Nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode with core-shell structure and preparation method thereof

Abstract

The invention discloses a nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode with a core-shell structure and a preparation method of the nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode, and relates to the technical field of material engineering and manufacturing. The preparation method comprises the following steps: adding nickel nitrate hexahydrate, cobalt nitrate hexahydrate, sodium citrate, potassium hydroxide and ammonia water into deionized water, and stirring for reaction to obtain nickel-cobalt hydroxide; putting sodium hypophosphite and nickel-cobalt hydroxide into a tubular furnace, and performing high-temperature treatment under argon to obtain a nickel-cobalt-phosphorus composite material; and uniformly mixing the nickel-cobalt-phosphorus composite material, PVDF and carbon black to obtain slurry, coating carbon cloth with the slurry, and performing electrochemical activation to obtain the composite electrode. The composite electrode provided by the invention has a large specific surface area and strong adsorption capacity to OH <->, and has abundant reaction active sites; the nickel-cobalt hydroxide and the nickel-cobalt phosphide synergistically participate in the electrode reaction, so that the electrode has extremely high mass specific capacity; due to the high conductivity of the heterostructure and the nickel-cobalt phosphide, the electrode has excellent conductivity and excellent rate capability.

Description

Technical Field

[0001] The present invention relates to the technical fields of materials engineering and manufacturing technology, and particularly relates to a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode and a preparation method thereof. Background Art

[0002] Due to the gradual depletion of non-renewable energy sources and the intermittent and decentralized problems of new energy sources such as wind energy and solar energy, it is of great significance to develop advanced and efficient energy storage devices. As an excellent electrochemical energy storage device, rechargeable secondary batteries have received extensive attention due to their advantages such as high stability, high energy efficiency, and excellent scalability. Currently, lithium-ion batteries dominate the portable electronic device and electric vehicle markets due to their high energy density, stable discharge platform, etc.; however, the low abundance, high cost of lithium, and safety problems such as the toxicity and flammability of electrolytes severely limit the further development of lithium-ion batteries. Aqueous zinc batteries have received much attention in the energy storage field and have great development potential due to their advantages such as high safety, low cost, relatively high energy density, and environmental friendliness. However, in aqueous zinc batteries, the research and optimization of efficient cathode materials have always been one of the cores and difficulties in realizing their large-scale application.

[0003] Currently, common cathode materials in zinc batteries include manganese oxides, vanadium oxides, Prussian blue analogs, nickel-based hydroxides, etc. Among them, MnO 2 is limited by three major problems: unstable structure, serious manganese dissolution, and poor conductivity, and the zinc storage mechanism involved in the reaction process is complex and there is some controversy. Vanadium oxides always have problems of poor ion diffusivity and slow kinetics. More seriously, the strain generated by the repeated insertion / extraction of Zn 2+ in its lattice will eventually induce the destruction and failure of the microstructure of vanadium oxides. Prussian blue analogs face problems of insufficient redox sites, unstable structure, and very limited capacity, and the capacity decay during the cycling process is also quite serious. Nickel-based hydroxides have pseudocapacitive characteristics, a large specific surface area and interlayer spacing, and two-dimensional ion diffusion channels, showing great application potential, but currently they are mainly limited by limited energy supply and slow reaction kinetics.

[0004] Therefore, modifying nickel-based cathodes to improve their energy supply and accelerate their reaction kinetics to obtain modified nickel cathodes with high mass specific capacity and excellent rate performance is of great significance for the further application of aqueous zinc batteries. Summary of the Invention

[0005] To solve the above technical problems, the object of the present invention is to provide a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode and a preparation method thereof, to obtain a core-shell structured NiCo-LDH@NiCo-P nanocomposite material directly used for aqueous zinc batteries on a carbon cloth substrate, which has a high mass specific capacity and excellent rate performance.

