Synthesis And Hybrid Supercapacitors Energy Stotage Properties Of Nickel-based Nanosheets

Supercapacitors（SC）are considered as a critical member for wind power system,new energy vehicles,smart grid,military power supply and other fields,and have been widely concerned.Hybrid supercapacitors（HSC）are new electrichemical energy storages composed of battery type cathode and capacitor type anode.Therefore,HSCs combine the advantages of high-capacity batteries and fast charge-discharge SCs and are expected to improve the energy density of SCs without decreasing power density.However,the capacitive performance of battery type materials for HSCs is still limited by the inappropriate structure design and sluggish reaction kinetics.More seriously,its unbalanced development trend at home and abroad has become the main bottleneck for the development of high power energy storage devices.Foreign enterprises lead in technology and occupy most of the global market,while domestic enterprises are in the initial and development stage.Therefore,it is necessary to improve the energy storage capacity and cycle life of hybrid supercapacitors from the design of electrode materials.Nickel-based battery-type materials are considered as potential positive electrodes.However,as reported nickel-based materials show low capacity,poor rate performance and extremely limited cycle life.Herein,this paper is aimed to develop efficient new synthesis strategys for battery type materials with high energy density based on nanostructure engineering and phase regulation of promisin g two dimension（2D）materials,which would maximize the advatanges of high surface area,high surfactivity and developed ion migration channels.And therefore,they can deeply accelerate the ion transform rate and ion diffusion rate i,resulting in high power and high capacity HSCs.The main research content of this dissertation is as follows:Firstly,in view of complex controllable synthesis process and closed structure of existing nickel-based materials,a concept of developing a synthesis strategy of2D battery type materials with whole compound process at normal pressure and room temperature is proposed to realize the energy saving and emission reduction.Typically,a mild ion exchange method using ZIF-67 as a self-sacrificial template was applied and porous hollow nanocages that self-assembled by Ni-Co LDH nanosheets were obtained based on Kendall effect.The height（～4 nm）and interlayer spacing（～1.3 nm）of nanosheets as well as porosity of nanocages could be optimized by controlled local Co²⁺supply and reduced charge density of the laminate.As a consequence,the max specific capacitance of porous hollow nanocages is 2344.1 F g^–1 at 0.5 A g^–1.Even when the current density increases 100 times to 50 A g^–1,the specific capacitance also retained 1811.1 F g^–1（77%）,showing excellent rate performance.Moreover,the capacity of powder-shaped two-dimensional nanosheets tends to decay rapidly due to continuous chemical changes during charging and discharging.However,there is no simple and effective way to synthesize stable nanosheets with nanometer thickness.In view of the above problem,a novel systhesis strategy named"continuous ion sustained release method"was employed to fabricate ultrathin Ni（OH）₂ nanosheet arrays.Inspired by the flood water regulating functions of natural lakes,Mn（OH）₂ plays a similar role in maintaining OH^-concentration for its higher Ksp compared with Ni（OH）₂.Therefore,the growth rate of nickel hydroxide along c axis and thichness of nanosheets can be limited by their reaction competition.The nanosheets were～5.7 nm thick and 4μm in length.Simulation results based on the first principles also show the nickel hydroxide prepared has better electrical conductivity.As prepared Ni（OH）₂ nanosheets arrays obtained 80%capacitance retain after 3000 cycles.Last but not the least,to further improve the ion diffusion rate,conductivity and porosity,an unique topological transformation synthesis method combining ion change and ordered etch to compound porous and ultrathin Ni Se nanosheet arrays for higher energy density was raised based on the ultrathin Ni（OH）₂ nanosheets arrays.The as obtained Ni Se nanosheet arrays with a thickness of～4.8 nm exhibited developed porosity and open structure.Their ion diffusion rate increased 25 times than Ni（OH）₂ nanosheets.Besides,crystal models of Ni Se and Ni（OH）₂ were built to verify the outstanding conductivity of Ni Se by density functional calculation.In consequence,the specific capacitance of porous ultrathin Ni Se nanosheet arrays increased to 3546.0 F g^–1(3 A g^–1),much higher than that of the other previously reported battery type materials.The energy density of the as assembled HSC based on the porous ultrathin Ni Se nanosheet arrays reached up to 69.8 Wh kg^–1(425 W kg^–1)and the HSC device achieved a max power density of 12750 W kg^–1(20.8 Wh kg^–1).More than that,the capacitance of the HSC device maintained for 94.8%of the initial capacitance after 5000 cycles.And even when the cycle numbers reached up to 10000 cycles,the capacitance also maintained for 80%of the initial capacitance,indicating excellent commercial application value.