Synthesis And Lithium-storage Performance Of Two-Dimensional Transition Metal Disulfides-Based Electrode Materials

The applications of rechargable lithium-ions batteries（LIBs）in electric vehichles and portable electronic devices contributes to the reduction of not only the consumption of fossil fuels but also the environmental pollution,which makes them to be one of the most important development directions in the field of new-energy resources.The improvement of electrochemical performance of electrode materials is an important precondition for the realization of the large-scale applications of LIBs.However,the storage capacity and lifetime of the current LIBs are not enough high due to the limit of electrode materials.Thus,the development of high performance electode materials for LIBs is of particular importance.Two-dimensional（2D）transition metal disulfides（TMDs）such as MoS₂ and SnS₂ are regared as one of the most important candidates for the LIB electrode materials due to their unique structures and high storage capacity.However,the pure TMD electodes for LIBs show the rapid capcity degeneration and poor electrical conductivity,which greatly limit their applications in LIBs.Previous studies have found that the composites composed of 2D TMDs and good conducting（e.g.,carbon-based nanomaterials）and highly stable materials（e.g.,transition-metal oxide nanomaterials）can effectively improve the cycling stability and conductivity of LIBs.In this thesis,we propose to combine with the advantages of 2D TMDs,graphene,and metal oxides,design and synthesize the MoS₂/graphene composites and MoO₃/SnS₂ core-shell nanowires,make systematic study of electrochemical performance of these composite nanostructures.Moreover,the electronic structures and charge/discharge mechanisms of these composite nanostructures are revealed by combining with first-principle calculations.The main research contents and results are as follows,1.MoS₂/graphene composite nanostructures with different molar mass ratios have been systhesized by using the two-step method,and the microscopic structures,morphology and electrochemical performance of these composites have been studied.The structural and morphological characterization results indicate that the synthesized MoS₂/graphene composites are composed of wrinkle MoS₂ and graphene nanosheets.Moreover,the composites will display the flower-like microscopic morphology when the ratio of graphene in the samples decreases into a specific value.The result of electrochemical characterizations suggest that the electode material presents the biggest first capacity（1110.4 mAh/g）when the molar mass ratio of MoS₂ and graphene in the sample is 5:1.After 100 charge-discharge cycles,it has a reversible capacity of 737.4 mAh/g.Moreover,these samples remain higher reversible capacity even if they work at larger current densities,and they present superior electrochemical performance as compared to that of pure MoS₂ electrode.By means of first-principles calculations,we find that the introduction of graphene can suppress effectively the rapid dissociation of MoS₂ nanosheets in the charge/discharge process.Moreover,the introduction of graphene can improve the diffusion of Li ions and conductivity of the electrode materials.Hence,the electrochemical performance of the composite electrodes can be promoted.2.MoO₃/SnS₂ core-shell nanowires have been fabricated by using hydrothermal and water-bath methods,and the structural composition,microscopic morphology,and electrochemical performance of these core-shell nanowires have been studied.Our results indicate that the MoO₃/SnS₂ core-shell nanowires are composed of vertically aligned SnS₂ nanosheets on MoO₃ nanobelts.The result of electrochemical characterizations suggests that the MoO₃/SnS₂ nanowires possess reversible capacity of 504 mAh/g after 100 cycles,which is far superior to that of pure MoO₃ and SnS₂electrodes.Based on ab initio molecular dynamics simulations,we find that the MoO₃core in core-shell nanowires acts as backbone to support the polysulfide and Sn-based alloys during the conversion reaction,and the formed MoO₃/SnS₂ heterointerface can slow down the dissociation rate of SnS₂ nanosheets,facilitate the diffusion of Li ions and charge transfer,and reduce the volume expansion in the charge-discharge process,which contributes to the excellent electrochemical performance of the MoO₃/SnS₂electrodes during the charge and discharge.The study of this thesis can provide solid basis for the design and fabrication of TMD-based LIBs and supply a reference for the understanding of charge-discharge mechanisms of other composite electrodes.