Structure-activity Relationships Of Electrocatalysts In Lithium-Sulfur Battery Cathodes

Lithium-sulfur batteries are one of the most promising next-generation highenergy density battery systems.However,the battery performance still falls far short of expectations.One of the key challenges is the sulfur cathode.The conversion reaction kinetics among the sulfur species(sulfur,LiPSs,Li2S2/Li2S)involved in the sulfur cathode determines the battery performance.Previous studies have demonstrated that electrocatalysis have a significantly effect on accelerating the Li-S redox reactions kinetics,thus enhancing the performance of Li-S batteries.Atomically dispersed metal catalysts(ADMCs)can maximize the utilization of metal atoms without sacrificing the energy density,and are perfect for catalyzing the redox reactions kinetics as additives in sulfur cathodes.Meanwhile,atomically dispersed metal catalysts with well-defined catalytic active sites help to unnderstand the structure-activity relationships of electrocatalysts in sulfur cathode and further regulating and optimizing the catalytic activity of catalyst materials.In this thesis,we have taken this as the starting point to do relevant research,and the specific research results are as follows:(1)The complex relationship between the catalytic behavior and the structure of ADMCs makes it quite challenging to exclusively determine the critical role of the central metal atoms in Li-S chemistry.By selecting group VIII elements Fe,Co,and Ni as metal atoms,we prepare three kinds of catalysts having similar physicochemical characteristics and containing similar coordination and electronic structure of active centers.We systematically study the catalytic behavior and active-site geometry,oxidation state,and the electronic structure of different metal centers(Fe/Co/Ni).It is revealed that the order of electrocatalytic activities of metal centers,i.e.,Co>Fe>Ni.And the coordination-geometry and oxidation state of the metal atoms are modified when interacting with sulfur species.This interaction is strongly dependent on the hybridization of metal 3d and S p-orbitals.A moderate hybridization with the Fermi level crossing the metal 3d band is more favorable for Li-S redox reactions.(2)The identity of central metal atoms plays a critical role in determining the interaction strength with different sulfur species over active sites.Integrating two kinds of metal centers in one catalyst would offer more possible interacted sites,so as to optimize the sulfur redox reactions kinetics.On the basis of the poor intrinsic electroactivities of Pt and Cu,we develop an dual-atom catalyst with active sites containing both Pt and Cu atoms.It is shown that Pt/Cu dual-atom catalyst enables a rapid sulfur species conversion kinetics,outperforming the catalysts with individual Pt or Cu atom.Further studies show that during the Li-S redox reactions,Pt/Cu atoms in the active site will undergo reversible changes in their oxidation states with the same trend but different degree,and Pt atom will coordinate with S generating a dynamic coordination structure,while Cu atom remains the original coordination structure,indicating the existence of the synergistic effect between Pt/Cu atoms.Li-S coin cells with Pt/Cu dual-atom catalyst in the cathodes can deliver small capacity decay of 0.053%per cycle for 500 cycles at 1 C.The assembled Li-S single-electrode pouch cell exhibits a reversible gravimetric capacity of 720 mAh g-1 after 100 cycles at 0.2 C.(3)Dual-atomic active site with defined coordination structures offer alternative choices to optimize the electronic structure of the catalytic active center,and understand the correlation between the structure of the active center and the electrocatalytic activity for polysulfide conversion reactions.We fabricate two kinds of dual-atom catalysts with different structures of active sites by changing the sequence of doping Pt and Cu atoms,which exhibit different catalytic activities towards the sulfur redox reactions.It is revealed that the differences in catalytic activities between the two catalysts mainly stems from the distinct catalytic behavior of Pt and Cu atoms towards the sulfur species.The sulfur conversion reactions are more favorable when there is a large difference in the interaction strength between Pt and Cu atoms with the sulfur species.Li-S coin cells with higher-catalytic-activity dual-atom catalyst in the cathodes can deliver an initial areal capacity of 5.4 mAh cm-2 at a mass loading of 6.4 mg cm-2 at 0.2 C.Li-S pouch cell based on single-sided coated cathode can deliver a reversible gravimetric capacity of 800 mAh g-1 at 0.2 C for 100 cycles.