Preparation Of Modified Separator Based On V₂CT_x And Its Application In Li-S Batteries

To meet the increasing demand of power equipment,it is urgent to develop high energy storage system.Lithium-sulfur(Li-S)batteries have attracted much attention due to their high energy density(2567 Wh kg^-1),abundant sulfur(S)resources and low price.Li-S batteries are becoming the promising new generation of rechargeable batteries.However,the commercial implementation of Li-S batteries is retarded by the low S utilization,rapid capacity decay and poor rate performance that are caused by the insulating properties of S,serious shuttling of lithium polysulfides(Li PSs)and sluggish redox kinetics.In order to improve the performance of Li-S batteries,we aims to design an anchoring-catalysis dual-functional modified layer for the separator modification,which can trap the Li PSs and promote the Li PSs transformation.V₂CT_xhas abundant active sites and electrocatalysis,which can trap Li PSs through the strong chemical bond of metal-S and promote the redox reaction of Li PSs through electrocatalysis.Therefore,V₂CT_xis regarded as an ideal material for separator modification.In this dissertation,V₂CT_x/CNT and V₂CT_x/Ti₃C₂T_xcomposite structures are designed based on V₂CT_xnanoribbons for the separator modification.The specific research contents are as follows:The V₂CT_x/CNT structure was constructed by introducing polar V₂CT_xnanoribbons into carbon nanotubes(CNT)framework.The anchor-catalysis modified separator(V₂CT_x/CNT-PP)was prepared by coating it on commercial PP separator using a simple vacuum filtration method.Among them,CNT has good electrical conductivity,continuous structure and unique mechanical stability of one-dimensional structure,which can provide enough conductivity for the redox reaction and prevent the structure from collapsing during the process of volume expansion in the cycling process.V₂CT_xnanoribbons can strengthen the chemical anchoring of Li PSs,catalyze the transformation of Li PSs at different stages,induce uniform nucleation of Li₂S,and achieve the inhibition of shuttle effect and rapid redox reactions.Thanks to the anchor-catalytic function of V₂CT_x/CNT,the specific capacity of the battery reaches893.6 m Ah g^-1at the current density of 2 C.Under the high current density of 1 C,the discharge specific capacity of the first cycle reaches 1055.3 m Ah g^-1,and the average capacity decay rate of each cycle is only 0.047%after 1000 cycles.V₂CT_xnanoribbons were introduced to Ti₃C₂T_xby cationic driving method to successfully compound V₂CT_x/Ti₃C₂T_xoverlapping layered structure and applied it in separator modification of Li-S batteries.Ti₃C₂T_xnanosheets with high electrical conductivity,abundant active sites and high mechanical properties provide a stable structure for the material and ensure the rapid transmission of electrons.However,the self-stacking of Ti₃C₂T_xnanosheets greatly inhibits the exposure of adsorption sites,leading to the reduction of adsorption capacity,which is not conducive to the reduction of shuttle effect.Small-sized V₂CT_xnanoribbons have rich Li PSs adsorption sites and can catalytic Li PSs conversion,which can effectively inhibit shuttle effect and accelerate battery dynamics.Therefore,V₂CT_xnanoribbons was inserted between Ti₃C₂T_xnanosheets to form a overlapping structure of MXene.At this time,a stable and functional substrate was obtained for V₂CT_xnanoribbons,and the self-stacking of Ti₃C₂T_xnanosheets was inhibited.Therefore,the V₂CT_x/Ti₃C₂T_xmodified separator achieves the dual functions of high adsorption and catalysis under the synergistic effect of Ti₃C₂T_xnanosheets and V₂CT_xnanoribbons.The Li-S battery with V₂CT_x/Ti₃C₂T_xmodified separator achieves excellent capacity of 935.3 m Ah g^-1at 2 C current density.After 600 cycles at 1 C,the cycling stability of capacity decay rate per cycle is 0.062%.