Preparation Of Transition Metal-based Sulfur Cathodes And Their Application In Lithium-sulfur Batteries

In the past few decades,the field of rechargeable secondary batteries has been greatly developed,and lithium-sulfur batteries（LSB）have attracted extensive attention due to their low cost and environmental friendliness.However,the shuttling effect of polysulfides and the inherently low conductivity of sulfur limit the practical application of LSB.In order to solve the above problems,this paper conducted the structure design of transition metal-based sulfur cathode to improve the sulfur loading and the electrochemical performance of LSB.The main research contents are as follows:（1）The metal organic framework（MOF）of ZIF-67 with a uniform cubic structure was synthesized at room temperature by a simple solvent method,and then etched with a weak acid to obtain a hollow cube with a large cavity structure,along with cobalt nanoclusters uniformly decorated inside.Subsequently,sublimed sulfur was loaded into the matrix material via melt diffusion method.The large cavity structure and well-dispersed cobalt nanoclusters with a uniform size～11 nm enable the main structure to maintain a high sulfur loading.At the same time,it can effectively release stress,accelerate polysulfide conversion and provide excellent physical confinement and chemical adsorption capability,exhibiting superior rate performance.The capacity retention is 70%after 500 cycles at a current density of 2 C,and the sulfur loading is as high as 6.5 mg cm^-2.Further density functional theory calculations revealed that well-dispersed cobalt nanoclusters play a key role in enhancing Li PS_n adsorption and catalytic conversion.（2）The uniformly mixed ZIF-67,graphene oxide,thioacetamide,and pyrrole were reacted at 180℃ for 12 h via a hydrothermal method.Then the obtained sample was freeze-dried,and finally subjected to high-temperature heat treatment to obtain pyrrole-modified graphene and in situ growth of carbon nanotube composites（denoted as Co₉S₈/CNTs-Gr）.The S@Co₉S₈/CNTs-Gr composite was obtained through loading elemental sulfur into Co₉S₈/CNTs-Gr by melting method.It was shown that the in situ growth of CNTs and the introduction of pyrrole-modified graphene facilitated the electron transfer,thereby significantly improving the rate performance of LSBs.Meanwhile,the Co₉S₈ electrocatalyst shows high catalytic activity,which can promote polysulfide conversion.Benefiting from this unique structural design,the S@Co₉S₈/CNTs-Gr electrode exhibits excellent electrochemical performance,and the battery has a high reversible specific capacity of 950 m Ah g^-1 at a current density of 1 C.The decay rate is only 0.01%.In addition,the S@Co₉S₈/CNTs-Gr electrode still exhibits good cycle stability at a high sulfur loading of 7.2 mg cm^-2,and the specific areal capacity is as high as～6 m Ah cm^-2.This work provides an ingenious structural design for constructing high-performance cathode materials for LSBs.（3）A bimetallic MOF containing Fe and Co,which was served as the precursor,was synthesized by a simple solvent method,and melamine was used as nitrogen source.After high-temperature heat treatment in an inert atmosphere,as well as combination of the doping of non-metallic N element and the controllable carbonization reaction,the FeCo-NC material was achieved.N doping is beneficial to the adsorption of polysulfides,and the uniform distribution of FeCo bimetallics on the carbon skeleton provides abundant reactive active sites,which accelerates the catalytic conversion of polysulfides and significantly improves the cycle stability of LSB.After 400 cycles at a current density of0.2 A g^-1,a high specific capacity of 560 m Ah g^-1 can be maintained.