Study On Preparation Of Transition Metal Compounds And Its Application In Lithium-Sulfur Batteries

For the next generation of energy storage devices,lithium-sulfur batteries have received widespread attention due to their low cost and the multi-electron transfer reaction from sulfur.Due to the insulation of sulfur,the shuttle effect of the soluble long-chain lithium polysulfide created during the chemical process,the utilization rate of sulfur in its practical application is low.Although the loss of lithium polysulfide is limited by physical methods(such as the structure design of the positive electrode material),the problem of capacity degradation has not been fundamentally solved.Considering the polarity of the polysulfide species,certain polar substances should be used as the sulfur positive electrode.Compared with the physical limitation,the polar adsorption can effectively alleviate the shuttle effect.Aiming at the problem of low active material utilization,ZIF-8 in-situ derived polyhedral carbon-coated Zn Se was designed and successfully prepared as the sulfur carrier in the positive electrode and used in lithium-sulfur batteries.The physical and chemical characteristics of material synthesis were observed through a series of physical characterizations such as SEM,TEM,XRD,XPS,etc.It was assembled into a half-cell to test its electrochemical performance,and the mechanism of its adsorption and catalytic conversion of lithium polysulfide was further analyzed and discussed.Aiming at the problem of weak polarity of transition metal selenide,Fe Se@NC/S composite cathode material was designed and successfully prepared.In this section,the carbon-derived framework provides an excellent conductive network and can physically capture lithium polysulfide.Fe Se forms Fe-S bonds and Li-Se bonds during discharging,which regulates the polarity between transition metal selenide and lithium polysulfide.At the same time,the inherent high conductivity and electrocatalytic ability of selenide can effectively improve the efficiency of lithium-sulfur batteries during charging and discharging,and can also induce rapid solid-solid conversion in the discharge process.Through GITT and other reaction kinetics tests and first-principles calculations to simulate the adsorption catalytic effect in the actual system,and related electrochemical performance tests,the mechanism of its adsorption and catalytic conversion of lithium polysulfide has been further analyzed and discussed.In view of the inevitable shuttle effect in the ether electrolyte and the slow conversion kinetics of the liquid-solid process,the PP diaphragm was modified to replace the vanadium in the vanadium oxide mixture with an average valence of less than tetravalent with Co.The method is increased to tetravalent,which triggers the generation of thiosulfate during the discharge process,which greatly improves the reaction kinetics of lithium polysulfide conversion.It was assembled into a half-cell to test its electrochemical performance,and the mechanism of its adsorption and catalytic conversion of lithium polysulfide was further analyzed and discussed through a variety of electrochemical methods.