The Effects Of Fe@C And FeS_x On The Lithium Storage Performance Of VS₄/RGO Composites And Mechanism Analysis

Due to the soaring requirements for lithium ion batteries?LIBs?with high energy density,it is urgent for researchers to explore new electrode materials with high capacity and long lifespan.Transition metal sulfides are promising electrode materials owing to the abundant resource,low price and high theoretical specific capacity.Nevertheless,it also possesses several disadvantages,such as the large volume change during the Li insertion/extraction,the dissolution of polysulfide as well as the poor electronic conductivity.In this work,we synthesized the VS₄/RGO electrode modified by FeS_x and Fe@C nanoparticles.The electrochemical properties and redox mechanisms of FeS_x-VS₄/RGO and Fe@C-VS₄/RGO were systematically investigated as cathode and anode in LIBs,respectively.Firstly,the electrochemical performance and redox mechanism of FeS_x-VS₄/RGO cathode in ether-based electrolyte were studied.VS₄ cathode with discharge plateau close to 2.0 V presented the unsatisfactory performance in LIBs,which was mainly ascribed to the dissolution of the polysulfide in ether-based electrolyte.Therefore,we prepared the FeS_x-coated VS₄/RGO composites via solution method.When used as cathode,FeS_x-VS₄/RGO exhibited enhanced electrochemical properties than the pristine VS₄/RGO.After100 cycles,it could deliver a capacity of 456 mAh g^-1 at the current density of 100 mA g^-1,and at 1.0 A g^-1,a capacity of 372 mAh g^-1 was maintained.The mechanism analysis demonstrates that FeS_x can chemically absorb the polysulfide and suppress the dissolution and shuttle of polysulfide to alleviate the self-discharge behavior of VS₄.Besides,FeS_x can facilitate the Li migration to improve the reaction kinetics.Hence,the lithium storage performance of VS₄/RGO cathode was significantly improved.Secondly,the effects of Fe@C nanoparticles on the lithium storage performance of the VS₄ anode in carbonate-ester electrolyte were studied.The VS₄/RGO exhibited poor cyclic stability as anode in LIBs.At the current density of 500 mA g^-11 for 150 cycles,the capacity decreased from 796 mAh g^-1 to 452 mAh g^-1.After the incorporation of Fe@C nanoparticles,the electrochemical performance of Fe@C-VS₄/RGO composites was dramatically improved.At 500 mA g^-1 for 150 cycles,the capacity still retained at 746 mAh g^-1,and at a high rate of 4 C,a capacity of 564 mAh g^-1 was maintained.The better electrochemical stability was mainly attributed to the positive effects of Fe@C nanoparticles.On one hand,Fe nanoparticles could strongly bind the sulfur atoms and effectively restrict the irreversible reaction between the polysulfide and carbonate-ester electrolyte.On the other hand,Fe nanoparticles could effectively catalyze the conversion of Li₂S into S.This work demonstrates that the lithium storage performance of VS₄ with Li/S redox reactions can be greatly enhanced via the modification by FeS_x or Fe@C,which will provide a new strategy to improve the electrochemical stability of sulfur/sulfides electrodes.