Controllable Synthesis,Modification Of Porous TiO₂ And Application For Lithium-sulfur Battery

Lithium-Sulfur Batteries?LSBs?have a high specific capacity(1672 mA h g^-1),which is five times of Lithium-Ion Batteries(LIBs,<350 mA h g^-1).They are low in cost and environmentally friendly and are expected to be used in new power batteries and large-scale grid energy storage.However,elemental sulfur has poor conductivity,large volume change during charging and discharging?⁸0%?,and the intermediate polysulfides are very soluble in ether electrolytes and undergoes a"shuttle"effect,resulting in low utilization ratio of cathode active materials,poor rate performance and poor cycle stability,and severely restricting the industrialization process.The composite positive electrode is prepared by dispersing or encapsulating sulfur on a porous or conductive polar carrier,which is expected to improve its electrochemical activity,reduce polarization,and relieve volume expansion during charge and discharge,especially reduce shuttle effect.Therefore,we can significantly increase the actual capacity,rate and cycle stability.In this thesis,titanium dioxide is used as the research object to explore its application in lithium-sulfur batteries.Regulating the crystal form,morphology,structural defects?oxygen vacancies?,building chemical bonds?Ti-C bonds?and heterojunctions improve the electrochemical performance.The main results are summarized as follows:?1?Considering that monoclinic TiO₂?B?has a relatively open crystal structure,we first explored its application in lithium-sulfur batteries.Through the hydrothermal reaction in alkaline solution and oleylamine as the structure-directing agent,we successfully prepared a flower-like structure composed of TiO₂?B?nanosheets.The material has a hierarchically porous structure and a high surface area(154 m²g^-1).Compared with standard cards,the?001?crystal plane has a larger interlayer spacing?0.8 nm?,which facilitates the adsorption and rapid insertion/extraction of lithium ions on the surface.For comparison,we also synthesized TiO₂?B?nanoribbons.Electrochemical performance tests show that TiO₂?B?nanosheets have higher lithium storage capacity and rate performance than nanobelts.Next,we loaded sulfur on two materials and assembled lithium-sulfur batteries.We also found that the TiO₂?B?nanosheets/sulfur composite positive electrode has higher electrochemical performance.We further studied the adsorption properties of two TiO₂?B?materials on Li₂S₆ electrolyte and found that the TiO₂?B?nanosheets have better adsorption capacity than TiO₂?B?nanoribbons for lithium polysulfide,indicating that the former has more active sites.?2?In order to improve the electrical conductivity of the electrode material,increase the utilization of sulfur and reduce the polarization of the electrode during charge and discharge,we have successfully designed and synthesized a porous TiO₂/carbon material with oxygen vacancies as a host using a simple hydrothermal reaction combined with in-situ carbonization.The presence of oxygen vacancies and a small amount of carbon?about 2%of the total mass?effectively enhances the conductivity of the material,while the porous structure can effectively load sulfur and relieve the micro strain caused by the volume change of the sulfur electrode during charge and discharge.What'more,the presence of polar TiO₂,especially oxygen vacancies,can effectively adsorb and immobilize lithium polysulfide and promote its electrochemical redox conversion and accelerate the electrode reaction kinetics.Therefore,the porous TiO₂/carbon/sulfur cathode has more excellent overall electrochemical performance than the pure porous TiO₂ material and the sulfur electrode.The above conclusion was further confirmed by comparing the adsorption experiments of Li₂S₆ electrolyte with TiO₂/carbon and TiO₂ materials and disassembling and subsequent characterization of the battery after charge and discharge test.?3?In order to further enhance the inhibitory effect of lithium polysulfide shuttle and its redox reaction kinetics,we used a porous TiO₂ material as a precursor and successfully designed and prepared a porous TiO2/BaTiO3 composite material by solvothermal method and annealing treatment as a host.In this material,besides the chemisorption effect of polar TiO₂ on lithium polysulfide,the spontaneous polarization induced by ferroelectric material BaTiO₃ can induce internal electric field during charge and discharge,and achieve effective adsorption and fixation of lithium polysulfide.What'more,the formation of TiO2/BaTiO3 heterojunction can promote the diffusion and migration of lithium ions on the heterojunction surface and accelerate the redox conversion of lithium polysulfide.Compared with pure porous TiO2 and BaTiO3,the porous TiO2/BaTiO3/sulfur electrode exhibits more excellent electrochemical performance.