88B-NH₂(FMN)and the C=O in SP,which could improve the cycling stability of the composties.The structure and electrochemical performances of FMNSPC were regulated by different mass of FMN,where FMNSPC-0.8,FMNSPC-1.0 and FMNSPC-1.2 were fabricated from the reactants with different FMN ratios of 80%,100%and 120%,respectively.When used as anode for LIBs,FMNSPC-0.8,FMNSPC-1.0 and FMNSPC-1.2 show outstanding reversible specific capacities with 496.1,883.3 and 539.5 m A h g^-1at 0.05A g^-1after 80 cycles,respectively.Compared with FMNSPC,Fe-MIL-88B-NH₂and SPC show low reversiable storage capacities of 92.7 and 131.2 m A h g^-1,respectively.Moreover,FMNSPC show robust rate performances and stable long-term cycling performance,which are ascribed to its unique architecture.