Study On Preparation And Electrochemical Performance Of NiCo-LDH-based Flexible Core-shell Structure Composite Electrode

With the rise of flexible wearable electronic products,the demand for flexible energy storage devices is also increasingly urgent.Among so many flexible energy storage devices,flexible supercapacitors have become a hotspot due to its high power density,excellent cycle stability,environmental friendliness,fast charge and discharge time in recent years.The performance and cost of flexible electrodes are decided by two key factors（material and preparation）.Duo to the tunable chemical composition,high redox activity,large specific area and so on,Layered Double Hydroxides（LDH）have attracted wide attention as one kind of good pseudocapacitor materials.Among the so many LDHs,Ni-based LDHs have been studied more.Among them,Ni Co-LDH has the advantages of strong electrochemical activity and high theoretical specific capacitance,but its poor conductivity,easy agglomeration,low cycle stability and structure is easy to collapse during charge and discharge,which limits its practical application.So,this paper uses hydrothermal and electrodeposition methods to prepare Ni Co@Ni Co-LDH and Ni Co₂O₄@Ni Co-LDH two kinds of core-shell structure composite electrodes in a flexible substrate（carbon fiber cloth）.In addition,their morphology,structure and composition were characterized.What’s more,the electrochemical performance of them was stduied systematically and deeply.The fabrication of asymmetric supercapacitors and the application experiments of lighting LEDs were also carried out.The main obtained results are as follows:（1）NiCo@Ni2Co1-LDH core-shell composite electrode were grown on a carbon fiber cloth（CFC）substrate using a hydrothermal and electrodeposition methods.The one-dimensional Ni Co-LDH nanowires（average diameter about 100 nm and average length of 1 um）act as core and grow almost perpendicularly and uniformly on the surface of CFC.The two-dimensional Ni Co-LDH nanosheets grow on the Ni Co-LDH nanowires as shell.Ni Co@Ni2Co1-LDH composite electrode has a specific surface area of 23.311m²/g and a volume specific surface area of 0.08173 cm³/g.It is much higher than that of Ni Co-LDH nanowires,indicating that the specific surface area increases significantly after compounding.（2）The effects of different Ni and Co ratios of NiCo-LDH nanosheet shells and the introduction of Ni Co-LDH nanowires on the electrochemical performance of the Ni Co@Ni Co-LDH composite electrode were studied.It is found that the electrode has the best electrochemical performance when the Ni and Co ratio of shells is at 2:1 and achieves a high areal capacitance of 9.67 F/cm² at 5 m A/cm²,which is about 1.2 times of the electrode without the introduction of Ni Co-LDH nanowires（8.194 F/cm²）;The coulomb efficiency reaches to 89.2%.This is due to the introduction of Ni Co-LDH nanowires accelerates the electron transmission and the core-shell structure exposes more active sites,and increases the specific surface area.This electrode retains 75%of the capacitance after 2000 consecutive cycles,however,the retention rate of the electrode without the introduction of Ni Co-LDH nanowires is only 43.2%.It shows that the cyclic stability of the core-shell structure electrode has also been significantly improved.（3）NiCo-LDH nanowire arrays were grown on CFC substrate using a hydrothermal method.Then,Ni Co₂O₄ nanowire arrays were formed by high temperature annealing of the Ni Co-LDH nanowire arrays.Ni Co-LDH nanosheets were electrodeposited on the Ni Co₂O₄ nanowire arrays to form Ni Co₂O₄@Ni Co-LDH core-shell structure composite electrode.The one-dimensional Ni Co₂O₄ nanowires（average diameter about 100 nm and average length of 1 um）act as core and grow almost perpendicularly and uniformly on the surface of CFC.The two-dimensional Ni2Co1-LDH nanosheets grow on the Ni Co-LDH nanowires as shell.（4）The effect of the introduction of NiCo₂O₄ nanowires on the electrochemical performance of the Ni Co₂O₄@Ni Co-LDH composite electrode were studied.It is found that the composite electrode has a high area specific capacitance of 11.1 F/cm² at a current density of 3 m A/cm².The improvement in performance is due to:Ni Co-LDH nanosheets deposited on Ni Co₂O₄ nanowires which increase the specific surface area of electroactive materials and expose more active sites to participate in the reaction;Ni Co₂O₄ nanowires can also provide some capacitance;the introduction of Ni Co₂O₄ nanowires enhances the electron transport capability and rate between Ni Co-LDH nanosheets and CFC substrate;the core-shell structure can shorten the transmission distance of electrolyte ions and improve the cycle stability of the material.In this thesis,two kinds of composite flexible electrodes with NiCo-LDH nanowires and Ni Co₂O₄ nanowires as core and Ni Co-LDH nanosheets as shell were constructed by hydrothermal and electrodeposition methods,respectively.Their electrochemical properties and related mechanism were studied.The research results have opened up new ideas for improving the electrochemical performance of Ni Co-LDH nanomaterial and provided basic data for their practical application in the field of flexible supercapacitor.