Preparation And Properties Investigation Of MOFs-derived Selenides/carbon Composites For Hybrid Ion Capacitors

With the rapid development of society,the low power density of secondary batteries and low energy density of superacapacitors cannot meet the requirements.Therefore,metal ion hybrid capacitors with high power density and energy density have been paid more and more attention in recent years.Transition metal selenides（TMSes）as battery electrode materials have attracted much attention due to their high theoretical specific capacity,but their severe volume changes during the cycling tests,result in structural damage,The leads to poor rate performance and rapid capacity degradation.So far,common methods,such as reducing particle size,constructing core-shell structure and compounding with some high conductive materials,are applied to improve the electrochemical performance of TMSes as electrode materials.Besides,the heteroatom doping（nitrogen,phosphorus）is considered to be an effective strategy to improve the electrochemical properties of materials.MOFs have the advantages of high specific surface area,high porosity,and controllable porous structure.MOFs can be used to prepare metal-carbon composites via a one-step solvothermal or pyrolysis process,therefore,they’re getting extensive attentions in the energy storage area.In this thesis,the MOFs precursor was prepared by using a multi-heteroatom（N,P and O）containing ligand（HCTP-COOH）and metal ions（Niand Sb）.The futher selenization of the as-prepaered MOFs precursor provides metal selenides/carbon substrates doped with multi-heteroatoms,and the application of the composite materials as electrodes for hybrid-ion capacitors was investigated.1.Firstly,six-（4-carboxyphenoxy）cyclotriphosphazene ligands contained nitrogen,phosphorus and oxygen heteroatoms were prepared.A sandwich-like Ni-MOF/GO composite was synthesized by solvothermal method in the presence of graphene oxide（GO）with the obtained ligand and nickel nitrate hexahydrate.The Ni-MOF@GO composite with a double sandwich structure was prepared by in-situ encapsulation through GO self-assembling.By a further one-step selenization process,the confined growth of NiSe particles can berealized to prepare the NiSe composite（NiSe@C/rGO）with ultra-small sizes encapsulated in multi-heteroatom carbon matrix.The composite combined the following strategies including downsizing,carbon encapsulation and heteroatom doping.The as-prepared NiSe@C/rGO composite exhibits excellent properties for sodium ion storage applications.The assembled sodium ion capacitor（SIHC）NiSe@C/rGO//AC shows an ideal energy density(60.5 Wh kg^-1)and power density(13340W kg^-1).After 5000 cycling tests at current density of 2 Ag^-1,the as-fabricated SIHCstill remains 76%of the initial capacity.2.First,Ni-MOF/GO composite was prepared.Then,NiSe₂/C/rGO composite was obtained by selenization of Ni-MOF/GO.At last,the NiSe₂@NC/rGO composite was prepared by carbonization of polymerized dopaminewrapped NiSe₂/C/rGO composite.The NiSe₂@NC/rGO is composite contianed NiSe₂ particles coated with double carbon layers.This structure is believed that it can improve the electrochemical performance of as-prepared electrode materials,which was confirmed by applying NiSe₂@NC/rGO for lithium storage studies.In lithium-ion half-cell applications,the NiSe₂@NC/rGO electrode didn’t show apparent capacity decay after 2000 testing cyles at 1 Ag^-1.And when the current density was increased from 0.1 Ag^-1 to 10 Ag^-1,the rate performance performed well.All these results show that the double-layer carbon coating can effectively improve the conductivity and cycle stability of the as-prepared electrode materials.3.Sb-MOF/MWCNTs composites were synthesized by a solvothermal method using six-（4-carboxylphenoxy）cyclotriphosphazene ligand and SbCl₃ in the presence of N-doped multi-walled carbon nanotubes（MWCNTs）.By adjusting the loading amount of MWCNTs and the selenization temperature A sereies of Sb₂Se₃/C/MWCNTs composites,Sb₂Se₃/C/MWCNTs-7-550,Sb₂Se₃/C/MWCNTs-7-650,Sb₂Se₃/C/MWCNTs-7-750,Sb₂Se₃-550 and Sb₂Se₃/C/MWCNTs-14-550 were prepared.Among those the Sb₂Se₃/C/MWCNTs-7-550composite exhibited the best performance,and the assembled SIHC（Sb₂Se₃/C/MWCNTs-7-550/AC）exhibited a high energy density of 68 kg Wh^-1.