Sodium Ions Storage Properties Of The Supermolecule Derived Carbon-Based Materials

Among the many anode materials for sodium ion batteries,carbon materials have the advantages of low voltage platform,high rate performance and high cycle stability,which have attracted wide attention from researchers.However,the carbon materials have low specific capacity and can’t meet the increasing demand for high capacity.In order to achieve higher electrochemical performance of carbon materials,many modification strategies include morphology engineering,interlayer engineering,and heteroatom doping engineering have been utilized.However,it is difficult to achieve the above three types of modification by one-step method.Self-assembled supermolecules have the characteristics of adjustable topological structure and the selectivity of monomers,which can achieve high-content element doping,as well as achieving the purpose of pore modification,interlayer spacing modification and defect modification in one step.In addition,as the conversion mechanism material of sodium storage,MoS₂ has the problem of serious volume expansion,which leads to serious capacity dacaly.Through the selection of supermolecule monomers and the design of the topological structure,the surface of the derived carbon material can be equipped with abundant oxygen-containing functional groups.This design provides abundant binding sites for MoS₂ loading,thus alleviating the accretion of MoS₂,which can improve the electrochemical performance of the material.Therefore,in this thesis,supermolecules and their derivatives as the research object are studied.Firstly,the transformation process of supermolecule topological structure through the gradual introduction of monomers is discussed.In addition,through the adjustment of solvent polarity,we explored the supermolecule assembly mechanism.Secondly,supermolecule with a specific topological structure can achieve high nitrogen content and phosphorus dual-doped.This cracking process enables the supermolecule-derived carbon material to equip abundant pores and defects,and at the same time expands the carbon layer spacing,which are able to improve the sodium storage performance.In addition,through the selection of monomers and the control of solvent conditions,the annealed supermolecules are used as the substrate of the MoS₂ composite material to realize the few layer arrangement of the MoS₂.In addition,the nitrogen doping of the substrate also increases the conductivity of the composite material.The main specific research content and results are as follows:（1）Supermolecules gradual assembly to realize two-dimensional nitrogen and phosphorus dual-doped porous carbon nanosheets（NP-PCN）.Benefiting from the abundant hydrogen bond acceptor in the monomer structure,supermolecules can be assembled in a reasonable topology structure.With the introduction of assembled monomers,the topological structure and morphology of supermolecules change gradually.PVP and melamine are assembled through hydrogen bonds to form sheet-layer supermolecules.When cyanuric acid is introduced,the morphology of the material changes to small size lamellar stacked structure.Furthermore,phytic acid is introduced through hydrogen bonds,thus achieve the leaf-like morphology.When the obtained supramolecular is annealed at high temperature,the rational design of the topological structure avoids agglomeration and realizes the uniform distribution of nitrogen and phosphorus atoms.In addition,the produced gas through the thermal decomposition of supermolecules enables the enlarging of the interlayer spacing and makes the defects more abundant.Finally,the NP-PCN reached a nitrogen doping level of 25.00 at.%and a phosphorus doping level of 6.37 at.%.The interlayer spacing is expanded to 0.47 nm.The specific surface area of the material is 435.34 m²g^-1and the average pore size is 3.12 nm.When NP-PCN is used as anode material for sodium ion batteries,it exhibits high specific capacity,excellent rate performance and stable cycle performance.In the 10th charge and discharge cycle,the discharge capacity can reach 247 m Ah g^-1 at a current density of 100 m A g^-1.After 4000 cycles at a current density of 2000 m A g^-1,NP-PCN can still reach 92.6%capacity retention.The excellent electrochemical performance of NP-PCN can be attributed to the following points:the synergistic effect of nitrogen and phosphorus dual doping enhances the storage performance of sodium ions;the larger interlayer spacing of NP-PCN is beneficial for the interlayer insertion and extraction of sodium ions.Also,nitrogen and phosphorus dual doping make the carbon materials richer defects,which is more conducive to the storage and adsorption of sodium ions.In addition,the rich pore structure accelerates the diffusion of sodium ions.These results show that the reasonable design of supermolecules can make the derived carbon materials achieve excellent sodium storage performance.（2）Formation of MoS₂ array arrangement composite material based on supermolecule-derived nitrogen-doped carbon substrate（MoS₂@NC）.Trimellitic acid（TMA）and melamine derivatives（CN-Me Na）are selected as the monomers to achieve supermocules in different solvents.Due to the difference of solvent polarity,the competitive relationship ofπ-πstacking and hydrogen bonding of the monomer assembly is influenced,leading to obvious differences in the morphology and topological structure.In the weakly polarity mixed solvent of acetone and ethanol,the two monomers are mainly assembled by hydrogen bonding,and the final supermolecules achieve flower-like morphology.When the weakly polar DMF and strong polar water are used as the assembly solvent,water molecules play a bridging role in the assembly of monomers,weakening the hydrogen bond between monomers.Finally,the nanoplate supermolecules（TCM）withπ-πstacking as the dominant force are achieved.After annealing treatment,a nitrogen-doped carbon material with rich oxygen on the surface is obtained.When the carbon materials are used as the substrate,the form MoS₂@NC material can arrange in an array,due to the abundant binding sites delay the rapid growth of MoS₂nanosheets.In the tenth cycle of charge and discharge process,the discharge capacity reaches 374 m Ah g^-1,and the coulombic efficiency is close to 100%,which are significantly better than that of pure MoS₂materials performance.In addition,the nitrogen-doped carbon base material in the composite material can enhance the conductivity of the composite materials and accelerate the transfer of Na⁺.The MoS₂ in the composite achieves a small-layer stack,which effectively reduces the volume expansion and structural collapse during the cycle process.These results indicate that the supermolecule-derived nitrogen-doped carbon material has abundant oxygen-containing functional groups on the surface,which can realize the growth of MoS₂ arrays on the substrate thus improving the electrochemical performance of the composite material.