Research On Synthesis Of Mesoporous Ti₄O₇ Hollow Spheres And Its Performance Of Lithium-sulfur Batteries

Lithium-sulfur batteries have attracted much attention due to their high theoretical discharge specific capacity(1675 m Ah·g^-1),energy density(2600 Wh·kg^-1),low cost and environmental friendliness.However,the poor conductivity of sulfur and lithium polysulfide,the volume change of sulfur during charging and discharging,the shuttle effect of lithium polysulfide,and the dendrite growth of the lithium negative electrode have caused that cycle performance of lithium-sulfur batteries can not meet actual application requirements.Studies have shown that the application of cathode materials with excellent electrochemical activity and special morphology is an effective means to improve the performance of lithium-sulfur batteries.Ti₄O₇ has a periodic oxygen defect structure,excellent conductivity and a special polar surface,and is widely used as the positive electrode carrier for lithium-sulfur batteries.In this thesis,three different cathode carrier materials,mesoporous Ti₄O₇ hollow spheres（H-Ti₄O₇）,Ti₄O₇ hollow spheres and multilayer graphene composite materials（H-Ti₄O₇/MLG）and H-Ti₄O₇/graphite carbon hollow spheres（H-Ti₄O₇/GC）are designed and prepared to study the influence of the changes in the phase composition and morphology of the materials on the capacity and cycle stability of the lithium-sulfur battery.The specific research content is as follows:（1）The mesoporous hollow Ti₄O₇ with large specific surface area and excellent electrical conductivity was prepared by the high-temperature carbothermic reduction method and applied to the cathode carrier of lithium-sulfur battery.At current densities of 0.1 C,0.2 C,0.5 C,1 C,and 2 C,the H-Ti₄O₇/S composite cathode has first-turn discharge capacities of1040.4,730.1,530.0,379.6,and 247.3 m Ah·g-1,respectively.Cycling 200 times under a large current density of 0.5 C,the capacity retention rate is still 70.2%..In addition,XPS analysis shows that the unsaturated Ti and S elements in H-Ti₄O₇ can form Ti-S bonds,which further indicates that H-Ti₄O₇ has a strong chemisorption effect on polysulfide ions.（2）The H-Ti₄O₇/MLG composite material was prepared based on ball milling and mixed and used as the positive electrode carrier of lithium-sulfur battery.It was found that the addition of multilayer graphene can greatly improve the electron and ion transmission rate of the positive electrode.At current densities of 0.1 C,0.2 C,0.5 C,1 C and 2 C,the discharge capacities of the first turn of the H-Ti₄O₇/MLG/S composite electrode are 1110.9,805.9,620.8,496.8 and 305.7 m Ah·g-1,respectively,and compared with H-Ti₄O₇,it has a greater improvement,which further illustrates the electrochemical advantages of graphene composite.（3）The H-Ti₄O₇/GC composite material was prepared by the carbon coating method.The fully crystallized Ti₄O₇ nanoparticles in the material were uniformly coated by the ultra-thin graphitic carbon layer to form a porous conductive network.At the same time,the graphitic carbon layer can effectively buffer the volume change during charging and discharging.Using this material as a sulfur carrier,at current densities of 0.1 C,0.2 C,0.5 C,1 C and 2C,the discharge capacities of the first circle of the H-Ti₄O₇/GC/S composite electrode are1317.1,922.6,781.1,594.2,and 494.1 m Ah·g-1,respectively.At the same time,at a current density of 0.5 C,the initial discharge capacity of the battery is 833.4 m Ah·g-1,and after 200cycles,the capacity retention rate is still 71.8%.The electrochemical performance of H-Ti₄O₇/GC cathodes is better than H-Ti₄O₇/S and H-Ti₄O₇/MLG/S cathodes.