Theoretical Study On Cathode Reaction Of Lithium Sulfur Battery Catalyzed By Graphene

It is very important to understand the reaction mechanism of lithium sulfur battery in charge and discharge at the cathode.Therefore,to understand the mechanism of charge and discharge is indispensable since there are few relevant reports.In addition,it is difficult to observe the reaction mechanism from the experiment due to the complex electrochemical/chemical reactions in the lithium sulfur battery.Therefore,we use graphene as a cathode material to study the discharge mechanism of lithium sulfur battery by the first principles calculations.Firstly,we used CALYPSO and Gaussin find the stable clusters of S₈ and Li₂S_n（n=1-8）.Secondly,build the possible initial adsorption models of the bridge,top and hollow of each cluster on graphene,the adsorption energy is calculated to find the optimal adsorption configuration of each cluster on the surface of graphene.then,The reaction energy of polysulfide conversion is studied from the thermodynamic point of view,and use the theoretical voltage of lithium sulfur battery 2.287 V as a reference to analyze the possible intermediate products and the possible reaction path of cathode discharge mechanism.finally,From the dynamic point of view,the reaction energy barrier of each possible path is calculated by the Nudged Elastic Band（NEB）method.It is show that the intermediate products of cathode reaction of lithium sulfur battery catalyzed by graphene are LiS₅,LiS₄ and LiS₃,Path A（S₈→LiS₅→LiS₄→LiS₃→LiS₂）and Path B（S₈→LiS₄→LiS₃→LiS₂）are feasible,in which Path B is the optimal path.Path C（S₈→Li₂S₈→Li₂S₇→Li₂S₆→Li₂S₅→Li₂S₄→Li₂S₃→Li₂S₂→Li₂S）is not feasible because the reaction of Li₂S₇→Li₂S₆ needs to span an energy barrier of up to 2.48 eV.In summary,the theoretical calculation results show that graphene is an excellent cathode matrix material for lithium sulfur battery,which has good catalytic activity for the cathode reaction of lithium sulfur battery,and the final products are LiS₂ and Li₂S.This conclusion can provide theoretical guidance for the rational design of cathode materials for lithium sulfur batteries.