Study On The Design Of High Performance Lithium-sulfur Battery Materials And The Mechanism During Charge And Discharge

The large consumption and demand of energy have led to the rapid development of electrical energy storage technology in recent years.Lithium-sulfur batteries are considered to be one of the more promising energy storage because of their environmental friendliness and high energy density.Although lithium-sulfur batteries have high application value,they still have important problems to be solved,such as poor conductivity,large volume change during charging and discharging,shuttle effect of lithium polysulfide and so on.From the current research status,electrodes combined with two-dimensional materials which have nanostructures,high specific surface area,good electrical conductivity and sulfur have high battery performance.Therefore,this paper selects some 2D material with good conductivity as the sulfur's host material,and studies the above problems and its internal reaction mechanism by means of first-principles calculation.The research content is divided into the following sections:1)Firstly,the adsorption of sulfur(S8)and its discharge products(Li2S8,Li2S6,Li2S4,Li2S2,Li2S)on borophene was analyzed,and the feasibility of using borophene as a host material for sulfur was discussed.The results show that the adsorption energy of lithium polysulfides on borophene are mainly concentrated in the range of-1 eV?-3 eV,indicating that borophene can stably adsorb lithium polysulfides.At the same time,we also calculate the Li-S bond length and charge transfer.The partial density of states proves that the adsorption of lithium polysulfides on borophene materials is strong,and mainly based on chemical adsorption.The analysis of band structure and local charge density show that the adsorption system is metallic,and its conductive source is mainly from borophene,indicating that electrons can be rapidly transferred between borophene materials and adsorbed molecules.In summary,borophene as a host material for sulfur cathode can alleviate the current problems of lithium-sulfur batteries in some degree,and is a suitable host material.2)The design principle of chemical co-doping graphene as a host material to achieve both chemical capture and catalysis is proposed.Seven different co-doped series of MN4?graphene(M=V,Cr,Mn,Fe,Co,Ni,Cu)have been discussed.Studies have shown that FeN4?graphene and CrN4?graphene have good performance among them on lithium-sulfur batteries,which can not only stabilize adsorption,but also achieve rapid transfer of lithium ions.The study also found that for the absorption of most lithium polysulfides,two bonding type(S bonding and Li bonding)can be achieved,while the competition of M(transition metal)-S and N-Li shows the main difference of co-doped structure.In addition,for medium-strength chemical trapping and strong catalytic effects,systems with S-bonding exhibit better performance.Binding energy and lithium diffusion properties are considered to be the key descriptors for the rational design of sulfur positive electrodes.3)Finally,the performance of the suitable Li2S host materials V2CTx were compared and the internal causes were analyzed.Combining the adsorption capacity,overvoltage and transfer barrier of Li2Sn on various host materials,we can conclude that V2CO2 is the most suitable Li2S host material for lithium-sulfur batteries among the three host materials.The reason for this is mainly the direct impact of special adsorption methods on performance.The adsorption mode of Li2S on V2C is S bonding,the adsorption mode of Li2S on V2CF2 is Li bonding,and the adsorption of Li2S on V2CO2 is accompanied by Li bonding and S bonding.The S bonding method enables Li2S to have a more stable adsorption on the host material and increase the load ratio,but the simple S combination also causes the lower open circuit voltage.Although the lithium bonding method can obtain a suitable open circuit voltage,it will cause a high migration barrier of lithium ions during charging and discharging.Only the structure in which S-bonding and Li-bonding coexist can take strength all,both stable adsorption,suitable open circuit voltage and low lithium ion migration barrier.Therefore,when looking for a suitable material for use as a Li2S host material,it is necessary to consider whether the material has both Li-bonding ability and S-bonding ability.This provides a direction for future research and provides a basis for experimental selection of battery materials.