Preparation And Electronchemical Performance Of Sulfur-based Composite Cathode Materials With High-sulfur Loading For Li-S Batteries

The elemental sulfur cathode has a very high theoretical specific capacity(1675m Ah g^-1),much higher than the current commercial lithium iron phosphate and lithium nickel/cobalt/manganate cathode.Moreover,sulfur is a kind of high-capacity cathode material with abundant resources and friendly environment.However,issues such as low conductivity of active substances in lithium-sulfur batteries,leaching and loss of polysulfides,and changes in electrode volume greatly hinder the industrial application of lithium-sulfur batteries.Especially under the practical condition of high positive sulfur surface load,how to improve the reversible capacity and cyclic stability of positive sulfur electrode is still an important challenge.In this paper,aiming at the problems existing in high surface load sulfur cathode,the influence of carrier structure,sulfur recombination mode and“core-shell type”cathode structure characteristics on the electrochemical performance of sulfur based cathode materials was studied,and a series of high performance high sulfur load composite cathode materials were obtained.The main contents are as follows:（1）Preparation of MnO₂/KB composite carrier and study on the performance of high sulfur load composite cathode.MnO₂/KB composite support material is obtained by in-situ loading MnO₂on commercial black KB.After melting and impregnating with high sulfur content,MnO₂/KB/S composite cathode with high sulfur load is obtained.MnO₂is uniformly loaded in KB pore structure,which can provide abundant polar adsorption sites,inhibit the dissolution and diffusion of polylithium sulfide,and improve the discharge capacity of the composite cathode with high sulfur load.The initial discharge capacity of MnO₂/KB/S composite positive electrode at 0.2C is 1044 m Ah g^-1,and the reversible capacity remains at 499 m Ah g^-1after 70 cycles,when the sulfur surface load is 4 mg cm^-2.（2）Preparation and electrochemical performance of S/C composite cathode materials with high sulfur load.The S/C composite cathode material with high sulfur load is obtained by secondary sulfur impregnation method.The effects of sulfur impregnation method and carbon carrier structure on the properties of sulfur based cathode material are studied.The results show that the S/C composite cathode（R-S/C）prepared by the secondary sulfur impregnation method has more porous structure and smaller charge transfer impedance,so it has higher discharge capacity and cycle stability.The composite carrier KB/CNT of Ketjen Black and carbon nanotubes can form a point-wire connection conductive network structure,which can further improve the recycling capacity of R-S/C composite cathode materials.The reversible capacity of R-S/KB/1.6CNT cathode material remains 550 m Ah g^-1after the initial discharge capacity of 975 m Ah g^-1at 0.2 C under the condition that cathode load is 4mg cm^-2.（3）Preparation and electrochemical performance of core-shell type S/KB@MnO₂composite cathode with high sulfur load.By the REDOX reaction of KMnO₄and S/MnO₂is coated on the surface of S/KB composites,and the core-shell type S/KB@MnO₂composite cathode with high sulfur load is prepared.The in-situ MnO₂coating is composed of porous nanosheet structure,which can not only physically block the dissolution diffusion of polysulfide,but also provide abundant polar adsorption sites to inhibit the flying shuttle of polysulfide.Under the joint action of MnO₂nanosheet shell and three-dimensional conducting carbon carrier core,the“core@shell”type S/KB@MnO₂composite cathode has excellent electrochemical performance.Under the condition of 4 mg cm^-2positive electrode load,the first discharge capacity of S/KB@MnO₂-16 composite cathode material at 0.2 C is as high as 1010 m Ah g^-1,100 cycles,and the reversible capacity remained 609 m Ah g^-1.