Preparation And Performance Of CeO₂/carbon-based And MXene-based Cathode Materials For Lithium-Sulfur Batteries

The lithium-sulfur batteries provide new impetus for the application of new energy batteries since their high specific capacity and high energy density.However,lithium-sulfur batteries have some problems of the low cycle life and poor stability due to poor electrical conductivity of sulfur,volume expansion of lithium sulfide and polysulfide shuttle effect.To solve the above problems,CeO₂/carbon-based and MXene-based cathode materials of lithium-sulfur batteries were designed and prepared in this thesis.In addition to ensuring the conductivity of the cathode material,it usefully relieves the volume expansion of the cathode material and inhibits the shuttle effect of the polysulfide.It significantly improved the battery cycle performance.With cerium nitrate hexahydrate（Ce（NO₃）₃·6H₂O）and cellulose acetate（CA）as the cerium source and carbon source,the core-shell cerium oxide/porous carbon（CS-CeO₂/PC）was prepared by the template method and the high-temperature carbonization method,and further loaded sulfur to obtain S/CS-CeO₂/PC lithium-sulfur battery cathode material.The transmission electron microscopy（TEM）and the scanning electron microscopy（SEM）characterized the core-shell structure of CeO₂and the existence of porous carbon.X-ray diffractometer（XRD）and Infrared spectroscopy（IR）were used to measure the crystallinity and structure of carbonization of the material.X-ray photoelectron spectroscopy（XPS）analyzed the electronic shift between CS-CeO₂ and S,which proved that CS-CeO₂ adsorbed sulfur.The electrochemical performance tests of S/CS-CeO₂/PC contained Cyclic voltammetry（CV）,Rate performance,Charge-discharge,and Electrochemical impedance spectroscopy（EIS）.The initial specific discharge capacity of S/CS-CeO₂/PC at 0.25C was 1242.6 m Ah·g^-1 with an average coulombic efficiency of 92.36%for 500 cycles.The cycle life was long and the electrochemical performance was steady and perfect.To further improve the battery performance,this thesis used organ-like MXene as the matrix,CeO₂ doped and loaded with S to prepare the S/CeO₂/MXene lithium-sulfur battery cathode material.The synergistic adsorption of lithium polysulfide between the metal oxide CeO₂ and the MXene matrix effectively inhibited the shuttle effect.The electrochemical performance was tested by CV,EIS,Tafel test,Deposition experiment,Rate performance and Constant-current charge-discharge methods.The chemical state,morphology and structure of S/CeO₂/MXene were analyzed by XPS,SEM and TEM.CV and Tafel tests showed that S/CeO₂/MXene had good dynamic behavior.The deposition of Li₂S on CeO₂/MXene-based cathode material reached 125.68 m Ah·g^-1.The first discharge specific capacity of S/CeO₂/MXene at 0.5C was 1051.1 m Ah·g^-1,and it is maintained at 921.9 m Ah·g^-1 after 200 cycles.The average coulombic efficiency reached 97.75%,and the capacity retention rate was 87.71%.XPS analysis showed that the Ce and Ti peaks shifted positively,and the S peak shifted negatively.The atoms gained electrons and the binding energy decreased,indicating that CeO₂and MXene acted as synergistic adsorption of sulfur.The adsorption experiments showed that CeO₂/MXene adsorbed lithium polysulfide.The synergistic effects inhibited the shuttle effect to keep S/CeO₂/MXene at a high level of electrochemical performance.The stability and durability of the Li-S battery cycle had significantly improved.The SEM and TEM test showed that the organ-like structure of MXene,that is,the interlayer structure,can effectively alleviate the volume expansion.Comparing the cathode and anode SEM images after cycling,the structure of S/CeO₂/MXene electrode was stable.The surface of the lithium sheet was flat and there were few lithium dendrites on the lithium sheet.S/CeO₂/MXene has excellent electrochemical performance and structural stability,which opens up new research and development prospects for lithium-sulfur battery cathode materials.