Preparation And Sodium Storage Properties Of ZIFs-derived Cobalt Selenide Core-shell Materials

With the development and progress of technology,the development of lithium-ion batteries tends to mature.But because of the high cost,researchers turned to sodium-ion batteries.Although the sodium-ion batteries has the advantages of low cost and uniform sodium distribution,there is still a distance from the practical application.Because graphite anode can not show good electrochemical performance in sodium-ion battery,the researchers focus to find a suitable anode material for sodium ion battery.Cobalt selenide material has a high theoretical specific capacity,but due to the expansion effect and poor electrical conductivity in the process of charge and discharge,it is necessary to construct special morphology and element doping to increase the stability and electrical conductivity of the material.Zeolite imidazole framework(ZIFS)material has a stable structure and simple synthesis method,which is an excellent precursor for the preparation of cobalt selenide materials.In this paper,using ZIF-67 as the precursor,the core-shell structure material was prepared by vertical growth or etching and ion exchange,and then the cobalt selenide electrode material was obtained by high temperature annealing and calcination.Different types of core-shell structure materials were prepared,such as core-shell structure CoSe₂/ZnSe/NC@ZnSe/NC,yolk shell structure CoSe₂/CoNiSe₄/NC and multi-layer core-shell structure CoSe₂.The electrochemical properties of core-shell structure materials and non-core-shell structure materials were compared,and the properties of heterostructure and non-heterostructure were compared,and the following results were obtained:(1)Core-shell structure CoSe₂/ZnSe/NC@ZnSe/NC materials can be achieved 308.6 m A h g^-1 specific capacity under the current density of 1 A g^-1,cycle capacity remain 89%after 300cycles.Compared with core-shell structure CoSe₂/ZnSe/NC and CoSe₂/NC,the electrochemical performance of the core-shell structure CoSe₂/ZnSe/NC@ZnSe/NC is better.The core-shell structure CoSe₂/ZnSe/NC@ZnSe/NC can still maintain the morphology of dodecahedron after cycling,which has no serious volume expansion and a high pseudocapacitance effect,proving that both core-shell structure and heterostructure have positive effects on improving the electrochemical properties of materials.(2)Yolk-shell CoSe₂/NC@CoNiSe₄/NC showed specific capacity of 577.8 m A h g^-1 under the current density of 0.1 A g^-1,which is better than heterostructure CoSe₂/NiSe₂/NC and CoSe₂/NC.It may be due to the unique structure of the material and the synergy of double metal,as well as carbon and nitrogen doping improves the stability and conductivity of the material.By comparing the sodium storage properties of CoSe₂/NC@CoNiSe₄/NC,CoSe₂/NiSe₂/NC and CoSe₂/NC,it was found that CoSe₂/NiSe₂/NC exhibited the best electrochemical properties,which proved that CoSe₂/NiSe₂/NC had a positive effect on the rate stability and cycle stability of the materials,and the heterogeneous structure materials could also improve the properties of the materials.(3)The discharge capacity of multilayer core-shell CoSe₂ is 352.9 m A h g^-1 at the current density of 1 A g^-1,and can maintain 2000 cycles,which is better than that of core-shell hollow CoSe₂/NC and CoSe₂/NC electrodes.It is proved that the multilayer core-shell structure is beneficial to improve the stability of the material.The multilayer core-shell CoSe₂/NC electrode exhibits a high pseudocapacitance ratio,which is 92.21%at a sweep rate of 1 m V s^-1,demonstrating a highly reversible reaction.