Preparation Of Molybdenum Disulfide-carbon Based Heterostructure For The Modification And Mechanism Of Lithium Sulfur Batteries Separator

In recent years,lithium sulfur（Li-S）batteries is expected to become a substitute for lithium-ion batteries,because of its high energy density(2600 Wh kg^-1).However,some of problems hinder the commercialization process of Li-S battery,especially serious is the shuttle effect caused by the dissolution of polysulfide intermediates in the electrolyte grievous impact of the practical application performance of Li-S batteries.Compared with other improvement methods,separator modification is a more effective and economical method to inhibit the shuttle effect.Among many modified materials,molybdenum disulfide（MoS₂）is one of the ideal separator modified materials.However,the low conductivity and limited adsorption/catalytic sites of MoS₂have always been the obstacles to its application in Li-S batteries.Therefore,combination of MoS₂and carbon materials,making use of the advantages of high conductivity and large specific surface area of carbon materials to make up for the shortcomings of MoS₂,is a more extensive improvement method at present.In this work,we firstly focus on the connection between MoS₂and carbon materieal,combined with theoretical calculation,thus design the composite material of MoS₂coupled on 3D carbon substrate as separator modification material to improve the electrochemical performance of Li-S batteries.Then,using the MoS₂quantum dot-nitrogen doped graphene composite as the medium,study the failure process of the separator modified layer under high concentration of polysulfide,which can provided theoretical help to improve the cyclic stability of the modified layer.The specific work and research results of this paper are as follows:（1）Design,preparation and research electrochemical properties of a robust MoS₂covalently coupled with 3D porous nitrogen doped carbon composites.DFT theoretical calculation discover that the formation of Mo-O-C bond between MoS₂and nitrogen doped carbon can effectively improve the electronic interaction between their,so as to significantly enhance the catalytic ability of MoS₂edge sites for the dissociation of Li₂S.Based on theoretical calculations,design a MoS₂with abundant active sites covalent coupled with 3D carbon matrix composed of nitrogen-doped reduced graphene oxide and carbonized melamine foam（MoS₂@CF-NRGO）.3D nitrogen doped carbon matrix has rich specific surface area and high conductivity.The nanoflower like MoS₂can fully expose the edge sites and improve the catalytic activity of the material.More importantly,the covalent coupling at the interface between MoS₂and carbon can enhance the electron conduction between them and improve the catalytic ability of MoS₂.As a separator modified layer,the Li-S batteries with modified separator has an initial reversible capacity of 1274.9m Ah g^-1at 0.2 C and 615.1 m Ah g^-1at high charge discharge rate of 5 C.Even under high sulfur load of 8 mg cm^-2,the initial capacity remains at 890.1 m Ah g^-1.（2）Preparation of MoS₂ quantum dots/nitrogen doped reduced graphene oxide composite and optimization of cyclic stability of separator modified layer under high sulfur load.The MoS₂quantum dot-nitrogen doped graphene composite（MoS₂QDs-NRGO）was prepared by simple electrostatic adsorption.Experiments show that as a separator modified material,it can effectively improve the electrochemical kinetics of Li-S batteries.After that,combined with in-situ and non in-situ analysis,the failure process of separator under different sulfur loads are characterized.The results show that the failure of separator modified layer may due to the uneven deposition of Li₂S on the lithium side of the modified layer,resulting in the lithium ion migration of the modified layer is hindered and injure the cycle performance of the modified layer.Then experiments prove that appropriate enhancement of the thickness of the modified layer can better alleviate the formation of Li₂S layer.Under the sulfur load of 6 mg cm^-2,the capacity decline rate of modified separator with 0.6mg cm^-2mass load is only 0.22%after 1000 cycles,significantly better than the modified separator of 0.3 mg cm^-2（0.29%）.