[0006] The technical solution of the present invention to solve the above technical problems is as follows: A preparation method of a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode is provided, including the following steps: (1) Nickel nitrate hexahydrate, cobalt nitrate hexahydrate, sodium citrate, potassium hydroxide and ammonia water are added to deionized water, reacted under mechanical stirring conditions, cooled to room temperature, the supernatant is removed, and after washing and drying, nickel-cobalt hydroxide NiCo-LDH is obtained; (2) Sodium hypophosphite and the nickel-cobalt hydroxide obtained in step (1) are respectively placed in a porcelain boat, then placed in a tube furnace, and the porcelain boat containing sodium hypophosphite is placed upstream of the gas, and kept warm in an argon environment, and cooled to room temperature with the furnace, to obtain a nickel-cobalt phosphide composite material NiCo-Px; (3) The nickel-cobalt phosphide composite material, polyvinylidene fluoride and carbon black obtained in step (2) are mixed evenly to obtain a slurry; then the slurry is coated on a carbon cloth and dried to obtain a carbon cloth coated with the nickel-cobalt phosphide composite material; (4) Using the carbon cloth coated with the nickel-cobalt phosphide composite material obtained in step (3) as a working electrode, three-electrode CV electrochemical activation is carried out to obtain a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P.

[0007] Further, in step (1), the molar ratio of nickel nitrate hexahydrate, cobalt nitrate hexahydrate, sodium citrate, potassium hydroxide and ammonia water is 4:1:8:16:40.

[0008] Further, in step (1), in step (1), under mechanical stirring conditions, the reaction is carried out at a temperature of 50-60 °C for 10 h.

[0009] Further, in step (2), the mass ratio of sodium hypophosphite to nickel-cobalt hydroxide is 0.5-1.5:1.

[0010] Further, in step (2), in an argon environment, it is heated to 350 °C at a rate of 1 °C / min and kept warm for 3 h.

[0011] Further, in step (2), for the nickel-cobalt phosphide composite material NiCo-Px, x represents the mass ratio of sodium hypophosphite to nickel-cobalt hydroxide, and can be 0.5, 0.75, 1.0, 1.25, 1.5.

[0012] Further, in step (3), the mass ratio of the nickel-cobalt-phosphorus composite material, polyvinylidene fluoride, and carbon black is 8:1:1.

[0013] Further, in step (3), the working electrode is a platinum sheet electrode, the reference electrode is a mercury / mercuric oxide electrode, and the electrolyte is a 3 mol / L potassium hydroxide solution.

[0014] Further, in step (3), during the three-electrode CV electrochemical activation, the voltage is 0 - 0.9 V, the scanning rate is 10 mV / s, and the cyclic scanning is performed 100 times.

[0015] The present invention also provides a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode prepared by the preparation method of the above-mentioned core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode.

[0016] The present invention has the following beneficial effects: 1. Compared with NiCo-LDH, the core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P exhibits a large specific surface area and a higher adsorption capacity for OH - , enabling the composite material to have abundant reactive sites.

[0017] 2. The core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P not only benefits from the high conductivity of the metalloid-like property of NiCo-P but also has a large number of heterostructures, which can improve the electronic structure through charge transfer, ultimately resulting in good electronic conductivity and fast reaction kinetics of the material.

[0018] 3. In the core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P, both NiCo-LDH and NiCo-P exhibit electrochemical activity and can participate in the electrode reaction synergistically. Compared with NiCo-LDH, it has a larger specific capacity, improving the energy supply of the electrode.

[0019] 4. The nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode of the present invention can be directly used as the electrode of an aqueous zinc battery. When used as an electrode, the mass specific capacity at a current density of 1 C (1 C = 289 mA / g) can reach 286.64 mAh / g, approaching its theoretical capacity of 289 mAh / g. And when the current density is increased to 40 C, it still maintains a mass specific capacity of 72.22%; when cyclically charged and discharged 1000 times at a current density of 5 C, the capacity retention rate is as high as 70.1%. Description of the Drawings

[0020] Figure 1 It is the SEM image of the product obtained in Example 3; Figure 2 TEM image of the product obtained in Example 3; Figure 3 CV curves of the products obtained in Example 3 and Comparative Examples 1-2 at a scanning rate of 1 mV / s; Figure 4 CV curves of the product obtained in Example 3 at different scanning rates; Figure 5 GCD diagram of the product obtained in Example 3. Detailed implementation mode

[0021] The principles and features of the present invention are described below. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention. For those not specified in the examples, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified for the manufacturer, they are all conventional products that can be obtained by purchasing in the market.

