Controllable Construction Of Defective Vanadate/rGo Functional Surface Interface And Its Energy Storage Performance

Lithium-sulfur（Li-S） batteries have become one of the current research hotspots in electrochemical energy storage due to their high theoretical energy density(2600Wh kg^-1)and specific capacity(1675 m Ah g^-1).However,the long-chain polysulfides（Li PSs）generated during the charge-discharge process are easily soluble in organic electrolytes and pass through the separator to the lithium anode,resulting in irreversible loss of active materials and affecting the cycling stability of Li-S batteries.In addition,the reversible conversion of the active intermediates is also the key to determine the rate performance of the battery.Therefore,in view of the irreversible dissolution and sluggish kinetic reaction of Li PSs,this paper solved above problems by controllable construction strategy for the modified separator layer,the main research strategies including:one is to build active sites that efficiently immobilize and inhibit the dissolution of Li PSs and further improve the cycle life of batteries;the other is to improve the interfacial charge transfer ability,which can enhance the redox kinetic reaction and finally achieve the controllable construction and mechanism exploration of modified separator layer.The specific research contents and conclusions are as follows:1.Construction and properties of vanadate composites doped with different metal ions.A general synthetic strategy is proposed to prepare ion-doped vanadates（NVO,KVO,Mg VO,and Ca VO）and use them to construct the modified functional separators.Vanadium provides the active center for chemical reactions due to its abundant oxidation states,which are beneficial to forming reversible chemical bonds to limit the migration of Li PSs and inhibit the shuttle effect,thereby significantly improving the battery’s electrochemical stability performance.Taking the Li-S battery assembled with NVO modified separator as an example,the reversible initial specific capacity of the battery is 1637.4 m Ah g^-1 at 1.0 C,which is much higher than the initial separator-based battery(633 m Ah g^-1).Even after 500 cycles,it still exhibited a high reversible specific capacity(744.3 m Ah g^-1,with a capacity retention rate of 45.5%).Therefore,the construction of V active sites and the design of cross-linked network structure can effectively immobilize the Li PSs to slow down the shuttle effect and improve the electrochemical stability of Li-S batteries.2.Research on the construction and properties of reduced graphene oxide-wrapped defective sodium vanadate material（G@HNVO）.Based on the construction strategy of ion-doped vanadate proposed in the previous chapter,we further prepared G@HNVO by hydrothermal method to improve the reversible conversion kinetics of Li PSs.It improves the interfacial charge transfer ability of the modified layer,which can accelerate the redox kinetic reaction process and finally improve the performance of batteries.Firstly,the proton doping induced during hydrothermal reduction process not only stabilizes the layered vanadium oxide through van der Waals forces,but also acts as the active site to enhance the redox reaction.Secondly,the rGO layer with good conductivity is beneficial to reducing the interfacial resistance,which can accelerate the electron transfer and improve the rate capability of the battery.Meanwhile,the formation of Li-O chemical bonds between defective sodium vanadate and Li PSs effectively limits the dissolution and improves the utilization of active materials.Furthermore,the result shows that when the GO content accounts for 10 wt%of the NVO precursor,G@HNVO can not only improve the conductivity of the modification layer,but also maximize the performance of HNVO to immobilize Li PSs.The Li-S battery with G@HNVO（10%）modified separator exhibits the high reversible specific capacity(1494.8 m Ah g^-1)at 0.2 C,and the capacity decay rate is only 0.083%per cycle after 700 cycles.Therefore,the rational construction of functional material active centers and interfacial electron transfer is conducive to the immobilization of Li PSs,which can suppress the shuttle effect and significantly improve the electrochemical performance of Li-S batteries.It shows that the construction of functional interface layer has potential application prospects in optimizing the electrochemical performance of Li-S batteries.