Synthesis And Performance Study Of Vanadium Sulfide Electrode Materials For Sodium（potassium） Ion Batteries

The high cost,low reserves and uneven geographical distribution of lithium resources in the earth’s crust have curbed the continued development of lithium-ion batteries for large-scale energy storage.Therefore,the research attention is being turned to sodium（potassium）-ion batteries,which have the advantages of low cost and abundant sodium（potassium）resources.However,compared to Li⁺,the larger radius of Na⁺（K⁺）ions presents numerous problems during electrochemical reaction processes,such as higher diffusion potentials and more severe volume changes during insertion/extraction of the existed electrode materials,which resulted in poor cycling stability and low rate performance.Therefore,the search for adaptable,high-performance electrode materials for sodium or potassium-ion batteries has become an urgent priority.Among the many anode materials,vanadium sulfide is considered to be the ideal electrode material for reversible de-embedding of Na⁺（K⁺）ions due to its high theoretical capacity,and its various crystal structures（1D chain-like,2D layer-like and 3D Ni As typed structures）,which provided abundant active sites for sodium or potassium storage and facilitated the diffusion of Na⁺（K⁺）ions.However,the common problems of vanadium sulfide,namely poor conductivity,high cyclic volume expansion,and easy crushing of the active material,have hindered its positive development.Herein,based on the modification strategies such as morphological construction and compounding with carbon materials,two types of vanadium sulfides materials(VS₄ and V_5.45S₈)were designed to address the inherent drawbacks of vanadium sulfides.When VS₄ and V_5.45S₈were applied to sodium and potassium-ion batteries,both exhibit excellent electrochemical properties,as follows:（1）To address the problem of large cyclic volume expansion when vanadium tetrasulphide（VS₄）was used as an anode material for sodium ion batteries,3D self-assembled curly nanosheet VS₄ hierarchitectures（VS₄-CN-Hs）were constructed by a one-step solvothermal method.With the curly nanosheets exposing the large specific surface area and the hierarchical structure with abundant buffer space to mitigate volume expansion,the VS₄-CN-Hs electrode,therefore,exhibited exceptional sodium storage performance,including excellent rate performance（10 A g-~1@444 m Ah g-~1）,high initial Coulombic efficiency（81%@829/1019 m Ah g-~1）and ultra-long cycle life（1000 turns）.In addition,the morphology and phase of the products at different reaction times were also investigated,which led to the determination that the growth process of the material conforms to the Ostwald ripening mechanism.Meanwhile,the optimum reaction time（36 h）for the preparation of the material was also specified.Finally,the reaction mechanism of the anode material was investigated by ex-situ TEM test,which verified that VS₄ underwent a conversion reaction in the voltage window of 0.01-3 V.（2）Vanadium sulfide(V_5.45S₈ nanoparticles)was encapsulated in N,S co-doped multi-channel carbon nanofibers by electrostatic spinning combined with in-situ sulfurization technology（CB-VS@NSCNFs）to form a strong chemical bond（V-C bond）under strong anchoring effect.The structure not only facilitated the alleviation of volume expansion,but also improved the conductivity of the active material.Compared to bulk V_5.45S₈/C（B-VS/C）without the V-C bond,CB-VS@NSCNFs showed more excellent electrochemical properties in potassium-ion batteries:high reversible capacity（0.1 A g-~1@411 m Ah g-~1）and good cycling stability（500 cycles at 1 A g-~1）.Furthermore,the CB-VS@NSCNFs with V-C bonding possessed the faster ion/electron transfer rate and greater pseudocapacitance contribution as demonstrated by CV and EIS analyses.Moreover,the potassium storage mechanism of CB-VS@NSCNFs had also been verified by ex-situ TEM and XPS measurements.