Functional Structure Design Of Carbon-Based Cathode And Its Application In Lithium-Sulfur Batteries

Lithium-sulfur batteries（LSBs）have gained increasing attention because of their significant advantages of high energy density(2600 Wh kg^-1).However,the insulating nature of elemental sulfur and its reactants（Li₂S）,the large volume changes of the active species in redox reactions,and the"shuttle effect"of polysulfides have hindered the commercialization of LSBs.Carbon materials have the advantages of relatively low price,good electrical conductivity,and diverse structural types,which make them suitable for use as matrix materials for the active substance sulfur.Based on this,this paper adopts the surface modification of carbon nanotubes with nickel-phosphorus alloy particles,internal reinforcement of graded porous carbon materials and core-shell structure design to prepare the positive electrode material and investigate its electrochemical properties and mechanism.The specific research contents and conclusions are as follows.（1）In order to improve the slow reaction kinetics and the"shuttle effect"of the elemental sulfur cathode.The nanoscale nickel-phosphorus alloy particles（Ni-P）were coated on the surface of carbon nanotubes（CNTs）by chemical plating,and the"shuttle effect"of polysulfides was effectively suppressed by the synergistic effect of physical adsorption of CNTs and chemisorption of Ni-P alloy particles.At the same time,the amount of Ni-P alloy particles plated is controlled by adjusting the concentration of nickel sulfate（Ni SO₄）in the plating solution.The electrochemical performance test results showed that the CNTs/（Ni-P）/S-2 cathode material exhibited optimal cycling performance（Ni-P alloy coating≈5 at%）with an initial discharge specific capacity of1344.6 mAh g^-1 at 0.2 C,and a reversible specific capacity of 885.6 mAh g^-1 after 200cycles,with a single cycle specific capacity decay rate is 0.076%.（2）Compared with one-dimensional CNTs,three-dimensional hierarchical porous carbon has a larger internal space and a certain pore size distribution,but the expansion of the active substance sulfur in the process of lithium is likely to cause damage to the complete electrode structure.In this paper,three-dimensional porous carbon/carbon nanotubes/sulfur composites（HPBC/CNTs/S）are prepared by high shear,freeze-drying and chemical activation processes to internally strengthen the mechanical properties of the matrix skeleton and improve the long-life stability of LSBs under high current density conditions.The initial discharge specific capacity of HPBC/CNTs/S cathode is1348.4 mAh g^-1 at 0.2 C,and the discharge specific capacity was still maintained at1018.5 mAh g^-1after 200 cycles.The initial discharge specific capacity is 988.4 mAh g^-1 at 1.0 C.After 1000 cycles,a reversible specific capacity of 482.4 mAh g^-1 is still available with a low single cycle specific capacity recession rate of 0.051%.（3）The active material attached to the surface of HPBC/CNTs/S material has a large contact area with the electrolyte,which leads to the excessive loss of active material.In this paper,porous N-doped micron carbon sphere@sulfur cathode material（PNCM@S@Al F₃）wrapped with aluminum fluoride is prepared by liquid phase method.The stable polar Al F₃ coating structure can hinder the dissolution of polysulfide by physical protection and chemisorption.The PNCM@S@Al F₃ cathode has an initial discharge specific capacity of 1196.2 mAh g^-1 at 0.2 C,and remains stable at 727.7mAh g^-1 after 200 cycles.Notably,in the rate performance test,the PNCM@S@Al F₃cathode still has a reversible specific capacity of over 1185.0 mAh g^-1 when recovered from 2.0 C to 0.1 C,with a recovery rate of 93.6%.