Preparation Of Transition Metal Selenide Cathode Materials And Their Application In Lithium-sulfur Batteries

Lithium-sulfur(Li-S)batteries have the characteristics of high energy density,large theoretical capacity,abundant sulfur resources and no pollution,becoming the replacement of traditional lithium battery energy storage equipment.However,due to the poor conductivity of cathode materials,"shuttle effect",volume expansion and other problems,the poor cycle life of the battery is difficult to commercialize.Therefore,the development of transition metal selenide nanomaterials with polarity,porosity,electrochemical catalytic activity and high conductivity can be used as a good host for sulfur cathode,and will be used to manufacture high-performance Li-S batteries.In this paper by the metal organic framework(MOF)material as precursor,mix with anionic selenium regulation after high temperature calcination with hollow dodecahedron structure transition metal selenides,conducting polymers by polypyrrole(PPy)coating,double metal synergies and carbon nanotube(CNT),can improve the electrical conductivity of the material and conducting channel for electron transfer.Transition metal selenide as half metal has good catalytic activity and electrical conductivity,the electronic properties of transition metal is a combination of the different,can be cut down by chemical adsorption and catalysis of polysulfide"shuttle effect",so as to improve the electrochemical performance of S cathode.As the precursor of ZIF-67 with high specific surface area pore structure,Co Se₂ with dodecahedron structure was synthesized after selenization at high temperature,and then PPy was coated to prepare Co Se₂@PPy-S composite with special structure.Co Se₂ material has abundant active sites,which adsorb and catalyze the conversion of lithium polysulfide in the charging and discharging process,making it difficult for polysulfide to dissolve and diffuse in the electrolyte.Coating PPy improves the conductivity of composite material,promotes electron migration in the reaction process,and speeds up the redox reaction.The discharge capacity of Co Se₂@PPy-S composite electrode is 690 m Ah g^-1 at 0.2 C current density.After200 cycles,the discharge capacity is still 376 m Ah g^-1,which is significantly higher than that of bare sulfur and Co Se₂-S electrodes.Even at the high current density of 3C,the composite electrode still has a discharge capacity of 341 m Ah g^-1.After 200 cycles,the reversibledischarge capacity of 292 m Ah g^-1 can still be maintained,and the reduction of each cycle is only 0.072%.Compared with monometallic selenides,heterogeneous bimetallic selenides have rich phase boundaries and superior electrical conductivity.Through the synergistic effect between Zn and Co,Zn Se/Co Se₂ composite was prepared by selenization with ZIF-8/67 as the precursor.The electrocatalytic conversion of polysulfide was accelerated by Zn Se through chemical adsorption and catalytic effect.The conductive Co Se₂ surface provides a rapid diffusion path for lithium ions,accelerating polysulfide conversion.Through their respective advantages,Zn Se and Co Se₂ can jointly promote the smooth adsorptive-diffuse-catalytic conversion process of polysulfide and guide the growth of lithium sulfide around its heterogeneous interface,thus improving the electrochemical performance of lithium sulfur battery cathode materials.The optimal Zn:Co=1:1,Zn Se/Co Se₂-S electrode has 790.06 m Ah g^-1 initial specific capacity at 0.2 C and good cycling stability at 1 C high current density test.The initial capacity of the electrode is 363.76 m Ah g^-1.After 300 cycles,the final capacity is 300.85 m Ah g^-1,the capacity retention rate is 82.71%,and the capacity decay rate per cycle is 0.0576%.In order to further accelerate the conversion of lithium polysulfide and enhance the electrical conductivity of the material,the bimetal selenides were combined with carbon nanotubes to form a positive material with strong adsorption capacity and fast catalytic conversion rate to ensure the electrochemical stability of lithium sulfide battery.The surface of Zn Se/Co Se₂is connected by CNT,and a conductive network is formed between the polyhedra.Among them,CNT provides a conductive channel for electron transport,enabling high sulfur utilization and preventing structure collapse during the cycle,while the Zn Se/Co Se₂ tightened polysulfide lithium chemical anchoring effect,promoting the further transformation of polysulfide lithium in reaction,induction of Li₂S all other nuclear,the shuttle effect was inhibited and the redox reaction was accelerated.The results show that the Zn Se/Co Se₂-CNT-S-4 electrode exhibits excellent electrochemical performance when CNT content is 50 mg(the total content was 11.1 wt%).The initial discharge capacity was 1021.46 m Ah g^-1 at the current density of 0.05 C,and the initial capacity of the electrode is 461.35 m Ah g^-1and the reversible capacity was 313.08 m Ah g^-1 after 200 cycles at the high current density of 1 C,and the capacity retention rate was 67.86%.