Preparation Of Modified Separator Based On VS₂ And Its Application In Li-S Batteries

With the increasing energy demand for energy storage applications,the rapid development of new power sources with high energy densities has become an important research.Among the reported energy storage systems,lithium-sulfur（Li-S）batteries have attracted much attention due to their extremely high theoretical energy density,environmental friendliness,and low cost.However,the industrialization of Li-S batteries has been hampered by poor cycle stability and rate performance.During the discharge of Li-S batteries,the soluble intermediate lithium polysulfides（Li PSs）produce shuttle effects,while sulfur insulation leads to retarded redox kinetics.The shuttle of Li PSs and retarded kinetics are the essential causes of the poor electrochemical performance.Recent research has shown that separator modification is a strategy that can effectively suppress the shuttle effect,thereby improving the rate performance and cycling stability.Carbon materials with high conductivity and porous channel structure can physically adsorb Li PSs,however,the non-polar nature can only inhibit a small fraction of Li PSs from shuttling,so polar materials need to be as sulfur hosts.Among them,metal sulfides,which have strong chemisorption and catalytic conversion properties,become an ideal candidate material,but metal sulfides have poor electrical conductivity.Therefore,to address this issue,in this thesis,MWCNT/VS₂and r GO/VS₂composites based on VS₂were designed and used for separator modification based on the synergistic effect of the excellent electrical conductivity of carbon materials and the high adsorption and catalytic properties of metal sulfides.These two separators effectively suppress the shuttle effect,promote the reaction kinetics and enhance the electrochemical performance.The specific contents are as follows:VS₂nanosheets were synthesized by a hydrothermal method and vacuum-filtered with MWCNT onto the surface of one side of PP separator to obtain a multi-walled carbon nanotube/vanadium sulfide coated modified separator（MWCNT/VS₂-PP）.MWCNT has high electrical conductivity and high specific surface area and VS₂have a high number of chemisorption and catalytic active sites,enhancing kinetics.The cells show good long-term cycling stability due to structural constraints and the chemisorption of VS₂synergistically mitigating the shuttle effect.The MWCNT/VS₂--PP cells exhibit a high reversible capacity of 1226.5 m Ah g^-1at a current density of 0.1 C.Even at a current density of 1 C,the specific capacity after 500 cycles is still 727 m Ah g^-1and the rate capacity is substantially increased(862.4 m Ah g^-1at 5C).VS₂was compounded with r GO as a modified separator to improve the electrochemical performance.Among them,the porous and conductive r GO with a high specific surface area can act as a physical barrier to intercept Li PSs,while the highly polar VS₂can facilitate the redox reaction of Li PSs through catalytic conversion and mitigate the shuttle of Li PSs.The r GO/VS₂modified separator was assembled into a Li-S cell with a current density of 0.2 C.The r GO/VS₂-PP cells,with a current density of0.2 C,have a specific capacity of 1309.7 m Ah g^-1on the first cycle and after 150 cycles the specific capacity remained at 981 m Ah g^-1,showing good cycling stability.At the same time,a discharge capacity of 1202.3 m Ah g^-1is achieved at 1 C,with a capacity retention rate of 72.5%after 400 cycles.In addition,a rate capacity of up to 995.3 m Ah g^-1can be achieved at 2 C.