Structure Design Of Transition Metal (Ni,Co) Compounds And Its Application For Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries have the advantages of high energy density(2600Wh kg^-1)and low cost,showing the potential for huge high-energy storage application.However,its practical application still suffers from many problems,mainly due to the poor conductivity of sulfur,the"shuttle effect"of polysulfide,and the volume expansion during the process of charge and discharge.In view of the defects above the sulfur positive electrode,this paper mainly improves the electrochemical performance of cathode materials for lithium-sulfur batteries from two aspects:on the one hand,the transition metal compound is used as host material of sulfur,which is better to improve the conductivity of the sulfur electrode and remit the dissolution of polysulfide;on the other hand,the reasonable structural design effectively alleviate the volume expansion of the electrode material and contain the diffusion of the polysulfide during the cycling.（1）The uniform particle Co（OH）₂ material is synthesized by direct precipitation method,and a coral-like sulfur carrier material CoP is obtained by phosphating.It is applied to a lithium-sulfur positive electrode,and the influence of the material CoP on the performance of the battery is analyzed by comparing the sulfur electrode.The carrier material CoP has interconnected structures that physically limit polysulfide and shorten the effective path of electron/ion transport,and utilize the polarity of the material to chemical adsorption polysulfide.The results show that the CoP@S electrode material has a high specific capacity of 826 mAh/g,and the specific capacity still has 438 mAh/g after 500 cycles at 0.1 C.The electrode material shows a good cycle performance compared to the pure sulfur electrode.It is indicated that the CoP material has a certain degree of inhibition effect on the“shuttle effect”of the lithium-sulfur batteries,which can significantly improve the cycle stability of the CoP@S composite.（2）In view of the problem that the cycle performance is not ideal of CoP@S composite material,NiCoP material with excellent performance and relatively better adsorption performance,so,it was used as carriers for sulfur.The experimental results show that the NiCoP-S composite has a first discharge capacity of 815.3 mAh/g at 0.1C,and the capacity of 620 mAh/g after 200 cycles,and the coulombic efficiency of about 100%.The analysis show that:first,the hollow structure of NiCoP material provides a buffer space for the volume expansion of sulfur,and the outer interconnect-ed sheets can limit the diffusion of polysulfide;second,the rich oxide layer on the surface of NiCoP material with high specific surface area（1539.2 m²/g）has strong adsorption to polysulfide.Therefore,its cycle stability is obvious improved compared with single metal phosphide.（3）CoNi₂S₄ material is prepared by template method and reflux treatment,and applied to the cathode electrode of lithium-sulfur batteries.The XPS characterization shows that the polar CoNi2S4 material can bond with polysulfide,and then the“shuttle effect”can be well suppressed.Moreover,CoNi2S4 is rich in active sites and conductivity,shows a good electrochemical performance.The experimental results show that the CoNi₂S₄/S has the first discharge specific capacity is as high as 1246.46mAh/g,and the specific capacity still has 642.42 mAh/g even after 300 cycles at a current density of 0.1 C.