Electrocatalyst Design And Kinetic Modulation For Bidirectional Reaction In Lithium-Sulfur Batteries

With the rapid development of electrified transportation and portable electronics,commercial lithium-ion batteries(LIB s)that suffer from inferior energy density will be hardpressed to satisfy future markets.It is indispensable to explore reliable substitutes for LIBs.Of note,lithium-sulfur(Li-S)batteries represents a potential quantum leap for nextgeneration energy storage systems owing to the superb theoretical capacity(1672 mAh g-1)and energy density(2600 Wh kg-1),as well as the cost-effectiveness and environmentalfriendliness of sulfur resource.However,a suite of troublesome problems,mainly pertaining to sluggish redox kinetics and severe poly sulfide(PS)shuttle,has hampered the pathway for pursuing Li-S commercialization.Recent years have witnessed a burgeoning interest in designing electrocatalysts for catalyzing PS conversion in Li-S realm,in order to essentially expedite redox kinetics and inhibit polysulfide shutting.Although strenuous efforts have been devoted to exploring various types of electrocatalysts,their unsatisfactory catalytic effect caused by insufficient active sites remains a huge barrier for obtaining favorable sulfur chemistry.Therefore,rational modulation of electrocatalysts is vital to boosting sulfur redox kinetics.Of note,defect engineering has received extensive application,which can optimize the electronic structure and catalytic effect.However,its stucture-effect relationship remains elusive.In addition,dual-directional electrocatalytic reaction of Li-S batteries has received insufficient attention,where the multistep conversion reaction need to be decoupled and electrocatalysts with selectivity are urgent to be developed.To circumvent these formidable challenges,this article will focus on the mediator design and kinetic modulation of bidirectional catalytic reactions for Li-S batteries with the aid of defect engineering,the specific research contents are as follows:(1)We designed a CoFe alloy decorated mesoporous carbon sphere(CoFe-MCS)as an electrocatalyst for Li-S batteries.Such bimetallic alloy can elevate the conductivity of metalbased electrocatalyst,optimize the electronic structure,and expose more active sites.It was found that electrocatalytic effect toward dual-directional polysulfide conversion can be boosted throughout detailed electroanalytic characterization,theoretical calculation,and operando instrumental probing.Accordingly,the S@CoFeMCS cathode harvests a capacity 698 mAh g-1 at a current density of 3.0 C.Meantime,a stable cycling with a low capacity decay rate of 0.062%per cycle over 500 cycles at 2.0 C can be achieved.(2)We designed a Co-doped VN electrocatalyst that loaded on N-doped carbon substrate(Co-VN@NC).Additionally,we proposed a promising strategy with respect to electrocatalyst design via concurrent optimization of morphologic architecture and electronic structure in Li-S system.This concurrent optimization stratergy endows CoVN@NC with multifarious advantages,including abundant three phase interface,excellent catalytic activity,and ultrafast ion diffusion.We also proposed a universal principle for the design of bidirectional electrocatalyst in Li-S batteries.Encouragingly,Co-VN@NC showcases excellent electrocatalytic effect toward bi-directional polysulfide conversion kinetics,which were evidenced by a series of electro-kinetic measurements.As a result,the S/Co-VN@NC harvests a stable cycling with a low capacity decay rate of 0.07%per cycle over 500 cycles at 1.0 C.(3)We designed a MoSe2 electrocatalyst harnessing concurrent N-doping and Sevacancy(N-MoSe2-x/C),thus enabling effective modulation of Li2S redox kinetics.It should be noted that we provide an insight for designing sulfur bidirectional electrocatalyst from random acceleration to directional catalysis.Throughout theoretical calculation and electrokinetic analysis,we revealed the selective electrocatalytic effect of this dual-defect engineering toward bidirectional sulfur chemistry.The DN and Vse are prone to catalyze the nucleation and dissociation process of Li2S.Consequently,the thus-derived S@N-MoSe2x/C cathode can achieve stable cycling performance at a current density of 2.0 C with a low capacity decay rate of 0.04%per cycle over 1000 cycles.Finally,we also implemented an advanced sulfur cathode a high areal capacity(7.3 mAh cm-2)along with a pouch cell with excellent mechanical strength,holding great potential for practical application.