Modified Separators Based On Transition Metal/Nitride Composite For Lithium-Sulfur Batteries

Lithium-sulfur（Li-S）batteries have shown great potential for the development of nextgeneration high-energy-density secondary batteries because of their high energy density,environmental friendliness and low price of sulfur.However,the severe shuttle effect of soluble polysulfides and sluggish redox kinetics seriously hinder the implement of Li-S batteries.To effectively address the above-mentioned issues,many methods have been proposed.Particularly,modification of the separator by functional coating has been considered as one of the most convenient and effective strategies.In this thesis,the author aimed to addresses the problems of shuttle effects and slow reaction kinetics in Li-S batteries by modifying the separator,with the following main research:（1）To effectively inhibit the shuttle effect of polysulfides and improve the electrochemical properties of Li-S batteries,the separator decorated by VN quantum dots（QDs）with enriched active adsorption sites has been constructed.Despite the strong adsorption and high conductivity of nanosized transition metal nitrides,the insufficient active sites on their surface are unable to significantly anchor the polysulfides and improve the electrochemical performance.In this work,VN QDs in the size range of 2-5 nm were prepared and evenly dispersed on the surface of nitrogendoped graphene by a simple in-situ chemical reaction.And the as-prepared VN QDs-NG was decorated onto the surface of polypropylene membranes through vacuum filtration procedure.Due to the quantum size effects,the prepared VN QDs with rich adsorption active sites,can effectively suppress the effect of shuttle effects,while the nitrogen-doped graphene as a conductive backbone provides abundant electron and ion transport paths and promote the reactivity.Compared with the separators modified by VN nanosheets or VN nanowires,Li-S batteries employed VN QDs decorated separator exhibits much better cycling stability and rate performance.（2）To address the problem of slow kinetics of redox reaction in Li-S batteries,this work further introduced an electrocatalytic component into the VN QDs functional coating for separator.Uniformly dispersed Co/VN QDs with size less than 5 nm were fabricated via an in-situ phase separation strategy and coated onto the surface of polypropylene membrane as a bifunctional modified separator.The heterogeneous structure of Co and VN enables efficient adsorption and catalytic conversion of polysulfides,which improves the reaction kinetics,suppress the shuttle effects,increases the utilization rate of the active material,and protects the lithium cathode during cycling,enhancing the safety of the battery.The battery assembled with the Co/VN QDs modified separator still has a high capacity of 659.6 mAh g^-1 at 5 C,and the capacity decay rate is less than0.005% per cycle after 1000 long cycles,significantly improves the performance of Li-S batteries.