Green Fabrication And Electrochemical Energy Storage Research Of Chitin-based Electrode Materials

The ever-growing energy demands have driven a research wave on alkaline metal ion batteries.Biomass,as an abundant and renewable resource on earth,has become increasingly attractive in the development of sustainable functional materials.Natural polymers play an important role in developing sustainable carbon and carbon-confined composites for alkali metal ion batteries anodes.In recent years,natural polysaccharides including cellulose,sodium alginate,carrageenan,and others have been widely exploited to construct carbon and carbon-confined composites,showing high specific capacity,superior rate performance and long cycle stability.Chitin is the second earth-abundant natural polysaccharide that contains rich N/O functional groups and be regarded as an important precursor for preparing carbon-based storage materials.Nevertheless,limited by its disadvantages of unfavorable processing ability and relatively poor metal cation chelating capability,it is unable to achieve effective precursor structural regulation,and thus fail to synthesize high-performance carbon and carbon-confined materials.In this work,a series of chitin-based carbon and carbon-confined composites were successfully fabricated by harnessing chitin and their derivatives as carbonaceous precursors,focusing on the effective utilization of their typical features,including chitin-Ca CO₃ mineralized structure in shrimp shells,the strong metal ion chelating ability and water solubility of carboxymethyl chitosan,and the easy assembly and processing capabilities of low molecular weight chitosan oligosaccharides.The structure and properties of these carbon-based materials were characterized by X-ray diffraction(XRD),Raman spectroscopy(Raman),X-ray photoelectron spectroscopy(XPS),infrared spectroscopy(FTIR),thermal gravimetric analyzer(TGA),scanning electron microscope(SEM),transmission electron microscope(TEM),nitrogen adsorption isotherm instruments,and so on.The battery testing system and electrochemical workstation measurements were performed to evaluate their potential applications in lithium/sodium/potassium-ion batteries(LIBs/SIBs/PIBs).These basic researches provide important scientific basis for the development and application of chitin-based carbon and carbon-confined composites.The innovation points of this thesis are as follows:(1)N/O co-doped hard carbon porous nanobelts were effectively constructed via self-template assisted pyrolysis strategy by utilizing the chitin-Ca CO₃ mineralized structure in discarded shrimp shells,and it was proved that they can be used as anode materials of PIBs and potassium dual-ion hybrid batteries(PDIBs);(2)3D nanofibrous carbon-confined metal oxides were constructed by assembling metal-carboxymethyl chitosan frameworks and annealing,which could be extended as an universal synthesis strategy;(3)The 3D nanofibrous carbon-confined Co₃O₄ constructed by above strategy was used as the candidate to evaluate their potential applications in LIBs/SIBs;(4)Two-dimensional(2D)carbon-confined Fe-Co Se₂ were constructed by microwave-assisted catalytic carbonization of mixed solution of chitosan oligosaccharide and metal salts,and proved that they show great potential in PIBs anodes.The main research contents and conclusions of this project consist of the following four parts.Using shrimp shells as raw materials,the protein and pigment were removed by5%Na OH and 10%H₂O₂aqueous solutions in sequence to obtain chitin-Ca CO3mineralized skeleton.Subsequently,the purified mineralized skeleton was carbonized at 650?under argon atmosphere.After the Ca CO₃ templates were removed,the NOCNBs were synthesized.The natural nano Ca CO₃ act as the self-templates to obtain robust and porous hard carbon frameworks,and the abundant nitrogen-and oxygen-containing functional groups contributed by chitin could provide the N and O resources for in-situ heteroatom doping.The NOCNBs consisted of oriented carbon nanotube arrays possess multiple structure with hierarchical micro/meso/macro-pores,high level N/O dual doping,and large interlayer spacing(0.40 nm).The density functional theory(DFT)calculation and quantitative kinetics analysis indicate that the N/O dual-doped hard carbon with abundant active sites can strengthen K adsorption and diffusion,leading to capacitive-adsorbed K⁺storage.Therefore,when NOCNBs used as anode materials of PIBs,they show excellent reversible capacity,superior rate capability and long cycle stability.Chitin-derived carboxymethyl chitosan(CMCh)was used as the raw material,dissolved in deionized water to obtain a transparent solution,and then regenerated through an acetic acid coagulation bath to obtain a 3D nanofibrous CMCh hydrogel.Subsequently,it served as a template and was further cross-linked by metal cations to create the nanofibrous metal-carboxymethyl chitosan coordination hydrogel.After freeze drying and carbonizing,3D nanofibrous carbon-confined metal oxides were successfully constructed.Such a strategy of nanofiber assembly and confined metal-polysaccharide coordination for fabricating 3D nanofibrous MO@NCFs aerogels is facile without complex technical equipment requirements and can be used in scalable production.The ultrafine metal oxide quantum dots are in-situ generated in the nanofibers,and finally are uniformly embedded in the nanofiber networks,which has a strong coupling effect with the carbon matrix.The metal oxides loading is as high as49.3-56.0wt%.The 3D nanofibrous carbon-confined Co₃O₄ composites were used as the candidates to evaluate their potential applications in lithium/sodium ion batteries.The in situ and confined coordination in the Co-CMCh framework not only generate the Co₃O₄quantum dots with ultrafine size,but also endow the quantum dots to be robustly embedded in N-enriched 3D carbon nanofiber networks.The diffusion kinetics experiments demonstrate that the designed 3D nanofibrous Co₃O₄@NCFs aerogels with abundant accessible active surface areas and enhanced electrical conductivity show higher ion diffusion coefficient and lower charge transfer resistance than those of the conventional carbon-confined Co₃O₄(Co₃O₄@NCBs),leading to fast ion and electron transport.After 400 discharge/charge cycles under high current density of 1000m A g^-1,Co₃O₄@NCFs still maintain a good structural stability.As expected,they exhibit high capacity,excellent rate performance,and long cycling stability when used as anode materials of LIBs/SIBs.The chitosan oligosaccharides obtained by deacetylation and degradation of chitin were used as raw materials to make full use of their water solubility,weak metal ions chelation ability and low molecular weight characteristics.After dissolving them in deionized water and homogeneously blending with metal salt solutions,the highly fluffy 2D sheet-like hybrid precursors were quickly synthesized by microwave heating.Afterwards,they were catalytic carbonized at high temperature to obtain the carbon-based metal hybrids(Fe-Co@NCs).Finally,carbon-confined Fe-Co Se₂ nanosheets(Fe-Co Se₂@NCs)with mesoporous structure were fabricated by sunsequent low temperature selenization.The Fe-doped Co Se₂ nanoparticles were uniformly embedded in 2D nitrogen-enriched carbon nanosheets.The results show that Fe-Co Se₂@NCs nanosheets possess large specific surface area,abundant mesoporous structure,high level pyridine/pyrrole-N doping,and good conductivity.Therefore,they exhibit high specific capacity and good rate performance when used as anode materials of PIBs.This thesis successfully breaks the bottlenecks of chitin to construct porous carbon and carbon-confined composite electrode materials.A series of high-performance and nanostructured heteroatom-doped carbon and carbon-confined hybrid materials were constructed,and their properties and functions were evaluated,which hold great applications in LIBs/SIBs/PIBs anodes.This work provides new inspiration for constructing 2D and 3D nanostructured carbon-based electrode materials from biomass resources,which satisfy the development needs of sustainable energy.