Preparation Of Porous Composites Sulfur Cathode Materials And Their Energy-Storage Performance

Lithium-sulfur?Li-S?batteries have been attracted much attention as the next generation energy storage system because of their theoretical specific energy of 2500 Wh kg^-1,theoretical specific capacity of 1675 mAh g^-1,low cost and high abundance of sulphur.However,due to the low conductivity and severe shuttle effect of sulfur,Li-S batteries have the following challenges:low Coulomb efficiency,poor cycle stability,fast decaying discharge capacity,shuttle effect of polysulfides and low utilization of active sulfur materials.In order to solve these problems,researchers have developed and designed various three-dimensional?3D?porous nanomaterials,which can be used as sulfur main materials because of their large surface area and electrochemical stability.These materials can promote Li⁺diffusion,shorten the electronic transmission path,alleviate the change of electrode materials during charging/discharging,and increase sulfur utilization rate by large specific surface area.In addition,conductive polymers are also widely used in lithium-sulfur batteries.For example,polypyrrole?PPy?was a strong interaction with polysulfides.Because of its unique long chain structure and lone pair electrons in nitrogen atoms in PPy,which could reduce the dissolution of polysulfides.In addition,PPy layer with excellent mechanical elasticity,and rough surface can adapt to volume expansion,and enhance the adhesion of active substances,and further capture dissolved polysulfides,improving the cycle stability of batteries.In this paper,the problems of lithium-sulfur batteries have been greatly improved by designing composites cathode materials with hollow porous structure and sulfur wrapped in polypyrrole.The research contents are as follows:1.Hollow zeolite microspheres?HZS?were synthesized by alkali etching and loaded with elemental sulfur in the cavity.Conductive polymer polypyrrole coated zeolite microspheres.HZS have abundant surface area and pore volume.They act as efficient Nano-sulfur carriers.PPy nanoparticles are modified on the surface of HZS as conductive networks.The core-shell structure HZS/S@PPy has the following advantages:?1?porous HZS spheres provide sufficient sulfur storage space and act as the intermediate of the barrier chemisorption layer of polysulfides,thus reducing sulfur loss,?2?HZS can adapt to the volume expansion of sulfur and promote structural stability,?3?Conductive PPy particles are evenly distributed on the surface of HZS/S to enhance the conductivity ofHZS/S and further improve the structural stability.2.Three-dimensional hollow porous CoMn₂O₄ was synthesized by one-step hydrothermal synthesis of ZIF-67 and KMnO₄.CoMn₂O₄/S was prepared by melt diffusion method.Polypyrrole conductive layer was further coated on the surface of CoMn₂O₄/S composite material to further enhance the cycle stability and high discharge capacity of Li-S battery.The three-dimensional hollow structure can accommodate sulfur adsorption,increase sulfur load and reduce polysulfide.The conductive polymer enhances the conductivity of the composites and improves the conductivity,and the cycle stability and rate performance are greatly improved.3.Nitrogen-doped porous carbon materials were successfully prepared using starch and melamine as raw materials.Carbon materials and sulfur powder were mixed in a certain proportion and sulfur-coated nitrogen-doped porous carbon composites were prepared by melt infiltration.Composite of sulfur and nitrogen-doped carbon materials and uniform combination of nitrogen atoms into carbon materials with controlled chemical properties can greatly change the properties of bulk carbon structure.Porous structure can increase sulfur load and reduce the dissolution and transfer of polysulfides,thus increasing the capacity of Li-S battery and enhancing the cycle stability of battery.The results show that the surface chemical modification with carbon as the main body is a good method to improve the electrochemical performance of carbon/sulfur based cathode materials.