Application Of Vanadium Oxide And Nitride Hollow Sphere In Lithium-sulfur Battery

Since the commercialization of lithium ion batteries（LIB）,rechargeable batteries have become the main energy storage equipment in various fields such as industrial society,electric vehicles,energy storage systems and mobile devices.Lithium ion batteries have become the leading industry in the secondary battery market.However,the theoretical specific capacity of lithium-ion batteries is only 200m A hg^-1,which cannot meet the further needs of social development.Lithium-sulfur batteries have gradually entered the field of vision of scientists.Because sulfur（S）has the advantages of high theoretical capacity,low cost,and wide distribution,lithium-sulfur batteries have become a very promising energy storage device.Although significant progress has been made in the past two decades,so far,due to the existence of numerous defects in lithium-sulfur batteries,their capacity has been severely reduced,and large-scale production of lithium-sulfur batteries has not yet been achieved.In order to solve the above problems,It has been found that the application of polar materials in lithium-sulfur batteries can generate chemical affinity through polar-polar interactions to anchor polysulfides that are also polar.Vanadium oxide and vanadium nitride are both extremely polar metal compounds and can be applied to the positive electrode host material of lithium-sulfur batteries to improve the coulombic efficiency and cycle stability of the battery.The conductivity of vanadium oxide and vanadium nitride is better than that of sulfur,which can improve the conductivity of the positive electrode.It can also physically restrict sulfur and polysulfide by adjusting the morphology.In this dissertation,the hollow sphere structures of vanadium oxide and vanadium nitride are designed and prepared.Carbon tubes are added on the basis of vanadium nitride to create a complete conductive network at the positive electrode of the battery,and the problem of excessive polarity of vanadium nitride is improved by carbon tubes,which gives the possibility of greatly improving the capacity of lithium-sulfur batteries.The prepared spheres are stable in structure and extremely restrictive to polysulfides and sulfur.They have successfully and effectively improved the performance of lithium-sulfur batteries.The specific research results are as follows:（1）Based on V₂O₅ hollow sphere,a high-efficiency sulfur host is designed and synthesized.V₂O₅ hollow ball is used as the positive electrode material of lithium-sulfur battery.It is used to store sulfur and limit the shuttle effect of polysulfide.The average diameter of the hollow sphere is about500nm,which provides more space for sulfur storage and adapting to the volume change of the sulfur electrode.At the same time,vanadium pentoxide has a strong chemical adsorption for polysulfide,which can effectively limit the shuttle effect of polysulfide,thereby improving the utilization rate of sulfur and the cycle stability of the battery.The V-S bond effectively encapsulates the polysulfide in V₂O₅ and plays a key role in improving the electrochemical performance.Thanks to the unique structure,the sulfur content is as high as 70 wt%,and the lithium-sulfur battery with V₂O₅/S as the positive electrode shows a high reversible capacity of 1439 m Ah g^-1 at a rate of 0.1 C and a current density of 1 C After 300 cycles,it still maintains a high capacity of 584 m Ah g^-1.（2）Explore the advantages of VN hollow spheres in the positive electrode of lithium-sulfur batteries,and further improve the positive electrode material by adding multi-wall carbon tubes.The structure of MWCNT-VN can not only store and limit sulfur and polysulfide through the hollow structure of vanadium nitride,but also form a conductive network through the carbon tube shuttle,which further improves the conductive performance of the positive electrode.Compared with the simple vanadium nitride hollow spheres,the prepared vanadium nitride-carbon tubes used in lithium-sulfur batteries all showed more excellent performance.Its structural excellence is manifested in high lithium ion mobility,stable rate performance and excellent long-cycle stability.It has an excellent performance of 809.3 m Ah g^-1 at 1C.After 350 charge-discharge cycles,the capacity remains at 519.6m Ah g^-1,and only about 0.1%attenuation per cycle is calculated.