Synthesis And Electrochemical Performance Of Vanadium Sulfides Electrode Materials

As one kind of electrode materials for lithium-ion battery,transition metal sulfide has attracted much attention due to its high theoretical specific capacity and good electrochemical activity.In addition,transition metal sulfide has many advantages,such as abundant reserves,low preparation cost,and fast ion diffusion speed etc.However,further research is needed before practical application due to the problems of volume expansion and poor rate performance of sulfide electrode materials in the cycling process.In this thesis,vanadium disulfide?VS₂?based on intercalation reaction mechanism and vanadium tetrasulfide?VS₄?based on conversion reaction mechanism were chosen.With the electrochemical performance testing of lithium-ion batteries,the optimum ratio of VS₂ to PEDOT-PSS and the synthesis procedures of VS₄ were obtained.Specific contents are as follows:?1?VS₂ nanosheets was synthesized by solvothermal method,and its surface was coated by PEDOT-PSS.Cycle performance tests show that PEDOT-PSS coating can significantly improve the cycle stability of lithium-ion batteries.It indicates that the high conductivity PEDOT-PSS coating will not destroy the original sheet structure of the material.The high ductility of PEDOT-PSS can effectively alleviates the volume change caused by Li⁺insertion/desertion while providing fast electronic transmission channel.On this basis,the performance of lithium-ion batteries assembled with three coating ratios,10VS₂@PEDOT-PSS,5VS₂@PEDOT-PSS and 1VS₂@PEDOT-PSS,were optimized.The results show that 5VS₂@PEDOT-PSS has the best cycle performance and rate performance,and the corresponding charge transfer electrochemical impedance is also the lowest.After 100 cycles at current density of0.1 A g^-1,5VS₂@PEDOT-PSS still has 569.0 mAh g^-1 reversible capacity.Even if the current density increases to 2 A g^-1,the capacity of 5VS₂@PEDOT-PSS is still186.8 mAh g^-1 after 1000 cycles.The comprehensive results show that the capacity decay of lithium ion batteries can be effectively alleviated by inhibiting the volume expansion of VS₂ nanosheets during the cycle processes due to the combined effects of high ion transfer characteristics of VS₂ nanosheets and the protection of conductive PEDOT-PSS coatings.?2?Using sodium orthavanadium and thioacetamide as vanadium and sulfur sources,respectively,high crystallinity hc-VS₄ and low crystallinity lc-VS₄ were synthesized with solvothermal reaction method by controlling the amount of hexadecyl trimethyl ammonium bromide?CTAB?.Structural observation showed that hc-VS₄ was nanoflower-like while lc-VS₄ was hollow sphere-like.With the specific surface area measurment,the nanoflower-like hc-VS₄ has an apparent higher specific surface area than hollow sphere-like lc-VS₄.The results of electrochemical performance tests show that hc-VS₄ has better cycle performance and rate performance,and the corresponding electrochemical impedance is the lowest.After100 cycles with current density of 0.1 A g^-1,hc-VS₄ shows 554.2 mAh g^-1 reversible specific capacity.After 1000 cycles with current density of 0.5 A g^-1,hc-VS₄ still shows 381.9 mAh g^-1 reversible capacity.It can be attributed to the fact that hc-VS₄nanoflower enlarges the contact area of electrolyte with VS₄ and effectively reduces the volume change during Li⁺insertion/desertion through variable free space and fast electron transport channels.For the two kinds of vanadium sulfide materials investigated in this thesis,the theoretical specific capacity of VS₂ based on intercalation reaction mechanism is lower,however,the volume change of VS₂ is weaker than that of VS₄ during the cycle processes.The cyclic performance of VS₂ can be improved by simple coating modification.The low reversible capacity of lithium-ion battery with present VS₄ is mainly caused by the rapid capacity decay in the initial cycles.Hence,a high actual reversible capacity of VS₄ due to its much higher theoretical value should be expected by reasonable morphology design and coating with corresponding high conductivity materials.