Synthesis Of TiO₂ Based Composites And Their Applications In Lithium-sulfur Batteries

In recent years,with the increasingly serious environmental problems caused by the extensive use of fossil fuels,it is imperative to develop clean and renewable energy source.The development of secondary batteries with safety,stability,high energy density and long cycle life is one of the most crucial missions to realize the used of clean energy.Lithium-ion secondary batteries were commercialized in 1990s and have occupied a dominant positon in the market,and become the most successful energy storage devices used in small portable electronic devices.However,the energy densities of lithium-ion batteries are unable to meet the increasing demand of large-scale energy storage systems.Among the numerous secondary batteries,lithium-sulfur batteries?LSBs?have been considered as one of the most promising next-generation energy storage system because of its high specific capacity(1675 mAh g^-1)and high theoretical specific energy(2500 Wh kg^-1).In addition,sulfur also have the advantages of abundant reserves and low-cost.However,intrinsic properties of LSBs are thought to be responsible for many technical problems in the development of LIBs,including the shuttle effect of polysulfide intermediates?Li₂S_n;4?n?8?during cycling,the poor conductivity of sulfur and its discharge product Li₂S,and the volume change between sulfur and Li₂S,which seriously affect the practical applications of LSBs.Designing the structure of cathode materials and modified the separator can effectively solve these problems and improve the cycling performance of LSBs.In this paper,we attempt to utilize TiO₂ as the adsorbent of lithium polysulfide and combine TiO₂ with carbon materials to preparing kinds of TiO₂/carbon composite.The prepared TiO₂/carbon composites were applied as functional cathodes and separators for high performance LSBs.The main contents are shown as following:1.Porous carbon materials were prepared by KOH etching at high temperature using cheap conductive carbon as precursor.TiO₂ nanoparticles uniformly deposited on porous carbon materials as efficient sulfur host.The TiO₂ nanoparticles interspersed graphene sheets was also used to prepared a sulfur electrode with dual adsorption structure.When used as the cathode of LSB,it delivered a high reversible capacity of 717 mAh g^-1 after 200 cycles at 0.5 C,displaying excellent cycling performance.2.Further simplifying the preparation of the cathode electrode,the commercial TiO₂ nanoparticles and CNTs were used to prepared the cathode with a sandwich structure by a simple coating method towards the industrial production.The conductive CNTs network can facilitate electrons transport and TiO₂ can limit the shuttle of polysulfides as an adsorbent in LSBs.When used as the cathode of LSBs,a discharge capacity as high as 817 mAh g^-1 was obtained after 200 cycles at 0.5 C,which proves the feasibility of this simple and controllable cathode preparation method.3.A modified separator by a composite TiO₂ nanosheets and graphene has been developed by vacuum filtration method,where the TiO₂ nanosheets were prepared by hydrothermal method.The modified layer of the separator can be used as an adsorption layer to suppress the shuttle effect,and on the other hand,it can serve as an upper current collector to provide more reaction sites for the electrochemical reactions during charge-discharge processes.As a result,LSBs with such a separator exhibit excellent cycling stability.4.Further simplify the preparation method in the previous section,commercial TiO₂ nanoparticles were used to replace the TiO₂ nanosheets,and modify the separator by the same method.It was found that commercial TiO₂ nanoparticles also improve the cycle performance of LSBs,and exhibit better rate performance than TiO₂ nanosheets,which is determined by the morphology of the TiO₂ we used.The granular TiO₂ can be used as a scaffold to support the graphene sheet,making the modified layer has a porous structure,which can ensure the rapid passage of electrolyte molecules and lithium ions.At the same time,the modified layer has a larger electrochemical reaction area,which accelerated the charge and discharge rate,improving the rate performance of LSBs.