Preparation And Properties Of Ni-Co Sulfides And Selenides As Electrode Materials For Supercapacitors

With the rapid development of consumer electronics and hybrid electric vehicles,the development of environmental-friendliness and high-performance energy storage devices has become a research hotspot.Supercapacitors have the advantages of both secondary batteries and traditional physical capacitors,such as high power density,fast charging/discharging rate,long cycle life,wide operating temperature range and environmental friendliness.Electrode material is the most important component of supercapacitor.The specific capacitance and power density of carbon-based electrode materials are limited.Conducting polymers tend to swell and shrink in the process of charging/discharging,leading to the deterioration of electrochemical properties and performance.The toxicity and high cost of RuO2 limit its large-scale application.MnO2 can partially dissolve in the electrolyte,which affects its cycling stability and restricts its application.Therefore,the development of electrode materials with low cost,high specific capacitance,long durability,high energy and power density is the key to promote the development of supercapacitor.Battery-type electrode materials have attracted attention and become a research hotspot because their high electrical conductivity and theoretical specific capacity.The main research object is nickel-cobalt sulfides and selenides.In this paper,nickel-cobalt sulfides and selenides with high performance are prepared controllably,by introducing the second metal element and modulating synthesis method and condition.For the nickel-cobalt sulfides and selenides,the influence of the above measures on crystal phase,composition,microstructure and improvement mechanism of electrochemical performance are investigated.For the above electrode materials,the structure-activity relationship between microstructure and electrochemical properties is established.The electrochemical behavior regulation mechanism and energy storage mechanism is explored.The research results provide theoretical guidance and experimental basis for the design and synthesis of transition metal sulfides and selenides,and have a positive role in promoting the application of transition metal sulfides and selenides in the fields of electronic equipment,energy storage and so on.First,nickel-cobalt metal sulfides are prepared by vulcanizing the transition metal alkoxide through controlling the nickel cobalt molar ratio in the precursor and the reaction time.The successful introduction of cobalt changes the microstructure and electron configuration of sulfides,resulting in the synthesis of nanomaterials with plentiful edge active sites and improving the electrochemical properties.Among the obtained sulfides,Ni0.75Co0.25S2 has abundant edge active sites.And it shows the highest specific capacitance(2141.9 F g-1 at 2 A g-1)and considerable stability.A hybrid supercapacitor is assembled with Ni0.75Co0.25S2 as anode material and activated carbon as cathode material,whose energy density can reach 65.5 W h kg-1 at power density of 1806.9 W kg-1 and 85.1%of the initial capacitance is retained after 4000 cycles.So its electrochemical performance is outstanding and has promising application.Secondly,nickel-cobalt metal selenides are prepared,using ZIF-67 as the precursor.In order to introduce nickel into the product,α-NixCo(1-x)(OH)2 based on ZIF-67 is synthesized through solvothermal reaction.The introduction of nickel improves the specific capacitance of the electrode materials.Effects of the solvothermal reaction temperature on the composition,microstructure and electrochemical properties of α-NixCo(1-x)(OH)2 are investigated,and the synthesis mechanism is speculated.Among the obtained hydroxides,the sample NS-120℃ exhibits high specific capacitances of 2329.2 and 1795.6 F g-1 at the discharge current densities of 1 and 20 A g-1,respectively.In order to improve the cycling stability,α-NixCo(1-x)(OH)2 with different nickel-cobalt molar ratios is synthesized and then selenized to prepare selenides.Effects of Ni-Co molar ratio on the crystal phase,composition,morphology and electrochemical properties of selenides are investigated.Although the stability of selenide is improved compared with hydroxide,the specific capacitance decreases and the synthesis steps are complicated.Then,nickel-cobalt selenides are synthesized by one-step solid-state reaction and one-step solvothermal selenization,respectively.The transition metal selenides obtained by latter exhibit better electrochemical performance.Finally,NixCo1-xSe2 with three-dimensional hierarchical structure is prepared by hydrothermal/solvothermal method using(Ni,Co)(CO3)0.5(OH)as precursor.The effects of the NH4F on the structure,morphology,crystal phase and electrochemical properties of the precursor and corresponding selenides are investigated.Influence of Co on the crystal phase,electron configuration,exposed electro-active sites on the surface and electrochemical properties of selenides are studied.Among the obtained selenides,the sample Ni0.6Co0.4Se2 with ordered nano-micro inserted structure shows specific capacitances of 1580 and 1205 F g-1 at the current densities of 1 and 20 A g-1,respectively,and 90%of its initial capacitance is remained after 20000 cycles.The changes of microstructure,composition and crystal phase of Ni0.6Co0.4Se2 during the process of charging/discharging are investigated.The results show that the Ni0.6Co0.4Se2 is gradually transformed into the highly active phase MOOH(M=Ni,Co)/Ni0.6Co0.4Se2,which acts as the actual active material in charge storage/release process.The systematic study strongly reveales that lots of active regions accessible to ions,distorted selenides formed on the surface and highly conductive and reactive MOOH(M=Ni,Co)/Ni0.6Co0.4Se2 active phase contribute to the excellent electrochemical performance.The hybrid supercapacitor based on Ni0.6Co0.4Se2 is assembled,whose practical application performance and prospect are evaluated by electrochemical test.Energy densities of the device can reach 43.5 and 36.8 W h kg-1 at power densities of 1489.6 and 15070.6 W kg-1 respectively,and 89.2%of the initial capacitance is remained after 20000 cycles.