Controllable Preparation And Sodium/potassium Storage Performance Of Cobalt-based Sulfur/selenide And Its Hybrid Material

With the development of science and the improvement of life demand,people’s need for energy is also growing.The researchers began to focus on sodium/potassium ion batteries,because the relatively mature lithium-ion batteries that had been commercialized on a large scale began to fail to meet people’s requirement as a result of their high cost.Sodium/potassium ion battery has the advantages of relatively low cost and uniform distribution of sodium resources,but there are still some challenges to be solved in the application of actual commercialization.Compared with lithium ion battery,sodium/potassium has a much larger ion radius than lithium,and the deintercalation resistance of sodium/potassium ion in graphite is greater,which makes the graphite negative electrode unable to continue to show excellent electrochemical performance in sodium/potassium ion battery.Therefore,the suitable negative electrode material of sodium/potassium ion battery is the focus of research to be solved.Cobalt based compound material has excellent theoretical capacity,but it has the disadvantages of volume expansion and terrible conductivity in the process of charging and discharging.It needs the improvement of material structure and doping elements to make up for its structural stability and conductivity.Zeolite imidazole framework（ZIFs）has become a high-quality precursor for cobalt-based material due to its stable structure and convenient synthesis method.In this paper,the ZIF-67 was used as a Co source,on the basis of the preparation of a simple core-shell structure,the structure was optimized and modified by the surface growth and attachment of carbon spheres or electrospinning,and then the electrode materials of cobalt-based sulfide compound were obtained by high temperature sulfide/selenization in tube furnace.The core-shell structure materials with different morphology types were prepared,such as the CoSe₂@Zn Se@Carbon sphere which has the core-shell structure coated with carbon spheres on the surface,the double core-shell structure Bi₂S₃@Co₉S₈@C and the carbon nanofiber structure Co₃S₄/Ni₃S₂-CNF.The electrochemical properties of these materials were compared with those of the corresponding core-shell structure materials,and the research results below were obtained:（1）The cored-shell structure CoSe₂@Zn Se@Carbon sphere electrode material with carbon sphere attachment can achieve 328.98 mAh g^-1 specific capacity under the large current density of 2 A g^-1,and after 350 cycles,the capacity was still achieved 270.87mAh g^-1,and the capacity retention rate was 82.34%.CoSe₂@Zn Se@Carbon sphere electrode material has better rate performance and cycling performance than the core-shell structure material without carbon sphere attachment CoSe₂@Zn Se which’s specific capacity of the first turn of the material is only 223.94 mAh g^-1 at the current density of 2 A g^-1.It is proved that selenization and carbon sphere coating have a positive effect for the material in the aspect of the electrochemical properties.（2）The double core-shell structure Bi₂S₃@Co₉S₈@C electrode material has good cycling stability.It can achieve 421.11 mAh g^-1 specific capacity at the current density of 2 A g^-1,and its capacity can keep at 337.25 mAh g^-1 after 220 cycles of cycling,with a capacity retention rate of 80.09%.Compared with the Bi₂S₃@Co₉S₈ materials without carbon coating,Bi₂S₃@Co₉S₈@C materials have better sodium ion storage performance,indicating that the carbon coating is conducive to the improvement of sodium storage performance.The double core-shell structure Bi₂S₃@Co₉S₈@C electrode material also shows a high proportion of pseudocapacitance,which reaches 87% at the scanning rate of 1.0 mV s^-1,indicating that the material with double core-shell structure has good redox reversibility and fast ion transport rate.（3）The Co₃S₄/Ni₃S₂-CNF was prepared by electrospinning with dispersive coating of the active material into carbon nanofiber（CNF）,which showed good magnification performance in tests.Its high sodium storage capacity and stability are far superior to those of Co₃S₄/Ni₃S₂ without carbon nanofiber,under the current density of 0.1 A g^-1,the first-cycle discharge/charge capacity could be achieved 790.19 mAh g^-1 and 754.07mAh g^-1 specific capacity,and the coulomb efficiency was 95.43%.Electrospinning not only provides a carbon nanofiber to improve conductivity,but also enables the active material to be dispersed uniformly and increase its active sites,so that the cycling performance can be improved in the long cycle test.