Synthesis,Carbon-coating And Electrochemical Performance Of FeS₂ And CoS As Cathode Materials For Rechargeable Magnesium Batteries

Rechargeable magnesium batteries are inexpensive,safe and environmentally friendly.As green secondary power supplies,they may be used in the electric vehicles or the large-scale energy storage systems.Mg and Li have similar chemical properties,but compared with lithium ion batteries,rechargeable magnesium batteries are still far from practical application.One of the key problems is the lack of suitable cathode materials.The main content of this article is to search and analyze the new kinds of cathode materials that can be used in rechargeable magnesium batteries.Some characterization methods,such as X-ray diffraction,field emission scanning electron microscopy?SEM?and transmission electron microscopy?TEM?,were used to characterize the crystal structure and morphology of the transition metal sulfides FeS₂ and CoS.Their feasibility of being cathode materials was investigated by cyclic voltammetry?CV?and galvanostatic charge-discharge tests.Their electrochemical performance affected by carbon coating was also discussed.The main research results are presented as follows:FeS₂ is synthesized by hydrothermal method and applied as a kind of new cathode material in rechargeable magnesium batteries for the first time.The powder of FeS₂ consists of spherical particles which are about 10?m in diameter.They have uniform shape and there are bulges on the surface.The average specific discharge capacity of FeS₂ in 40 cycles is 43.7 mAh/g,showing that it is a promising candidate as cathode material.After carbon coating,the surface of particles becomes more regular.The voltage of discharge platform gets higher and the capacity is greatly increased to 70.7mAh/g.The coulombic efficiency of batteries is close to100%and their cycle performance is also improved.Flower-like CoS with nanostructures is synthesized by solvothermal method.The microspheres with a diameter of about 1.5?m are self-assembled by nanoflakes whose thickness is about 40nm.The high crystallinity of CoS is proved by the XRD patterns.As cathode material,the coulombic efficiency of CoS is between 80% and 90%.The average specific discharge capacity is as high as 157.1 mAh/g,which is superior to most reported cathode materials.Meanwhile,the possible mechanism of charge-discharge reactions is proposed and discussed.The rate capability of the flower-like CoS is determined to further test its electrochemical properties.After carbon coating,both the coulombic efficiency and the cycle performance of batteries have been improved.