Solvothermal Preparation Of Ni₃S₂ Materials In Eutectic System And Their Capacitive Properties

Ni₃S₂ exists in hexagonal crystal ore and is widely used in supercapacitors,hydrogenation catalysts,dye-sensitized solar cells and lithium-ion batteries and so on.It has various advantages for supercapercitoor material,such as large theoretical capacity,excellent rate performance,good conductivity,and so on.In particular,Ni₃S₂has a high theoretical capacitance of 2412 F g^-1,which has attracted more and more research and development of high-performance supercapacitors using Ni₃S₂ as the electrode material.Deep eutectic solvents(DESs)are non-aqueous solutions,with high conductivity and wide electrochemical window,good compatibility and thermal stability,low melting point,good solubility and low viscosity.DESs are good medium for preparing electrode materials,and has good application potential in the field of supercapacitors.In this paper,using choline chloride-ethylene glycol(Ethaline)combined DES as the solvent system,the Ni₃S₂ electrode material was synthesized by the solvothermal sulfurization method,and its formation mechanism and capacitance performance in the system were studied.The main research work of the article is as follows:(1)In Ethaline solvent,a simple solvothermal sulfurization method was used to prepare Ni₃S₂ electrode material on foamed nickel(NF).The sulfurization mechanism of its in-situ growth on the NF current collector was studied,and the law of the NF surface structural evolution with different times.Studies have shown that in the high chlorine environment created by Ethaline,NF tends to dissolve,which can provide Ni^II for the formation of Ni₃S₂;Ni₃S₂ achieves in-situ growth on NF and forms an integrated structure to ensure the good conductivity of the electrode;Thiourea(TU)provide S ions,and its strong complexation can form ligand compounds[Ni(TU)₄]²⁺and[Ni Cl₄(TU)₂]^2-with Ni^II in Ethaline,thereby participating in the reaction process;the formation of Ni₃S₂ is mainly affected by the sulfurization time.As the sulfurization time goes on,the surface structure of Ni₃S₂ changes from nanoplatelets to nanoparticles.This change in structural characteristics directly affects the electrochemistry performance of the electrode.(2)Combining the method of preparing Ni₃S₂ material by solvothermal sulfurization,an intermediate layer of Ni(Ni/NF)is introduced on the NF substrate by electrodeposition method to increase the surface roughness of the substrate to provide more reaction interfaces.Thereby improve the active specific surface area and electrochemical performance of Ni₃S₂ electrode.Studies have shown that the introduction of the Ni intermediate layer,after sulfurization,forms a nano-sheet stacked porous spherical cluster-like Ni₃S₂ material(Ni₃S₂/Ni/NF),and its electrochemically active specific surface area(ESCA)is 10.48 m F cm^-2,comparing with the Ni₃S₂/NF electrode(1.215 m F cm^-2)that obtained by direct solvothermal sulfurization,the increase is 8 times,and the electrochemical performance is significantly improved.Ni₃S₂/Ni/NF is used as the electrode material of supercapacitors,at a current density of 5?40 m A cm^-2,the area specific capacitance/capacitance is 7,370 m F cm^-2(1,126?Ah cm^-2)and 5,633 m F cm^-2(860.6?Ah cm^-2),3,858 m F cm^-2(589.4?Ah cm^-2),3,244 m F cm^-2(495.6?Ah cm^-2)and2,636 m F cm^-2(402.7?Ah cm^-2),respectively,with good electrochemical performance.(3)The long-cycle stability of the Ni₃S₂/Ni/NF electrode was studied,and the changes in the electrode surface structure and phase during the charge and discharge process were explored.Studies have shown that the stability change of the electrode involves three stages.In the initial stage(0?20 cycles),the Ni₃S₂/Ni/NF electrode is in full contact with the electrolyte,which induces its surface pores to become larger and fully exposes its own active sites.So its electrochemical performance rises sharply;in the second stage(20?100 cycles),with the continuous dissolution of S in Ni₃S₂/Ni/NF material,and the phase transition of Ni₃S₂ to Ni(OH)₂,the surface of the electrode changes from porous nanosheet spherical clusters to large nanosheet structure,making the electrochemical performance of the electrode drops abruptly;in the third stage(100?1000 cycles),in order to adapt to the volumetric strain,the surface structure of the electrode agglomerates and collapses,resulting in a slow decline in performance.