| The advantages of high specific capacity,safety and non-toxicity of lithium-sulfur batteries make them attract great attention in the field of secondary batteries.However,the poor conductivity of the sulfur cathode,the volume expansion during charging and discharging,and the"shuttle effect"of polysulfides are the difficulties that need to be overcome in the large-scale commercialization of lithium-sulfur batteries.In view of the above problems,this paper will mainly improve the electrochemical performance of lithium sulfur battery from the graphite-like phase carbon nitride（g-C3N4）based cathode materials.The main research content is as follows:First,to explore the effect of g-C3N4/g-C3N4 materials prepared by double precursors on the adsorption properties of polysulfides;Second,study the effect of different methods on the improvement of the conductivity of g-C3N4/g-C3N4 materials by composite porous carbon materials;Third,Explore the role of transition metal and non-metallic double-doped g-C3N4materials on accelerating the migration rate of lithium ions.（1）g-C3N4/g-C3N4 composites are prepared by simply heat treatment by dissolving urea and thiourea to form molecular hybrid precursors,which serve as sulfur host materials.The experimental results show that g-C3N4/g-C3N4/S cathode material has strong adsorbability,so that g-C3N4/g-C3N4/S cathode material still has a reversible specific capacity of 350 m Ah/g after 500 cycles at 2.0 C,showing excellent cyclic stability.The appearance of the heterojunction not only enhances the ability of g-C3N4to inhibit the shuttle effect,but also creates conditions for the rapid transmission of lithium ion/electron,which significantly improves the cycling stability of the composite cathode material..（2）In order to enhance the conductivity of g-C3N4,the g-C3N4/g-C3N4 was compounded with porous carbon material.The porous carbon enhances the electrical conductivity of the material,and the abundant void structure provides space for the bulk expansion of sulfur.In addition,nitrogen doping occurs during the sintering process of porous carbon,which synergies with g-C3N4/g-C3N4 to enhance the anchoring ability of the material to polysulfide.The experimental results show that the composite formed by adding porous carbon material into the molecular mixture precursor and then sintering exhibits the best electrochemical performance when the mass ratio of g-C3N4/g-C3N4 to porous carbon material is 1:2.At a current density of 2.0 C,the reversible specific capacity of 468 m Ah/g remains after 500 cycles,showing excellent cycling stability.（3）The transition metal Co and non-metallic element P double-doped g-C3N4 were prepared by a two-step doping method.The results show that Co and P can increase the number of active sites on the surface of g-C3N4,which can anchor polysulfide,and accelerate ion and electron transfer.The composite cathode material has excellent charge-discharge performance at a large rate.At a current density of 1.0 C,the composite cathode material still has a reversible specific capacity of 580 m Ah/g after300 cycles,and also has excellent ladder rate performance. |
