Study On Sulfurization Modification And Electrochemical Performance Of Nickel-based Oxides/Hydroxides

The monoclinic wolframite structure NiWO₄（NWO）powders was synthesized by the simple hydrothermal method,and then the NWO powders was treated using the different Na₂S content via the solvothermal approach.The multiphase sulfides are generated on the surface of the sulfurized samples（NWSs）in addition to maintain the matrix structure of NWO.With the increased Na₂S level and reaction temperature,the more sulfides are produced in NWS samples and the crystallinity of the generated sulfide increases.However,the reaction time has no obvious effect on the crystallinity of NWS samples.The specific surface areas of NWS samples are smaller than that of the NWO sample.Also,the wettabilities of NWS samples are significantly improved.NWS samples contain mainly Ni²⁺,O^2-,W⁶⁺and S^2-ions.The electrochemical performance of NWS electrodes is obviously improved compared with the NWO electrode.NWS-5 electrode obtained by using 5 mmol Na₂S via the solvothermal reaction at 150℃for 6 hexhibits the better electrochemical performance.The specific capacitance is 1288 F×g^-1 at the current density of 0.5 A×g^-1 for NWS-5 electrode,only 292 F×g^-1 for NWO electrode.When the current density increases to 7 A×g^-1,the specific capacitance retention rate is～17%for NWS-5electrode,declining to 0 for NWO electrode.At the current density of 5 A×g^-1,for NWS-5electrode the specific capacitance can remain～11%of the initial specific capacitance after1000 continuous charge-discharge cycles,declining to 0 for NWO electrode after 380cycles.The mechanism of the improved electrochemical performance of NWS electrode is discussed.α-Ni（OH）₂（NOH）powders was obtained by the simple hydrothermal method,and then the NOH powders was treated using the different thiourea level via the solvothermal approach.A new phase ofα-NiS is formed in the sulfurized samples（NOSs）in addition to keep the matrix structure of NOH.With the increase of thiourea level and reaction time,the crystallinities of NOS samples change scarcely,but the more sulfides are produced in NOS samples.In addition,α-andβ-NiS with good crystallinity are produced in the NOS sample at high temperature of 150℃.The wettabilities of the NOS samples are improved compared with the NOH sample.However,the NOS sample obtained at a low reaction temperature and a short reaction time displays the bigger specific surface area.NOS samples mainly include Ni²⁺、O^2-and S^2-ions.The electrochemical performance of NOS electrodes is obviously improved compared with NOH electrode.NOS-3 electrode obtained by using 3 mmol thiourea via solvothermal reaction at 120℃for 4 h shows the better electrochemical performance.The specific capacitance is 1206 F×g^-1 for NOS-3electrode at the current density of 0.5A×g^-1,929 F×g^-1 for NOH electrode.When the current density increases to 15 A×g^-1,the specific capacitance retention rate still keeps～39%for NOS-3 electrode,declining to 0 for NOH electrode.At the current density of 5 A×g^-1,for NOS-3 electrode the specific capacitance maintains～17%of the initial specific capacitance after 1000 continuous charge-discharge cycles,only～13%for NOH electrode.The mechanism of the improved electrochemical performance of NOS electrode is discussed.The spinel structure NiCo₂O₄（NCO）powders was prepared by the simple hydrothermal method and annealing process,and then the NCO powders was treated using the different Na₂S level via the solvothermal approach.A spinel structure NiCo₂S₄ phase is generated in the sulfurized samples（NCOSs）in addition to keep the matrix structure of NCO.The crystallinity of NCOS samples decreased gradually with the increase of the Na₂S level,reaction temperature and time.The wettabilities of NCOS samples are improved compared with NCO sample.NCOS samples contain mainly Ni²⁺、Co³⁺、O^2-and S^2-ions.The electrochemical performance of NCOS electrodes is significantly improved compared with NCO electrode.NCOS-0.5 electrode obtained by using 0.5 mmol Na₂S via solvothermal reaction at 120℃for 3 h exhibits better specific capacitance and rate performance.The specific capacitance is 989 F×g^-1 for NOS-0.5 electrode at the current density of 0.5A×g^-1,733 F×g^-1 for NCO electrode.When the current density is increased to10 A×g^-1,the specific capacitance retention rate achieves～69%for NCOS-0.5 electrode,only～3%for NOH electrode.At the current density of 5A×g^-1,for NCOS-0.5 electrode the specific capacitance keeps～73%of the initial specific capacitance after 1000 continuous charge-discharge cycles,which is lower than that of the NCO electrode（～83%）.The mechanism of the improved electrochemical performance of NCOS electrode is discussed.