Preparation And Electrochemical Performance Of Porous Lithium-Sulfur Battery Cathode Materials

Lithium-sulfur batteries are regarded as the most promising next-generation lithium-ion batteries due to their high theoretical specific capacity and low raw material prices.However,several problems in lithium-sulfur batteries seriously restrict the commercial application of lithium-sulfur batteries.First,elemental sulfur and discharge products Li₂S are poor conductors about electrons and ions,resulting in low utilization rate of active materials.Second,the lithium polysulfide will dissolve in the electrolyte and cause the shuttle effect between the positive and negative electrodes during the discharge process and resulting in a loss of active material.The third is the complicated phase change of the lithium-sulfur battery during the charge and discharge process.The electrochemical kinetics are slow,resulting in rapid charge and discharge at high current density.In order to improve the electrochemical performance of lithium-sulfur battery cathode materials,this paper starts with the above three problems and prepares lithium-sulfur battery cathode materials.The main research contents are as follows:1.For the low conductivity of elemental sulfur and discharge products,NH₄Cl and sucrose are simply mixed and ground in a certain rat io to obtain a rich porous carbon by using the same decomposition temperature of NH₄Cl and the melting temperature of sucrose.The effects of different carbon sources,different ratios of sucrose and NH₄Cl on the pore structure of the material were studied.It was found that sucrose as a carbon source made it easier to prepare porous carbon materials.T he effects of different ratios of sucrose and NH₄Cl on porous carbon materials were further studied.From the microscopic morphology analysis,when the mass ratio of sucrose to NH₄Cl was 1:2,the pore structure was honeycomb-shaped and the pore walls were thin.It is exhibits excellent electrochemical properties as a carrier of sulfur.On the basis of this work,the preparation process of the above porous carbon is improved,and the three-dimensional porous carbon material with richer porosity is prepared by the method of activation after KOH etching.Compared with the without KOH etching activated carbon.Porous carbon materials exhibit superior rate performance and cycle stability after sulfur loading,which is mainly due to the rich three-dimensional multi-porous pore structure,which can fully contact carbon and sulfur,and provide a good electronic conduction network for electroc hemical reactions,enhance the electrochemical reaction kinetics of the electrode.2.For the shuttle effect of lithium polysulfide in lithium-sulfur batteries,Metal-Organic Frameworks MIL-101?Cr?large surface area of 1919.7 m²/g was successfully prepared as a host material for sulfur.After electrochemical tests,it was found that although the MIL-101?Cr?/S electrode material attenuated the specific capacity of the electrode with increasing charge and discharge current density,the first discharge platform at 2.3 V only attenuated 66.71 mAh/g.It is indicated that the metal organic framework material MIL-101?Cr?as a carrier material for sulfur can significantly inhibit the shuttle effect of lithium polysulfide.3.Based on the problem of the shuttle effect of the cathode and the slow kinetics of electrochemical reaction.CoS₂ was successful to in situ-grow on the carbon nanotube substrate forming CNT-CoS₂-n composites by simple hydrothermal method.It is found that in situ-grow CoS₂ significantly enhances the polysulfide redox reaction process after CNT-CoS₂-n is used as the host material of the sulfur of lithium-sulfur battery.This research find that CoS₂ catalysis the conversion of Li₂S₆-Li₂S₄ and Li₂S₄-Li₂S₂ process,and reduce the polarization in the charge and discharge.improves cycle stability.CoS₂ proved to be an effective sulfur cathode catalyst process.When the content of CoS₂ is 20%of CNT,it exhibits the most excellent electrochemical performance.