A boron-containing, low-crystallinity NiCo2O4 electrode material and its preparation method

Abstract

The application discloses a boron-containing low-crystallinity NiCo2O4 electrode material and a preparation method thereof. First, a nickel source, a cobalt source, ammonium fluoride and urea are mixed and dissolved in deionized water to perform a hydrothermal reaction to obtain a precursor M(OH)2; the M(OH)2 is mixed and ground with boric acid at a certain mass ratio, and the mixture is calcined to obtain the boron-containing low-crystallinity NiCo2O4 electrode material; in the preferred scheme, the prepared electrode has an internal resistance as low as 0.49 Ω, a faster ion diffusion rate and an ultra-low charge transfer resistance; when the current density is 1 A·g ‑1 , the specific capacitance can reach 712.5 F·g ‑1 ; after 8000 charge-discharge cycles, the sample can still maintain 98.4% of the initial capacitance; the application has the advantages of simple operation, low material cost, good supercapacitor performance and good application prospect as a supercapacitor electrode material.

Description

Technical Field

[0001] This invention belongs to the field of supercapacitor electrode material technology, specifically involving the preparation of a boron-containing, low-crystallinity NiCo2O4 electrode material by adding boric acid. Background Technology

[0002] The widespread use of fossil fuels has led to increasingly severe environmental pollution problems. To reduce dependence on fossil fuels, my country is vigorously developing new energy vehicles, which in turn places higher demands on energy storage materials. Currently, electric vehicles mainly use lithium-ion batteries as their power source, but these batteries have safety hazards such as low charge / discharge power, limited cycle life, and flammability. Furthermore, as a complex system engineering project, electric vehicles involve high-power applications such as kinetic energy recovery, urgently requiring supercapacitors (SCs) that combine high charge / discharge power, long cycle life, and high safety as supplementary or replacement devices. Compared to traditional capacitors, SCs have a higher specific capacitance and are expected to play a crucial role in the energy storage field by combining them with batteries to build hybrid energy storage systems.

[0003] Binary metal oxides have been widely studied in the field of energy storage due to their enhanced charge storage capacity through bimetallic synergy. Among them, NiCo2O4 stands out due to its low price, environmental friendliness, ease of synthesis, and excellent capacitance performance. However, intrinsic NiCo2O4 electrode materials suffer from poor conductivity and low effective specific surface area, leading to structural instability and low specific capacitance during actual charge and discharge processes. Therefore, advanced NiCo2O4 electrode materials with unique nanostructures and high performance have attracted widespread attention from researchers. Patent CN114604906A discloses "a dual-defect process for constructing molybdenum-doped R-Mo-NiCo2O4 reduced by sodium borohydride, and its preparation method and application," which prepares Mo-NiCo2O4 by Mo doping, and then immerses the Mo-NiCo2O4 in a sodium borohydride solution to prepare R-Mo-NiCo2O4. Patent CN118156059A discloses "A Se-doped NiCo2O4 material and its preparation method and application." First, a NiCo precursor is obtained by reacting hexamethylenetetramine, Co(NO3)2·6H2O, and Ni(NO3)2·6H2O. Then, Na2SeO3 and N2H4·H2O are added to react and obtain the Se-doped NiCo2O4 material. Crystallinity is also a key indicator affecting the performance of electrode materials. Generally, low-crystallinity materials exhibit excellent electrochemical performance (such as high specific capacitance) in SCs due to their abundant grain boundaries and ion diffusion channels; while high-crystallinity materials generally have better structural stability. The literature "Reagent-assisted hydrothermal synthesis of NiCo2O4 nanomaterials as electrodes for high-performance asymmetric supercapacitors," *New Journal of Chemistry*, 2021, 45, 9230, reports a method for preparing highly crystalline NiCo2O4 nanoneedle arrays, with samples exhibiting high specific surface area and electrochemical performance.

