Preparation And Electrochemical Performance Of Sulfur-based Cathode With High Energy Density

Lithium-sulfur battery with sulfur as cathode and lithium as anode is the most promising next-generation energy storage system due to the high theoretical energy of2.6 kWh kg^-1,as well as the natural abundance,low cost and environmental friendliness for elemental sulfur.Unfortunately,the low sulfur utilization,short cycling life and low coulombic efficiency,resulting from the dissolution of the intermediate long-chain lithium polysulfide in ether-based electrolyte as well as the inherent electrical and ionic insulation of elemental sulfur and discharged products?Li₂S₂/Li₂S?,great hinder its practical application.Tremendous efforts have been made to solve above problems,most of which are focused on incorporating sulfur into various carbon nanomaterials.However,the low sulfur content and tap density are the common feature mostly based on carbon materials,which are intrinsic factor for poor gravimetric and volumetric capacities of lithium-sulfur battery,resulting in the practical energy density usually far below the theoretical value.Thereof,exploring the stable sulfur cathode with high specific capacity is the pivotal issue to obtain the lithium-sulfur battery with high energy density.In this work,the high-density,polar and conductive metal oxide with electrocatalytic property is desired as the sulfur matrix in order to achieve both the high energy density and good electrochemical performance.Firstly,conductive cobalt oxyhydroxide?CoOOH?sheets are obtained after the dehydrogenation reaction of Co?OH?₂ by the oxidation with NaClO solution.The sulfur-based composites are prepared by liquid method,and the sulfur content is up to91.8 wt%.The structure characterization shows that CoOOH sheets cross over each other to form three-dimensional conductive frameworks,where sulfur active material is distributed uniformly.The adsorption and XPS tests reveal that CoOOH can delieve strong adsorption capacity and good electrocatalytic activity towards polysulfide species.In lithium polysulfide environment,Co-O bonds are partially cleaved,accompanied with the oxidation of polysulfides to insoluble thiosulfate/polythionate complex and the formation of Co-S bands.In addition,the electrochemical tests indicate the good electrochemical performance of the S/CoOOH composite.The initial gravimetric capacity of the composite is 1199.4 mAh g^-1_-composite at 0.1C rate,accompanied with sulfur utilization of 78.1%.Meanwhile,the volumetric capacity of the S/CoOOH composite is up to 1511.3 mAh cm^-3 with the tap density of 1.26 g cm^-3,much higher than that of the S/C composite.The composite presents satisfactory cycle stability with a slow capacity decay rate of 0.09%per cycle within 500 cycles at 1C rate.Owing to the high conductivity and electrocatalytic activity,CoOOH sheets contributes much to the sulfur utilization and the suppression of shuttle effect.Secondly,the conductive ruthenium dioxide?RuO₂?stacking microspheres are obtained via the solvothermal-calcination method with glucose as template and RuCl₃as ruthenium source.The S/RuO₂ composite with sulfur content of about⁸2 wt%is obtained with a facile melt-diffusion method.The structure characterization indicates that amounts of RuO₂ microspheres with inhomogenous size of a few hundred nanometers and rough surface are gathered together,presenting three-dimentional conductive frameworks.The specific surface areas of RuO₂ spheres increase as the usage of glucose template,and when the glucose usage is 15 mmol,the composite shows the highest capacity retention rate.The S/RuO₂ composite deliver the maximum gravimetric capacity of 985.2 mAh g^-1_-composite,and 788.4 mAh g^-1_-composite after 100cycles at 0.2C rate.The composite also present stable cycling performance with 0.08%per cycle within 500 cycles at 1C rate.Remarkably,the S/RuO₂ composite delivers outstanding rate performance at large current density of 5C rate with 479.2 mAh g_-^-1_composite obtained.The conductive polar RuO₂ microspheres with high specific surface areas are stacked together,providing not only good conductive framework and ion diffusion paths,but also abundant absorption and electrocatalytic sites towards polysulfide,which are in favour of entrapping soluble polysulfide and accelerating redox reaction rate.the stacked RuO₂ microspheres with inhomogenous sizes maximize the tap density of the S/RuO₂ composite with 1.38 g cm^-3,thus the volumetric capacity of the S/RuO₂ composite(1359.6 mAh cm^-3)is obvious higher than that of the S/C compositeIn summary,the polar electrocatalytic and conductive metal oxide is prepared as the sulfur immobilizer to fabricate sulfur-based composite for lithium-sulfur battery in this thesis.Composites deliver high gravimetric and volumetric capacities,as well as good cycle stability.Overall,this work unfolds a promising strategy for the lithium-sulfur battery with high energy density.