Modified Synthesis And Electrochemical Performance Of Sulfur/carbon Cathodes For Lithium-sulfur Batteries

Lithium-sulfur batteries(LSBs)based on sulfur cathodes possess overwhelming advantages in terms of theoretical capacity and energy density,and elemental sulfur is abundant,low-cost,eco-friendly and reliable.Therefore,LSBs are recognized as the most promising next-generation energy storage systems.Whereas,LSBs are puzzled by several drawbacks impeding their practical application.The insulating nature of sulfur and sulfides results in ultralow sulfur utilization,the huge volume expansion of leads to the collapse of sulfur cathode,and the high solubility of lithium polysulfides gives rise to severe capacity degradation during cycling.Restraining the dissolution of polysulfides and improving the conductivity of sulfur cathode are the key tasks for the enhanced development of LSBs.In this paper,conductive carbon,conductive polymer and metal compound are engaged to modify the sulfur-carbon black composites.And biomass carbon of special structure is composited with sulfur for fabricating advanced sulfur cathodes.The specific research contents and results are as follows:(1)The S@CB/VGCF composites of bridged structure are fabricated by solvent dispersion and molten sulfurizing.VGCF is tightly embedded in the S@CB clusters and connect the clusters of decreased size together.The constructed highly-conductive structure supplies electrons rapid transfer paths for sulfur and optimizes the electrochemical kinetics of the electrode.The modified S@CB/VGCF electrode exhibits improved electrochemical performance.An initial discharge capacity of 1062/758 mAh g-1 is achieved at 0.2/1 C and the capacity retention is 53.9%/76%after 200 cycles.The discharge capacity at 5 C is achieved 500 mAh g-1.(2)The S/C@pDA composites of core-shell structure are fabricated by molten sulfurizing and polymerization reaction.Both inside and outside blocking layers are realized for the polysulfides in the designed core-shell composites.In such a novel structure,conductive carbon black serves as the matrix for loading sulfur to facilitate rapid electrons transfer,and pDA acts as a protective layer to hinder the dissolution of polysulfides.The modified S/C@pDA electrode exhibits improved electrochemical performance.A maximum discharge capacity of 1185 mAh g-1 is achieved at 0.2 C and the capacity retention is 85%after 200 cycles.The discharge capacity at 5 C is achieved 533 mAh g-1.(3)The CB/Co3O4/S composites of self-assembly microspheres are fabricated by solution combustion synthesis,solvent dispersion and thermal treatment.Sufur is well dispersed in the microspheres with slightly open morphology and rough surface,which helps the active materials to contact with electrolyte adequately.The relatively unstable P-S produced in the thermal process is easier to dissolve in the electrolyte participating in electrochemical reaction.The increased graphitization degree of CB caused by the catalytic action of Co3O4 can improve the conductivity of sulfur cathode.The modified CB/Co3O4/S electrode exhibits improved electrochemical performance.An innitial discharge capacity of 1173 mAh g-1 is achieved at 0.2 C and the capacity retention is 79.7%after 300 cycles.The discharge capacity at 5 C is achieved 630.6 mAh g-1.(4)The CHPC/CoS2/S composites of hierachical porous structure are fabricated by KOH activation,carbonization,hydrothermal treatment and molten sulfurizing.The composites can load more sulfur and produce dual effect of "physical confine and chemical entrapment" on polysulfides.The conductive and porous carbon frame provides adequate space to accommodate more sulfur and physically supress the shuttle effect of polysulfides.The embedded half-metallic CoS2 sites can chemically anchor the polysulfides and enhance the electrochemical reaction activity as well.The designed CHPC/CoS2/S electrode exhibits improved electrochemical performance.An innitial discharge capacity of 1230.9 mAh g-1 is achieved at 0.2 C and the capacity retention is 85.2%after 250 cycles.The discharge capacity at 5 C is achieved 506.1 mAh g-1.(5)The PGCF/Ni/S composites of porous fibrous structure are fabricated by hydrothermal treatment,NiCl2 pore-forming and supercritical CO2 fluid sulfurizing.The PGCF/Ni is filled with interconnected pores,and Ni nanoparticles are firmly embedded in the porous graphitic carbon fibers.The high porosity and large surface area in the PGCF/Ni can supply more space of sulfur loading and benefit rapid ion transfer.The metal-catalyzed graphitic carbon and embedded Ni nanoparticles can construct a high conductive network,affording rapid electrons transfer channels.The designed PGCF/Ni/S electrode exhibits improved electrochemical performance.An innitial discharge capacity of 1198/856 mAh g-1 is achieved at 0.2/1 C and the capacity retention is 86%/83%after 200/300 cycles.The discharge capacity at 5 C is achieved 746 mAh g-1.