Research On Adsorption And Catalytic Of Polar Sites In Cathode Carbon Host Materials For Lithium Sulfur Battery

Lithium-Sulfur Battery with high theoretical energy density(2600 Wh·kg^-1)and theoretical specific capacity(1675 m Ah·g^-1)has great application value.However,sulfur cathodes suffer from problems such as lithium polysulfide severe shuttle effect,and slow electrochemical reaction rates.In view of the above problems,carbon material was designed and constructed with small scale polarity sites in order to study its adsorption and catalytic behavior on lithium polysulfide.Based on the first principles calculations and various experiment characterization results,the preparation and electrochemical properties of materials were studied,meanwhile,the mechanism of polar sites on the conversion process of lithium polysulfide was explored.The main research work in this article is as follows.First-principles calculations were used to reasonably predict the adsorption of lithium polysulfides and their catalytic effects on the conversion process of lithium polysulfides by polar active sites（single metal atom polar sites and diatomic metal atoms polar sites）in carbon supports.It was found that single metal atoms in different coordination environments have great differences in the adsorption capacity of lithium polysulfides;different metal atom sites in the same coordination environment have different adsorption capacities for lithium polysulfides.When Mn atom and N,O are coordinated as MN1O3,it shows strong adsorption capacity,Mn-（N,O）site has a lower decomposition energy barrier for Li₂S,but Fe-（N,O）and Co-（N,O）polar site have a significant enhancement effect on the decomposition reaction of Li₂S₄.The analysis of the calculation results of the bimetallic phosphide shows that NiCoP exhibits better adsorption capacity for lithium polysulfide,and CoP exhibits a stronger promotion effect on the decomposition process of Li₂S₄.The precursor material was synthesized by the room temperature self-assembly process,and the in-situ supported carbon host material with Mn atom in coordination with N and O atoms（Mn-N_pO/C）was prepared by combining the calcination-etching method.As an etchant and nitrogen source,dicyandiamide can improve the conductivity of the material while doping nitrogen with carbon;As polar sites,N and Mn atoms in the carbon structure can interact with lithium polysulfides to form N-Li and Mn-S interactions to anchor lithium polysulfides and promote the reduction reaction kinetics of lithium polysulfides.Mn-（N,O）synergistic sites can reduce the decomposition energy barrier and deposition energy barrier of Li₂S during the charge-discharge process,which promotes the decomposition and nucleation growth of Li₂S,and improves the utilization of active materials.Therefore,the S@Mn-N_pO/C electrode exhibits good long-cycle stability,with a capacity decay rate of 0.06%per cycle after 600 cycles of charge and discharge at a current density of 1 C.The ion doping strategy was adopted to construct bimetallic atomic synergistic nitrogen and oxygen supported graphitic carbon matrix composites.Further testing of the synergy of different metal atoms describe that Fe Mn-NO/C possesses a better promotion effect on the decomposition process of Li₂S₄,the fastest deposition rate and the largest Li₂S nucleation capacity,and Li₂S nucleation is the easiest to achieve in Fe Co-NO/C,and CoMn-NO/C is provided with a lower decomposition energy barrier of Li₂S.the electrochemical performance shows that S@CoMn-NO/C cathode exhibited a high rate performance and high active material conversion.The kinetic test results demonstrate that CoMn₈-NO/C has a better promotion effect on the conversion reaction of liquid-liquid lithium polysulfide and the deposition process of lithium sulfide.The initial discharge capacity of S@CoMn₈-NO/C electrode at a current density of 0.5 C is 918.6 m Ah·g^-1,meanwhile,the capacity remains at 601.5m Ah·g^-1 after 500 cycles,and the capacity decay rate per cycle of S@CoMn₈-NO/C is 0.07%.Bimetallic phosphide in-situ supported nitrogen-doped carbon nanocages（NiCoP@CoP/NC）were prepared by two-step ion etching and heat treatment as the sulfur support material.The theoretical calculation results show that NiCoP has a stronger adsorption capacity for soluble lithium polysulfide,and there is more electron transfer between CoP and Li₂S₄,which activates Li₂S₄ molecules,so that reduces the decomposition energy barrier of Li₂S₄,and realizes the rapid transformation of Li₂S₄ and the rapid deposition of Li₂S.Kinetic test analysis depicts that NiCoP@CoP/NC has a stronger catalytic effect on the lithium polysulfide conversion reaction of liquid-liquid phase and liquid-solid phase,meanwhile,make the oxidation process of lithium sulfide own a lower conversion activation energy,which is beneficial to improve the utilization rate of active materials.The efficient electronic and ionic conducting network is constructed by the conductive NiCoP@CoP supported in nitrogen-doped graphitic carbon,which is beneficial to promote the electron and ion transport during the transformation of lithium polysulfide,and improve the rate capability of the electrode.Therefore,the S@NiCoP@CoP electrode exhibits a high initial discharge capacity of 1624 m Ah g^-1in the first cycle at a current density of 0.1 C,and can obtain a reversible capacity of763 m Ah g^-1 when the current density is increased to 3 C.Moreover,the initial discharge capacity is 1026.8 m Ah g^-1,and the capacity decay rate per cycle is 0.05%after 1300 cycles at a current density of 1 C.