Preparation And Characteristics Of Layered Double Hydroxides As Electrode Material For Supercapacitors

Supercapacitors are expected to become a new generation of energy storage equipment due to their advantages of good cycle stability,high power density,fast charge and discharge,good safety and low cost.However,the low energy density of supercapacitors severely hinders large-scale practical applications.Therefore,the development of electrode materials with high performance has become the focus of research.Layered double hydroxides（LDHs）stand out from many electrode materials for supercapacitors owing to their high theoretical specific capacitance and adjustable structure.Nevertheless,LDHs present some disadvantages such as poor conductivity and cycle stability.In this thesis,in order to effectively improve the electrochemical performance of electrode materials,three strategies include the introduction of third metal element,the optimization of the material morphology and the control of the metal ion ratio have been used.The main works of this thesis are as follows:（1）The NiCo₂-OH and NiCo₂Al_x-LDHs（x=0.5,1,2）materials were successfully synthesized by hydrothermal method.The effects of Al content on the morphology,crystal structure and electrochemical properties of NiCo₂Al _X-LDHs were systematically studied.The results show that the addition of appropriate amount of Al is beneficial to stable the structure of LDH,and the NiCo₂Al_0.5-LDHs electrode exhibits the best electrochemical performance when x=0.5.The specific capacitance of NiCo₂Al_0.5-LDHs electrode is 1260 F g^-1at the current density of 0.5 A g^-1,and the specific capacitance can reach 1027 F g^-1at 10 A g^-1.The assembled NiCo₂Al_0.5-LDHs//AC ASC device possesses a maximum energy density of 39 Wh kg^-1at the power density of 801 W kg^-1,and an excellent cycle stability with the capacitance retention of 91.7%after 6000 cycles at 5 A g^-1.（2）The NiCo-LDH was prepared by hydrothermal method using dodecahedral ZIF-67 as precursor and Co source.A series of ternary LDH materials were synthesized by introducing different third metal elements（Mn,Fe,Zn）,and the effects of introducing different metals on the LDH materials were discussed.It was found that the prepared Ni Co Mn-LDH has a hollow dodecahedron structure constructed by nanosheets,which can provide large specific surface area and abundant active sites,shortening the charge transfer path,thus obtaining the fast ion transfer rate.The specific capacitance of Ni Co Mn-LDH electrode can reach 1242.5 F g^-1at 0.5 A g^-1.The assembled Ni Co Mn-LDH//AC ASC achieves a maximum energy density of 23.9Wh kg^-1at the power density of 752.0 W kg^-1.（3）The NiCo Mn-LDH with different Ni and Mn introduction amounts were prepared by employing ZIF-67 as a precursor,and the effects of different metal element ratio on the properties of Ni Co Mn-LDH materials were studied in detail.The results suggest that the Ni Co Mn-LDH can obtain a maximum synergistic effect of different transition metals when the mass ratio of Ni source and Mn source is 2:1.Benefiting from the unique morphology and optimal composition superiorities,the Ni₂Co Mn₁-LDH electrode has a high specific capacitance of 1634.4 F g^-1at 0.5 A g^-1,the specific capacitance can reach 1040.2 F g^-1when the current density increases to20 A g^-1.Moreover,the assembled Ni₂Co Mn₁-LDH//AC ASC device has a high specific capacitance of 123.4 F g^-1at 1 A g^-1and delivers a high energy density of 43.9Wh kg^-1at 800 W kg^-1.