Research On The Cathode Properties Of Transition Metals And Their Compounds For Lithium-Sulfur Batteries

Lithium-sulfur（Li-S）batteries are regarded as one of the most promising candidates for lithium batteries due to their high theoretical specific capacity,low price of elemental sulfur,green environmental protection and sufficient reserves in the earth’s crust.Due to the poor conductivity of the active material sulfur and the solid reaction product,the volume contraction and expansion of the positive electrode during the battery reaction,and the"shuttle effect"caused by lithium polysulfide,the battery has low positive sulfur utilization rate,poor cycle stability and high-rate performance and other problems.In order to solve the above problems,this paper starts from the design direction of cathode materials for lithium-sulfur batteries.By modifying the cathode with transition metals and their compounds,such composite materials can adsorb lithium polysulfide and accelerate the conversion of lithium polysulfide to improve the comprehensive performance of Li-S batteries,the specific work is as follows:（1）A nitrogen-doped carbon polyhedron with CoS₂and FeS₂nanoparticles was prepared to form CoS₂-NC@Co-FeS₂composite,which was investigated as a positive electrode modification material.This composite material restricts the shuttle behavior of lithium polysulfides,catalyzes and adsorbs polysulfides,and improves the electrochemical performance of the battery.S/CoS₂-NC@Co-FeS₂achieved a capacity of 938.9 m Ah·g^-1at0.2 C,and exhibits 394.8 m Ah·g^-1after 200 cycles,with an average decay of 0.28%per cycle.The initial discharge capacities of S/CoS₂-NC and bare sulfur electrodes is 82.3%and 63.8%of those of S/CoS₂-NC@Co-FeS₂electrodes.S/CoS₂-NC@Co-FeS₂achieved the capacity of 795.6 m Ah·g^-1at 0.5 C,and exhibits 354.4 m Ah·g^-1after 200 cycles.It can be seen that the battery using S/CoS₂-NC@Co-FeS₂electrode has superior electrochemical performance.（2）Using Co-Co PBA as the precursor,Co₉S₈-Mo₂C nanoparticles were synthesized by carburizing method and assembled in lithium sulfur batteries.Co₉S₈-Mo₂C nanoparticles exhibit strong sulfur fixation and catalytic effects on liquid-liquid phase and liquid-solid phase transformation.Electrochemical tests showed that the cathode of Co₉S₈-Mo₂C nanoparticles with the optimal ratio（280 mg of Co-Co PBA and 100 mg of molybdenum salt）emerge a high specific capacity(0.2 C,1010.8 m Ah·g^-1)and excellent rate performance(1 C,609.4 m Ah·g^-1and 2 C,590 m Ah·g^-1),while the cathode with Co₉S₈nanoparticles was inferior to that of Co₉S₈-Mo₂C nanoparticles in all aspects.This demonstrates that the adsorption-catalytic function of Co₉S₈-Mo₂C improves the capture and conversion efficiency of polysulfides.When the mass ratio of Co-Co PBA to molybdenum salt is 2.8:1,the adsorption-catalytic effect of Co₉S₈-Mo₂C is the most effective for polysulfides.（3）Mo₂C nanospheres with Ni nanoparticles（Ni-Mo₂C）were synthesized by the introduction of Ni,which was used as the host of cathode sulfur,which solved the problems of the few electrochemically active sites of Mo₂C nanospheres and poor catalytic lithium polysulfide（Li PS）,and the optimal ratio of Ni to Mo₂C was explored（the mass ratio of nickel salt to Mo-PDA is 1:0.6）.The comparison of electrochemical tests revealed that the Ni-Mo₂C nanospheres with the optimal ratio exhibit high discharge capacity(0.2 C,1101.3m Ah·g^-1)and excellent rate performance(692.9 m Ah·g^-1at 2 C),which showed the best catalytic and adsorption effect on Li PS.In addition,the optimal proportion of Ni-Mo₂C nanospheres has a high diffusion coefficient of Lithium ions,which can accelerate the transportion of Li⁺and accelerate the electrochemical reaction kinetics.