[0004] As can be seen from the examples above, there are already some methods for preparing composite materials by transition metal doping or boron doping of NiCo2O4, and there are also studies on controlling the crystallinity of NiCo2O4. However, the transition metals used in these methods are expensive, and sodium borohydride used for boron doping is a hazardous chemical that is easily explosive. The high-crystallinity NiCo2O4 nanoneedle arrays obtained by controlling the morphology in the literature have cumbersome preparation processes and poor cycle stability. This invention aims to solve the above problems by innovatively proposing the addition of boric acid during the calcination of M(OH)2 (M=Ni / Co), thereby obtaining boron-containing NiCo2O4 electrode materials with lower crystallinity. Studies have shown that NiCo2O4 obtained by calcination without the addition of boric acid is prone to severe agglomeration, leading to a significant reduction in specific surface area and poor specific capacitance performance. The method of this invention only requires the simple addition of boric acid during calcination to effectively control the crystallinity of NiCo2O4, inhibit agglomeration, and significantly improve material performance. Compared with methods such as adding other materials for composites or elemental doping, this invention has a simple process, convenient operation, and low cost, and has good development prospects.

[0005] According to research, those skilled in the art generally believe that higher crystallinity is beneficial to the structural stability of materials, and therefore tend to promote grain growth through high temperatures or the addition of mineralizers. Thus, while boric acid is used as a dopant source in existing technologies, it is mostly used to improve crystallinity or form specific crystalline phases, such as the boric acid coating modification of LiNiO2. This invention, however, does the opposite, utilizing boric acid to inhibit grain growth, obtaining a low-crystallinity material, while simultaneously achieving superior electrochemical performance. Currently, there are no reports on preparing boron-containing low-crystallinity NiCo2O4 materials by adding boric acid during calcination. Summary of the Invention

[0006] This invention proposes a method for preparing boron-containing, low-crystallinity NiCo2O4 electrode materials by adding boric acid during the calcination of M(OH)2 (M=Ni / Co). The addition of boric acid significantly reduces the crystallinity of the material and inhibits agglomeration. During calcination, the B2O3 or borate produced by the decomposition of boric acid interacts with the surface of NiCo2O4 to form an amorphous interface layer, hindering further grain growth and agglomeration. This allows the material to maintain low crystallinity and a high specific surface area, thereby increasing the ion diffusion channels of the NiCo2O4 electrode material and ultimately achieving a simultaneous improvement in the specific capacitance and cycle stability of the electrode material.

[0007] The present invention discloses a method for preparing a boron-containing, low-crystallinity NiCo2O4 electrode material, comprising the following steps: (1) Preparation of M(OH)2 (M=Ni / Co) Weigh 1-2 mmol of nickel source, 2-4 mmol of cobalt source, 1-3 mmol of NH4F, and 4-8 mmol of urea and dissolve them in 30-50 mL of deionized water to obtain solution A. Stir solution A for 15-30 min and then transfer it to a reaction vessel. Incubate the reaction vessel at 120-130℃ for 4-6 h. After the reaction is complete, wash the reactants three times with deionized water and once with anhydrous ethanol by centrifugation to obtain the sample. Place the sample in a vacuum oven at 50-60℃ and dry it for 10-12 h to obtain the precursor M(OH)2.

[0008] (2) Preparation of NiCo2O4 electrode material The precursor M(OH)₂ and boric acid were mixed and ground at a certain mass ratio. The ground mixture was then placed in a quartz boat and placed in a muffle furnace at 5 °C·min. -1 The temperature was increased at a rate of 350-500℃ and held at that temperature for 2-4 h. After calcination, the product was washed three times alternately with hot water at 60-80℃ and anhydrous ethanol by vacuum filtration. After washing, the product was placed in a vacuum oven at 50-60℃ for 10-12 h to obtain boron-containing low-crystallinity NiCo2O4 electrode material.

