The Design Of Nickel-cobalt Layered Double Hydroxide Electrode Materials And Energy Storage Properties Of Supercapacitors

In recent years,supercapacitors have attracted extensive attention due to their fast charge rate,high power density and long service life,which have been considered as a candidate energy storage device.However,compared to batteries,their energy densities are still at a low level,making it difficult for large-scale practical applications.At present,the development of electrode materials with high capacity is one of the most effective strategies to improve the energy density of supercapacitors.As a typical two-dimensional material,during charge/discharge process,the layered structure of nickel-cobalt layered double hydroxide（Ni-CoLDH）can provide fast intercalation/deintercalation channels for ions.It has the features of good hydrophilicity,ultra-high theoretical specific capacitance(>1600 C g^-1)and high redox activity.In addition,due to the synergistic effect between Ni and Co,it can exhibit better electrochemical performance than single metal hydroxides.These advantages have made it one of the hot electrode materials in the field of supercapacitors for a long time.However,some problems of Ni-CoLDH,such as low mass loading,serious agglomeration/stacking,poor electrical conductivity and unclear understanding for the energy storage mechanism,still limit the development of Ni-CoLDH.In this dissertation,I will combine experimental analysis and density functional theory（DFT）calculation.Firstly,the effect of Ni/Coratio on the electrochemical performance of Ni-CoLDH is studied,and the optimal Ni/Coratio is determined;then the Ni-CoLDH is grown on the surface of NiCo₂S₄ nanoarray to form a heterostructured three-dimensional core-shell nanoarray electrode,which can utilize the good conductivity,open structure and ultra-high theoretical capacitance of NiCo₂S₄ to suppress the agglomeration and stacking of Ni-CoLDH.The intrinsic relationship among the charge transport,mass loading and capacity of the NiCo₂S₄@Ni-CoLDH electrode is analyzed;subsequently,to adjust the properties of the interface between NiCo₂S₄ and Ni-CoLDH,a small amount of Mn is doped into NiCo₂S₄ and the possibility of further improving the rate capability and capacity of the electrode is explored.Finally,the electrodes are assembled into supercapacitors to verify their feasibility for practical application.The main research results are as follows:（1）Ni-CoLDH nanosheets are in-situ grown on carbon cloth by hydrothermal method,and a series of Ni-CoLDH electrodes with different Ni/Coratios are prepared.The effects of Ni/Coratio on the morphology,mass loading,crystal structure,ion adsorption ability and reactivity of Ni-CoLDH have been systematically studied.The results show that the size of Ni-CoLDH nanosheets grown on carbon cloth became smaller and smaller with the increase of Cocontent,resulting in a decrease in mass loading 2.3 mg cm^-2 of Ni（OH）₂ to 1 mg cm^-2 of Co（OH）₂;The interplanar spacing is increased and the M-OH（M is Ni or Co）bonds of the Ni-CoLDH is stretched.In addition,it shows that the content of Co³⁺in Ni-CoLDH becomes increase and then decrease.The content of Co³⁺reaches up to 70.8%in the sample of Ni:Co=3:1,which is the highest value in all samples.DFT calculations indicate that Co²⁺will lose part of their charges and spontaneously transfer to Ni²⁺and O^2-in Ni-CoLDH,resulting in the appearance of Co³⁺in Ni-CoLDH.This charge transfer effect can improve the adsorption ability and reactivity of Ni-CoLDH for OH^-.