Preparation And Application Of Lithium/Sodium Secondary Batteries Based On ZIF Derivatives

The high theoretical capacity of silicon-based anode can well break the commercial limitation of graphite anode for high-performance Li-ion batteries and become the optimal choice for next-generation Li-ion batteries,but its severe volume expansion and low electrical conductivity during cycling limit its overall electrochemical performance.And in recent years,the application of metal organic frameworks（MOFs）in the field of energy storage has gradually emerged.The rigid controllable structure of nano-porous zeolitic imidazolate frameworks（ZIFs）material of its branch can well solve the volume expansion effect of silicon-based anode.After calcination,in-situ doping of nitrogen atoms can be achieved,which can significantly improve the defects of poor electrical conductivity of ZIF while increasing the capacity of the material.Meanwhile,the advantages of graphene such as high electrical conductivity and large specific surface area led to an extraordinary amount of research on it.In this project,the three are designed into a coating structure with different coating order,which can significantly improve the electrochemical properties and conductivity of the composite while alleviating the volume expansion of silicon particles.When they are used as the negative electrode of lithium/sodium-ion batteries,their electrochemical properties are as follows:（1）ZIF-67 dodecahedrons with a particle size of about 700 nm were synthesized by a simple one-step coprecipitation method,and obtained the pure Co@CNTs intermediate phase by high-temperature carbonization reduction under a H₂/Ar mixed gas,then oxidized and sulfided the intermediate phase under oxygen and argon atmospheres,respectively.The oxidized product,Co₃O₄@CNTs,can maintain a reversible capacity of 1037.6 m Ah g^-1under 1 A g^-1after 200 cycles,and still has a excellent capacity of 581 m Ah g^-1at a high current density of 5A g^-1due to its unique stacking structure that generates a large number of voids and provides sufficient channels for lithium ion transport.However,due to the severe volume effect of the oxide during cycling,its performance in sodium ion batteries is significantly worse than that of the sulfided product Co S₂@CNTs.The sulfided Co S₂nanoparticles are dispersed on the polyhedral carbon framework,and the large amount of nitrogen-doped carbon nanotubes it retains also significantly mitigates its volume change during cycling,after 50 cycles at 0.1 A g^-1,the reversible capacity is 801.2 m Ah g^-1.When cycled for 200 turns under 1 A g^-1,it still maintains a high capacity of 505.3 m Ah g^-1.（2）To improve the capacity of the composites and considering that ZIF-67 can form a desirable hollow structure after carbonization,silicon nanoparticles were added during the step of synthesizing ZIF-67.Meanwhile,to mitigate the effect of oxides on the capacity retention of the material due to bulk effect,compounded the prepared Si@ZIF-67with graphene oxide by electrostatic self-assembly.The three-carbon system of one-dimensional carbon nanotubes,two-dimensional reduced graphene oxide and three-dimensional dodecahedral ZIF carbon shell formed by the whole material after calcination provides sufficient buffering space for the volume expansion of the silicon particles,and also limits the pulverization of the oxide during the cycling process to a great extent.Better silicon addition,heat treatment temperature,ZIF cladding selection,and graphene concentration can provide a higher reversible specific capacity under the premise of stable cycling.After cycling at the high current of 5 A g^-1for 300 cycles,the specific capacity of 1432.6m Ah g^-1is still retained.What’s more,the coulomb efficiencies all remain above 99%.（3）To further improve the capacity retention of the composites at small current densities and reduce the chalking of the materials after multiple cycles,controlled growth of carbon nanotubes at different positions of the structural units by simply changing the type of ZIF precursors and the coating order,limiting the carbon nanotubes to where they need to grow,which effectively slowed down the capacity decay of the materials during the cycling process.After 100 and 500 cycles under0.1 A g^-1and 1 A g^-1,respectively,the capacity can be maintained at1744.2 m Ah g^-1and 1498.3 m Ah g^-1,showing excellent cycling stability under the protection of the double-layer carbon shell.The high Li⁺diffusion coefficient(3.4×10^-11)and pseudocapacitance ratio also confirm the excellent reaction kinetic performance of the material.