Synthesis Of Electrocatalytic Materials（CoSe₂,Mo₂N） For High Performance Lithium-sulfur Batteries

Owing to the high theory theoretical density(2600 Wh kg^-1)and cost-effective raw materials,lithium-sulfur batteries（LSBs）have drawn extensive research in recent years.The energy storage mechanism of LSBs is based on the lithium polysulfides（Li PSs）related redox reaction which usually occurs at the interface between the electrode and electrolyte.Though the reaction of Li PSs delivers a high capacity,it provides obstacles in the commercialization of LSBs.Specifically,the soluble long-chain Li PSs prefer to transport into electrolytes under the control of the electric field,leading to the inevitable shuttle effect.To address the issue,the most popular strategy in recent research is to introduce functionalizing separators with electrocatalytic materials,which can block the shuttle behavior of Li PSs and accelerate the redox kinetics of the transformation between the soluble Li PSs and Li₂S₂/Li₂S.To alleviate the shuttle effect and improve the electrochemical performance of the LSBs,two kinds of electrocatalytic materials,composite of Co Se₂ nanoparticles and carbon skeleton（Co Se₂/C）and the composite of nitrogen-doped analogous graphene and Mo₂N（Mo₂N/NG）,were prepared and used to modify the separators.The main research contents are as follows.（1）Co Se₂/C composite was obtained by a simple two-step calcining method with the rhombohedral dodecahedral ZIF-67 as the template.And the Co Se₂ nanoparticles were dispersed in the carbon skeleton.The morphology of Co Se₂/C nanoparticles was characterized by SEM,TEM and XPS.And the catalytic capability was systematically studied by combining various electrochemical methods with infiltration experiments.The results indicate that Co Se₂/C can not only guide the deposition of Li₂S during the discharge process but can also boost the oxidation of Li₂S.In addition,the Co Se₂/C has a good adsorption capacity for Li PSs.Therefore,the cells with Co Se₂/C-modified separator,on which the mass loading of the modified layer is only 0.15 mg cm^-2,delivered an excellent electrochemical performance.The cells with a sulfur loading of 4.8 mg cm^-2 gave an initial specific capacity of 756 m Ah g^-1 at 0.2C and maintained at 715 m Ah g^-1 after 180 cycles,with a per attenuation rate of only 0.031%.（2）Mo₂N/NG composites was obtained by mixing glucose,（NH₄）₆Mo₇O₂₄·4H₂O,and dicyandiamide and followed in-situ pyrolysis process.To manipulate the Lewis acidity,Ni-Mo（or Co-Mo）based Anderson-type polyoxometalates were synthesized and used to prepare nickel（or cobalt）doped Mo₂N/NG composites（labeled as Ni-Mo₂N/NG or Co-Mo₂N/NG）,the preparation method of which is the same as Mo₂N/NG except that the（NH₄）₆Mo₇O₂₄·4H₂O was replaced by the polyoxometalates.The electro-microscopy images showed that the morphology of Ni-Mo₂N/NG and Co-Mo₂N/NG was in good agreement with Mo₂N/NG.However,the NH₃-TPD revealed that the surface acidity of Ni-Mo₂N/NG is the strongest among the three samples,indicating the strongest Lewis acid-base interaction between Ni-Mo₂N/NG and Li PSs.The catalytic capability of the electrochemical materials was systematically studied by various electrochemical methods.Owing to the Lewis acid-base interaction,Ni-Mo₂N/NG has a better catalytic ability towards various Li PSs involved solid-liquid-solid conversion reaction.When the cells with Co/Ni-Mo₂N/NG-modified separator,Ni-Mo₂N/NG-modified separator delivered an excellent rate performance(4 C,935 m Ah g^-1)and long-range cyclic stability at high rate（3C,after 800 cycles with a per attenuation rate of only 0.048%）.The cells with Ni-Mo₂N/NG-modified separator also delivered a fantabulous electrochemical stability at high sulfur load(the maintained at 769 m Ah g^-1 at 0.5 C after two cycles of 0.05 C to 0.5 C at 4.8 mg cm^-2,and then can be stable to the 200th cycles at 0.5 C rate).