[0022] Example 1 A core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode, and its preparation method includes the following steps: (1) Add 50 mmol nickel nitrate hexahydrate, 12.5 mmol cobalt nitrate hexahydrate, 100 mmol sodium citrate, 200 mmol potassium hydroxide, and 500 mmol ammonia water into 100 mL deionized water. Under mechanical stirring conditions, react at 50 °C for 10 h, cool to room temperature, remove the supernatant, centrifuge and wash the lower layer of the green solution, and dry at 90 °C to obtain nickel-cobalt hydroxide NiCo-LDH; (2) Place 0.5 g sodium hypophosphite and 1 g of the nickel-cobalt hydroxide obtained in step (1) in porcelain boats respectively, then place them in a tube furnace, and the porcelain boat containing sodium hypophosphite is placed upstream of the gas. In an argon atmosphere, heat to 350 °C at a rate of 1 °C / min and hold for 3 h, and cool to room temperature with the furnace to obtain a nickel-cobalt phosphide composite material NiCo-P0.5; (3) Add the nickel-cobalt phosphide composite material, polyvinylidene fluoride, and carbon black obtained in step (2) to an N-methylpyrrolidone solution in a mass ratio of 8:1:1, mix evenly to obtain a slurry; then coat the slurry on carbon cloth and dry at 60 °C to obtain carbon cloth coated with the nickel-cobalt phosphide composite material; (4) Use the carbon cloth coated with the nickel-cobalt phosphide composite material obtained in step (3) as the working electrode, the counter electrode is a platinum sheet electrode, the reference electrode is a mercury / mercuric oxide electrode, and the electrolyte is a 3 mol / L potassium hydroxide solution. Perform three-electrode CV electrochemical activation, the voltage is 0-0.9 V, the scanning rate is 10 mV / s, and cycle scan 100 circles to obtain a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P0.5.

[0023] Example 2 A core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode, and its preparation method includes the following steps: (1) Add 50 mmol nickel nitrate hexahydrate, 12.5 mmol cobalt nitrate hexahydrate, 100 mmol sodium citrate, 200 mmol potassium hydroxide, and 500 mmol ammonia water into 100 mL deionized water. Under mechanical stirring conditions, react at 50 °C for 10 h, cool to room temperature, remove the supernatant, centrifuge and wash the lower green solution, and dry at 90 °C to obtain nickel-cobalt hydroxide NiCo-LDH; (2) Place 0.75 g sodium hypophosphite and 1 g nickel-cobalt hydroxide obtained in step (1) in porcelain boats respectively, then place them in a tube furnace, and the porcelain boat containing sodium hypophosphite is placed upstream of the gas. In an argon environment, heat up to 350 °C at a rate of 1 °C / min and keep it warm for 3 h, and cool down to room temperature with the furnace to obtain nickel-cobalt phosphorus composite material NiCo-P0.75; (3) Add the nickel-cobalt phosphorus composite material, polyvinylidene fluoride, and carbon black obtained in step (2) to an N-methylpyrrolidone solution in a mass ratio of 8:1:1, mix evenly to obtain a slurry; then coat the slurry on carbon cloth and dry it at 60 °C to obtain carbon cloth coated with nickel-cobalt phosphorus composite material; (4) Use the carbon cloth coated with nickel-cobalt phosphorus composite material obtained in step (3) as the working electrode, the counter electrode is a platinum sheet electrode, the reference electrode is a mercury / mercuric oxide electrode, and the electrolyte is 3 mol / L potassium hydroxide solution. Perform three-electrode CV electrochemical activation, with a voltage of 0 - 0.9 V and a scanning rate of 10 mV / s, and cycle scan 100 circles to obtain a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P0.75.

