Preparation And Properties Of Molybdenum-based Compound Cathode Materials For Lithium-Sulfur Battery

The superior capacity and high energy density of lithium-sulfur battery make it become a research hotspot,and sulfur is abundant in nature,non-toxic and environmentally friendly.However,the actual application of lithium-sulfur batteries still confronts many obstacles,including the low utilization rate of active substances due to sulfur insulation,the intermediate polysulfide which will not only dissolve in the electrolyte but also“shuttle”back and forth between the electrodes,resulting in serious decrease of the active substance,the damage of cathode material caused by volume expansion caused by density difference of sulfur and its reduction product.In this paper,molybdenum-based compound materials:molybdenum disulfide and molybdenum trioxide and modifying them as cathode materials to overcome the difficulties faced by lithium-sulfur batteries.The main research contents and results were as follows:（1）Effect of molybdenum disulfide（MoS₂）as monolayer and interlayer on performance of lithium-sulfur batteries.Molybdenum disulfide/graphene（MoS₂/G）and molybdenum disulfide/carbon nanotubes（MoS₂/CNTs）composites were prepared by simple hydrothermal synthesis method.The MoS₂/G and MoS₂/CNTs composite materials were combined with sulfur as the cathode of lithium-sulfur battery.Meanwhile,MoS₂/G and MoS₂/CNTs as interlayers were coated on the carbon nanotubes/sulfur（CNTs/S）material as the cathode to explore the influence of MoS₂ as a monolayer and interlayer on the performance of lithium-sulfur battery.The results reveal that MoS₂/G has excellent electrochemical performance as an interlayer.This is because the MoS₂/G intermediate layer can be used as a secondary collector to recycle sulfur,and graphene（G）has excellent conductivity and a large specific surface area,which is an ideal material for modifying electrode of lithium-sulfur batteries.Meanwhile,in the MoS₂/G interlayer,the edge of MoS₂ has insufficient Mo-S bonding coordination and has adjustable active sites to absorb polysulfide effectively.Additionally,the discharge of the carbon nanotubes/sulfur@molybdenum disulfide/graphene（CNTs/S@MoS₂/G）cathode is 984.9 m Ah·g^-1 and there is a reversible capacity of 713.8 m Ah·g^-1 after 450 cycles with a lower capacity decay of 0.061%per cycle at 0.5 C,exhibiting an ideal rate performance.（2）Molybdenum trioxide（MoO₃）was synthesized by heat treatment.MoO₃and carbon nanotubes（CNTs）were mixed in a certain proportion.MoO₃ is not only a polar transition metal oxide,but also has a large band gap,and there are oxygen vacancies between the band gaps,which can adsorb polysulfide and alleviate the shuttle effect.At the same time,flakes of MoO₃ can effectively immobilize sulfur as the sulfur carrier of lithium-sulfur batteries.However,due to the low electrical conductivity of MoO₃ nanosheets,they cannot be used as electrode materials in the absence of conductive materials,so CNTs were introduced.Finally,it was determined that the optimal ratio of MoO₃ and CNTs was 3:7,which had the best electrochemical performance.The cathode containing sulfur with a ratio of 3:7 was denoted as 3-MoO₃/CNTs/S.The initial specific discharge capacity of 3-MoO₃/CNTs/S cathode is 1237.0 m Ah·g^-1 at 0.1 C.The initial specific discharge capacity of 3-MoO₃/CNTs/S cathode is 976.9 m Ah·g^-1and there is a reversible capacity of 391.4 m Ah·g^-1 after 800 cycles and the capacity decay rate is 0.075%at 0.5 C.（3）Polyethylene glycol（PEG）was coated on MoO₃/CNTs/S by solvent evaporation method and the optimal coating amount was determined.PEG has ether bond and ether electrolyte wettability is good,so that the electrolyte fluidity increased to increase the dissolved capacity of active substance sulfur in the electrolyte,to improve the role of capacity.The results showed that the best electrochemical stability was obtained when the coating amount was 2.5%PEG.The cathode containing sulfur at 2.5%PEG coating amount is denoted as the 2.5-PEG@MoO₃/CNTs/S.The initial discharge capacity of the 2.5-PEG@MoO₃/CNTs/S cathode at 0.1 C is 1327.4 m Ah·g^-1,the coulombic efficiency is up to 99%.The discharge capacity of the 2.5-PEG@MoO₃/CNTs/S cathode is 992.9 m Ah·g^-1 at 0.5 C.After 800 cycles,the capacity decreases to462.5 m Ah·g^-1 with an ultralow capacity decay of 0.067%per cycle.