Preparation And Electrochemical Properties Of Vanadium-based Materials

Sodium ion batteries（SIBs）are considered to be one of the most attractive alternatives to lithium ion batteries（LIBs）for grid energy storage due to the rich sodium resources,low cost,and similar electrochemical reaction mechanism to lithium ion batteries.In the past few years,many materials（such as alloy materials,metallic chalcogenides,carbon-based materials,etc.）have been developed as high efficiency anode materials for SIBs.Among them,metal sulfides anode electrodes have attracted much attention because of their high capacity and remarkable electrochemical reversibility.Vanadium-based materials have become a hotspot in the field of energy storage due to their outstanding advantages such as abundant raw materials,low cost,and variable valence state.This paper specifically studied on the preparation of vanadium disulfide and vanadium tetrasulfide materials,and at the same time investigated their sodium storage performance in SIBs.Meanwhile,the influence of different morphology and structure on the sodium storage performance was studied.The specific work contents are as follows:（1）Study on preparation and sodium storage properties of three-dimensional vanadium tetrasulfide:three-dimensional VS₄ composed of uniform nanosheets was synthesized by one-step solvothermal method.Due to its perfect 3D structure,VS₄-20 h electrode exhibits excellent electrochemical performance as anode material for sodium ion batteries.The VS₄-20 h electrode was tested at 0.2 A g^-1and the reversible specific capacity of 472 m A h g^-1 was obtained after 100 cycles.The reversible specific capacities of 345 m A h g^-1 and 288 m A h g^-1 were obtained at 5 A g^-1 and 10 A g^-1.The VS₄-20 h electrode also shows excellent long cycle performance.When tested at 1 A g^-1,after 400cycles,it can maintain a stable reversible specific capacity of 410 m A h g^-1 with a capacity retention rate of 91%.（2）Study on the preparation and sodium storage properties of vanadium tetrasulfide modified nitrogen doped carbon nanotubes（NCNTs）:using nitrogen doped carbon nanotubes（NCNTs）as template,the three-dimensional structure of poplar flower vanadium tetrasulfide modified nitrogen-doped carbon nanotubes（NCNt@VS₄）composite was successfully prepared by solvothermal method.Thanks to the synergistic effect of hollow nitrogen doped carbon nanotubes and coral-like VS₄ nanorods,the NCNt@VS₄ electrode presents an excellent electron/sodium ion transfer rate in the solid phase,and the special structure supresses the volume expansion in the electrochemical reaction process,so it demonstrates an excellent sodium storage performance.When it is used as an anode material in SIBs,NCNt@VS₄ electrode shows excellent long cycle stability(430 m A h g^-1 reversible capacity can be obtained after 2000 cycles at 1 A g^-1test)and excellent rate performance(tested at 5 A g^-1,it can obtained 460 m A h g^-1reversible capacity)and has a high tolerance to current changes.（3）Preparation of vanadium tetrasulfide and vanadium disulfide materials（nitromethylpyrrolidone as solvent）and study on their sodium storage properties:VS₄（NMP）and VS₂（NMP）materials were successfully synthesized by a solvothermal method using NMP as solvent.The synergy effect between the nanosheets and the three-dimensional architecture can not only serve as a repository of ions and electrolytes,reducing diffusion length,but also provide sufficient space to mitigate the volume change during electrochemical reactions.Both materials exhibit excellent electrochemical properties when used as anodes in SIBs.The VS₄（NMP）material can maintain a reversible specific capacity of 534 m A h g^-1 after 500 cycles at 1A g^-1.In addition,the specific discharge capacities at 0.2 A g^-1 and 5 A g^-1 are 743 m A h g^-1 and 550 m A h g^-1,respectively.VS₂（NMP）material can maintain a high reversible specific capacity of 558m A h g^-1 after 280 cycles(at 2 A g^-1).At 5 A g^-1,it can still reach a discharge specific capacity of 432 m A h g^-1.（4）Preparation and sodium storage properties of vanadium disulfide nanosheets:the layered VS₂ nanosheets were directly prepared by a solvothermal method.Due to the layered structure of the material,the large layer spacing provides channels for the diffusion of sodium ions and electrons.VS₂ electrode as the anode material for SIBs displays good sodium storage performance.The VS₂ electrode was tested at 5 A g^-1.After1500 cycles,it can maintain a reversible specific capacity of 204 m A h g^-1.In addition,when tested at 10 A g^-1 and 20 A g^-1,reversible specific capacities of 185 and 164 m Ah g^-1 can still be obtained.