The Study Of Electrochemical Performance Of Ni₄OHF₇ Electrode Materials With Vacancy And Co Dopant

Supercapacitor is an excellent energy storage device with high power density,long cycle life and wide operating temperature range,which is expected to meet the energy supply needs of hybrid vehicles,smart grids and other fields.The core factor affecting the performance of supercapacitors is the electrode material.Electrode materials for supercapacitors are mainly classified into carbon materials,conductive polymers,and transition metal compounds.Among them,transition metal compounds,especially nickel-based and cobalt-based metal compound electrode materials,have received extensive attention due to their high theoretical specific capacitance.Ni₄OHF₇ is a new type of electrode material in nickel-based metal compounds,the metal-fluorine chemical bond contained in it increases the polarity of the material surface,which is beneficial to the adsorption of electrolyte ions on the surface of the electrode material during the redox process and improves the electrochemical performance.However,due to the limited active sites and low electronic conductivity of Ni₄OHF₇,it is difficult to obtain high specific capacity and excellent rate performance in practical applications.To address these key issues,electrode materials can be modified by defect engineering strategies,such as introducing vacancies or heteroatom doping.The introduction of vacancies can provide additional active sites and further enable fast faradaic reactions.The introduction of heteroatoms can adjust the intrinsic electrical conductivity of the electrode,effectively increase the carrier concentration and electron mobility of the system,which can significantly improve the rate capability of the electrode material.In this paper,large quantities of vacancies were controllably introduced into Ni₄OHF₇ by adjusting the surfacial microstructure under electrochemical activation to improve the specific capacitance.In addition,the electronic structure and electrochemical performance of the system were regulated by doping the Ni₄OHF₇ electrode material with cobalt element to further improve the rate capability.The specific research content is as follows:1.In this paper,a series of Ni₄OHF₇ containing carbon chains of different lengths were synthesized in ethanol,ethylene glycol,1-propanol and 1-butanol by a one-pot solvothermal method,respectively denoted as F-Ni-O-R₂,F-Ni-O₂-R₂,F-Ni-O-R₃ and F-Ni-O-R₄.Both theoretical and experimental results showed that the carbon chains on the surface of the electrode material connected with F ions through hydrogen bonds,and play a role in stabilizing the structure of Ni₄OHF₇ during the synthesis process.During the subsequent electrochemical preactivation,the hydrogen bonds can be broken rapidly,resulting in the escape of fluorine ions and the formation of abundant vacancies.In addition,the adjustment of the carbon chain length can effectively control the amounts of hydrogen bonds,further realizing the controllable preparation of electrode materials with different vacancy contents.The electrochemically reconstructed F-Ni-O₂-R₂ electrode contains abundant active sites such as vacancies,which improves the electrical conductivity and accelerates the kinetics of surface redox reaction,with 2975 F g^-1 ultra high specific capacitance at a current density of 1 A g^-1.2.Based on above work,we adjusted the electronic structure of Ni₄OHF₇ by doping with heterogeneous elements,which improved the rate capability.We synthesized Co²⁺doped Ni₄OHF₇（denoted as Ni₄OHF₇,Co-Ni（OH）F（Co=10%）,Co-Ni（OH）F（Co=20%）and Co-Ni（OH）F（Co=30%））with different Co:Ni molar ratios（0:1,1:9,1:4 and 3:7）,and explored the optimal Co doping ratio.The calculated density of states show that dopant Co induces the more electronic states appearing near the Fermi level,thereby improving the electrical conductivity of system.Compared with pristine Ni₄OHF₇,the optimized Co-Ni（OH）F（Co=20%）electrode has a larger specific surface area and more defects,and exhibits an ultra-high capacitance of 3380.2 F g^-1 at1 A g^-1.Even at current density of 20 A g^-1,there is 78.4%capacitance retention.