Synthesis And Applications Of Multifunctional Composites In Lithium-Sulfur Battery Cathode

Benefiting from the advantages of high theoretical energy density,low-cost sulfur cathodes and environmental benignity,lithium sulfur（Li-S）batteries are considered as the most promising candidate of new-generation electrochemical energy storage systems.Unfortunately,the practical application of Li-S batteries is greatly impeded by their low sulfur utilization,poor rate performance and short cycle life.In this paper,we came up with the strategy that combined different materials to construct multi-functional composites for sulfur cathodes.Specifically,we designed and prepared two kinds of composites:"Ti O-Ti O₂ heterostructures-polypyrrole"and"Mo₂C-carbon",aiming to combine the different advantages of polar metal compounds and conductive polymer/carbon materials to improve the performances of Li-S batteries synergistically.The main contents of this paper are as follows:（1）Initially,titanium dioxide（Ti O₂）was partially reduced to Ti O-Ti O₂heterostructure materials（H-Ti O_x,x=1,2）by a facile magnesiothermic reduction method.After that,sublimed sulfur was loaded onto the H-Ti O_x host by a modified“CS₂ volatilization+sulfur melt-diffusion”method.Finally,the as-prepared H-Ti O_x@S particles were coated by polypyrrole（PPy）layers via the in-situ polymerization of pyrrole to afford the H-Ti O_x@S/PPy composites.The experimental results show that H-Ti O_x and PPy can not only improve the conductivity of the electrode,but also synergistically adsorb lithium polysulfides（Li PSs）via"Lewis acid-base"and"polar-polar"interaction,respectively.Benefiting from the synergistic effect of H-Ti O_x and PPy layers,H-Ti O_x@S/PPy composites show excellent electrochemical performances when applied to the cathodes of Li-S batteries.For example,the Li-S battery with H-Ti O_x@S/PPy cathode delivers high specific capacities at different current rates（1130,990,932,862 and 726 m Ah/g at 0.1,0.2,0.3,0.5 and 1 C,respectively）and obtains an ultra-low capacity decay rate of0.0406%per cycle after 1000 cycles at 1 C.（2）Novel“molybdenum carbide-copper-carbon nano-octahedrons”（Mo₂C-Cu-C NOs）were prepared via a metal-organic frameworks（MOFs）assisted strategy.Subsequently,Mo₂C-C NOs with Mo₂C nanoparticles embedded in three-dimensional porous carbon matrices were obtained by removing Cu particles with Fe Cl₃ solution.After that,Mo₂C-C NOs@S composites with a sulfur content of 72.15 wt%were prepared by infiltrating sulfur into the as-obtained Mo₂C-C NOs via a melt-diffusion method.The test results show that the existence of Mo₂C nanoparticles can not only immobilize Li PSs via the formation of Mo-S bond,but also electrocatalytically accelerate the conversion between sulfur and Li PSs.Meanwhile,the interconnected porous carbon matrices contribute to effective sulfur/electrolyte infiltration,fast electron/Li⁺ion transportation and sufficient buffer for volume change.As a result,the Mo₂C-C NOs@S cathode demonstrates a high initial specific capacity of 1396 and1050 m Ah/g at 0.1 C and 1 C,respectively,with an ultra-low average specific capacity decay rate of 0.0457%per cycle within 600 cycles at 1 C.Moreover,the Mo₂C-C NOs@S cathode with high areal sulfur loading of 4.2 mg_sulfur/cm²can deliver a high initial discharge specific capacity of 807 m Ah/g,and a capacity of 623 m Ah/g after 100 cycles at 0.5 C.