Research On Catalytic Conversion Mechanism Of Polysulfides And New Catalytic Materials For Li-S Batteries

Lithium-sulfur batteries are considered to be a powerful competitor for the next generation of energy storage technology due to their high theoretical energy density(2600 Wh kg^-1)and the low cost of sulfur.However,the intrisic defect and electrochemical behavior of sulfur itself cause many unfavorable challenges,especially the shuttle effect of polysulfides,which seriously hinders the commercialization of Li-S batteries.Recently,a new strategy has been developed to accelerate the redox kinetic of sulfur by applying electrocatalysts,which can effectively mitigate the shuttle effect.Among them,transitional metal catalysts have attracted much attention due to their large specific surface area and high active catalytic sites.For instance,Fe-or Co-based catalysts have performed well in Li-S batteries.In fact,the redox process of S involves solid-liquid-solid multiphase conversion,which is also a complex multi-step reaction involving various intermediate products.The precise design of highly efficient catalysts must take into account these actual complex environments in a cell.We have carried out the construction of highly efficient catalytic sulfur host materials from the following two aspects:（1）Size-dependence on catalytic conversion of lithium-sulfur battries.In classical heterogeneous catalysis,the catalytic activity of metal catalysts is generally affected by their sizes,but there is no definite theory to guide the size dependence of different catalysts in different catalytic reactions.We studied the size dependence of metal Cocatalyst on the catalytic conversion of sulfur kinetic firstly.By adjusting the molar ratio of Zn/Coin bimetallic ZnCo-ZIFs precursors,the control sample NC（nitrogen-doped carbon）was obtained,and Cocatalysts with different state of aggregation were prepared:Cosingle atoms（Co-SAs/NC）,Coatomic clusters（Co-ACs/NC）and Conanoparticles（Co-NPs/NC）.According to the experimental results and theoretical calculations,the electrocatalytic activity of sulfur redox reactions followed the order:Co-SAs/NC>Co-ACs/NC>Co-NPs/NC,because more active sites can be exposed as the size decrease,and the surrounding coordination atoms will change the electronic structure of the active center atoms.Thus,the catalyst with smaller size will undergo stronger geometric deformation when interacting with sulfur species,which can reduce the Gibbs free energy changes.（2）Catalytic precise regulation of multi-step conversion reactions of polysulfides.The catalytic selectivity of different metal catalysts for multi-step conversion of sulfur was also studied.PC（porous carbon）,FeNC-PC and CoNC-PC catalysts were prepared by a simple hierarchical anchor strategy.The experimental results show that FeNC-PC has more advantages in catalyzing the conversion of short-chain polysulfides,such as the reduction of Li ₂S ₄→Li₂S and the oxidation of Li ₂S,whereas CoNC-PC is more conductive to the catalytic conversion of long-chain polysulfides,such as Li ₂S ₈→Li₂S ₆.Such difference is demonstrated in evaluating the performance of the battery.Since the conversion of Li ₂S occupies 3/4 of the theoretical capacity,the Li-S cell with FeNC-PC decorated separator showed a slightly better capacity and cycling performance.These works will strengthen the understanding of the size dependence of metal catalysts on heterogenous reactions in Li-S batteries,and contribute to the exploitation of catalysts materials with higher activity based on the size of catalysts.At the same time,catalysts with dual catalytic sites can be developed to synergistically accelerate the multi-step kinetic conversion of sulfur,so as to improve the performance of lithium-sulfur batteries.