The Modification Of Sulfur Host And Separator And Their Application In Lithium-sulfur Batteries

With the rapid development of the economy,the available fossil fuels are increasing exhaustion,the development of clean and renewable energy is very important.Therefore,the rechargeable battery as an electrical energy storage device has received more attention.Among them,lithium-sulfur batteries with high theoretical capacity are one of the most promising energy storage devices.However,lithium-sulfur batteries have several problems that restrict their development and application.For example:the poor conductivity of the active material sulfur leads to low battery discharge capacity and poor rate performance;the shuttle effect of the intermediate product polysulfide in the electrolyte,resulting in severe capacity attenuation;and volume expansion problem,destroying the electrode structure.Aiming at the problem of the shuttle effect of lithium-sulfur batteries,this dissertation designs and synthesizes some composite materials,such as titanium dioxide（TiO₂）nanosheets/carbon nanotubes（CNTs）andTiO₂（B）/reduced graphite oxide（RGO）composite to modify the separator.The dual polysulfides adsorption of physical and chemical theories enhanced the adsorption ability effectivety.On the one hand,the porous polyaniline（PANI）/reduced oxide graphene（RGO）was designed as sulfur carrier with the flexible network structure to solve the problems of low sulfur loading,poor conductivity,and electrode volume expansion.The material characterization and electrochemical performance tests were performed on the modified separotor and sulfur host materials.The specific research contents are as follows:（1）The TiO₂ nanosheets with high specific surface area were prepared by stripping method and composited with carbon nanotubes to prepare the multifunctional modified coating on the separator by vacuum filtration.TiO₂ can chemically anchor polysulfides by forming Ti-S bonds and has excellent catalytic performance.The two-dimensional layered TiO₂ nanosheets and CNTs are intertwined to form a conductive network structure,which provides more reaction sites to adsorb the polysulfides.At a current density of 0.2 C,when the mass ratio of TiO₂ nanosheets to carbon nanotubes is 1:4,the first discharge capacity of the battery using the modified separator reaches 1247 mAh g^-1,which has the best discharge specific capacity and cycle performance.The results of the examinations show that the modified separator has a strong polysulfide adsorption ability and significantly improves the performance of the Li-S battery.（2）The RGO and TiO₂（B）composite material was synthesized by using the in-situ hydrothermal method and modified coating the separator by vacuum-deposited method.The SEM image showed that TiO₂（B）was uniformly dispersed on the RGO sheet.The RGO has the high conductivity and increases the conductive interface between the separator and the positive electrode.At the same timeMoreover,the oxygen-containing functional groups on the RGO sheets and TiO₂（B）adsorb the polysulfides by physical and chemical adsorption.Nanosized TiO2（B）provides a more open channel along the[010]direction,providing a fast channel for the migration of lithium ions.The first discharge specific capacity of the battery using RGO/TiO₂（B）modified separator is 1097 mAh g^-1.The results indicate that RGO/TiO2（B）relieves the polysulfide shuttle effectively,significantly improving the cycle performance and rate performance of lithium-sulfur batteries.（3）The porous polyaniline（PANI）are synthesized by using a salt template synthesis method,and it has the porous and flexible network structure.The RGO/porous PANI composite was obtained through self-assembly hydrothermal and reduction treatments,which was used as the sulfur host material.The flexible network structure of PANI provides the space for slowing down the volume expansion of sulfur during the charge and discharge process of the rechargeable battery.PANI as the conductive polymer,it can improve the conductivity of the electrode.The porous structure shortens the lithium ion transmission path.The connected network structure can also effectively adsorb polysulfides.RGO with the high surface area layered structure was able to uniformly disperse sulfur in the host material,providing more reaction sites for active substances and improving sulfur utilization.Synergistic effect of the two materials after hydrothermal treatment,the sulfur-carrying material RGO/porous PANI increased the sulfur loading and inhibited the polysulfide shuttle.