Research On Inorganic Polysulfide And Organic Polysulfide Composite Cathode Materials

Due to population and economic growth and advances in lifestyles,the demand for energy has steadily increased over time.As a high energy density system,the lithium-sulfur battery has a theoretical energy density of 2600 Wh kg-1,which is much higher than the current commercial lithium-ion batteries.However,in order to realize its commercial application,there are still many technical problems that need to be overcome,such as the low conductivity of sulfur and lithium disulfide,the shuttle effect of lithium polysulfide,and the dendritic growth of lithium metal negative electrodes.While the lithium organic battery has a considerable specific capacity,it can avoid the shuttle effect and the volume expansion of sulfur caused by the lithium polysulfide in the lithium-sulfur battery.Organic compounds include organic carbonyls,free radicals and organic sulfides,because they have the advantages of large capacity,abundant resources,renewable,degradable,and adjustable structure,so they are very promising.In the 1980s,some organic sulfides were studied to a certain extent in rechargeable lithium batteries,especially organic disulfides as positive electrode materials.However,they show low capacity and poor cycle performance,which makes them unattractive compared to transition metal oxide cathode materials.As a result,organic sulfides have not been studied as extensively as other cathode materials(including organic carbonyls and free radicals).In recent years,with the development of lithium-sulfur batteries,organic sulfides with long sulfur chains in the structure(such as trisulfides,tetrasulfides,pentasulfides,etc.)have received more and more attention.As a major class of sulfur derivatives,compared with elemental sulfur,they have a general structure and unique properties.Organosulfide molecules have precise lithiation sites and adjustable capacity.Organic functional groups can provide additional benefits,such as increased discharge voltage and energy efficiency through phenyl groups,and cycle stability through N heterocycles.Organic sulfides have better advantages than elemental sulfur as cathode materials for lithium-ion batteries.Because of its special structure and abundant resources,transition metal sulfide is an important part of lithium ion battery cathode material.They can be made into nanosheets or nano-powders.They have good plasticity and adsorption properties,and are not easy to explode and burn.They have obvious advantages in improving the safety performance of the battery.This study is based on the inorganic cathode materials vanadium tetrasulfide(VS4),vanadium disulfide(VS2)and organic sulfide diphenyl disulfide(DPDS)to make composite cathodes,and explore the interaction of inorganic sulfides and organic sulfides in lithium batteries.In this paper,the vacuum filtration method is used to prepare the composite electrodes CNTs/VS2 and CNTs/VS4 consisting of carbon nanotubes(CNTs)and transition metal sulfides(VS2 and VS4).In the composite positive electrodes prepared by this method,the advantages of the two materials can be gathered.The interwoven carbon nanotubes can not only disperse the transition metal sulfide nanoparticles and evenly weave them in the network,but also improve the utilization of active materials due to the uniform dispersion of transition metal sulfide.DPDS is added to the positive electrode of the battery by means of electrolyte additives.1)In this paper,transition metal sulfide vanadium tetrasulfide(VS4)is used as the cathode material of lithium ion batteries,and carbon nanotubes are used as current collectors to explore the cycle performance and capacity retention rate of the composite electrode CNTs/VS4 with and without DPDS.Through comparison,it is found that the battery performance of VS4 and DPDS composite electrode is better than their performance as a battery cathode material alone.After studying the electrochemical performance of the VS4 and DPDS composite electrode,we found bis(trifluoromethanesulfonyl)imide(TFSI")anion in the electrolyte can be intercalated into VS4,leading to evolution of elemental sulfur.DPDS can capture the evolved sulfur to form diphenyl trisulfide(DPTS)to participate in the battery cycle,avoiding sulfur from participating in the battery cycle to form lithium polysulfide,and significantly improve the composite electrode energy density and cycle performance of CNTs/VS4/DPDS.2)This paper uses VS2 as the positive electrode material,and the lithium battery assembled with CNTs/VS2 electrode as the positive electrode has poor cycle stability,and the coulombic efficiency is extremely unstable.After 20 cycles at a current density of 120 mA g-1,almost no capacity can be released.After adding DPDS to the electrode,the cycle stability of the electrode has been greatly improved.After optimizing the ratio of VS2 and DPDS,the CNTs/VS2/DPDS composite electrode has an initial specific capacity of 232.7 mAh g-1 at a current density of 120 mA g-1,and it still has a capacity of 164.4 mAh g-1 after 500 cycles.The retention rate is as high as 70.6%.This work shows that the composite electrode of transition metal sulfide and organic sulfide can combine the advantages of the two materials:VS2 has a certain adsorption effect on the organic sulfide DPDS.The addition of DPDS increases the migration rate of lithium ions and accelerates the internal conversion reaction of the battery.The process improves the utilization rate of the positive electrode active material,reduces the attenuation of the battery capacity,and further improves the electrochemical performance of the composite electrode material CNTs/VS2/DPDS.