Preparation And Sodium Storage Performance Of Nickel-based Chalcogenides

Rechargeable lithium-ion batteries?LIBs?have been widely investigated and successfully commercialized due to their high energy density and outstanding cycling stability.However,with the advent of the era of hybrid vehicles and electric vehicles,the increase in the consumption of lithium resources has led to higher and higher prices,which is bound to limit the further development of lithium-ion batteries.Recently,sodium-ion batteries?SIBs?have attracted extensive attention,because of abundance of sodium resource and similar properties in comparison to lithium.Nevertheless,the ionic radius and molar mass of sodium are larger than those of lithium,which lead to larger volume change and poorer electrochemical performance.Thus,it is urgent to exploit new electrode materials with high performance for suitable Na-host materials to accommodate reversible sodium ion insertion and deinsertion.In this article,we chose nickel-based chalcogenides as the research objects,and successfully synthesized NiSe₂ nanooctahedra and NiS₂ nanospheres.The structure,morphology and other characteristics were characterized by advanced characterization methods.The electrochemical properties of these two materials as anode materials for SIBs were studied.The main research contents and results are as follows:?1?NiSe₂ nanooctahedra were successfully synthesized by a hydrothermal method,the size of which was between 200 and 350 nm,and explained the formation mechanism of octahedron.NiS₂ nanospheres were also synthesized by a solvothermal method.The diameters of nanospheres were in the range of 80-150 nm and it had mesoporous structure and large specific surface area.?2?NiSe₂ nanooctahedra and NiS₂ nanospheres were used as anode materials for SIBs,respectively,and both of them exhibited excellent electrochemical performance.The discharge capacity of NiSe₂ nanooctahedra maintained at 330 mAh g^-1 after 4000cycles at the current density of 5 A g^-1 and it still 175 mAh g^-1 even at 20 A g^-1.The discharge capacity of NiS₂ nanospheres maintained at 450 mAh g^-1 after 100 cycles at the current density of 0.1 A g^-1.?3?Such remarkable performances are attributed to good conductivity and nanoscale particle size of NiSe₂ which can increase the contact area with the electrolyte,shorten the sodium ion transmission pathway and effectively relieve the volume expansion.?4?NiS₂ nanospheres have such excellent electrochemical properties because nanoscale particle size and mesoporous structure give it a larger specific surface area and active sites,which can not only increase the contact area with the electrolyte,but also shorten the sodium ion transmission pathway.?5?Through the in-situ and ex-situ XRD characterization methods,the mechanism of sodium storage of NiSe₂ nanooctahedra was revealed.During the discharge process,there are three platforms,corresponding to the formation of Na_xNiSe₂,NiSe and Na₂Se,Ni and Na₂Se,respectively.There are two platforms in the charging process,corresponding to the formation of Na_xNiSe₂ and NiSe₂,Se and Ni₃Se₄,respectively.?6?The reasons that the discharge specific capacity of NiSe₂ nanooctahedra decayed gradually during the first 50 cycles were explained.Due to generation of Se and Ni₃Se₄ at charged 2.9 V,the discharge specific capacity decayed.When NiSe₂,Se,and Ni₃Se₄ reach a balance,the capacity almost remains unchanged.