[0024] Example 3 A core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode, and its preparation method includes the following steps: (1) Add 50 mmol nickel nitrate hexahydrate, 12.5 mmol cobalt nitrate hexahydrate, 100 mmol sodium citrate, 200 mmol potassium hydroxide, and 500 mmol ammonia water into 100 mL deionized water. Under mechanical stirring conditions, react at 50 °C for 10 h, cool to room temperature, remove the supernatant, centrifuge and wash the lower green solution, and dry at 90 °C to obtain nickel-cobalt hydroxide NiCo-LDH; (2) Place 1 g of sodium hypophosphite and 1 g of the nickel-cobalt hydroxide obtained in step (1) in porcelain boats respectively, then place them in a tube furnace, with the porcelain boat containing sodium hypophosphite placed upstream of the gas. In an argon atmosphere, heat to 350 °C at a rate of 1 °C / min, hold for 3 h, and cool to room temperature with the furnace to obtain the nickel-cobalt phosphorus composite material NiCo-P1.0; (3) Add the nickel-cobalt phosphorus composite material, polyvinylidene fluoride, and carbon black obtained in step (2) to an N-methylpyrrolidone solution in a mass ratio of 8:1:1, mix evenly to obtain a slurry; then coat the slurry on carbon cloth and dry it at 60 °C to obtain carbon cloth coated with the nickel-cobalt phosphorus composite material; (4) Use the carbon cloth coated with the nickel-cobalt phosphorus composite material obtained in step (3) as the working electrode, the counter electrode is a platinum sheet electrode, the reference electrode is a mercury / mercuric oxide electrode, and the electrolyte is a 3 mol / L potassium hydroxide solution. Perform three-electrode CV electrochemical activation with a voltage of 0 - 0.9 V, a scanning rate of 10 mV / s, and cycle scan 100 times to obtain the core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P1.0.

[0025] Example 4 A core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode, and its preparation method includes the following steps: (1) Add 50 mmol of nickel nitrate hexahydrate, 12.5 mmol of cobalt nitrate hexahydrate, 100 mmol of sodium citrate, 200 mmol of potassium hydroxide, and 500 mmol of ammonia water to 100 mL of deionized water. Under mechanical stirring conditions, react at 50 °C for 10 h, cool to room temperature, remove the supernatant, centrifuge and wash the lower green solution, and dry it at 90 °C to obtain nickel-cobalt hydroxide NiCo-LDH; (2) Place 1.25 g of sodium hypophosphite and 1 g of the nickel-cobalt hydroxide obtained in step (1) in porcelain boats respectively, then place them in a tube furnace, with the porcelain boat containing sodium hypophosphite placed upstream of the gas. In an argon atmosphere, heat to 350 °C at a rate of 1 °C / min, hold for 3 h, and cool to room temperature with the furnace to obtain the nickel-cobalt phosphorus composite material NiCo-P1.25; (3) Add the nickel-cobalt phosphorus composite material, polyvinylidene fluoride, and carbon black obtained in step (2) to an N-methylpyrrolidone solution in a mass ratio of 8:1:1, mix evenly to obtain a slurry; then coat the slurry on carbon cloth and dry it at 60 °C to obtain carbon cloth coated with the nickel-cobalt phosphorus composite material; (4)Using the carbon cloth coated with the nickel-cobalt-phosphorus composite material obtained in step (3) as the working electrode, the counter electrode is a platinum sheet electrode, the reference electrode is a mercury / mercuric oxide electrode, and the electrolyte is a 3 mol / L potassium hydroxide solution. Perform three-electrode CV electrochemical activation with a voltage of 0 - 0.9 V, a scanning rate of 10 mV / s, and cycle scan 100 times to obtain a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P1.25.

[0026] Example 5 A core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode, and its preparation method includes the following steps: (1) Add 50 mmol nickel nitrate hexahydrate, 12.5 mmol cobalt nitrate hexahydrate, 100 mmol sodium citrate, 200 mmol potassium hydroxide, and 500 mmol ammonia water to 100 mL of deionized water. Under mechanical stirring conditions, react at 50 °C for 10 h, cool to room temperature, remove the supernatant, centrifuge and wash the lower green solution, and dry at 90 °C to obtain nickel-cobalt hydroxide NiCo-LDH; (2) Place 1.5 g of sodium hypophosphite and 1 g of the nickel-cobalt hydroxide obtained in step (1) in porcelain boats respectively, then place them in a tube furnace, and the porcelain boat containing sodium hypophosphite is placed upstream of the gas. In an argon environment, heat to 350 °C at a rate of 1 °C / min and hold for 3 h, and cool to room temperature with the furnace to obtain a nickel-cobalt-phosphorus composite material NiCo-P1.5; (3) Add the nickel-cobalt-phosphorus composite material, polyvinylidene fluoride, and carbon black obtained in step (2) to an N-methylpyrrolidone solution in a mass ratio of 8:1:1, mix evenly to obtain a slurry; then coat the slurry on the carbon cloth and dry at 60 °C to obtain a carbon cloth coated with the nickel-cobalt-phosphorus composite material; (4) Using the carbon cloth coated with the nickel-cobalt-phosphorus composite material obtained in step (3) as the working electrode, the counter electrode is a platinum sheet electrode, the reference electrode is a mercury / mercuric oxide electrode, and the electrolyte is a 3 mol / L potassium hydroxide solution. Perform three-electrode CV electrochemical activation with a voltage of 0 - 0.9 V, a scanning rate of 10 mV / s, and cycle scan 100 times to obtain a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P1.5.

