Preparation Of Highly Dispersed Transition Metal/Heteroatom Doped Carbon Composite Materials And Their Properties In Lithium-sulfur Batteries

With the rapid development of a variety of electronic equipments and new energy vehicles in recent years,people’s demand for high energy density secondary batteries has gradually increased.Compared with traditional lithium-ion batteries,lithium-sulfur batteries have higher theoretical mass specific capacity(1675 m Ah g^-1).And due to the advantages of abundant surface reserves of the active material sulfur,environmental friendliness and low cost,it is considered to be one of the most promising next-generation commercialized lithium secondary batteries.However,the drawbacks,including the low conductivity of the active material and its discharge products,the serious shuttle effect and the sluggish redox kinetics of the lithium polysulfides（Li PSs）intermediates during the charging and discharging process,hindered their commercialization.In this paper,four kinds of highly dispersed transition metal/heteroatom-doped carbon composites were designed and prepared,as the separator modification layer,to solve the above problems.By adjusting the structure and chemical composition of the materials,the electrochemical performance of corresponding lithium-sulfur batteries was significantly improved.The main research contents are as follows:Titanium carbide with mixed dimensional morphology（maintaining two-dimensional layered architecture with one-dimensional fibrous interweaving morphology within layers）was prepared by acid-base etching.Based single-atom Zr were loaded on the mixed-dimensional titanium carbide by ion exchange method,to construct composite catalyst materials（Zr@MD-Ti₃C₂T_x）with Zr-O/C-Ti coordination structure.On the one hand,due to the high conductivity,the high specific surface area and pore volume of the mixed dimensioned titanium carbide materials,more active sites are exposed,thus the utilization of active materials and discharge products are greatly improved.On the other hand,the active sites of Zr-O/C-Ti can chemisorb lithium polysulfide,lower the activation energy of the reaction through catalytic process during charging and discharging to accelerate the conversion of lithium polysulfide,thus suppress the shuttle effect and improve the cycle properties.Therefore,Zr@MD-Ti₃C₂T_x modified separator leads to excellent rate performance and cycling stability.At 10 C current density,the reversible capacity of 447 m Ah g^-1 is displayed,and a specific capacity of 861.8 m Ah g^-1 is still maintained after 2000charge/discharge cycles at a high current density of 2 C.The average decay ratio of each cycle is 0.02%,while the coulomb efficiency remains close to 100%.Highly dispersed La-loaded catalyst materials（La@MD-Ti₃C₂T_x）were prepared by ion-exchange method with mixed-dimensional titanium carbide materials as the support.Benefit from the unique 4f electronic structure of rare earth elements and the lanthanide contraction effect,the conductivity of the catalyst was improved and the loading of the active metal was increased.The highly dispersed La active sites can effectively anchor lithium polysulfide and mitigate the shuttle effect.At the same time,the interaction between the active center and the active substance lowers the activation energy of the lithium polysulfide conversion reaction,improves its slow kinetic properties,and effectively reduces the over potential of nucleation/decomposition of Li₂S molecules,thus promoting the liquid-solid-liquid phase transformation process during charge and discharge.As a result,the La@MD-Ti₃C₂T_x modified separator exhibits excellent rate performance and cycling stability.At 3 C current density,La@MD-Ti₃C₂T_x can demonstrate a reversible capacity of 864.5 m Ah g^-1.At 1 C current density,the average decay rate is 0.05%per cycle after 500charge/discharge cycles.Titanium carbide materials loaded with highly dispersed Fe,nitrogen and sulfur co-doping were constructed by a freeze drying-calcinating two-step method.The nitrogen and sulfur atoms can synergistically regulate the electronic structure of the active center,provide different lithium polysulfide adsorption sites,enhance the interaction between the active site and lithium polysulfide,and improve the atomic utilization,selectivity and stability of the active center,thus achieving rapid conversion of lithium polysulfide during the discharge process.Therefore,Fe@N,S-Ti₃C₂T_x as a separator modification material can significantly improve the rate performance and cycling stability of lithium-sulfur batteries.At 3 C current density,Fe@N,S-Ti₃C₂T_x can exhibit a reversible capacity of 927.3 m Ah g^-1.At 1 C current density,the average decay rate is 0.09%per cycle after 500 charge/discharge cycles.Highly dispersed Co,Ni bimetallic loaded nitrogen doped graphene materials（Co,Ni@NG）were constructed by the molten salt method.The highly dispersed double transition metal active centers can bond with two S atoms in lithium polysulfide and change the bond lengths and bond angles of Li-S and S-S bonds in lithium polysulfide,thus activating lithium polysulfide,reducing the reaction energy barrier and realizing the rapid conversion of lithium polysulfide.Therefore,Co,Ni@NG as separator modification material can significantly improve the cycle stability of lithium-sulfur batteries.With 200 charge/discharge cycles at 1 C current density,the average capacity decay rate is 0.07%per cycle.As a conclusion,a series of composite materials with single-atom or quasi-single-atomic dispersed transition metal loading on heteroatom-doped carbon were prepared to address the existing problems of lithium-sulfur batteries by combining the advantages of high conductivity from the conductive carbon matrix and high catalytic activity from highly-dispersed catalyst.The long cycle stability and the rate performance of the corresponding lithium-sulfur batteries were greatly improved by using as-modified separators.The work in this thesis provides both theoretical and technical references for the design of high performance lithium-sulfur batteries.