[0009] Steps for testing electrochemical properties: The prepared boron-containing low-crystallinity NiCo2O4 electrode material was used as the positive electrode material of a supercapacitor, and its capacitance performance was tested using 6 MKOH as the electrolyte solution. Experiments included cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) plots, and electrochemical impedance spectroscopy (EIS). Using a Hg / HgO electrode as the reference electrode, a boron-containing low-crystallinity NiCo2O4 electrode material coated on nickel foam as the working electrode, and a platinum wire electrode as the counter electrode, the electrochemical performance of the electrode materials was tested on an electrochemical workstation. The nickel source of the reactant is one of nickel acetate tetrahydrate, nickel nitrate hexahydrate, and nickel chloride hexahydrate; The cobalt source of the reactant is one of cobalt acetate tetrahydrate, cobalt nitrate hexahydrate, and cobalt chloride hexahydrate. The reaction vessel is a 100 mL polytetrafluoroethylene high-pressure reactor; The mass ratio of M(OH)₂ to boric acid is M(OH)₂ : boric acid = 1:0.1 to 1:1.5, preferably 1:0.5. The electrochemical workstation mentioned is the Shanghai Chenhua Electrochemical Workstation (CHI600E). Attached Figure Description Figure 1 This is a scanning electron microscope (SEM) image of the boron-containing low-crystallinity NiCo2O4 electrode material prepared in Example 1. Figure 2 and Figure 3This is a transmission electron microscope (TEM) image of the boron-containing low-crystallinity NiCo2O4 electrode material prepared in Example 1. Figure 4 The X-ray powder diffraction (XRD) patterns of the samples from Examples 1-4 and M(OH)2 (M=Ni / Co) are shown. Figure 5 The total X-ray photoelectron spectroscopy (XPS) spectrum of the boron-containing low-crystallinity NiCo2O4 electrode material prepared in Example 1 is shown. Figure 6 The NiCo2O4 electrode materials prepared in Examples 1-4 were scanned at a rate of 10 mV·s. -1 Comparison of cyclic voltammetry (CV) curves for time-varying tests; Figure 7 Cyclic voltammetry (CV) curves of the boron-containing low-crystallinity NiCo2O4 electrode material prepared in Example 1 at different scan rates. Figure 8 The NiCo2O4 electrode materials prepared in Examples 1-4 were used at a current density of 1 A·g -1 A comparison of constant current charge-discharge (GCD) curves tested over time; Figure 9 The galvanostatic charge-discharge (GCD) curves of the boron-containing low-crystallinity NiCo2O4 electrode material prepared in Example 1 were tested at different current densities. Figure 10 This is a rate performance diagram of the boron-containing low-crystallinity NiCo2O4 electrode material prepared in Example 1; Figure 11 The electrochemical impedance spectroscopy (EIS) spectrum of the boron-containing low-crystallinity NiCo2O4 electrode material prepared in Example 1 is shown below. Figure 12 The diagram shows the cyclic stability of the NiCo2O4 electrode materials prepared in Examples 1-4. Detailed Implementation The present invention will be specifically described below with reference to the embodiments, which include the preferred preparation method of the present invention, including reagent amounts and reaction conditions. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Example 1: Preparation method of boron-containing low-crystallinity NiCo2O4 electrode material with added boric acid (addition ratio 1:0.5) (preferred scheme): Weigh out 1.5 mmol Ni(NO3)2·6H2O, 3 mmol Co(NO3)2·6H2O, 2 mmol NH4F, and 6 mmol urea, and dissolve them in 40 mL of deionized water to obtain solution A. Stir solution A for 30 min and then transfer it to a reaction vessel. Incubate the reaction vessel at 130 °C for 5 h. After the reaction is complete, wash the reactants three times with deionized water and once with anhydrous ethanol by centrifugation to obtain the sample. Place the sample in a vacuum oven and dry it at 60 °C for 12 h to obtain the precursor M(OH)2. M(OH)₂ and boric acid were mixed and ground at a mass ratio of 1:0.5. The ground mixture was then placed in a quartz boat, which was placed in a muffle furnace at 5 °C·min. -1 The temperature was increased rapidly and held at 400 °C for 3 h. After calcination, the product was washed three times alternately with hot water at 80 °C and anhydrous ethanol by vacuum filtration. After washing, it was dried in a vacuum oven at 60 °C for 12 h to obtain boron-containing low-crystallinity NiCo2O4 electrode material.