[0027] Comparative Example 1 A composite electrode, and its preparation method includes the following steps: (1) Add 50 mmol nickel nitrate hexahydrate, 12.5 mmol cobalt nitrate hexahydrate, 100 mmol sodium citrate, 200 mmol potassium hydroxide, and 500 mmol ammonia water into 100 mL deionized water. Under mechanical stirring, react at 50 °C for 10 h. Cool to room temperature, remove the supernatant, centrifuge and wash the lower green solution, and dry at 90 °C to obtain nickel-cobalt hydroxide NiCo-LDH; (2) Add the nickel-cobalt hydroxide obtained in step (1), polyvinylidene fluoride, and carbon black into an N-methylpyrrolidone solution in a mass ratio of 8:1:1, mix evenly to obtain a slurry; then coat the slurry on carbon cloth and dry at 60 °C to obtain carbon cloth coated with nickel-cobalt hydroxide; (3) Use the carbon cloth coated with nickel-cobalt hydroxide obtained in step (2) as the working electrode, the counter electrode is a platinum sheet electrode, the reference electrode is a mercury / mercuric oxide electrode, and the electrolyte is 3 mol / L potassium hydroxide solution. Perform three-electrode CV electrochemical activation with a voltage of 0 - 0.9 V, a scanning rate of 10 mV / s, and cycle scan 100 circles to obtain the composite electrode NiCo-LDH.

[0028] Comparative Example 2 A composite electrode, and its preparation method includes the following steps: (1) Add 50 mmol nickel nitrate hexahydrate, 12.5 mmol cobalt nitrate hexahydrate, 100 mmol sodium citrate, 200 mmol potassium hydroxide, and 500 mmol ammonia water into 100 mL deionized water. Under mechanical stirring, react at 50 °C for 10 h. Cool to room temperature, remove the supernatant, centrifuge and wash the lower green solution, and dry at 90 °C to obtain nickel-cobalt hydroxide NiCo-LDH; (2) Place 1 g of the nickel-cobalt hydroxide obtained in step (1) in a porcelain boat, then place it in a tube furnace, and the porcelain boat is placed downstream of the gas. In an argon atmosphere, heat to 350 °C at a rate of 1 °C / min and hold for 3 h, and cool to room temperature with the furnace to obtain NiCo-P0; (3) Add NiCo-P0 obtained in step (2), polyvinylidene fluoride, and carbon black into an N-methylpyrrolidone solution in a mass ratio of 8:1:1, mix evenly to obtain a slurry; then coat the slurry on carbon cloth and dry at 60 °C to obtain carbon cloth coated with NiCo-P0; (4)Using the carbon cloth coated with NiCo-P0 obtained in step (3) as the working electrode, the counter electrode is a platinum sheet electrode, the reference electrode is a mercury / mercuric oxide electrode, and the electrolyte is a 3 mol / L potassium hydroxide solution. Perform three-electrode CV electrochemical activation with a voltage of 0 - 0.9 V, a scan rate of 10 mV / s, and cycle scan 100 times to obtain the composite electrode NiCo-LDH@NiCo-P0.

[0029] Test Example 1. Obtain the SEM and TEM images of the product obtained in Example 3 (core-shell structured nickel cobalt hydroxide / nickel cobalt phosphide composite electrode NiCo-LDH@NiCo-P1.0) respectively, as shown in Figure 1-2 shown.

