Performance And Catalytic Mechanism Investigation Of Transition Metal Selenide/MXene-based Bifunctional Catalysts Via Tailoring Their Electronic Structure In Lithium-sulfur Batteries

The spread of electric vehicles and modern mobile electronic devices has put forward stricter requirements with the rechargeable battery systems of safety,environmental friendliness,cost and stability.Lithium-sulfur batteries is regarded as one of the most promising candidates for the next-generation high-energy batteries owing to the overwhelming theoretical specific energy of 2600 Wh kg^-1 and impressive specific capacity of 1675 m Ah g^-1,respectively.However,the poor conductivity and“shuttle effect,”which have been regarded as one of the crucial issues obstructing the commercialization and industrialization of LSBs.The above situation becomes more serious under the condition of lean electrolyte,high sulfur loading,and thick electrode.In this work,a series of transition metal selenide/MXene-based materials with high conductivity,abundant active sites,and strong adsorption capacity were developed by phase engineering and doping engineering.These materials can accelerate the electron/ion transport in the redox process.More importantly,which can quickly catalyze the conversion of lithium polysulfide to Li₂S,and reduce the decomposition barrier of Li₂S,simultaneously reverse catalyze the conversion of Li₂S to lithium polysulfide and even S₈molecules,indicating that the use of catalyst promotes the two-way redox kinetics of lithium sulfur battery,Indicating that the catalysts can bidirectionally promote the redox kinetics of sulfur.As a result,the sulfur cathodes with high sulfur loading and lean electrolyte still maintain high sulfur utilization rate,and effectively improves the specific capacity and areal capacity of the Li-S battery.Through the combination of a large number of thermodynamic and kinetic experiments and DFT theoretical calculations,the catalytic mechanism of the new electrocatalyst in lithium-sulfur battery is profoundly explained from the aspects of atomic level and electronic structure,aiming to provide some theoretical reference and guidance for catalyst design and mechanism interpretation of lithium-sulfur battery.The major synopsis and results are summarized as follows:（1）A dual-functional conductive 1T-Mo Se₂/MXene bidirectional catalyst as a high-efficiency sulfur host was constructed by few-layer 1T-Mo Se₂ in-situ growth on MXene nano-flakes through one-step solvothermal reaction.Experimental and DFT theoretical analysis reveal that 1T-Mo Se₂/MXene possess a high electronic conductivity,strong adsorption ability,and abundant active sites,which can provide the strong capture and catalytic conversion ability for Li PSs and uniform deposition and dissolution of Li₂S,thus effectively inhibiting the Li PSs shuttle behavior in LSBs.Benefiting from the enhanced bidirectional sulfur redox kinetics and adsorption capacity,the S/1T-Mo Se₂/MXene cathode delivers a high areal capacity of 6.9 m Ah cm^-2 and good capacity retention of～73.1%were obtained after 200 cycles at 0.1 C.This work provides a novel way for the development of dual-functional conductive catalyst to accelerate the catalytic conversion of Li PSs.（2）A cation-doping strategy has been developed to tailor the electronic structure and catalytic activity of Mo Se₂ that in-situ hybridized with conductive Ti₃C₂T_x MXene,thus obtaining Co-Mo Se₂/MXene bifunctional catalyst as a high-efficient sulfur host.Combining a smart design of the dense sulfur structure,the as-fabricated highly dense S/Co-Mo Se₂/MXene monolith cathode achieves a high reversible specific capacity of 1454 m Ah g^-1 and an ultrahigh volumetric energy density of 3659 Wh L^-1 at routine electrolyte,and a high areal capacity of～8.0 m Ah cm^-2 under extremely lean electrolyte of 3.5μL mg_s^-1 at 0.1 C.Experimental and DFT theoretical results uncover that introducing Co element into Mo Se₂plane can form a shorter Co-Se bond,impel Mo 3d band to approach Fermi level,and provide the strong interactions between polysulfides and Co-Mo Se₂,thereby enhancing its intrinsic electronic conductivity and catalytic activity for the fast redox kinetics and uniform Li₂S nucleation in the dense high-sulfur-loaded cathode.This deep work provides a good strategy in constructing high-volumetric-energy-density,high-areal-capacity LSBs with lean electrolytes.（3）The effects of metal cations（M=Co,V,Mo,Ni,Zn,Cu）doping WSe₂on its catalytic activity and adsorption performance were systematically investigated from the aspect of atomic level and electronic structure.The following results are verified by a combination of systematic thermodynamic and kinetic experiments and DFT theoretical calculations:the adsorption energy of metal cation-doped WSe₂ for polysulfides depend on the amount of charge transfer between the catalyst and the polysulfides.The more charge transfer between polysulfides and the catalyst,the stronger adsorption capacity,the order follows:V-WSe₂>Co-WSe₂>Mo-WSe₂>Zn-WSe₂>Ni-WSe₂>Cu-WSe₂>WSe₂.More importantly,the decomposition barrier of Li₂S was found to be positively correlated with the ratio of electron affinity and ionic radius of the doped metal cations.The order of decomposition barrier of Li₂S follows:V-WSe₂>Co-WSe₂>Mo-WSe₂>Zn-WSe₂>Ni-WSe₂>Cu-WSe₂>WSe₂.In addition,it is also found that as the ratio of electron affinity to ionic radius of the doped metal cations decreases,the Li-S bond length of Li₂S increases,and it is easier to decompose into Li S and Li.Based on the above analysis,the S/V-WSe₂/MXene cathode deliver a high areal capacity of 4.69 m Ah cm^-2 with a lean electrolyte of 5μL mgs^-1 and a high sulfur loading of 6mg cm^-2,the capacity retention rate remained at 62.0%after 200 cycles.Therefore,this design provides general rules for optimizing metal-cation doping into TMDCs to realize rapid catalytic conversion in constructing high performance Li-S batteries.（4）The effects of anions（X=B,N,P,S,Te）doping WSe₂on its catalytic activity and adsorption properties were systematically studied from the aspects of atomic level and electronic structure.The following results are testified by a combination of systematic thermodynamic and kinetic experiments and DFT theoretical calculations:the adsorption energy of anion-doped WSe₂ for polysulfides are decided by the outer electronic structure of the doped anion.When the number of electrons is odd,there will be one unpaired electron after doping,and when the number of outer electrons is even,the structure of electron pair is present after doping.The unpaired electrons are favorable for electron transfer with polysulfides,thereby improving the adsorption capacity and electrical conductivity.Therefore,the adsorption energy of B,N,P doping is greater than that of undoped and S,Te doping.Due to the difference between the electronegativity and covalent radius of the doping ions,the Se vacancies formed by doping on the surface of WSe₂ are also different,the order of Se vacancy follow:B-WSe₂>N-WSe₂>P-WSe₂>S-WSe₂>Te-WSe₂>WSe₂.Abundant vacancies are favorable for electron/ion transport,and the formation of vacancies also increases active sites,which is favorable for the catalysis of polysulfides and the deposition and decomposition of Li₂S.Based on above,the S/B-WSe₂/MXene cathode can achieve an initial capacity of 8.26m Ah cm^-2with a high sulfur loading of 7.6 mg cm^-2 and a lean electrolyte of 5μL mgs^-1.Therefore,the systematic study of different doping anions in this thesis will help to provide effective guidance for the future selection principles of doping strategies.Therefore,the systematic study of different doped anions is helpful to provide effective guidance for the selection principle of doping strategy.