| Lithium-sulfur batteries are widely regarded as one of the most promising secondary battery systems due to their high theoretical specific capacity and energy density.However,there are still many problems and challenges in lithium-sulfur batteries,such as low utilization rate of active materials,the shuttle effect of soluble polysulfides,the volume expansion caused by the different density of charge and discharge products,and the problems of excessive electrolyte,etc.Because the cathode material is pure sulfur,its low cycle life and poor rate performance seriously hinder the development and application of lithium sulfur battery.To overcome the above problems,it is necessary to find a matrix material that can suppress the polysulfide shuttle,improve the overall conductivity of the sulfur electrode and the polysulfide redox reaction power,and adopt a reasonable electrode structure design to improve the utilization rate of the sulfur cathode.In recent years,transition metal compounds have polar compound bonds that can chemically adsorb polysulfides to suppress the shuttle effect.However,the mechanism for the conversion reaction of transition metal compounds to polysulfides and the utilization of sulfur-enhancing active substances are not clear.Therefore,in this paper,three typical transition metal compounds(molybdenum disulfide,molybdenum diselenide,and molybdenum trioxide)are selected.Using carbon cloth as a flexible substrate,three metal compounds are grown in situ to obtain a flexible self-supporting electrode.Li2S8 as the active material,through in-situ catalytic deposition of Li2S8 on self-supporting electrodes,their reduction process on different polar host materials was studied,and their effect as a polar host matrix on improving the performance of lithium-sulfur batteries was investigated,and the mechanism of polysulfide conversion was discussed and analyzed.The main research contents are as follows:(1)preparation and electrochemical properties of carbon cloth/molybdenum disulfide/sulfur-based cathode materials.The flexible self-supporting CC/MoSe2 composite electrode was synthesized by one-step hydrothermal method using carbon cloth as the substrate.The structure was characterized by XRD,Raman and XPS.The CC/1T-MoS2@Li2S composite electrode was prepared by in-situ electrochemical deposition method,which greatly improved the utilization rate of sulfur,and its electrochemical performance was analyzed.The results showed that the CC/1T-MoS2 electrode exhibited strong redox kinetics and long electrochemical stability to the polysulfide,and contributed the initial capacity of 1238 mAh g-1 at the current density of 0.5 C.After 500 cycles,the average capacity attenuation of each cycle was only 0.051%.Even under the condition of high S load(3.2 mg cm-2)and very low 5.0 μL mg-1 electrolyte volume/sulfur mass(E/S)ratio,it can still maintain a high sulfur utilization rate and electrochemical activity,achieving a high initial capacity of 1246 mAh g-1 at 0.2 C current density and a capacity retention rate of 85.1%after 100 cycles.The combination of self-supporting electrode and in-situ electrochemical catalytic deposition provides a new idea for the commercialization of lithium sulfur batteries with high sulfur load and poor electrolyte.(2)preparation and electrochemical properties of carbon cloth/molybdenum selenide/sulfur-based cathode materials.CC/MoSe2 composite material was synthesized by one-step hydrothermal method,and then CC/MoSe2@Li2S8 electrode was prepared by adsorption of Li2S8.Then,Li2S were uniformly deposited on the surface of CC/MoSe2 by electrochemical reduction.Its electrochemical properties were studied,and high sulfur utilization rate and strong redox kinetics were obtained.When the sulfur load was 2 mg cm-2,the initial capacity reached 1142 mAh g-1 at 1 C current density.After 500 cycles,the average capacity attenuation per cycle was only 0.038%.Moreover,under high sulfur load(4 mg cm-2),the ratio of E/S(6.2 L mg-1)is very low,and it still has a high electrochemical activity.After 100 cycles at the current density of 0.2c,the specific capacity of 1204 mAh g-1 and the capacity retention rate of 93.3%are still maintained.This transition metal selenide layered polar material provides a new scheme for the development of cathode materials for lithium sulfur batteries.(3)preparation and electrochemical properties of carbon cloth/molybdenum oxide/sulfur based cathode materials.After step hydrothermal synthesis of CC/MoSe2 precursor,then through 400℃ muffle furnace,sintering,30 min to get CC/MoO3 composites,then through electrochemical reduction,get CC/MoO3@Li2S,studied the electrochemical properties of electrodes,it obtained the relatively excellent electrochemical performance,when sulfur load of 2.0 mg cm-2,under the current density of 1 C,after a 350 cycle,capacity remains in 804 mAh g-1.Moreover,under high sulfur load(4 mg cm-2),the specific capacity of 899 mAh g-1 and 84.1%capacity retention rate were still maintained after 100 cycles at 0.2 C current density.This metal oxide polar material provides a new direction for the development of cathode materials for lithium sulfur batteries.The above research shows that the three polar host materials of molybdenum disulfide,molybdenum diselenide,and molybdenum trioxide all have strong ability to chemically adsorb polysulfide,and their surface has rich edges,which can catalyze the acceleration of polysulfide conversion process,thereby inhibiting the shuttle effect of polysulfides.Li2S8 as the active material,in-situ catalytic deposition of Li2S can increase the distribution of the active material on the substrate and can improve the utilization rate of the sulfur active material.The effect is significantly better than the method of in-situ deposition of Li2S8,carbon material and sulfur in the carbon cloth substrate.Therefore,the use of polar host materials combined with in-situ catalytic conversion of active materials is an effective method to improve the electrochemical performance of lithium-sulfur batteries. |
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