[0030] At the same time, use the products obtained in Example 3 and Comparative Examples 1 - 2 as electrodes for electrochemical performance testing. The counter electrode is a platinum sheet electrode, the reference electrode is a mercury / mercuric oxide electrode, and the electrolyte is a 3 mol / L potassium hydroxide solution. The comparison diagram of CV at a scan rate of 1 mV / s is shown in Figure 3 shown.

[0031] And perform CV tests on the product obtained in Example 3 (core-shell structured nickel cobalt hydroxide / nickel cobalt phosphide composite electrode NiCo-LDH@NiCo-P1.0) at different scan rates. The results are shown in Figure 4 shown. And the GCD diagram of the product obtained in Example 3 is shown in Figure 5 shown.

[0032] It can be seen from Figure 1 that the material presents a nanosheet morphology with a coated surface.

[0033] It can be seen from Figure 2 that for the core-shell structured nickel cobalt hydroxide / nickel cobalt phosphide composite electrode NiCo-LDH@NiCo-P1.0 obtained in Example 3, the outer side is nickel cobalt phosphide Ni 2 P / Co 2 P, and the inside is NiCo-LDH, which proves that the obtained nickel cobalt hydroxide / nickel cobalt phosphide composite electrode material has a core-shell structure, where the shell is nickel cobalt phosphide NiCo-P and the inside is nickel cobalt hydroxide NiCo-LDH; in addition, there are a large number of heterostructures in the substance, which is beneficial to adjusting its electronic structure through charge transfer, thereby improving the electronic conductivity of the material.

[0034] It can be seen from Figure 3It can be seen that the core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode NiCo-LDH@NiCo-P1.0 obtained in Example 3 has the largest specific capacity (CV curve area), followed by the NiCo-LDH electrode in Comparative Example 1, and the NiCo-LDH@NiCo-P0 electrode in Comparative Example 2 has the smallest specific capacity. In addition, the CV curves of the NiCo-LDH electrode and the NiCo-LDH@NiCo-P0 electrode only show a pair of redox peaks, corresponding to the electrode reaction of nickel-cobalt hydroxide NiCo-LDH. However, two obvious pairs of redox peaks can be seen in the CV of the NiCo-LDH@NiCoP1.0 electrode. One pair is the electrode reaction of nickel-cobalt hydroxide NiCo-LDH, and the other pair is the electrode reaction of nickel-cobalt phosphide NiCo-P. It shows that the synergistic reaction of NiCo-LDH and NiCo-P in the NiCo-LDH@NiCoP1.0 electrode improves the specific capacity of the composite electrode.

[0035] It can be seen from Figure 4 that two obvious pairs of redox peaks can be seen. One pair is the electrode reaction of NiCo-LDH, and the other pair is the electrode reaction of NiCo-P; in addition, when the scan rate increases from 1 mV / s to 5 mV / s, the CV curve maintains a good shape, indicating that NiCo-LDH@NiCo-P1.0 has good rate potential.

[0036] It can be seen from Figure 5 that the NiCo-LDH@NiCo-P1.0 composite electrode has excellent specific capacity and rate performance.

[0037] 2. The products obtained in Examples 1-5 and Comparative Examples 1-2 were used as electrodes for electrochemical performance testing. The working electrode was a platinum sheet electrode, the reference electrode was a mercury / mercuric oxide electrode, and the electrolyte was 3 mol / L potassium hydroxide solution. The results are shown in Table 1.

[0038] Table 1 Electrochemical performance test results of Examples 1-5 and Comparative Examples 1-2

[0039] As can be seen from Table 1, when NiCo-LDH is used as an electrode, the specific capacity at a current density of 1 C is about 198.1 mAh / g, and the capacity retention rate is 59.5% when the current density increases to 40 C; when NiCo-LDH@NiCo-P0 is used as an electrode, the specific capacity at a current density of 1 C is about 169.8 mAh / g, and the capacitance retention rate is 61.0% when the current density increases to 40 C; in comparison, for the core-shell structured NiCo-LDH@NiCo-P1.0 electrode obtained in Example 3, the electrode material has a large specific surface area, abundant reactive sites, the metalloid characteristics of NiCo-P itself, and a large number of heterostructures between NiCo-LDH and NiCo-P, which greatly improves the electronic conductivity of the material, endows the electrode with ultrafast reaction kinetics and excellent rate performance. In addition, both NiCo-LDH and NiCo-P exhibit reactivity and synergistically participate in the electrode reaction, improving the mass specific capacity and energy density of the electrode. When used as an electrode, the mass specific capacity at a current density of 1 C is about 286.6 mAh / g, close to its theoretical specific capacity of 289 mAh / g; and the capacity retention rate is 72.2% when the current density increases to 40 C, far superior to the electrochemical activity of NiCo-LDH or NiCo-LDH@NiCoP0 as an electrode. In addition, NiCo-LDH@NiCo-P1.0 has good structural stability. Under the condition of cyclic charge and discharge 1150 times at a current density of 5 C, the capacity retention rate is as high as 70.1%.

