| Lithium-ion batteries(LIBs)have been widely used in industrial production and people’s life.Lithium-ion batteries are increasingly needed in power batteries,consumer batteries and energy storage batteries.However,the shortage and uneven distribution of lithium resources limit the large-scale application of lithium-ion batteries.Both lithium and potassium are the same main group elements and they have similar physical and chemical properties.Potassium ion batteries(PIBs)and lithium-ion batteries have similar working principles.Potassium ion batteries have the advantages of abundant potassium resources and low standard redox potential of potassium.Moreover,aluminum foil can be used to further reduce the cost of the anode collector fluid of potassium ion batteries.Potassium ion batteries are considered to be the next generation of battery technology that can replace lithium-ion batteries.At present,the main challenges of potassium ion batteries are the low diffusion rate of potassium ion in solid electrode,often resulting in sluggish kinetics and the volume expansion in the process of charge and discharge because of the excessive radius of potassium ion.Besides,potassium ion battery system has serious side reaction and electrolyte decomposition is irreversible.Potassium ion batteries anode materials are the key component of PIBs.It is of great significance to explore the preparation of high-performance potassium ion batteries anode materials.Metal-organic frameworks(MOFs)are porous crystalline materials with an adjustable structure,high specific surface area and adjustable porosity.The porous carbon materials,metals and metal compounds derived from MOFs meet the special requirements of high-performance potassium ion battery anode materials.The morphology control,heteroatom doping sites,pore distribution and carbon layer distribution can be adjusted by modifying MOFs,so that the prepared materials have better electrochemical potassium storage performance.Among them,the design of nanostructure and the introduction of carbon materials can effectively solve the problem of the volume expansion of potassium ion battery anode materials.Therefore,it is the research emphasis to modify MOFs materials from the perspective of nanostructure design and changing the electronic structure of carbon materials.The application of a series of MOFs material derivatives in the anode materials of potassium ion batteries are investigated in this dissertation.Through high temperature annealing,acid etching,surface coating,oxidation and phosphating,porous carbon materials,metal phosphating and metal oxides are prepared to achieve good electrochemical performance.The structural-activity relationship of materials is studied by typical characterization methods.Furthermore,the electronic structure of the material is investigated by density functional theory(DFT)calculations.The electrochemical reaction mechanism of the materials is revealed by electrochemical measurement methods.The details are as follows:1.High nitrogen doping can regulate the electronic structure of carbon materials by producing abundant defects,so as to achieve good electrochemical potassium storage performance.The nitrogen content of MOFs-derived carbon materials is closely related to the nitrogen content of organic ligands.Currently,ZIFs series MOFs materials are mostly used to prepare high-nitrogen doped carbon materials,but their nitrogen content is limited.In this dissertation,an organic ligand 1H-1,2,3-triazole(C2H3N3)with high nitrogen content is selected to form an energy metal-organic framework(MET-6)with zinc ions as metal center.After annealing,the high energy ligand released a large amount of gas to produce abundant pores.The optimal nitrogen content and nitrogen type are obtained by optimizing the annealing temperature.The N-doped porous carbon frameworks(NPCF-800)are annealed at 800℃ and achieves good rate performance and cycle stability,reaching 258.9 mA h g-1 after 2000 cycles at 1 A g-1.By using typical characterization methods and DFT calculations,it is found that the prepared NPCF-800 has high content of pyrrolic and pyridinic nitrogen.The pyrrolic and pyridinic nitrogen in the nitrogen doped carbon materials are potassium adsorption sites.Increasing the content of pyrrolic and pyridinic nitrogen is conducive to potassium adsorption,thus enhancing the electrochemical performance of the carbon materials.2.The design of nanostructured materials and the introduction of hollow confinement structure can effectively buffer the volume expansion,so as to improve the cycle performance of transition metal phosphide.MOFs tend to collapse and form irregular morphology after calcination treatment.Surface modification of MOFs materials is necessary to design and prepare hollow structure materials.In this work,MET-5,a Cu-based MOFs material with small particle size,is selected as the precursor.After the PDA coating,due to the PDA-MET-5 interface interaction,the hollow structure is formed by using the structural stress of MET-5 and PDA during the pyrolysis process.Then,after phosphating,nitrogen/phosphorus dual-doped carbon-coated layer is formed outside,and copper phosphide active material is formed inside.The internal void can buffer the volume expansion,and the nitrogen/phosphorus dual-doped carbon layer can improve the electrical conductivity of the material.The prepared Cu3P/CuP2@NPC achieves good electrochemical performance with a specific capacity of 100 mA h g-1 after 5000 cycles at 1000 mA g-1,effectively alleviate the volume expansion,and improve the cycle stability.The potassium storage mechanism of Cu3P/CuP2@NPC is analyzed as the solid-solution reaction mechanism by in-situ X-ray diffraction experiment,and the potassium adsorption energy and electronic structure of Cu3P and CuP2 are calculated and analyzed by DFT calculations.3.Bi2O3 material based on conversion-alloying reaction mechanism has a high theoretical specific capacity.However,little research has been attempted to investigate it as an anode material of the potassium ion batteries.The poor conductivity of metal oxides and the volume expansion of bismuth-based materials during the cycles are two major factors restricting the development of Bi2O3 as an anode material.The introduction of carbon materials with stable structure can effectively solve these two problems.Using bismuth-based MOFs(Bi-BTC)as precursor,Bi2O3 particles embedded in carbon matrix are obtained by carbonization and further oxidation strategies.The introduction of carbon matrix is equivalent to the introduction of a large "sponge",thus alleviating the volume expansion during the charge and discharge process.The prepared Bi2O3@C constructed an effective conductive network,effectively reducing the transmission of resistance.Based on this,Bi2O3@C has a specific charging capacity of 314 mA h g-1 after 100 cycles of 50 mA g-1.The hybrids containing carbon material and Bi2O3 particles effectively solves the problem of volume expansion and poor electrical conductivity.The ex-situ X-ray diffraction experiment reveals the mechanism of potassium storage at Bi2O3@C is conversion-alloying reaction.The diffusion of potassium in Bi2O3 lattice and the electronic structure of Bi2O3 are analyzed by DFT calculation. |
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