Studies On Metal Organic Frameworks-Derived Metal Chalcogenides/Carbon Composites And Sodium Storage Properties

Recently,the growing environmental pollution and serious energy problems have forced to pay more attention to the research and development of clean energy and new energy storage technologies.Sodium ion batteries（SIBs）as new energy storage batteries with broad application prospects are important for green sustainable developments.As the core component of energy storage in SIBs,anode plays an important role in improving of the battery performance.Among diverse anode materials for SIBs,metal chalcogenides composites have been deemed as a class of potential electrode materials due to their high theoretical capacity,abundant resources,as well as environmental friendliness.However,most of metal chalcogenides are semiconductors,whose low conductivity results in a poor ion diffusion rate and large volume expansion during cycling.Aiming at these drawbacks of metal chalcogenides,various metal chalcogenides/carbon composites are designed and prepared using metal organic frameworks（MOFs）as precursors（templates）in this dissertation,and distinct synthesis strategies are adopted including microstructure control,element doping,integration of multiple carbon materials,construction of heterogeneous interface and self-supporting structure,etc.The compositions,structures,and electrochemical properties of the as-prepared anode materials are characterized and investigated,with in-depth studies on the mechanism of electrochemical properties and sodium storage improvement.The main finding are as follows.The tin monosulfide/carbon nanocomposites（C/SnS@NC and SnS@C-500）are synthesized by two different strategies using MOFs precursors.The C/SnS@NC nanocomposites are prepared by the hydrothermal coating and high-temperature calcination on nitrogen-doped carbon（NC）polyhedrons that are fabricated by carbonization of ZIF 8 precursor.The MOFs-derived NC in C/SnS@NC meliorates the electrical conductivity of the electrode,alleviates the volume changes during the cycling process,and increases the active site of SnS nanosheets which facilitates the rapid diffusion of Na⁺ion.The outer carbon layer enhances the stability of the SnS structure during annealing process,and improves the electrical conductivity of whole electrode.As the C/SnS@NC is used for anode of SIBs,which displays a specific capacity of 379 m Ah g^-1 at 0.2A g^-1over 100 cycles.Even at 4 A g^-1,a reversible specific capacity of 264 m Ah g^-1 can be achieved.After that,we simplify the synthesis strategy and fabricate SnS@C-500 nanocomposites via one-step vulcanization reaction of Sn MOFs,and further investigate the effect of different vulcanization temperature on their sodium storage properties.The MOFs-derived carbon enhances the overall conductivity of electrode,and relieves the volume change on cycling process.When SnS@C-500 is used for SIB anode,it delivers a specific capacity of 538m Ah g^-1 at 0.5 A g^-1over 100 cycles.To accelerate charges transfer and transport as well as enhance the struc ture stability of the electrodes,the Zn/Mo bimetallic sulfide/carbon（ZnS@MoS₂/NC@rGO）nanocomposites are fabricated by loading of graphene oxide and high-temperature vulcanization of Zn/Mo BIFs precursor.The reduced graphene oxide component in the composite meliorates the overall conductivity of electrode,as well as relieves the volume expansion during the cycl e that enhancement the stability of the materials.Additionaly,due to the large band gap difference between ZnS and MoS₂,the constructed ZnS-MoS₂ heterostructure generates internal electric field to promote the ion transfer,thus enhancing the electrochemical reaction kinetic.The imidazole frameworks-derived NC in which nitrogen atoms regulating the electronic configuration of carbon material promotes electron transfer and boosts the conductivity.The as-prepared ZnS@MoS₂/NC@rGO displays a specific capacity of 309 m Ah g^-1 at 4 A g^-1,and a reversible specific capacity of 367 m Ah g^-1 can be preserved over 200 cycles at1.0 A g^-1,with 97.3%capacity retention.Consideration the metal selenides possess advantages of relatively smaller band gaps and higher electrical conductivity than metal sulfides.The self-supporting molybdenum selenide/carbon（NC/MoSe ₂@NC）nanosheet arrays are constructed using the Zn/Co ZIFs precursor.The self-supporting structure not only increases the contact between the electrode materials with electrolyte,but also shortens the transport route of electron and ion effectively.The ZIFs-derived two-dimensional NC arrays provide MoSe₂ nanosheet for a good dispersion and effectively alleviate the volume expansion during the cycling process.In addition,the outer carbon layer further meliorates the electrical conductivity and maintains the stability of the electrode.The DFT theoretical calculation reveals that the interfacial interaction between MoSe₂ and carbon is favorable to improve the electrical conductivity of composites,and enhance the adsorption/diffusion of Na⁺of MoSe₂.The as-prepared NC/MoSe₂@NC exhibits excellent sodium storage performance with a reversible capacity of 669 m Ah g^-1 at the current of0.2 A g^-1,and a specific capacity of 348 m Ah g^-1 can be achieved over 1000 cycles at 1.0 A g^-1,corresponding to approximately 94.3%capacity retention.The sodium ion full battery is fabricated by NC/MoSe₂@NC anode with Na₃V₂（PO₄）₃/C cathode,which delivers the highest energy density of 186.2 Wh Kg^-1.