Energy Storage Mechanism And Electrochemical Performance Of Nickel-cobalt Sulfide

With the increase of industrialization,the development of human society is increasingly dependent on fossil fuels,which further leads to environmental degradation and energy crisis.To alleviate environmental degradation,people have increased the development and utilization of green and clean energy,such as wind and solar energy.As an energy storage device suitable for variable current,electrochemical capacitor is beneficial to further improve energy conversion efficiency and the service life of power grid energy storage device.However,the development of an electrochemical capacitor with high energy density and power density lies in the study of electrode materials.Transition metal sulfides have been widely used as electrode materials for lithium-sulfur batteries and asymmetric supercapacitors.Among them,NiCo₂S₄?NCS?has become a popular electrode material because of its narrow band gap and multiple energy storage sites.However,the energy storage mechanism of NCS is not clear at present.In addition,its synthesis method mainly consists of high temperature solid phase method and anion exchange method.But the former synthesis will result in large particle sizes,requiring further ball grinding,and the latter preparation period is longer.Therefore,it is necessary to develop a method that can shorten the preparation period and improve the electrochemical performance of NCS.In addition,although NCS has a narrow band gap,it is still a semiconductor material,so further compounding with conductive materials is beneficial to further improve its electrochemical performance.In this paper,NCS was successfully prepared by one-step hydrothermal method and its electrochemical performance were studied.Meanwhile,the energy storage mechanism of NCS in alkaline electrolyte was pointed out.And NCS was composited with reduced graphene oxide?rGO?by one-step hydrothermal method and its electrochemical performance were further improved.The thesis is mainly divided into the following three aspects:?1?In this study,NCS was obtained through a one-step hydrothermal reaction,and its electrochemical performance was regulated by the adding amount of urea.Meanwhile,flower-like NCS-10 with sulfur vacancies was obtained when 10 mmol of urea and 1 mmol of thioacetamide were added,whose highest specific capacity performance was 453.50 C/g at 1 A/g.Additionally,the as-prepared NCS-10 electrode and activated carbon?AC?electrode were fabricated into hybrid supercapacitors.The energy density of such a device reaches the highest value of 27.77 Wh/kg,whereas the maximum power density is 13340 W/kg.Moreover,this hybrid supercapacitor maintains the 62.07% of its initial capacity after 10 000 cycles at 2 A/g,indicating that this one-step synthesized NCS-10 has potential to be applied to long-serving energy storage device.?2?NCS showed a low initial capacity and exhibited a rapid rise within first 500 galvanostatic charge-discharge cycles in three-electrode system.To further reveal the intrinsic energy-storage mechanism of NCS,we summarized previous electrochemical reaction formulas and investigated the interactions between hydroxide and NCS with sulfur vacancies through electrochemical testing,as well as surface and structure characterization.In addition,electrochemical desulfurization technology was first introduced in electrochemical charge storage to amend the intrinsic energy-storage mechanism of NCS in alkaline electrolyte.The highest specific capacity of NCS reached 276.50 C/g at 5 A/g,which originated from the irreversible phase transition during the activation process.The long cycle stability retained 65.29% of the highest specific capacity after 10 000 cycles,attributed to the transformation product of nickel-cobalt dihydroxide without further phase change after the activation process.In sum,these findings reveal the intrinsic energy-storage mechanism of NCS with sulfur vacancies as relevant anode material for alkaline supercapacitors,batteries,and hybrid devices.?3?NCS composited with rGO?NCS/rGO?was obtained through a one-step hydrothermal reaction.And it achieved the optimal electrochemical performance when we regulated the adding amount of graphene oxide reaching 20% of the total mass during hydrothermal reaction.Meanwhile,this composite electrode shows a specific capacity of 518.09 C/g at the current density of 1 A/g in the 2 M KOH electrolyte,which is higher than the specific capacity of NCS?453.50 C/g?.In addition,the composite electrode and AC electrode were fabricated into hybrid supercapacitors.The energy density of such a device reaches the highest value of 32.44 Wh/kg,whereas the maximum power density is 12987.69 W/kg.In addition,I participate in off-campus practice that Changchun Up Optotech Co.,ltd.cooperation projects with advisor,designing an emergency power supply in photoelectric instrument.The MnCo₂O₄?MCO?nano-needles were synthesized and its specific capacity of 79.20 C/g at 0.5 A/g.Moreover,the MCO composited with 200 mg AC further performed higher specific capacity of 477.5 C/g at the same current density.Such composite electrode exhibited excellent volumetric capacity of 527.13 C/cm³ under high quality load,which further improved the effective utilization of the actual electrode under high load.