Preparation Of Bimetallic MOF-derived Metal Compounds And Their Lithium-sulfur Battery Performance Study

With the widespread use of portable electronic devices and electric vehicles,people have put forward higher requirements on the performance of batteries.However,the energy density of traditional lithium-ion batteries has reached the theoretical limit,which cannot meet the higher demand of emerging industries for electrochemical energy storage systems.Under the background of energy technology revolution,it is necessary to develop energy storage system with higher energy density and lower costs to connect energy storage and terminal application more effectively.As a kind of high-energy chemical power source,lithium-sulfur batteries have a high theoretical specific capacity(1675 m Ah g^-1)and energy density(2600 Wh kg^-1).Meanwhile,as a cathode active substance,sulfur is cheap,abundant,and environmentally friendly.Therefore,lithium-sulfur batteries are considered to be the most promising energy storage devices for the next generation.However,due to the poor intrinsic conductivity of monolithic sulfur and lithium sulfide（Li₂S）,the severe volume changes during battery charging and discharging,and the shuttle effect caused by the dissolution and diffusion of intermediate products,all to lead to rapid battery capacity decay and poor cycling performance,resulting in lithium-sulfur batteries still face many challenges in the commercialization process.Therefore,exploring the cathode sulfur carrier material that can improve the above problems is an effective way to improve the battery performance.Metal-organic frameworks（MOFs）have been widely used in catalysis,adsorption,energy,optoelectronic and biological fields due to their advantages of adjustable pore size,surface modification,various frameworks and ultra-high specific surface area.Therefore,more and more researchers are applying MOFs materials in lithium-sulfur batteries to solve the above mentioned problems.In this paper,monometallic Zn S and bimetallic Zn S-Co S₂were prepared as sulfur carrier materials based on Zn-based MOFs and Co-based MOFs,and it was found that the bimetallic sulfide Zn S-Co S₂electrode has better electrochemical performance through material characterization and electrochemical performance testing;in the subsequent study,bimetallic MOFs were used as precursor materials to prepare different hollow degrees of bimetallic Zn Se-Co Se composites with different degrees of hollowness were prepared for application in the cathode of lithium-sulfur batteries using bimetallic MOFs as precursor materials.By comparing the performance of several cells,it was found that the 45@Zn Se-Co Se electrode with semi-hollow core-shell structure has the best multiplicity performance as well as cycling stability.And on this basis,we gave45@Zn Se-Co Se composite two-dimensional conductive material MXene,and finally obtained the sulfur carrier material that can comprehensively improve the performance of lithium-sulfur batteries.The specific research is as follows:（1）ZIF-8 and ZIF-8@ZIF-67 precursor materials were synthesized by the self-assembly method,followed by sulfidation reactions to obtain amorphous carbon compounded with monometallic Zn S and bimetallic Zn S-Co S₂,respectively.Further,Zn S/S and Zn S-Co S₂/S were synthesized as lithium-sulfur battery cathode materials by introducing sulfur into the two matrices at 155°C using the melt-diffusion method.Firstly,the organic ligands in the material MOFs were carbonized to form amorphous carbon that can conduct electrons effectively,while the nitrogen atoms in the ligands were successfully doped in the carbon material by carbonization to form a polar surface that can form strong interactions with polysulfides,thus reducing the diffusion of polysulfides.Secondly,bimetallic Zn S-Co S₂has better electrochemical performance than monometallic Zn S when used as sulfur cathode.The bimetallic sulfide has a stronger adsorption capacity for polysulfides during battery charging and discharging,which has a good improvement on the shuttle effect.（2）Since the bimetallic compounds showed some advantages in the application of lithium-sulfur batteries,we continued to use ZIF-8@ZIF-67 as the precursor material.The bimetallic selenide Zn Se-Co Se composites with different hollow degrees were prepared by regulating the time of ZIF etching by tannic acid and combining with the selenization reaction.Finally,the melt-diffusion method was used to fill the internal voids of the composites with sublimated sulfur to obtain the anode material（Zn Se-Co Se/S）.The semi-hollow core-shell structure of 45@Zn Se-Co Se composite with the best electrochemical performance was obtained by material characterization and electrochemical performance testing.Firstly,the percentage of microporous structure is a major factor affecting the electrochemical performance of the battery.The semi-hollow core-shell structure with the most micropores not only provides more storage sites for sulfur,but also alleviates the volume change during charging and discharging.And the semi-hollow core-shell structure can also shorten the ion/electron transfer path,which in turn improves the conductivity of the cell.Secondly,a moderate amount of bimetallic Zn Se-Co Se in the 45@Zn Se-Co Se composite can provide sufficient active sites to capture and catalyze polysulfides to improve the redox kinetics of the cell.To further improve the conductivity,we selected two-dimensional MXene materials and compounded them on the45@Zn Se-Co Se surface of the core shell by electrostatic self-assembly to form45@Zn Se-Co Se/M composites and used them as sulfur carrier materials.The two-dimensional MXene conductive material not only improves the conductivity of the material and accelerates the conversion to polysulfides,but also promotes the migration of lithium ions and reduces the electrochemical polarization.