Investigation On MoS₂-based Anode Materials For Na-ion Batteries

Since the twenty-first century,the contradiction between the supply and use of energy reserves has become more and more prominent.Under the challenge of social sustainable development,the current situation of relying on non-renewable energy sources such as oil and coal fossils has a profound impact on human development.Therefore,there is a urgent need to find renewable and clean-green energy that can instead of traditional fossil fuels,and develop the corresponding energy storage devices.Among the secondary batteries,the rapid development of lithium-ion batteries and sodium-ion batteries makes them new members of green energy.They have many advantages such as clean and environmental protection.The two-dimensional layered materials such as MoS₂ and WS₂ have huge development potential due to their unique properties.However,they still have the disadvantages of structural instability and poor conductivity during cycling.In this paper,aiming at the above shortcomings and problems,by designing and synthesizing the composite of molybdenum disulfide,and compound with carbon-based materials;molybdenum-doped metal phase tungsten disulfide,thus further optimize the electrochemical performance of the electrode materials as negative electrode materials for sodium ion batteries.A series of research results have been achieved through exploration:(1)Preparation of MoS₂/RGO composite material by introducing graphene oxide to hydrothermal reduction.The MoS₂ ultra-thin nanosheets are evenly distributed on the surface of the layered graphene substrate in an embedded manner.Under the reaction of low hydrothermal temperature,MoS₂ has a certain degree of interlayer expansion,the expanded Na⁺ deintercalation space can reduce the migration and diffusion resistance of Na⁺.As a carbon-based material,graphene has excellent physical and chemical properties,which not only improves the conductivity of the material,but also has a large specific surface area that can activate more reaction sites and reduce the diffusion barrier of sodium ions.In the negative electrode test of sodium ion batteries,the reversible capacity is still 428 mAh g^-1after 300 cycles at a low current density of 0.1 A g^-1.(2)The two-step process of organic solvent hydrothermal and high-temperature sintering can synthesize the MoS₂/C composite material with two-dimensional layer spacing extension.The introduction of DMF solvent in the hydrothermal process increases the two-dimensional layer spacing of MoS₂ which was generated by the reaction,this also can accommodate more sodium ions in the electrochemical reaction deintercalation.The addition of surfactant PVP can not only regulate the microscopic morphology of the material in hydrothermal,the spherical structure facilitates the contact between the material and the electrolyte,and alleviates the agglomeration effect.And after high-temperature sintering and carbonization,it can also be used as conductive carbon to compund with molybdenum disulfide,this solves the problem of materials insufficient conductivity.After 100 cycles at a current density of 0.1 A g^-1,the reversible capacity remains at considerable450 mAh g^-1.(3)WS₂ is synthesized by the high-temperature hydrothermal method of organic solvents,which has the conductivity of the metal phase and a very large two-dimensional layer spacing,the excellent sodium ion deintercalation ability is very suitable for sodium ion battery material.On this basis,a small amount of doping is carried out to form a heterogeneously doped metal phase 1T-W_0.9Mo_0.1S₂ composite material,which has better conductivity than the traditional semiconductor phase.The enlarged interlayer spacing is more conducive to the active deintercalation reaction of sodium ions,and through the heterogeneous atom doping,more fracture defects can be generated during the substitution process,more active sites are exposed for contact reaction.And at the same time,there is a certain ability to refine the particles,the small particle size can further improve the electrochemical activity of the material.Under the current density test of a small current 0.1 A g^-1in the sodium ion battery,after 180 cycles,the reversible capacity is maintained at 411 mAh g^-1,and it has a very high capacity retention rate.