Preparation Of ZIF-67 Derived Materials And Their Application In Lithium-sulfur Batteries

Lithium-sulfur batteries use sulfur or sulfur-containing compound as the positive electrode and lithium as the negative electrode.The mutual conversion of electrical energy and chemical energy is achieved through the sulphur-sulfur bond break/generation.Because of its advantages of low cost and high energy density,it has become the most promising new-generation energy storage device.However,the extremely low conductivity of elemental sulfur and its final product lithium sulfide（Li₂S）,leads to the low utilization rate of the active material and poor kinetic performance.During the charge and discharge process,the structure stability of positive electrode is damaged due to the volume change of sulfur,which leads to the degradation of battery performance.And the shuttle effect of lithium sulfide in charge and discharge intermediates hinders its commercialization.To solve these problems,some electrode materials with high performance and low cost are needed to improve the performance of lithium sulfur batteries.This thesis mainly studies how to solve the problems existing in the process of charging and discharging lithium-sulfur batteries.Metal organic framework materials（MOF）are obtained by self-assembly of metal ions and organic ligands,also known as porous coordination polymers（PCP）,because of their huge specific surface area（100-10,000 m²/g）,controllable pore size（3-100?）,morphology,various frame structures,high thermal stability and excellent chemical stability have become one of the research hotspots in the field of new energy batteries.Applications of MOF related materials in the field of batteries can be divided into two categories:（1）MOF and MOF composite materials loaded with active substances.（2）Derived materials obtained by using MOF as a sacrifice template.Among them,the MOF derived materials converted by heat treatment and other means,using MOF as sacrifice template,generally include:carbon materials,metal composite materials,carbon/metal composite materials.Compared with MOF,MOF derived materials have good electrical conductivity due to their ca rbon components,and have advantages in material transfer,material load and other aspects because they retain the porosity and high specific surface area of MOF precursor materials.In addition,the active substances in MOF derived materials can adsorb polysulfide and promote the electrode reaction,so they have more application prospects in batteries.Based on the ZIF-67 derivative material of Co-based imidazolate skeleton material（ZIF is a typical member of MOF）,this paper uses three methods to improve its performance.The specific research content is as follows:1.The metal organic framework material-ZIF-67 was synthesized by solvent heat method as the precursor and carbonized at 750℃to obtain the porous carbon material Co@NPC derived from ZIF-67 containing metal Co.As a conductive matrix,TiO₂ was coated on the surface by in situ hydrolysis of n-butyl titanate to obtain core-shell structure of Co@NPC@TiO₂ as adsorbent of lithium polysulfide.Then,after the sulfur was loaded by the traditional fusion-diffusion method,Co@NPC-S and Co@NPC@TiO₂-S composite materials were obtained.The two composites were characterized by XRD,SEM,TEM,EDS and other physical characterization,and used as cathode materials to assemble batteries for electrochemical performance testing.It was found that the porous carbon materials coated with TiO ₂ exhibited better rate performance and cycle stability after sulfur loading.2.In order to inhibit the dissolution and migration of polysulfides and the volume expansion during charge and discharge,through a simple heat transfer strategy,the synthetic bimetallic CoNi was embedded in the nitrogen rich porous carbon framework.Study the influence of the molar ratio of different metal sources on the electrochemical performance of the battery.Due to the catalytic effect of secondary metals,cobalt-based electroactive phases,the template effect of MOFs and the unique porous structure,it can effectively adsorb and catalyze the conversion of polysulfides,and the bimetallic embedded nitrogen rich porous carbon framework showed good electrochemical performance.Among them,when the metal source Co:Ni（molar ratio）is 3:1,the electrochemical performance of the Co_0.75Ni_0.25-NC/S composite material is the best.3.CNTs were added to ZIFs derived Ni-Co mixed oxide using a metal organic framework（MOF）based strategy.The main processes include depositing Co and2-methylimidazolate（2-Melm）on the surface of CNTs,replacing partially dissolved cobalt ions with nickel ions,and then heat treating the precursor to obtain Ni-Co oxide/CNTs composites.The nickel and cobalt mixed oxide has a hollow nanocage structure.This structure has a large specific surface area and a large number of active storage sites,so the energy density per unit volume is large.Its thin and permeable shell guarantees short ions and electron transport paths,and the hollow structure can effectively alleviate the volume expansion brought by th e ion circulation shuttle.At the same time,the conductivity of carbon nanotubes is good,so the composite material can significantly enhance the electrochemical reaction kinetics of the electrode.