Preparation And Electrochemical Properties Of Chitosan Derived Carbon-(Mo,W) Chalcogenides

Due to the rapid development of society,the massive use of traditional fossil energy has caused the depletion of nonrenewable energy and serious environmental problems.The development and use of energy storage technology with long service life and low cost has attracted extensive attention.However,the lack and uneven distribution of lithium resources lead to a significant increase in cost,which can not meet the application needs of large-scale energy storage.Therefore,it is particularly important to develop low-cost,safe and efficient energy storage batteries to replace and supplement lithium-ion batteries.The reserves of sodium（potassium）in the earth’s crust are significantly higher than lithium,and the price is cheaper.Although the physical and chemical properties of Na（K）are similar,the larger ion radius（Na:1.02（?）,K:1.38（?））leads to its slow kinetics,affects the migration of ions and causes huge volume changes of electrode materials.The performance of negative electrode of sodium or potassium-ion batteries（SIBs/PIBs）has become the key factor restricting its popularization and application.Therefore,it is significant and challenging to develop a safe,high storage capacity and long cycle life SIBs/PIBs anode material.A variety of renewable natural biomass materials have been proved to be raw materials and reactors for the fabrication of functional nano materials.The previous research on chlorella derived nitrogen and phosphorus doped carbon composite for sodium storage also confirmed that biomass derived carbon materials have superior conductivity,rich doping of heterogeneous elements and rich active sites.In natural biomass,chitosan molecules contain active groups such as amino and hydroxyl groups.Deacetylated chitosan has strong adsorption characteristics for metal salt ions,and has been proved to be an effective sewage treatment agent.These studies inspired us to use chitosan as a reactor and adsorbent to construct N-doped carbon nanocomposite.The research work of this dissertation mainly includes the following parts:Firstly,biomass chitosan was selected as adsorbent.After adsorbing molybdenum source,it was further selenided and calcined to form N-doped carbon molybdenum selenide composite（Mo Se₂/N-C）.The facile synthesis method not only makes full use of the adsorption characteristics of chitosan and transition metals,but also can effectively reduce the cost.Acting as the anode of SIBs,Mo Se₂/N-C-2 material shows a specific capacity of 505 m Ah g^-1after 100 cycles of charge and discharge at 1 A g^-1current density,and can still provide a specific capacity of 266 m Ah g^-1even after 1200 cycles at1 A g^-1.When it is used as the anode electrode of PIBs,it also maintains a specific capacity of 240 m Ah g^-1at 0.1 A g^-1after 100 cycles.On the basis of the first chapter,Sb doped Mo S₂chitosan derived nitrogen doped carbon composite（1T-Mo S₂-Sb）was prepared by sulfurization and calcination with chitosan as adsorbent.Antimony doping into the crystal lattice of Mo S₂induces the transformation of 2H phase Mo S₂into 1T phase Mo S₂.It makes full use of the adsorption capacity of chitosan,acts as an adsorbent and reactor,inhibits the agglomeration and stacking of molybdenum sulfide nano sheets,forms a nanocomposite structure between molybdenum sulfide and chitosan derived carbon,increases the storage sites of sodium/potassium ions and speeds up ion conduction.By introducing antimony source doping,the layer spacing is expanded,and the 2H phase Mo S₂is transformed into 1T phase Mo S₂,which improves the conductivity of the composite,makes the ions and electrons conduct rapidly,and improves the reaction kinetics.When 1T-Mo S₂-Sb electrode is used as the negative electrode of SIBs,it exhibits the reversible specific capacity of 493 m Ah g^-1after 100 cycles at 0.1 A g^-1,and it still maintains the specific capacity of 253 m Ah g^-1after 2200 cycles at high current density of 1 A g^-1.Paired with Na₃V₂（PO₄）₃cathode,the sodium ion full cell also shows a high reversible specific capacity of 242 m Ah g^-1after 100 cycles at 0.5 A g^-1.As the anode electrode of PIBs,the electrode also shows good performance,and still provides a reversible specific capacity of 343 m Ah g^-1after 100 cycles of 0.1 A g^-1.Finally,based on the second chapter,Sn doped WSe₂chitosan derived nitrogen doped carbon composite（1T-WSe₂-Sn）was prepared by selenium calcination after adsorption of tungsten source and tin source with chitosan as adsorbent.By doping tin into the lattice of WSe₂,2H phase WSe₂is induced to transform into 1T phase WSe₂to further improve the conductivity.At the same time,chitosan derived carbon as a buffer substrate is conducive to buffer the volume change during the process of charge-discharge,so as to enhance the cycle stability and improve the rate performance.As the anode electrode of SIBs,it delivers an excellent specific capacity of 460 m Ah g^-1after 100 cycles at 0.1 A g^-1.At the same time,it still maintains a specific capacity of 285 m Ah g^-1after 900 cycles at the high current density of 1 A g^-1.Paired with Na₃V₂（PO₄）₃cathode,the sodium ion full cell also showed a reversible capacity of 183 m Ah g^-1at 0.5 A g^-1after 200 cycles.When used as the anode PIBs,it also shows good electrochemical performance and remained the reversible capacity of 345 m Ah g^-1after 50 cycles at 0.1 A g^-1.