Preparation,Transformation And Electrochemical Properties Of Transition Metal Hydroxide-based Materials

Transition metal hydroxide has been widely used in supercapacitors due to its large specific surface area,rich active components and strong structure tunability.However,the inherent defects of transition metal hydroxide,such as easy agglomeration,poor conductivity and electrochemical activity,limit its development in supercapacitors.To solve these problems,the structure,physical or chemical properties of transition metal hydroxides were modified based on the controllable preparation technology and phase conversion method such as combination,acid salt method,oxidation,sulfidation and phosphorization.The corresponding electrochemical properties of the obtained materials were evaluated,and the key influence factors of electrochemical properties were proposed.The main results include:（1）In view of the unclear basic properties and controllable preparation mechanism of transition metal hydroxide-based electrode materials,montmorillonite@Ni Fe LDH（MMT@Ni Fe LDH）,Cl^-intercalaced Ni Al LDH and porous cobalt-aluminum layered double oxide@Mn O2（Co Al LDO@Mn O2）were prepared by hydrothermal method.The results of different modification method have been studied by the analysis of the basic physical or chemical properties of the materials:the layer spacing can be increased by increasing of the proportion of divalent metal cation or using the Cl^-to replace the CO3^2-in the gallery of LDH;the Cl^-intercalaced Ni Al LDH can be prepared simply and controllable by the acid salt method which is more advantageous than the one-step co-precipitation method;growing two-dimensional Mn O2 nanosheets on two-dimensional Co Al LDO nanosheets is helpful to improve the space utilization of materials;alkali etching treatment produced the porous structure of Co Al LDO without change the crystalline phase.（2）To solve the problems such as easy aggregation,poor conductivity and electrochemical activity of transition metal hydroxides,S and P co-doped graphene@CoNi2S4 nanocomposite（SPG@CoNi2S4）was prepared controllable by combining combination and sulfidation.The prepared material has a high specific capacitance of 1138.0 F g^-1(specific capacity:126.2 m Ah g^-1)at a current density of 2 A g^-1,and the desirable cycling stability of 70.1%after 5000 cycles.Compared with nickel-cobalt hydroxide（Ni Co-OH）nanowires,the specific capacitance of SPG@CoNi2S4 was improved by 2.28 times.In general,the electrochemical performance of Ni Co-OH nanowires can be improved significantly by combination,sulfidation and S and P co-doped graphene.（3）In view of unclear electrochemical reaction mechanism and the factors affecting the electrochemical properties of LDH at present,GO@Co Al LDH was used as a template to achieve the crystalline phase transformation from LDH to single metal hydroxide,sulfide and phosphide by combining the different chemical modification methods such as alkaline etching treatment,sulfidation and phosphorization.The results provide a perspective for evaluating the effect of crystalline phase component on the electrochemical performance and analyzing the key factors influencing the electrochemical properties and the corresponding electrochemical mechanism.In general,not all modification methods are beneficial to the improvement of electrochemical performance of electrode materials.The electrochemical performance of electrode materials are affected by many factors such as the crystal structure,electrical conductivity and exposure rate of active atoms.（4）Aiming at the current problems such as unclear regulatory mechanism between metallic components and electrochemical performance and poor application prospect of supercapacitors,GO@Ni Co P was prepared controllable using Ni Co-OH nanowires as the template.The influence of metallic components on electrochemical mechanism was analyzed by adjusting the Ni/Co atomic concentration ratio.Based on the outer/inner-component synergistic effect,the GO@Ni Co P showed the excellent rate capability of 39.77%capacitance retention at the current density of 60 A g^-1.By combining the solar self-charging system with three GO@Ni Co P//active graphene（AG）asymmetric supercapacitor device,25 LED lights can be turned on for 60 s.The result is benefit for expanding the application range of supercapacitors.In general,with the help of different modification methods such as combination,phosphorization and metallic component regulation,the rate capability of the material can be optimized without damaging the morphology.Based on the profound understanding of transition metal hydroxides,a set of controllable preparation techniques and crystalline phase transformation methods were developed in this paper to modify the micro-nano characteristics of transition metal hydroxides.The electrochemical properties were optimized.The key factors affecting the electrochemical performance and mechanism were proposed.In conclusion,this work plays a positive role in the controlled preparation,crystalline phase transformation and high-value application of transition metal hydroxides.