Synthesis Of Molybdenum-Based VIA Compounds/Carbon Composites And Their Application For Lithium-Sulfur Batteries

With the development of electrochemical energy storage stations and electric vehicles,the demand of high energy density batteries is expanding.Compared with the current commercialized lithium-ion batteries,lithium-sulfur batteries（LSBs）have a higher theoretical specific capacity of 1675 m Ah g^-1,a higher energy density of2600 Wh kg^-1,more obvious cost advantages,and environmental friendliness.They are considered to be one of the most promising new generation of rechargeable batteries,and have received a lot of attention.However,the practical application process of lithium-sulfur batteries is progressing slowly,and faces a series of difficulties.For example,the slow reaction kinetics caused by the insulation problems of sulfur and discharge product Li₂S₂/Li₂S lead to a low utilization of active sulfur;The huge volume expansion caused by the different density of S and Li₂S₂/Li₂S during discharge process;The shuttle effect of soluble intermediate lithium polysulfide between positive and negative electrodes leads to rapid capacity fade.To improve performance of LSBs,in this work,molybdenum-based VIA compounds are compounded with different carbon substrate materials.And the improvement of LSBs performance by the composite materials was investigated.The main research contents are as follows:（1）The amorphous carbon coated carbon nanotubes are obtained by introducing amorphous carbon on the surface of carbon nanotubes（CNTs@C）.Then the 1T phase molybdenum disulfide with good conductivity and catalytic performance is grown on its surface by solvothermal method to obtain CNTs@C@1T-MoS₂nanocomposite.XRD,XPS,Raman,TGA,BET,SEM and TEM were used to analyze the structure,component content and microscopic morphology of the composite.Then coated on the surface of polypropylene separator as a functional coating to inhibit the shuttle effect to improve the electrochemical performance of lithium-sulfur batteries.First of all,CNTs and 1T-MoS₂with high conductivity can promote electron migration,reduce polarization of batteries,and improve the utilization of active materials as the second current collector.Secondly,the extremely small lamellar 1T-MoS₂can expose more active sites,further enhancing the adsorption and catalytic conversion capabilities of polysulfides.Which can greatly reduce shuttle effect and enhance redox kinetics.Through electrochemical test,batteries with CNTs@C@1T-MoS₂-PP separator have good rate performance.At the current density of 0.2 C,the specific discharge capacity of the first cycle is 1373 m Ah g^-1,which can maintain 864 m Ah g^-1after 100 cycles,is much higher than the batteries using ordinary separator.In addition,battery with CNTs@C@1T-MoS₂-PP separator still has a good initial sulfur utilization rate and cycle stability at 1 C,（2）The nitrogen-sulfur co-doped mesoporous hollow carbon spheres（NSHC）were synthesized and used as the growth substrate of 1T/2H phase molybdenum selenide.After hydrothermal reaction,the nitrogen-sulfur co-doped mesoporous hollow carbon spheres with a large number of thin 1T/2H-MoSe₂lamellae on the inner and external walls composite（MoSe₂-NSHC）was obtained.XRD,XPS,Raman,TGA,BET,SEM and TEM were used to analyze the structure,component content and microscopic morphology of the composite.Applied MoSe₂-NSHC to both modified coating of separator and carrier of positive sulfur material.The first,large hollow structure of MoSe₂-NSHC can store a large amount of elemental sulfur,and buffer the violent volume expansion in the process of lithiation.Secondly,the nitrogen and sulfur elements doped into the carbon skeleton not only improve conductivity of carbon skeleton,but also adsorb polysulfides,and catalyze the conversion of polysulfides through the formation of Li-N and Li-S bonds.Finally,MoSe₂growing on the inner wall can chemically adsorb the polysulfide,inhibit its escape from the carbon spheres and cause the shuttle effect,while the MoSe₂on the external wall can be used as the"second line of defense"to capture polysulfide.Through the electrochemical test,the initial discharge capacity of the battery using both the MoSe₂-NSHC/S positive electrode and the MoSe₂-NSHC-PP separator was1155 m Ah g^-1at the current density of 0.5 C,and 908 m Ah g^-1was retained after 100cycles.At high current density of 1 C,it also has an initial discharge capacity of 882m Ah g^-1.Even under a high sulfur load of 3.2 mg cm^-2,the initial capacity still reach to 1212 m Ah g^-1at 0.1 C,and maintain excellent cycling stability.