[0040] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode, characterized in that: The following steps are involved: (1) adding nickel nitrate hexahydrate, cobalt nitrate hexahydrate, sodium citrate, potassium hydroxide and ammonia water to deionized water, reacting under mechanical stirring, cooling to room temperature, removing the supernatant, washing and drying to obtain nickel cobalt hydroxide; (2) placing sodium hypophosphite and the nickel-cobalt hydroxide obtained in step (1) in porcelain boats respectively, and then placing them in a tubular furnace, with the porcelain boat containing sodium hypophosphite placed upstream of the gas, keeping warm in an argon environment, and cooling to room temperature along with the furnace to obtain a nickel-cobalt-phosphorus composite material; (3) uniformly mixing the nickel-cobalt-phosphorus composite material obtained in step (2), polyvinylidene fluoride and carbon black to obtain a slurry; then coating the slurry on a carbon cloth and drying it to obtain a carbon cloth coated with the nickel-cobalt-phosphorus composite material; (4) Using the carbon cloth coated with the nickel-cobalt-phosphorus composite material obtained in step (3) as the working electrode, three-electrode CV electrochemical activation is performed to obtain a core-shell structured nickel-cobalt hydroxide / nickel-cobalt phosphide composite electrode.

2. The method for preparing the core-shell structure nickel cobalt hydroxide / nickel cobalt phosphide composite electrode according to claim 1, characterized in that: In step (1), the molar ratio of nickel nitrate hexahydrate, cobalt nitrate hexahydrate, sodium citrate, potassium hydroxide and ammonia water is 4:1:8:16:

40.

3. The method for preparing the core-shell structure nickel cobalt hydroxide / nickel cobalt phosphide composite electrode according to claim 1, characterized in that: In step (1), the reaction is carried out at 50-60 °C for 10 h under mechanical stirring conditions.

4. The method for preparing the core-shell structure nickel cobalt hydroxide / nickel cobalt phosphide composite electrode according to claim 1, characterized in that: In step (2), the mass ratio of sodium hypophosphite to nickel cobalt hydroxide is 0.5-1.5:

1.

5. The method for preparing the core-shell structure nickel cobalt hydroxide / nickel cobalt phosphide composite electrode according to claim 1, characterized in that: In step (2), in an argon environment, the temperature was raised to 350 °C at 1 °C / min and kept at that temperature for 3 h.

6. The method for preparing the core-shell structure nickel cobalt hydroxide / nickel cobalt phosphide composite electrode according to claim 1, characterized in that: In step (3), the mass ratio of the nickel-cobalt-phosphorus composite material, polyvinylidene fluoride and carbon black is 8:1:

1.

7. The method for preparing the core-shell structure nickel cobalt hydroxide / nickel cobalt phosphide composite electrode according to claim 1, characterized in that: In step (3), the counter electrode is a platinum electrode, the reference electrode is a mercury / mercury oxide electrode, and the electrolyte is a 3 mol / L potassium hydroxide solution.

8. The method for preparing the core-shell structure nickel cobalt hydroxide / nickel cobalt phosphide composite electrode according to claim 1, characterized in that: In step (3), during the three-electrode CV electrochemical activation, the voltage was 0-0.9 V, the scanning rate was 10 mV / s, and the cycle scan was 100 cycles.

9. A core-shell structured nickel cobalt hydroxide / nickel cobalt phosphide composite electrode prepared by the method for preparing a core-shell structured nickel cobalt hydroxide / nickel cobalt phosphide composite electrode according to any one of claims 1 to 8.

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