Investigation On Synthesis And Electrochemical Performance Of Transition Metal Sulfides As Cathode Materials For Magnesium-based Batteries

Since magnesium anode has the advantages of high volumetric capacity,security,and low cost,magnesium-lithium hybrid ion batteries are one of the current research hotspots of new energy storage systems.Magnesium-lithium hybrid ion battery not only has the rapid diffusion kinetics of Li⁺,but also has the advantages of magnesium metal anode.In this paper,two kinds of metal sulfides Co₃S₄ and VS₄ composite are designed and prepared by the liquid phase method and the solid phase method,which are applicated in magnesium-lithium hybrid ion batteries as cathode materials for the first time.X-ray diffraction,X-ray photoelectron spectroscopy,cyclic voltammetry,charge-discharge test and other methods are adopted to analyze the physical and chemical properties,electrochemical properties and energy storage mechanism of the material.First,“popcorn”Co3S4 particles.The“popcorn”Co₃S₄-F particles were prepared by a one-step solvothermal method,with a particle diameter of～500 nm and a nanosheet thickness of～10 nm.In the APC+Li Cl/THF electrolyte,the"popcorn"Co₃S₄-F shows a higher discharge capacity and better cycling stability.At100 m A g^-1,the first discharge specific capacity of Co₃S₄-F reaches 779.8 m Ah g^-1,and the sloped discharge voltage located at 0.4-0.8 V and 1.1-1.2 V vs.Mg²⁺/Mg.After 50 cycles,the discharge specific capacity is 566.2 m Ah g^-1,and the capacity retention rate is 72.6%.After 100 cycles,the discharge voltage is situated at 1.08 V,and the energy density is 431.46 Wh kg^-1,which is higher than that of the most cathode materials for magnesium-lithium hybrid ion batteries.During the charge and discharge process,the Co₃S₄ electrode first undergoes an intercalation reaction（x Li⁺+Co₃S₄（?）Li_xCo₃S₄）,and then a conversion reaction Li_xCo₃S₄（?）Co+1/2x Li₂S.The electrochemical process of Co₃S₄-F is controlled by diffusion,and the lithium ion diffusion coefficient reachs as high as 7.89×10^-10 cm² s^-1,which is camparable to lithium ion batteries.Therefore,the“popcorn”Co₃S₄-F is an important high-capacity cathode material for magnesium-lithium hybrid ion batteries.Second,the VS4/VOOH composite.The composites with special morphology of hydrangea-like and brain-flower-like shapes were prepared by one-step hydrothermal method and with stable electrochemical properties.In the APC+Li Cl/THF electrolyte,the VS₄/VOOH composite exhibits higher discharge specific capacity and better cycling stability.At 100 m A g^-1,the first discharge specific capacity of VS₄/VOOH achieves as high as 552.7 m Ah g^-1,and two sloped discharge voltage are situated at 0.69 and 0.11 V vs.Mg²⁺/Mg.After 200 cycles,the specific capacity still retains 276.1 m Ah g^-1 with discharge platform of 1.0 V and the capacity retention rate of 50.0%.During the charge/discharge process,the electrochemical reactions of the VS₄/VOOH composite are listed as VS₄/VOOH+x Li⁺+y Mg²⁺（?）Mg_yLi_x-aVS₄+Li_aVOOH and Mg_yLi_x-aVS₄（?）1/2（x-a）Li₂S+y Mg S+V.The electrochemical reactions of VS₄/VOOH composite is mainly controlled by diffusion process and the ion diffusion coefficient is 3.5×10^-11 cm² s^-1.In brief,compared with the VS₄electrode material reported in the literature,VS₄/VOOH shows excellent electrochemical performance for magnesium-lithium hybrid ion batteries.In summary,the sulfides——Co₃S₄ and VS₄/VOOH composite are potential cathode materials with high-capacity maganesium-lithium hybrid ion batteries.