Design Of Hierarchical Structures Of Cobalt-Based Composites And Application In Lithium-Sulfur Batteries

With the advantages of high theoretical specific capacitance(1675 m Ah g^-1)and energy density(2600 Wh kg^-1),environmental friendliness and low price,lithium-sulfur batteries are considered as one of the most promising energy storage devices.However,the practical application of lithium-sulfur batteries is still restrained by many problems,such as the shuttle effect of the lithium polysulfide（Li PS）and the poor conductivity of sulfur.According to research,the rational design of the cathode and interlayer of lithium-sulfur batteries is an effective strategy to improve the above problems.Among them,the cobalt metal is a very promising sulfur carrier material because of its ability to catalyze and adsorb lithium polysulfide;in addition,the material with porous structure is rich in pores,which can enhance the capture of polysulfide and thus improve battery perform.Thus,this paper is based on cobalt metal embedded N-doped carbon composite（Co-NC）,and considering its lack of electrical conductivity,both metallic tin and carbon nanofiber（CNF）are added,and finally design two kinds of cobalt-based composites with hierarchical structure,which are used for the cathode as well as the interlayer of lithium-sulfur batteries.Their microscopic morphology,specific surface area and electrochemical properties were investigated.The details of the study are as follows:（1）Using polystyrene（PS）spheres as a template,Sn O₂/Sn with three-dimensionally ordered macropores were first synthesized,and then zeolite imidazolyl ester skeleton-67（ZIF-67）was introduced into the pores,and calcined to obtain a cathode material with a multi-level structure named Co-NC@Sn（NH₃）.The 3D ordered interconnected macropores of the unique hierarchical structure can provide fast mass transport,and the plentiful micro-/mesopores can trap Li PS by physical adsorption.Meanwhile,the embedded Co nanoparticles and N-doped C further strengthen the chemical adsorption of Li PS and expedites its redox conversion kinetics.In addition,the high conductive 3DOM Sn skeleton accelerates charge transfer/transport and provides strong adsorption towards Li PS.Benefiting from these features,the Co-NC@Sn（NH₃）composite shows a relatively low capacity fading rate of 0.045%per cycle over 500 cycles at 1 C.Even with raised sulfur loading of 5.2 mg cm^-2,a high areal capacity of 4.65 m Ah cm^-2can be obtained.Pouch cell fabricated with Co-NC@Sn（NH₃）delivers a high discharge capacity of 701 m Ah g^-1 under high sulfur loading of 4.50 mg cm^-2.（2）The carbon nanofiber（CNF）obtained by electrostatic spinning was used as the substrate,and the CNF was then treated sequentially in a solution of PS spheres and ZIF-67,followed by calcination,and finally 3D Co-NC@CNF was obtained.Firstly,carbon nanofibers act as an interconnected conductive network,improving electrical conductivity and accelerating electron transport;secondly,the porous structure derived from ZIF-67 and PS templates,coupled with Co nanoparticles and nitrogen-doped carbon,can effectively trap lithium polysulfide by physical and chemical adsorption,which mitigated the shuttle effect,thus improving the electrochemical performance of the Li-S batteries.When the 3D Co-NC@CNF interlayer was applied to the lithium-sulfur battery,the initial discharge capacity was 1287 m Ah g^-1 at 0.2 C and maintained at 1078.08 m Ah g^-1 after 100 cycles,with a capacity retention rate of 83.8%;A low capacity degradation rate of 0.03%per cycle can be obtained at 1 C;and a discharge specific capacity of 708.6 m Ah g^-1can be achieved at 5 C.