[0010] The morphology of the final product, the boron-containing low-crystallinity NiCo2O4 electrode material obtained in Example 1, was characterized using a Japanese S-4800 field emission scanning electron microscope (SEM) and a Japanese JEM-2100 transmission electron microscope (TEM), respectively. The phase of the final product in Example 1 was characterized using a Japanese SmartLab 9 KW X-ray diffractometer (XRD), and the elemental composition of the final product in Example 1 was characterized using a British VGESCA X-ray photoelectron spectroscopy (XPS).

[0011] from Figure 1 It can be seen that M(OH)2, after calcination, exhibits a layered structure composed of interwoven fine rods, which is... Figure 2 and Figure 3 The TEM images clearly show that the addition of boric acid makes the lattice fringes and boundaries of the NiCo2O4 microspheres indistinct, indicating a significant decrease in crystallinity. Figure 4 The XRD results show that its composition is NiCo2O4 (JCPDS card number No. 20-0781). The comparison shows that the crystallinity of Examples 1, 2, and 3 after adding boric acid and calcining is significantly reduced compared to Example 4. Figure 5 The XPS full spectrum scan of Example 1 shows that the sample contains Ni, Co, O, C and B elements. The carbon element signal comes from the test results using carbon as a standard reference.

[0012] Example 2: Preparation method of boron-containing low-crystallinity NiCo2O4 electrode material with added boric acid (addition ratio 1:0.25): 1.5 mmol Ni(NO3)2·6H2O, 3 mmol Co(NO3)2·6H2O, 2 mmol NH4F, and 6 mmol urea were weighed and dissolved in 40 mL of deionized water to obtain solution A. Solution A was stirred for 30 min and then transferred to a reaction vessel, which was kept at 130 °C for 5 h. After the reaction was complete, the reactants were washed three times with deionized water and once with anhydrous ethanol by centrifugation to obtain the sample. The sample was then dried in a vacuum oven at 60 °C for 12 h to obtain the precursor M(OH)2. M(OH)₂ and boric acid were mixed and ground at a mass ratio of 1:0.25. The ground mixture was then placed in a quartz boat, which was placed in a muffle furnace at 5 °C·min. -1 The temperature was increased rapidly and held at 400℃ for 3 h. After calcination, the product was washed three times alternately with hot water at 80℃ and anhydrous ethanol by vacuum filtration. After washing, it was placed in a vacuum oven and dried at 60℃ for 12 h to obtain a low-doped boron-containing NiCo2O4 electrode material with added boric acid.

[0013] Example 3: Preparation method of boron-containing low-crystallinity NiCo2O4 electrode material with added boric acid (addition ratio 1:1): Weigh out 1.5 mmol Ni(NO3)2·6H2O, 3 mmol Co(NO3)2·6H2O, 2 mmol NH4F, and 6 mmol urea, and dissolve them in 40 mL of deionized water to obtain solution A. Stir solution A for 30 min and then transfer it to a reaction vessel. Incubate the reaction vessel at 130 °C for 5 h. After the reaction is complete, wash the reactants three times with deionized water and once with anhydrous ethanol by centrifugation to obtain the sample. Place the sample in a vacuum oven and dry it at 60 °C for 12 h to obtain the precursor M(OH)2. M(OH)₂ and boric acid were mixed and ground at a mass ratio of 1:1. The ground mixture was then placed in a quartz boat and placed in a muffle furnace at 5 °C·min. -1 The temperature was increased rapidly and held at 400℃ for 3 h. After calcination, the product was washed three times alternately with hot water at 80℃ and anhydrous ethanol by vacuum filtration. After washing, it was placed in a vacuum oven and dried at 60℃ for 12 h to obtain a boron-doped NiCo2O4 electrode material with added boric acid.

