Design Of Metal Modified Cathode Materials And Study Of The Mechanism Of Anchoring Lithium Polysulfide By These Materials

Having high theoretical energy density and specific capacity,lithium sulfur battery has become one of the most potential secondary batteries.However,the shuttle of lithium polysulfide（Li₂S_x）to electrolyte and the deposition of poor conductivity and insoluble Li₂S lead to the decrease of cycle life and insufficient dynamic performance.In view of the above problems,the key is to construct cathode materials with excellent conductivity,sulfur capture capacity and catalytic ability.Recently,metal modified materials can provide strong polar metal atoms and catalytic active centers,which have great research prospects in energy storage and conversion.Covalent organic framework materials（COFs）have these advantages of the easy control of connecting units and modification of heteroatom functional groups.In this paper,metal modified covalent organic framework materials were used as the host cathode to study the anchoring mechanism of lithium polysulfides,the catalytic ability for the dissociation of insoluble Li₂S,and the influence of solvents on lithium polysulfides capture performance of materials by density functional theory calculation.The main contents and results are as follows:（1）A covalent organic framework material with M-N4 coordination structure by the metal atoms modification（MPc-COF,M=Ti,V,Mn,Cu,Zn）was designed as the host cathode.Li₂S_x（x=1,4,6,8）was used as the key species in discharge to study the anchoring mechanism of materials and evaluate the adsorption performance.Through the screening and analysis of stable adsorption patterns,differential charge density,Bader charge and density of state,it is proved that M-N4 structure on the surface of the materials has synergistic adsorption interaction for S and Li atoms of Li₂S_x（M-S,N-Li）.Further study shows that N and O atoms in MPc-COF channel can chemically bond with Li atoms,which has good performance for lithium polysulfide capture.It is found that the M-N active sites on Mn Pc-COF and Zn Pc-COF have lower energy barrier for the dissociation of Li₂S.The results of anchoring of polysulfides and catalytic mechanism show that Mn Pc-COF modified by Mn atom is a potential cathode material.（2）A covalent triazine framework material with unsaturated M-N3 coordination was designed,and composited with graphene（M-CTF/G,M=Li,Ti～Zn）was used as the host cathode.It is found that M-CTF/G can exist stably and have good conductivity.Taking Li₂S₄as key species in shuttle effect,it was found that M-N3 coordination had synergistic adsorption capacity（N-Li,M-S）for Li₂S₄,Ti-CTF/G and Mn-CTF/G had the strongest adsorption capacity.The structure and polarity of solvents with different dielectric constants were further studied to clarify the solvent effect on the anchoring performance of M-CTF/G composites.By studying the effects of 1,3-dioxolane（DOL）,dimethoxyethane（DME）,dimethyl sulfoxide（DMSO）and tetramethylurea（TMU）on the anchoring performance of M-CTF/G（M=Li,Cu,Zn）,Li-CTF/G was not enough to inhibit the dissolution of Li₂S₄ to solvents,while Cu-CTF/G and Zn-CTF/G could effectively inhibit the dissolution of Li₂S₄ to DOL and DME,but not in TMU and DMSO.Finally,the relationship between the adsorption energy of Li₂S₄ on the surface of M-CTF/G materials and its solubility in solvents is established,and the standard of adsorption energy of Li₂S₄ on M-CTF/G material under the condition of simplified calculation model is proposed,which provides an effective theoretical guidance for screening high-performance cathode materials.