Study On Synthesis And Electrochemical Performance Of Transition Metal Sulfides Anode Material

Rechargeable lithium ion batteries?LIBs?have become one of the indispensable decives in modern social life.As traditional commercial anode material for LIBs,graphite holds low theoretical specific capacity of 372 mAh g^-1.LIBs'energy storage performance is increasingly unable to meet the huge demand.It is one of the crucial chananels to boost the performance of ion batteries through exploring anode materials which holds higher specific capacity and better stability.Due to their higher theoretical specific capacity and larger interlayer spacing,Layered transition metal sulfides?LTMS?have been successfully applied in the field of ion batteries.However,they also exhibit some disadvantages,such as inherent poor condectivity and irreversible huge volume expansion in ion uptakes,which ultimately leads to poor electrochemical performance.These greatly limit its commercial application.There are two ways which could be adopted to solve these problems.On the one hand,nanostructures with different morphologies and good structural stability were synthesized by ameliorating the synthetic methods.On the other hand,compound materials were prepared by introducing carbon-based materials?especially graphene?with excellent flexibility and conductivity.of spectroscopy technology makes it more convenient to analyze electrode materials,such as Raman spectrum,Infrared?IR?spectrum and Ultraviolet-visible?UV-Vis?spectrum.They can be widely applied to the crystal structure of electrode materials,the interaction between electrode and electrolyte,the formation of solid electrolyte interface film and energy storage mechanism of charge-discharge.In this paper,we constructed novel LTMS nanostructures combining with graphene to effectively strengthen the energy storage performance of anode materials.We adopted simple hydrothermal and solvothermal growth methods to prepare two advanced types of anode manterials,which is named as one-dimensional nanorod-like vanadium tetrasulfide?VS₄?and two-dimensional molybdenum disulfide-tin disulfide/reduced graphene oxide composites?MoS₂-SnS₂/rGO?,respectively.As-prepared materials were qualitatively/semi-quantitatively characterized by spectroscopy technology,such as Raman,IR and UV-Vis spectrum.The lithium/sodium storage performance and mechanism of anode materials were systematically explored.The chief scientific researchs are as follows:1?Nanorod-like VS₄ was prepared by a simple one-step solvothermal method.The microscopic geometric morphology,crystal structure and electrochemical performance of LIBs were systematically studied in detail.The characterization results showed that the micro-morphology of VS₄ is nanorod-like.The length of rod-like structure is about 1?m-2?m,and the diameter is about 20nm-60nm.It was confirmed that the phase composition of anode material is monoclinic VS₄ with high crystallinity.The electrochemical tests of LIBs showed that the reversible discharge specific capacity could still remain 439.3 mAh g^-1 after 120 cycles at current density of 200mA g^-1,which indicates that nanorod-like VS₄ possesses better cycling stability and longer cycling life.The specific capacity of nanorod VS₄ remains 267.3 mAh g^-1 at high current density of 2.0 A g^-1,which shows that nanorod-like VS₄ has excellent rate performance.In addition,the smaller internal impedance also exhibits that nanorod-like VS₄ holds higher conductivity and its electrochemical performance could be enhanced.2?Flake nanocrystalline MoS₂/SnS₂ Van der Waals heterojunctions grown vertically or horizontally on reduced graphene oxide nanosheets were fabricated by a convenient one-step hydrothermal method.MoS₂/SnS₂-rGO exhibits flaky nanocrystals of smaller size.As an anode material for LIBs,the residual specific capacity of the composite is 894 mAh g^-1 after 55 cycles at the current density of 200mA g^-1 and 590 mAh g^-1 at high current density of 1.0 A g^-1.In term of NIBs,the discharge specific capacity of the composite remains 288.9 mAh g^-1 after 200 cycles of at the current density of 300 mA g^-1.The electrochemical tests clearly state that the composite holds better cycling stability,longer cycling life and excellent rate performance.MoS₂/SnS₂ heterojunctions possess a unique interfacial synergistic coupling effect,which could effectively shorten the diffusion path of ions and improve the transport of lithium/sodium ions?Li⁺/Na⁺?between electrodes and electrolytes.This structure could also increase the contact area between electrodes and electrolytes,thus promoting the electrochemical activity of electrode materials.As a buffer matrix,graphene could effectively alleviate the volume expansion caused by Li⁺/Na⁺inserting into the electrode material,thereby improving the cycling stability and service life.Based on spectroscopy technology?such as Raman,IR and UV spectrum?and other microstructural characterization techniques,we qualitatively/semi-quantitatively analyze crystal structure and phase composition of as-prepared materials and explore their energy storage mechanism.It provides a reference for analysing the properties of LTMS and their energy storage mechanism.It also lays a foundation for designing and improving high-performance anode materials of ion batteries.