[0014] Example 4: Preparation method of NiCo2O4 electrode material without added boric acid: Weigh out 1.5 mmol Ni(NO3)2·6H2O, 3 mmol Co(NO3)2·6H2O, 2 mmol NH4F, and 6 mmol urea, and dissolve them in 40 mL of deionized water to obtain solution A. Stir solution A for 30 min and then transfer it to a reaction vessel. Incubate the reaction vessel at 130 °C for 5 h. After the reaction is complete, wash the reactants three times with deionized water and once with anhydrous ethanol by centrifugation to obtain the sample. Place the sample in a vacuum oven and dry it at 60 °C for 12 h to obtain the precursor M(OH)2. Transfer M(OH)2 into a quartz boat, and place the quartz boat in a muffle furnace at 5 °C·min. -1 The temperature was increased rapidly and held at 400℃ for 3 h. After calcination, the product was washed three times alternately with hot water at 80℃ and anhydrous ethanol by vacuum filtration. After washing, it was dried in a vacuum oven at 60℃ for 12 h to obtain NiCo2O4 electrode material without added boric acid.

[0015] Example 5: Electrochemical property testing of the electrode materials prepared in Examples 1-4: The electrode materials prepared in Examples 1-4 were coated onto nickel foam, dried, and the electrolyte was 6 M KOH. The coated nickel foam samples were clamped with electrode clips, connected to an electrochemical workstation, and their electrochemical properties were tested. The specific steps are as follows:

[0016] (I) Washing the foamed nickel: Cut the foamed nickel into a size of 1 cm × 1 cm for testing and place it in a beaker. Add acetone and deionized water to just cover the foamed nickel. Place it in an ultrasonic cleaner and ultrasonically clean for 15 min. Then add 8% dilute hydrochloric acid and ultrasonically clean for 25 min. Then rinse with deionized water several times until the pH of the cleaning solution is neutral. Finally, add anhydrous ethanol and ultrasonically clean for 30 min. Place the beaker containing the cleaned foamed nickel in a vacuum drying oven at 65℃ and take it out for use after 24 h.

[0017] (II) Sample Coating: Weigh 8 mg of the electrode material prepared in Examples 1-4, 1 mg of acetylene black, and 1 mg of polyvinylidene fluoride into a mortar, add 2-3 drops of N-methylpyrrolidone as a solvent, grind until a uniform paste is formed, and coat it onto a cleaned nickel foam with a size of 1 cm × 1 cm. The calculated mass of the active material coated on the nickel foam is approximately 2.5 mg. Place the coated nickel foam in a vacuum drying oven at 65°C and dry for 24 h. Take out the dried nickel foam and press it into a tablet using a tablet press to ensure that the sample adheres tightly to the nickel foam. Subtract the uncoated nickel foam from the nickel foam tableted with the coated sample to further calculate the mass of the electrode material actually used for testing.

