Study On Sulfur/Carbon Composite Cathode Materials For Lithium-sulfur Batteries

The capacity and energy density of lithium-sulfur battery is significantly higherthan the lithium-ion battery. However, lithium-sulfur batteries have poor electricalconductivity of cathode active substances, short life cycle and other shortcomings.Inthis paper, the composition and structure of the cathode materials has been optimized toimprove the performance of lithium-sulfur batteries. In terms of material composition, asuitable carbon material is selected. The structure of it is further optimized to provide abetter conductive network for the active material, as well as the adsorption forpolysulfides. In terms of the structure, activated carbon (AC) with high surface area,large pore volume and strong adsorption capacity is designed as framework foraccommodating sulfur. The sulfur/activated carbon composite (AC/S) are formed as thefirst level core-shell structure. The second level core-shell structure is conformed by thegraphene layer which is coated on the outside of sulfur/activated carbon. The core-shellstructure is conducive to restricted sulfur and its products.The preparation processes of sulfur/carbon cathode are explored. The binder,carbon material and the method of sulfur encapsulating into carbon material areoptimized. The specific capacity of the sulfur/carbon material with PTFE as binder at3C is9.9times higher than the PVDF binder. In addition, a hypothesis that suitableamount of electrolyte contained in the cathode can be automatically maintained due tothe usage of PTFE as the binder was proposed. Compared to the disproportionatingmethod, the melt-diffusion method is more beneficial for the uniform dispersion ofsulfur on the channel surface of the porous carbon material, and has betterelectrochemical properties.Compared with graphene, carbon nanotubes, CMK-3and vacuum expandedgraphene, the activated carbon attributes of high specific surface area, large porevolume and existence of small mesopores are proven important to achieve high specificcapacity, high-rate capability and long-term cycling stability of AC/S composites withsulfur contents as high as75%. On the basis of theoretical maximum sulfur contentanalyses, the AC/S composites with suitable sulfur contents were designed and preparedthrough two facile synthetic routes to completely encapsulate sulfur inside AC pores. Asa result, the AC/S composite delivers high initial discharge capacity of1334mAh g-1at0.1C.The two levels core-shell structure (S/AC@graphene) is formed by graphenecoating on the outside of AC/S composites, which facilitates the limition of lithiumpolysulfides between AC and graphene. The diffusion of lithium polysulfides from theAC pores is prevented. Metal oxide Li4Ti5O12and PVP is added into the S/AC@graphene composites to improve the cycling performance, respectively. Hydrothermal method is used to form3D conducting graphene layer to connectAC/S materials. And the conductivity of the graphene coated layer is also improved byhydrothermal reduction. The rate performance of the S/AC@graphene composites isreinforced. The initial discharge capacity is as high as1467mAh g-1at the rate of0.1C.Even at3C, the discharge capacity is630mAh g-1.