（2）The electrochemical properties of Ni-CoLDH electrodes are studied through three-electrode system.The effects of Ni/Coratio on the ion storage mechanism,specific capacitance,resistance,rate capability and cycle stability of Ni-CoLDH are analyzed.The results show that the increasing of Ni content is beneficial to increase the capacity of Ni-CoLDH,which is also beneficial to improve the rate capability and cycle stability.Ni-CoLDH electrode with the ratio of Ni:Co=3:1 has the best electrochemical performance.It has a mass loading of 1.73 mg cm^-2 and exhibits a specific capacity of 1438 C g^-1(2.48 C cm^-2)at a current density of 2 m A cm^-2.When the current density is increased to 15 m A cm^-2,75.6%of the specific capacity is retained(1087 C g^-1,1.87 C cm^-2),and the capacity retention is 88.3%after 5000 cycles at this current density.（3）A three-dimensional core-shell nanoarray electrode of NiCo₂S₄@Ni-CoLDH is synthesized via hydrothermal method and kirkendall effect.The structure,electrochemical performance and charge transport properties at the heterointerfaces are investigated.The results show that NiCo₂S₄ nanotubes have a hollow structure with a wall thickness of about 25 nm.Ni₃-CoLDH nanosheets are grown on the surface of NiCo₂S₄ nanotubes and the formed heterointerfaces can promote the spontaneous and rapid charge transfer from Ni₃-CoLDH to NiCo₂S₄,enhanced the ion adsorption ability and reactivity of Ni₃-CoLDH.The as-prepared electrode has high mass loading of 8.1 mg cm^-2,and its specific capacity is 8.14 C cm^-2(1005C g^-1)at a current density of 1 m A cm^-2.When the current density is increased to 15 m A cm^-2,the specific capacity is retained 57%(4.66 C cm^-2,575 C g^-1),and the capacity retention is 91.5%after 5000 cycles at this current density.（4）NiCo₂S₄@Ni₃-CoLDH electrode with Mn doping is prepared by hydrothermal method.The effects of Mn doping on the electrochemical performance（especially rate capability）of NiCo₂S₄@Ni₃-CoLDH electrode and the energy storage mechanism are studied.The result shows that the optimal doping amount of Mn is about 6.7%,namely Mn_0.2-NiCo₂S₄@Ni₃-CoLDH.The optimum hydrothermal reaction time for growing Ni₃-CoLDH is 2.5 hours.Mn doping can change the charge transport properties of the heterointerface locally,enhance the stability of the heterointerface,and improve the adsorption ability and reactivity of the Ni₃-CoLDH for OH^-.At a current density of 1 m A cm^-2,the specific capacitance of the Mn_0.2-NiCo₂S₄@Ni₃-CoLDH electrode is 9.67 C cm^-2(1151 C g^-1).When the current density is increased to 15 m A cm^-2,76.4%of the specific capacity is retained(7.39 C cm^-2,876 C g^-1).After 5000 cycles at this current density,the electrode capacity can be retained 94.3%.（5）Activated carbon（AC）,polyacrylate potassium salt（PAAK）/KOH and the designed three electrodes are used as the negative electrode,electrolyte and positive electrode,and three asymmetric supercapacitors of Ni₃-CoLDH//AC,NiCo₂S₄@Ni₃-CoLDH//AC and Mn_0.2-NiCo₂S₄@Ni₃-CoLDH//AC are assembled.The results show that the highest energy densities of 126.1μWh cm^-2(72.9 Wh kg^-1),880μWh cm^-2(108.8 Wh kg^-1)and 1046.9μWh cm^-2(123.1Wh kg^-1)are obtained at power densities of 2.4 m Wh cm^-2(1.39 k W kg^-1),1.5 m Wh cm^-2(185W kg^-1)and 1.5 m Wh cm^-2(176 W kg_-1),respectively.Their highest specific capacities are 328C g^-1,522 C g^-1,591 C g^-1,respectively.When the current density is increased from 3 m A cm^-2to 15 m A cm^-2,their capacitis are retained 61.9%,62.9%and 76.8%,respectively.After 5000cycles at the current density of 15 m A cm^-2,their capacity retentions are 82.8%,89.8%,and92.7%,respectively.In addition,when the assembled supercapacitor is used to light LED,it can work stably for more than 17 minutes,indicating that the supercapacitor designed in this work has promising practical application.