Syntheses And Performance Of Carbon/Sulfur Hybrids Cathode Materiais For Lithium Sulfur Batteries

Lithium-sulfur batteries with high theoretical capacity(1675 m Ah·g^-1),and high energy density(2600 Wh·kg^-1)is considered to be one of the most promising energy storage devices.In addition,Lithium-sulfur batteries also shows advantages of low cost,natural abundance,low toxicity,and environmental friendliness.However,the lithium-sulfur battery positive electrode still has the defects of fast capacity decay,short cycle life,and low Coulombic efficiency.The main problems are?1?the poor conductivity of elemental sulfur and the final discharge product Li₂S;?2?the shuttle effect caused by the dissolution and diffusion of intermediate polysulfides;?3?large volume expansion during charge and discharge.Therefore,the construction of new sulfur cathode material is of great significance to the development of lithium sulfur battery.In order to solve the above problems,we should solve the positive electrode conductivity of lithium sulfur battery,inhibit the shuttle effect and alleviate the volume expansion.The ideal sulfur anode material should have excellent conductivity to alleviate the poor conductivity of sulfur;Porous or hollow structure with a large specific area to solve the volume expansion during charge and discharge,physical limits,and full contact between the active material and the electrolyte;Abundant active sites inhibit the shuttle effect.Based on the above considerations,this paper design transition metal compounds@carbon composites with different structures and morphologies by using dopamine and glucose as carbon sources and transition metal compounds?Co P₃-Co₃O₄,Ni Co₂S₄,Fe₃C?as adsorbents.The specific research content is as follows:?1?NC@Co P₃-Co₃O₄ composite material is prepared by growing cobalt phosphide and cobalt oxide heterojunction on the surface of carbon material.Hollow carbon tubes provide abundant sulfur storage space,and can effectively alleviate the volume expansion of sulfur element during charge and discharge process,and the surface of Co P₃-Co₃O₄ and provide active sites during charge and discharge process,adsorb and promote the conversion of polysulfide.Thanks to the above advantages,the initial discharge specific capacity of the NC@Co P₃-Co₃O₄/S electrode material at 1 C rate is 789.9 m Ah·g^-1,and the discharge specific capacity after 700 cycles is 476.3 m Ah·g^-1,and the attenuation rate is 0.057%per cycle,which indicates that the NC@Co P₃-Co₃O₄ material has significantly improved the cycle stability and long cycle life of lithium-sulfur batteries.?2?Ni Co₂S₄@NC material was prepared by high temperature pyrolysis with Prussian blue analogues as template and dopamine as carbon source,and then sulfurization.The spinel structure of Ni Co₂S₄ has excellent electrical conductivity and higher electrochemical characteristics.Ni Co₂S₄ provides the active site for adsorption of polysulfide and promotes the conversion of polysulfide.The surface-coated carbon material improves the conductivity of the material.The cubic structure can also physically limit the polysulfides,effectively alleviate the volume expansion during the electrochemical reaction,so that the lithium-sulfur battery has good cycle stability and long cycle life.Ni Co₂S₄@NC has an initial capacity of 812.7 m Ah·g^-1at a rate of 1 C,a capacity of 579.3 m Ah·g^-1 after two hundred cycles,and a capacity retention rate of 71.3%;the specific capacity still maintains 480.1 m Ah·g^-1 after 350 cycles,the capacity retention rate is 59.1%,which is equivalent to a single-turn capacity decay of 0.168%per cycle.This shows that Ni Co₂S₄@NC protects the stability of Ni Co₂S₄ during cycling after dopamine carbonization coating,and can play a synergistic effect of physical limits and chemical adsorption on polysulfides,which significantly improves long-cycle performance.?3?We used glucose as the carbon source and iron nitrate as the iron source to generate Fe₃C@C materials with irregular hollow interconnected porous nanosheet structure,through NH₄Cl at high temperature.The discharge capacity of Fe₃C@C/S electrode material at 0.2 C is1190.2 m Ah·g^-1.When the current density increases to 2 C,the discharge capacity remains at587.7 m Ah·g^-1.When the current density returned to 0.2 C,the capacity of Fe₃C@C/S returned to 877.1 m Ah·g^-1,and the retention rate was 73.7%.The first discharge capacity of Fe₃C@C/S electrode material at 1 C rate is 789.5 m Ah·g^-1,the capacity is 583.3 m Ah·g^-1 after 200 cycles,the capacity retention rate is 74.1%,and the attenuation rate per cycle is 0.13%.Fe₃C not only can be used as an adsorbent for polysulfides to suppress the shuttle effect,but also can significantly improve the conductivity of sulfur cathodes with carbon materials.The porous and fluffy sheet structure can effectively alleviate volume expansion during charge and discharge,and effectively improve the cycle stability and rate performance of lithium-sulfur batteries.