Design Of Defect-Rich Molybdenum Disulfide-Based Heterostructures And Their Applications In Lithium-Sulfur Batteries

With the rapid development of electrical transportation vehicles in recent years,lithium-ion batteries（LIBs）,as the core energy storage devices,gradually failed to meet the people’s increasing range demand.In this context,lithium-sulfur batteries（LSBs）have been considered as one of the most promising candidates to replace LIBs due to their high energy density(>400 Wh kg^-1)as well as the low cost of the cathode active material sulfur.However,the commercial application of LSBs was severely hampered by the shuttle effect caused by the dissolution of polysulfide.To address this problem,many polar transition-metal chalcogenides have been introduced into the sulfur-containing electrode of LSBs,to enhance the electrodes’adsorption of lithium polysulfide,and simultaneously catalyze the redox reaction of lithium polysulfide.Unfortunately,the adsorption and catalytic performances of these two-dimensional layered compounds to lithium polysulfide mainly depends on their limited edge active sites.Therefore,in this dissertation,MoS₂/carbon nanofiber composites were used as research object,sulfur vacancy defects and heterostructures were introduced at the same time,to further improve the adsorption and catalytic performance of MoS₂ to lithium polysulfide,and then inhibit their shuttle effect,and finally realize the design of high-performance lithium-sulfur battery.The main research work of this dissertation was summarized as following:（1）Design and electrochemical properties of edge-defect-rich MoS_2-x/Mo₂C heterojunctionsIn this chapter,MoS₂ nanosheet arrays were first grown on the surface of carbon nanofibers by hydrothermal processes,followed by a heat treatment under reducing atmosphere（Ar/H₂）to obtain edge-defect-rich MoS_2-x/Mo₂C heterostructure decorated carbon nanofiber(CNF@MoS_2-x/Mo₂C)composite electrodes.The growth mechanism analysis indicates that the sulfur deficiency caused by the hydrodesulfurization reaction of MoS₂ at high temperature resulted in the formation of sulfur vacancy defects;meanwhile,a part of MoS_2-x edge transformed into carbide at a certain temperature,resulting in the formation of MoS_2-x/Mo₂C heterostructure.According to the analysis of visual adsorption experiments,cyclic voltammetry curve tests and Tafel curve results,the CNF@MoS_2-x/Mo₂C composites can combine the strong adsorption of S vacancies and the high catalytic performance of the heterogeneous structure to realize the rapid adsorption-catalytic conversion of polysulfides.At the same time,the introduction of high-conductance Mo₂C materials can effectively reduce the charge transfer resistance and accelerate the rate of charge transfer.Furthermore,the electrochemical test results showed that the CNF@MoS_2-x/Mo₂C-1000-S cathodes provided a total specific capacity of 1208 m Ah g^-1 and 740 m Ah g^-1 at current densities of 0.1 C and 3.0 C,respectively,exhibiting a high capacity and an excellent rate capability.And the CNF@MoS_2-x/Mo₂C-1000-S cathodes can retain a specific capacity of 790 m Ah g^-1after 250 cycles at 1.0 C,with a capacity retention rate of about 85%,showing a good cycling stability.This study demonstrates the rationality and effectiveness of the idea of constructing defective heterostructure sulfur hosts for fast adsorption-catalytic conversion to suppress the shuttle effect of LSBs.（2）Design and electrochemical properties of base-defect-rich MoS_2-x/ZnS and MoS_2-x/Ag₂S heterojunctionsIn the former work,although a certain amount of sulfur vacancies is created at the edge of MoS₂ by hydrodesulfurization reaction under thermal reduction conditions,these finite edge sulfur vacancies cannot meet the stable long-term work of the device at large current density.Thus,in this chapter,in order to further improve the adsorption and catalytic properties of MoS₂ towards lithium polysulfide,sulfur vacancies were accurately created on the MoS₂ inert base surface by doping metal atoms with different electronegativity,which can further increase the active site of MoS₂.This work utilized the strong cutting effect of active metal ions on chemical bonds in high-temperature treatment,the base-defect-rich MoS_2-x/ZnS heterostructure decorated carbon nanofiber(CNF@MoS_2-x/ZnS)composite electrodes were designed by annealing under an Ar atmosphere.The growth mechanism analysis indicates that Zn²⁺ions trap the S^2-ions in the MoS₂ basal plane to form tightly attached ZnS nanoparticles;meanwhile,MoS₂ nanosheets turn into MoS_2-x owing to the loss of S^2-ions.And the visual adsorption experiments,CV tests,symmetric cells and Tafel curves indicated that the large number of active sites and rich heterogeneous interfaces significantly accelerate the polysulfide redox kinetics.Moreover,the EIS test results illustrated that the heterogeneous structure between MoS_2-x nanosheets and ZnS nanoparticles further accelerated the electron transport and Li⁺ion diffusion.With a result,the CNF@MoS_2-x/ZnS-S cathode has a high reversible capacity of 1233 m Ah g^-1 at 0.1 C,maintains 944 m Ah g^-1 at 3.0 C,showing a high capacity and an excellent rate capability.At the same time,the CNF@MoS_2-x/ZnS-S cathode has an extremely low decay rate of 0.012%for 900 cycles at 2.0 C.Besides,the CNF@MoS_2-x/ZnS-S LSBs also show an ultralow self-discharge rate of 1.1%per day in voltage.More interestingly,in this chapter,using metal ion-assisted cutting method,a type of base-defect-rich CNF@MoS_2-x/Ag₂S heterostructures was also successfully prepared.And the electrochemical test results showed that this anode also has the effect of improving the electrochemical performance of lithium-sulfur batteries.The CNF@MoS_2-x/Ag₂S-S cathodes can provide a specific capacity of 1219 m Ah g^-1 and611 m Ah g^-1 at 0.1 C and 3.0 C,respectively;and still retain a capacity of 819 m Ah g^-1 after cycling at 1.0 C for 250 cycles,showing a capacity decay rate of 0.031%per cycle.This study will provide some novel ideas for the design of defect-rich heterostructure electrode materials for high-performance energy storage devices.