Design Principle And Mechanism Study On Functionalized Cathode Materials For Lithium Sulfur Battery

In the new energy storage system,lithium-sulfur batteries have been recognized as the potential energy storage systems for commercial applications in the future because of their exceptional theoretical capacity.However,The shuttle effect,which caused by the intermediate polysulfide products dissolved in the electrolyte during the charge and discharge processes,resulted in rapid capacity fading,low Coulombic efficiency and poor cycle life;hence hindering the commercialization process.To tackle this notorious issue,in this project,covalent organic framework materials?COFs?were employed as scaffold cathode materials due to their preferable characteristics in self-design geometry and easily-functionalized property.By using the Density Functional Theory method with van der Waals force correction,the effects of the pores,functional groups of COFs frameworks and solvent effect on inhibiting shuttle effect were studied,clarifying the anchoring mechanism and the structure-activity relationship between COFs composition and anchoring performance,the main contents are as follows:?1?The anchoring mechanism between two B-O COFs nanomaterials and Li₂S_x species on the surface and inside the pore was systematically investigated,using the density functional theory methods with van der Waals interactions.Firstly,two kinds of B-O COFs materials were built and optimized,which consisted of Boron,Oxygen atoms and Benzene group.The detailed analysis of the adsorption energy,difference charge density,charge transfer and atomic density of states can be used to determine that the anchoring performance of Boroxine is higher than that of Borobate Ester group,and the Benzene group of COFs structure has a synergistic effect on the adsorption of Li₂S_x molecules by boroxane groups in the process of surface adsorption;as for pore adsorption,small steric hindrance of active framework groups can help to improve the anchoring performance of COF materials.?2?The anchoring mechanism between COFs nanomaterials?consist of different kinds of functional linkages?and Li₂S_x species was systematically investigated,using the density functional theory methods with van der Waals interactions.First,eight representative linkages composited by high-electronegativity elements are selected and constructed to be 8linkage-benzene-linkage COFs?four functional linkages are constructed at the corner and four linkages are on the edge of COFs structures?.Through the analyses of adsorption energy,charge transfer,atomic density of states and so forth,the synergistic effect of double O atoms both make significant contributions to improve anchoring capacity on polysulfides.Meanwhile,the promising effect of nucleophilic group branches?-OCH₂CH₃?is demonstrated.Besides,the solvent effect is also considered to make a classification of 9 kinds of COFs into three types while selecting favorable COFs scaffoldings in 1,3-dioxolane?DOL?or Dimethoxyethane?DME?solvents.Through studying the anchoring mechanism of COFs materials and the structure-activity relationship between COFs composition and anchoring performance,it provides a theoretical basis for the synthesis of efficient COF/S composite cathode materials.