Design And Application Of Transition Metal-based Electrocatalysts For Lithium-sulfur Battery

The development of science,technology and civilization greatly improved the quality of human life,but the production and use of science and technology brought about social problems like energy crisis and environmental pollution.The development and use of clean and renewable energy is the mainstream strategy to solve the above problems.However,clean energy,represented by wind and solar energy,has the disadvantage of intermittency.Therefore,suitable energy storage devices are needed to collect it and then deliver it stably.Lithium-ion secondary batteries play an important role in energy storage,with the advantages of portability and long life.However,its specific energy has been difficult to meet the requirements of new high-power and longlasting devices due to the limited theoretical capacity of its anodes and cathodes.As a result,many new battery systems have emerged in recent years.Research into these systems is on the rise.Among the many emerging battery systems,lithium-sulfur batteries are considered very competitive candidates for the next generation of secondary batteries,with the advantages of high theoretical energy density,low environmental impact,abundant sulfur reserves and low price.However,the development of lithium-sulfur batteries faces many challenges that severely limit their commercialization.In the cathode,sulfur and discharge products have insulating properties,which leads to the problem of slow electrode reaction kinetics.In addition,since the lithium-sulfur battery is a multi-step conversion reaction,the reaction intermediate products(lithium polysulfides)are easily dissolved in the ether-based electrolyte and shuttle between the cathode and anode,resulting in the loss of active materials,reducing the battery capacity and Coulomb efficiency.In the anode,there are problems such as lithium dendrite growth and electrode corrosion caused by polysulfides shuttle.In this work,different transition metal electrocatalysts have been selected for the two key problems in the cathode of lithium-sulfur batteries.From the perspective of “inside” and “outside” of the cathode,sulfur hosts and interlayers with catalytic properties were designed,and a lithium-sulfur battery with excellent performance was obtained.Combined with experiments and theoretical calculations,the effects of different electrocatalysts on the kinetics of the sulfur redox reaction were studied,and the working principles of different electrocatalysts to inhibit the shuttle effect and promote the reaction kinetics were analyzed in depth.Finally,a lithium-sulfur battery using a host or an interlayer with an electrocatalyst was assembled to verify the actual effect,and its practical potential was analyzed by testing a battery with a high sulfur areal loading.The main research results of this thesis are as follows:(1)Enhanced catalytic activity of Mo S2 by heteroatom doping: V-doped Mo S2 nanosheets were prepared by a one-step hydrothermal method using a structurally stable polyoxometalate precursor containing both Mo and V atoms.The doping of V atoms regulated the surface properties of Mo S2 and increased the active sites.In addition,the doping of V atoms induced the formation of a more active 1T phase structure,which increased the adsorption capacity of Mo S2 for polysulfides and the catalytic ability for the sulfur redox process.With these advantages,batteries using V-Mo S2 electrocatalysts had faster reaction kinetics and better electrochemical performance.When the sulfur loading of the cathode ≥ 5 mg·cm-2,the discharge capacity of the battery still exceeded4 m A·h·cm-2.(2)Design electrocatalysts with amphiphilic surface: Porous carbon cage sulfur host Co Se2/C embedded with Co Se2 nanoparticles electrocatalyst was prepared by selenizing MOF precursor.The surface of Co Se2 electrocatalyst has both electrophilic and nucleophilic centers,which can simultaneously form Co-S bonds and Li-Se bonds with polysulfides,effectively anchoring the polysulfides.At the same time,conductive porous carbon can limit sulfur loss by physical confinement.In addition,the amphiphilic surface of Co Se2 can reduce the decomposition energy barrier of polysulfides and promote the reaction kinetics.The battery with Co Se2/C nanocages as the host still maintains a discharge specific capacity of 503.4 m A·h·g-1 after 400 cycles at a rate of 1.0 C,which is much higher than that of the battery without the host(217.2m A·h·g-1).(3)Inducing three-dimensional deposition of Li2 S to improve battery capacity:Nb B2 electrocatalyst was prepared by metathesis method.After being dispersed on the conductive skeleton,the Nb B2@CC interlayer with catalytic properties was obtained.Since the B atoms on the surface of Nb B2(001)can interact with both Li and S atoms in polysulfides,Nb B2 had a strong anchoring ability to polysulfides.At the same time,the CC can physically block the penetration of polysulfides into the anode.The Nb B2@CC interlayer can effectively suppress the shuttle effect.In addition,on the Nb B2 surface,the decomposition energy barrier of polysulfides was significantly reduced,and the kinetics was significantly improved.The strong adsorption and catalytic effect of Nb B2 on polysulfides enabled the discharge product Li2 S to be deposited around Nb B2 in a three-dimensional form,which solved the problem of premature termination of the discharge process due to rapid coverage of the insulating product on the electrode surface and improved the discharge capacity of the battery.As a result,the capacity degradation rate of lithium-sulfur batteries with Nb B2@CC interlayer was only 0.006% per cycle after 800 cycles,which was much lower than that of batteries without Nb B2 electrocatalyst(0.066% per cycle).Even when the sulfur load exceeded 9.4 mg·cm-2,the discharge capacity of the battery can be as high as 7.34 m A·h·cm-2.(4)Non-stoichiometric treatment to enhance the intrinsic activity of VS2: By adjusting the temperature and time,two types of VS2 materials with non-stoichiometric structure were prepared using VS2 as a precursor,namely SV-VS2 with sulfur vacancies and VI-VS2 with self-intercalated V atoms.After coating them on the surface of the separator,a functional separator with an active interlayer was obtained.Due to the lithiation and delithiation behavior of both electrocatalysts during the cycling,and the lithiated SV-LVS2 had stronger catalytic ability than the lithiated VI-LVS2,the battery using SV-VS2 coated separator had faster reaction kinetics during operation.In addition,SV-VS2 had stronger adsorption capacity and catalytic ability than VI-VS2.Therefore,SV-VS2 had higher activity than VI-VS2,and the battery using SV-VS2 coated separator had higher rate performance and cycling stability.Even when the sulfur areal loading was increased to 7.5 mg·cm-2,the remaining surface capacity of the battery with SVVS2 electrocatalyst still exceeded 4 m A·h·cm-2 after 20 cycles.In summary,this paper designs four effective electrocatalysts for the key problems of the cathode of lithium-sulfur batteries,which are used in the form of host and interlayer.A high rate and long life lithium-sulfur battery was realized.The working mechanism of transition metal selenides with amphiphilic surfaces and twodimensional transition metal boride electrocatalysts in lithium-sulfur batteries was studied by combining experimental and theoretical calculations.In addition,through the study of transition metal sulfide electrocatalysts,the effects of heteroatom doping,surface vacancies,and atomic self-intercalation on the intrinsic activity of the electrocatalysts have been researched.The working mechanism of different electrocatalysts was analyzed from molecular and atomic levels,which provided a reference for the subsequent design and optimization of transition metal electrocatalysts for lithium-sulfur batteries.