Study On Synthesis And Performance Of Functional Carbon-based Materials For High-energy Lithium-sulfur Batteries

Lithium-sulfur batteries（LSBs）have high theoretical energy density(2600 Wh kg^-1),high theoretical specific capacity(1675 m Ah g^-1),and low cost and environment friendly,which have attracted wide attention of many scientists,and are expected to become one of the most promising energy storage systems.However,the shuttle effect of polysulfide,the insulation of the active substance sulfur,and the slow dynamics of the polysulfide conversion process seriously hinder the practical application of lithium-sulfur batteries.More importantly,these questions are interrelated,so that many questions originate from the shuttling effect of polysulfide.Therefore,by alleviating the shuttle effect,the problems faced on the positive and negative poles can be solved to a large extent.In recent years,many studies have shown that the diaphragm modification of lithium-sulfur batteries is one of the important ways to solve this problem.The purpose of this paper is to design high-performance carbon-based functional composite materials for lithium-sulfur battery diaphragm,and study the process and method to improve the electrochemical performance of lithium-sulfur batteries.The specific research contents are shown as follows:1.A mesh material（V-ZIF-8）was prepared by high temperature calcination and modified for the diaphragm of lithium-sulfur batteries.Experimental data show that this prepared mesh material can inhibit the shuttling effect of polysulfide to some extent,serve as an excellent host material and the physical barrier of sulfur,and also provide an effective conductive network for the rapid transmission of electrons and ions.The diaphragm was modified with a V-ZIF-8 composite and assembled into a lithium-sulfur battery for performance testing.The results showed that the initial specific discharge capacity at 0.5C was 1588 m Ah g^-1,and after 100 charge-discharge cycles,the specific discharge capacity retained 905 m Ah g^-1.In addition,using the lithium-sulfur battery with V-ZIF-8 modified diaphragm,the initial discharge specific capacity of activation is 977m Ah g^-1.After 300 cycles,the capacity is maintained at 720 m Ah g^-1,and the average capacity attenuation rate of each cycle is 0.000685%,showing excellent cycle stability.The results show that the V-ZIF-8 modified diaphragm has excellent electrochemical properties,which can effectively alleviate the shuttling effect of polysulfide,but also reduce the charge transfer resistance of the battery and improve the utilization rate of sulfur.2.A three-dimensional functional carbon matrix composite（Fe C@C₃N₄）mixed with sodium chloride as a template,and used to modify the diaphragm of the lithium-sulfur battery.A catalytic reservoir was constructed to inhibit the shuttle effect,with the aim of facilitating polysulfide conversion.The test results show that the lithium-sulfur battery assembled with Fe C@C₃N₄-700 modified diaphragm has significantly improved the rate performance and cycle stability of the battery.In addition,the lithium-sulfur battery was cycle-tested after assembly with the Fe C@C₃N₄-700 modified diaphragm at 0.5C,with an initial discharge specific capacity of 1591 m Ah g^-1.After 300 charge and discharge cycles,the specific discharge capacity still retains 757 m Ah g^-1,and the attenuation rate of each charge and discharge cycle is only 0.0017%.The cell discharge ratio capacity of Fe C@C₃N₄-700 battery at the ratio of 2C is 1190 m Ah g^-1.After 300 cycles,the specific discharge capacity retains 688 m Ah g^-1,and the capacity attenuation rate of each cycle is only 0.001339%.The experimental results show that the Fe C@C₃N₄-700 modified diaphragm has higher Li⁺diffusion rate and lower activation energy during the transformation of Li PSs.The results of this chapter show that the Fe C@C₃N₄ modified diaphragm has the application prospect of long-life and high-performance lithium-sulfur batteries.3.Using carbon sphere as template,spherical bimetal oxide functional carbon matrix composite material（SMO）was prepared by hydrothermal method and high temperature calcination,which was used for the diaphragm modification of lithium-sulfur battery.Using carbon ball as a template to improve the conductivity of the battery,and carbon ball for three-dimensional structure for polysulfide provides more adsorption sites and transformation space,Sb atoms and Mo atoms embedded on the carbon sphere exposed more active site,promote the battery internal electrochemical reaction dynamics,thus the polysulfide shuttle effect got very good inhibition.The lithium-sulfur battery modified with SMO carbon matrix composite has a high discharge specific capacity of 1158 m Ah g^-1at 0.5C,and a discharge specific capacity of 687 m Ah g^-1 after 300 cycles of charge-discharge cycle.The capacity attenuation rate per cycle is only 0.001256%.The experiment show that the electrochemical performance of lithium-sulfur battery modified by the carbon matrix composite has been significantly improved.