First Principles Study For Cathode Anchoring Materials Of Lithium-Sulfur Battery

Lithium-sulfur batteries have become a hot research system in the field of secondary batteries because of high theoretical capacity,high energy density,and the cathode raw material sulfur with abundant reserves in the earth,low price and environmental friendliness.However,its industrialization process is facing a series of problems,such as low conductivity of sulfur,the shuttle effect caused by the dissolution of polysulfides in electrolyte,volume expansion during the reaction,etc.,which all cause the fast decay of the battery capacity and the short cycle life.At present,further efforts to improve the electrochemical properties of lithium-sulfur batteries from the experimental level are the main research focus,with the proposal of the“material genome”construction scheme,screening out potential materials by using computers to calculate the relevant properties has also become a new research method.Focusing on the two problems of polysulfides shuttle effect and poor conductivity of the cathode final product in lithium-sulfur batteries,the substance such as polymers and metal compounds combined with sulfur are used as battery cathode materials to chemically adsorb polysulfides.Taking the performance requirements of the positive electrode of lithium-sulfur batteries as the starting point,and based on the first principle,this paper is committed to screening materials,also called anchoring materials,that can be used as the positive electrode of lithium-sulfur batteries and exhibit good chemical adsorption for polysulfides（Li₂S,Li₂S₂,Li₂S₄,Li₂S₆,Li₂S₈and S₈）.The specific solutions are as follows:（1）The first principle based on density functional theory is used to explore the mechanism of action between polyethylene glycol and polysulfides.The adsorption energy,the shortest distance of adsorption,the number of charge transfer,the proportion of van der Waals force and the partial state density corresponding to the stable adsorption configuration were calculated.The results show that polyethylene glycol molecule presents physical effect to S₈,and presents moderate chemical adsorption to other polysulfides through Li-O bonds.The chemical adsorption for short-chain polysulfides（Li₂S,Li₂S₂）is strong,and adsorption efficiency for long-chain polysulfides（Li₂S₄,Li₂S₆,Li₂S₈）is comparable to that of electrolyte.It can be inferred that polyethylene glycol can appropriately alleviate the shuttle effect caused by soluble long-chain polysulfides,but the effect is poor.（2）Based on the poor effect of polyethylene glycol in alleviating the shuttle effect,the polyethylene glycol（PEG400）molecule was doped to introduce functional groups.The stable geometries of the three adsorption base molecules of S-doped PEG400,N-doped PEG400 and S-N co-doped PEG400were established,and the respective electrical properties were calculated.The stable adsorption configurations between the three adsorption substrates and the polysulfides were determined,and the properties of adsorption energy,shortest adsorption distance,charge transfer quantity,the proportion of van der Waals force,partial state density and differential charge density in the adsorption configuration were studied.The results show that the doped PEG400 has been greatly improved its chemical adsorption anchoring effect on polysulfides after the introduction of functional groups.Among these three modified PEG400adsorption substrates,N-doped PEG400 and S-N co-doped PEG400 molecules have a strong adsorption effect on polysulfides,while S-doped PEG400 has a weak adsorption capacity of polysulfides.（3）Using the first principle based on density functional theory,the adsorption behavior of Li₂S molecules on the surface of VS and VSe adsorption substrates was described in this paper.The stable adsorption configuration corresponding to the two substrates were determined,and the adsorption performance of adsorption energy,charge transfer,state density,differential charge density and the migration behavior of lithium ions on the surface of VS and VSe crystals were studied.The results show that,on the one hand,the migration energy barrier of lithium ions on the surface of VS and VSe is 0.18 e V and 0.26 e V,respectively;on the other hand,VS and VSe adsorption substrates exhibits strong chemical interaction to Li₂S molecules through V-S bonds,and the adsorption energy is-5.4 e V and-6.44 e V,respectively.The excellent adsorption effect is not only conducive to controlling the ordered deposition of Li₂S molecules on the surface of the two adsorption substrates,but also effectively alleviating the shuttle effect,and the low lithium ion migration energy barrier is conducive to accelerating electron and ion transport,thereby ensuring the utilization rate of the active material of the cathode material and the battery reaction speed.（4）The vanadium selenide exhibits excellent chemical adsorption effect to Li₂S and low surface migration energy barrier for lithium ions above-mentioned.Taking the selenide with two-dimensional layered junction and good conductivity as the research object,the first principle based on density functional theory was used to analyze the adsorption behavior of Li₂S molecules at different sites on the surface of Co Se₂and Ni Se₂this paper.Two stable adsorption configurations were determined and used to study the adsorption energy,charge transfer,state density,differential charge density,and migration behavior of lithium ions on the surface of Co Se₂and Ni Se₂crystals.The results show that the two adsorption substrates of Co Se₂and Ni Se₂exhibit a strong adsorption effect to Li₂S by Li-Se bond and S-X（X=Co,Se）bond,and the corresponding adsorption energy of the two adsorption configurations is-2.57e V and-2.8 e V,respectively.In addition,the migration energy barrier of lithium ions on the surface of Co Se₂and Ni Se₂adsorption substrate is 0.173 e V and0.144 e V,respectively.The strong chemical adsorption showed that the two metal adsorption substrates of Co Se₂and Ni Se₂are conducive to the ordered deposition of Li₂S molecules,and could also weaken the polysulfides shuttle,ensuring the utilization rate of active substances and the performance of battery magnification.The low migration energy barrier of lithium ion indicates that the adsorption substrate surfaces of Co Se₂and Ni Se₂are conducive to the rapid diffusion of lithium ions,which is convenient for improving the reaction kinetics of the battery.