Zeolitic Imidazolate Framework-Derived Carbon Nanocomposites For Potassium Storage

Lithium-ion batteries（LIBs）are widely used due to the high energy density.However,the research on the alternatives of LIBs has received extensive attention because of the limited and uneven distribution of lithium resources.Potassium-ion batteries（PIBs）are similar to LIBs in terms of working principle.Potassium is abundant and its redox potential is very close to that of lithium（K⁺/K:-2.93 V vs.Li⁺/Li:-3.04 V）.Whereas,the large radius of potassium ions（1.38?）results in severe volume changes of electrode materials,sluggish diffusion kinetics,poor cycling stability and low rate capacity during insertion/extraction process.The key factor of the development of PIBs is to exploit low-cost and mass-produced PIBs electrode materials with excellent electrochemical performance.In order to design and develop high-efficiency anode materials,the following research are carried out in this paper:（1）N doping improves the conductivity of carbon matrix;（2）hollow structure increases the effective contact area between the material and the electrolyte;（3）metal selenium/sulfide improves conductivity and reversible capacity;（4）layered double hydroxide（LDH）enriches the K⁺diffusion pathway.MoSe₂ has great potential in accelerating K⁺insertion/extraction,but pure MoSe₂ has low electrical conductivity and agglomerates during long cycling.In Chapter 2,MoSe₂nanosheets are grown on N-doped porous carbon polyhedra（NPCP）by a simple solvothermal method.The synergistic effect of MoSe₂and NPCP can prevent the agglomeration of MoSe₂nanosheets.N doping enhances the conductivity of the carbon matrix,expands the interlayer spacing,and provides potential binding active sites for K⁺.The NPCP@MoSe₂ core-shell nano-architectures display higher initial performance,better cycling stability and superior rate performance than pure MoSe₂.In Chapter 3,a cobalt selenide/hollow carbon polyhedron material with carbon nanotubes（CNTs）is designed and synthesized.Firstly,ZIF-8@ZIF-67 with polyhedral core-shell topology is synthesized as a precursor.Then the precursor is calcined at high temperature to obtain a hollow carbon polyhedron with CNTs on the surface（Co@CNNCP）.Finally,the Co-Se@CNNCP is obtained by selenization.The specific capacity is 410 m A h g^-1over 80 cycles at 0.1 A g^-1.At 0.5 A g^-1,the specific capacity is 253 m A h g^-1 after 200 cycles with a capacity retention of 100%.The kinetic analysis based on cyclic voltammetry proves the electrochemical process is mainly contributed by pseudocapacitance.The cycling stability of Co-Se@CNNCP mainly relies on carbon nanotubes which inhibit the agglomeration of metal selenide nanoparticles and maintain the structural integrity of hollow carbon polyhedra during the processes of K⁺insertion/extraction.In Chapter 4,ZIF-67 is etched by nickel ions via a self-template in situ transformation strategy to obtain dodecahedral nanocages with uniformly distributed LDH nanosheets on the surface.After sulfuration treatment,Co₉S₈ nanoparticles are embedded on the LDH nanocages（LDH-S）tightly.Finally,carbon dots（CDs）are anchored on LDH-S by a simple hydrothermal method（CDs@LDH-S）.The initial discharge/charge capacities of LDH-S at 0.1A g^-1 are 1387 m A h g^-1 and 582 m A h g^-1 respectively,with 33%capacity loss after 200cycles.The CDs@LDH-S exhibites a capacity of 200 m A h g^-1 after 1000 cycles at 0.5 A g^-1.The hollow porous structure not only ensures the close contact between the electrolyte and the electrode material,but also shortens the diffusion length of potassium ions.The uniformly distributed ultrafine Co₉S₈ forms a conductive network that accelerates the efficient transport of electrons and ions.The presence of CDs further improves the electrode conductivity and structural stability.Ex-situ characterizations prove the reversible valence changes of Co³⁺/Co²⁺and Ni²⁺/Ni³⁺in LDH-based electrodes and the structural stability during intercalation/extraction of K⁺.Favorable transfer paths between LDH layers with low energy barriers（<0.36 e V）are identified by density functional theory（DFT）calculations.