| The demand for energy in today’s society is increasing dramatically.That demand forces researchers to focus on different energy sources.Lithium-ion batteries have the advantages of high energy density,high specific capacity,and long cycle life.The various components of lithium-ion batteries have been deeply researched and developed,but they still cannot meet the requirements of large-scale energy consumption applications.Sodium-ion batteries have attracted much attention due to their high sodium storage.Graphite has a very stable potential and a long cycle life as a commercial lithium-ion battery anode.However,its low specific capacity and safety issues have caused people to worry about new energy sources.Among other negative electrode materials,transition metal sulfides are rich in reserves and have a high theoretical capacity,but they exhibit low actual capacities and poor stability as anode materials.There is a problem of poor cycle performance due to volume expansion and low electrical conductivity.In addition,polysulfides are dissolved in the electrolyte,which would form a passivation layer on the surface,resulting in lower ion transfer kinetics.Graphene has excellent conductivity,so the above problems can be solved by optimizing the material structure and compounding with the carbon material.Vanadium tetrasulfide(VS4)has a special chain structure and a large interlayer spacing potential for use in lithium/sodium-ion batteries.In this paper,pure VS4 and VS4/rGO composites with different graphene contents were prepared.The electrochemical properties of pure VS4 and composite materials were explored from different aspects.The main research content is as follows:(1)Using solvothermal method,pure VS4 was prepared at a lower temperature with a mixture of ethylene glycol and distilled water in a volume ratio of I:1.From the scanning electron micrograph,it can be seen that pure VS4 is a uniformly distributed submicron sphere with a size of 1～2 μm Then,a modified Hummers method was used to prepare graphite oxide(GO).VS4/rGO composites with different rGO contents were still prepared by solvothermal method.By using NH4VO3 as source of vanadium and CH3CSNH2 as source of sulfur and GO as a carrier,CH3CSNH2will be hydrolyzed to produce H2S,and then undergo oxidation-reduction reaction with VO3-,and the graphite oxide is reduced to reduced graphene oxide at the same time,a composite material of VS4 uniformly on the graphene sheet is generated.The prepared materials were characterized by scanning electron microscopy(SEM),X-ray powder diffraction(XRD)and Raman spectroscopy.And as the anode material assembled into a battery to test theirgalvanostatic charge and discharge,cyclic voltammetry and other electrochemical properties.When assembled into a lithium-ion battery,the galvanostatic charge-discharge test showed that the higher the content of rGO,the higher the charge-discharge specific capacity of the material,but the highest Coulomb efficiency of the material containing 13.3%rGO.Comprehensively,composites with rGO content of 13.3%have obvious advantages.When the control voltage range is 0.005-3V and the current density is 100 mA g-1,the initial discharge specific capacity of the composite is 1886.3 mAhg-1,the charge specific capacity is 1322.3 mAhg-1,and the Coulomb efficiency is 70.1%.The cycle performance and rate performance tests of the composites showed that after 50 cycles,the specific discharge capacity of the composites was 1181.9 mAhg-1.When the current density reaches 2000 mA g-1,the discharge specific capacity of VS4/rGO is still maintained at 719.3 mAhg-1,which shows good cycle performance and rate performance,indicating that the composite material is advantageous for lithium ion battery anode materials.(2)Based on the above work,pure VS4,VS4/rGO composites containing 13.3%rGO and pure rGO prepared by ourselves were selected to test their electrochemical performance in sodium ion batteries as a negative electrode material.Electrochemical performance tests show that when the voltage range is 0.005-3V and the current density is 100 mA g-1,the VS4/rGO material has an initial discharge specific capacity of 1203.8 mAh g-1 and a charge specific capacity of 781.4 mAh g-1.The Coulomb efficiency was 64.9%,and the reversible capacity was 200.6 mAh g-1 after 50 cycles.When the current density recovers to 100 mA g-1 in the rate test,the reversible specific capacity can be restored to 332.2 mAhg-1.It can be seen that the VS4/rGO composite exhibits good performance as a negative electrode material for sodium ion batteries.In addition,the inhibition of polysulfide dissolved in the electrolyte by the composite material with graphene was observed,and the presence of graphene was found to limit the dissolution of polysulfide. |
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