| The development of high energy-density(specific capacity=1672 mAh/g)and environmentally friendly lithium-sulfur batteries is of strategic importance for national energy transformation.On the commercialization of lithium-sulfur batteries,the shuttle effect of intermediate polysulfides is a major problem need be solved.Transition metal sulfides have the advantages of good electrical conductivity and better sulfophilicity to polysulfides,which play an important role in the adsorption and catalytic conversion of polysulfides in lithium-sulfur batteries.In addition,deficiencies can also affect the electrical properties of the material,so the introduction of deficiencies into transition metal sulfides is expected to further expand its advantages.In this paper,TiS2 was selected as a typical transition metal sulfide.The TiS2-x material containing sulfur deficiencies was successfully prepared by simple solid-phase reaction and vacuum annealing,and it was used as a cathode promoter to study the influences on the electrochemical performance of lithium-sulfur battery.The main research contents of this article are as below:1.Study on the effect of deficient TiS2-x on the polysulfide adsorption and redox reaction.Firstly,the TiS2 sheet material was synthesized by a simple solid phase reaction method,and the TiS2-x sample was obtained by vacuum heat treatment later.Then the existence of sulfur vacancies in TiS2-x was confirmed by electron spin resonance(ESR)and X-ray photoelectron spectroscopy(XPS).Next,it was further confirmed by lithium polysulfide adsorption experiments and electrochemical tests that sulfur deficiencies had a promoting effect on chemisorption and redox conversion of polysulfide.Thereafter,the material was applied to the cathode of the lithium-sulfur battery.Compared with TiS2,benefiting from the electron-rich active sites generated by the sulfur vacancies,the TiS2-x significantly improved the overall performance of the lithium-sulfur battery.For example,at a current density of 1 C,the specific capacity could still maintain 751 mAh/g after 500 cycles,and the average capacity fade rate is as low as 0.04%per cycle.In addition,the cathode with a sulfur loading of 4 mg/cm2 could display a specific capacity as high as 963 mAh/g over 50 cycles at 0.2 C,and a corresponding capacity retention as high as 91.7%.2.Study on the role of TiS2-x nanobelts and TiS2-x/CNTs composites in the cathode of lithium-sulfur batteries.The TiS2-x nanobelts were synthesized by taking TiS3 as precursor,which is more easily to synthesize and has a larger specific surface area(?16 m2/g).And the sulfur defect concentration in the material was controlled by adjusting the reaction time,which was determined by ESR tests.The better effect of TiS2-x-40 on the polysulfides redox conversion than other samples was preliminarily proved by the cyclic voltammetry of symmetrical cells.Then the TiS2-x-40/S composite was used as the cathode of lithium-sulfur battery and the corresponding electrochemical performance was investigated.The results showed that the specific capacity of TiS2-x-40/S electrode was 923 mAh/g after 80 cycles with the capacity retention rate of 84.2%at 0.2 C.After the introduction of the purchased multi-walled carbon nanotubes,the performance of the lithium-sulfur batteries changed in both capacity density and cycling stability.Specifically,under the same conditions,the initial specific capacities decreased by about 13%,while the cycling stability increased,manifesting as the average capacity fade rates decreased by about 40%.This may be because the inner diameters of the CNTs were too small,thus the active materials could not be fully utilized.Therefore,we could improve the cycling stability and keep the high capacity density of the Li-S batteries concurrently by appropriately increasing the inner diameters of the carbon nanotubes.And this work needs to be further completed. |
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