The Research On Solid-state Synthesis And Electrochemical Performance Of Ni-based Metal Sulfide Nanomaterials And Their Composites

The rapid development of high-performance electrochemical energy storage devices,such as lithium-ion batteries and supercapacitors,provides solutions for overcoming environmental problems caused by excessive consumption of fossil fuels.Supercapacitors have attracted wide attention due to their high specific power,long cycle life,low maintenance,excellent safety and reliability.Transition metal sulfides,due to its superior electrical conductivity,high reactivity,mechanical and thermal stability,transition metal sulfides have great potential in the field of energy storage.In particular,transition metal sulfides can undergo rapid and reversible redox reactions in alkaline electrolytes,so they have ultra-high specific capacity and specific energy and can be used as electrode materials for supercapacitors.In this paper,a simple,efficient and green solid-phase synthesis route was used to directionally synthesize nickel sulfide and its composite materials,and the structure-activity relationship between its morphology,specific surface area,element valence configuration and electrochemical performance was studied.Finally,the electrochemical performances of nickel sulfide and its composite materials,which was used as the electrode material of asymmetric supercapacitors,were systematically tested.（1）Hexagonalα-NiS and pyrite-type NiS₂ nanomaterials were successfully synthesized through a simple,efficient and green solid-phase synthesis method using nickel formate as the nickel source and sublimation sulfur as the sulfur source,and the preparation conditions were systematically optimized.The results show that the hexagonalα-NiS and pyrite NiS₂ nanomaterials with high crystallinity can be obtained when the molar ratio of Ni to S is 1:1 and 1:2 is strictly controlled at the synthesis temperature of 440℃.Due to the special nano morphology,they exhibit excellent electrochemical performance when used as a supercapacitor electrode material.In the three-electrode system,the pyrite-type NiS₂ and hexagonalα-NiS material electrodes show high specific capacities of 163.9 m Ah g^-1(1072.6 F g^-1)和133.8 m Ah g^-1(875.5F g^-1),respectively,at a current density of 2 A g^-1.The assembled asymmetric supercapacitor N-NiS₂//PCN and N-NiS//PCN showed the high specific energy of 54.20Wh kg^-1 and 38.77 Wh kg^-1 at the specific power of 0.94 k W kg^-1 and 1.08 k W kg^-1,respectively,and after 10,000 cycles,more than 83.7%and 79.1%of the initial specific capacity were retained,respectively.The proposed solid-phase synthesis route is a low-cost,eco-friendly and simple synthesis method,which can be used to synthesize nickel sulfide nanomaterials with excellent electrochemical energy storage performance.（2）NiS/rGO composites were successfully synthesized through a simple,efficient and green solid-phase synthesis method using nickel formate,sublimed sulfur and graphene as raw materials,and the preparation conditions were systematically optimized,and the effects of the synthesis temperature and the amount of graphene on the morphology and electrochemical properties of the composites were systematically optimized.The results show that the NiS/rGO-2 composites have the best electrochemical properties when the amount of graphene is 40 mg at 500℃（The content of graphene is about 11.37 wt.%）.The NiS/rGO-2 material electrodes prepared in this experiment show an ultra-high specific capacitance of 299.7 m Ah g^-1(2,157.8 F g^-1)at a current density of2 A g^-1 and more than 53.8%of the initial specific capacity were retained at the current density of 2 to 20 A g^-1.The assembled asymmetric supercapacitor has a high specific energy of 56.11 Wh kg^-1 at a specific power of 0.88 k W kg^-1,and after 30,000 charge-discharge cycles,the capacity retention rate is 92.4%of the initial capacity.The results show that the NiS/rGO-2 composite prepared by this method has an excellent performance in the practical application of asymmetric supercapacitors.（3）NiS/CNT composites were successfully synthesized through a simple,efficient and green solid-phase synthesis method using nickel formate,sublimed sulfur and carbon nanotube as raw materials,and the preparation conditions were systematically optimized,and the effects of the synthesis temperature and the amount of carbon nanotube on the morphology and electrochemical properties of the composites were systematically optimized.The results show that the NiS/CNT-2 composites have the best electrochemical properties when the amount of carbon nanotube is 40 mg at 500℃（The content of carbon nanotube is about 11.37 wt.%）.The NiS/CNT-2 material electrodes prepared in this experiment show an ultra-high specific capacitance of 282.1 m Ah g^-1(2,030.8 F g^-1)at a current density of 2 A g^-1,and the capacity retention of NiS/CNT-2electrodes is 51.3%at the current density of 2 to 20 A g^-1.The assembled asymmetric supercapacitor NiS/CNT-2//IHPC has a high energy density of 49.65 Wh kg^-1 at a power density of 0.85 k W kg^-1,after 20,000 charge-discharge cycles,the capacity retention rate is 85.0%of the initial capacity,and the coulombic efficiency is close to 100%.Therefore,the conductivity of the material can be effectively improved by adding carbon nanotubes,thereby improving the energy storage performance and rate performance of the composite material.The NiS/CNT-2 composite prepared by this method has excellent performance in the practical application of asymmetric supercapacitors.