[0018] (III) Electrochemical Properties Testing: Nickel foam coated with electrode material was used as the working electrode, platinum wire as the counter electrode, and an Hg / HgO electrode as the reference electrode. These electrodes were placed in a 6 M potassium hydroxide solution using a three-hole electrode holder. The electrochemical properties were tested using a Shanghai Chenhua electrochemical workstation (CHI600E) in Cyclic Voltammetry mode. Figure 6 The electrode materials used in Examples 1-4 were scanned at a rate of 10 mV·s. -1 The CV curves at different times are shown. It can be seen that the electrode material in Example 1 has the highest integral area of ​​the CV curve, indicating that it has a stronger energy storage capacity; Figure 7 The voltage scan rates measured for the electrode material in Example 1 within the voltage range of 0 to 0.6 V were 10, 20, 30, 50, 80, and 100 mV·s. -1 The cyclic voltammetry (CV) curves at different times show that the shape of the CV curves remains good as the scan speed increases, indicating that the electrode material has good capacitance performance. Figure 8 To test the electrode materials of Examples 1-4 in Chronopotentiometry mode at a current density of 1 A·g -1 The GCD curves at different times are shown in Table 1. It can be seen that the discharge times of the electrode materials in Examples 1-4 are 354.8 s, 275.9 s, 349.2 s, and 114.5 s, respectively. The calculated specific capacitance of the electrode materials in Examples 1-4 is 712.5 F·g. -1 554.8 F·g -1 701.3 F·g -1 With 231.1 F·g -1 Among them, the electrode material of Example 1 has the longest discharge time and the largest specific capacitance. Figure 9 The electrode material in Example 1 was subjected to current densities of 1, 2, 3, 5, 10, 15, and 20 A·g. -1 The constant current charge-discharge diagram (GCD) at different current densities can be obtained by calculating the specific capacitance at different current densities. Figure 10 ; Figure 10 This is a rate performance diagram of the electrode material in Example 1, showing the rate performance when the current density increases from 1 A·g. -1 Increased to 20 A·g -1 At that time, the capacitance retention rate of the electrode material reached as high as 92.0%, demonstrating excellent rate performance; Figure 11 The electrochemical impedance spectroscopy (EIS) of the electrode material in Example 1 was measured in ACImpedance mode. The impedance spectrum shows that the prepared product has a large slope and an internal resistance of 0.49 Ω. It has a fast ion diffusion rate and ultra-low charge transfer resistance in the electrochemical process, indicating that it has high capacitive behavior. Figure 12Table 1 shows the cycling performance of the electrode materials in Examples 1-4. As can be seen from Table 1, after 8000 charge-discharge cycles, the capacitance retention rates of the electrode materials in Examples 1-4 compared to the initial capacitance were 98.4%, 95.5%, 82.0%, and 91.0%, respectively. Among them, the electrode material in Example 1 had the best cycling stability and could still maintain an excellent capacitance value after long-term cycling.

[0019] Table 1: Comparison of Electrochemical Performance Example 1 (Preferred Solution) 354.8 712.5 98.4 Example 2 275.9 554.8 95.5 Example 3 349.2 701.3 82.0 Example 4 114.5 231.1 91.0

Claims

1. A boron-containing, low-crystallinity NiCo2O4 electrode material and its preparation method, characterized in that: Weigh 1-2 mmol of nickel source, 2-4 mmol of cobalt source, 1-3 mmol of NH4F, and 4-8 mmol of urea and dissolve them in 30-50 mL of deionized water to obtain solution A. Stir solution A for 15-30 min and then transfer it to a reaction vessel. Incubate the reaction vessel at 120-130℃ for 4-6 h. After the reaction is complete, wash the reactants three times with deionized water and once with anhydrous ethanol by centrifugation to obtain the sample. Place the sample in a vacuum oven at 50-60℃ for 10-12 h to obtain the precursor M(OH)2 (M=Ni / Co). Mix M(OH)2 with boric acid at a certain mass ratio and grind it. Place the ground mixture in a quartz boat and place the quartz boat in a muffle furnace at 5℃·min. -1 The temperature was increased at a rate of 350-500℃ and held at 350-500℃ for 2-4 h. After calcination, the product was washed three times alternately with hot water at 60-80℃ and anhydrous ethanol by vacuum filtration. After washing, it was placed in a vacuum oven and dried at 50-60℃ for 10-12 h to obtain boron-containing low crystallinity NiCo2O4 electrode material.

2. The method for preparing a boron-containing low-crystallinity NiCo2O4 electrode material as described in claim 1, characterized in that: The nickel source of the reactant is one of nickel acetate tetrahydrate, nickel nitrate hexahydrate, and nickel chloride hexahydrate; The cobalt source of the reactant is one of cobalt acetate tetrahydrate, cobalt nitrate hexahydrate, and cobalt chloride hexahydrate. The reaction vessel is a 100 mL polytetrafluoroethylene high-pressure reactor; The mass ratio of M(OH)2 to boric acid is M(OH)2 : boric acid = 1:0.1~1:1.

5.

3. The preparation method according to claim 2, characterized in that, The preferred mass ratio of the precursor M(OH)2 to boric acid is 1:0.

5.

4. The preparation method according to claim 1 or 2, characterized in that, Includes the following steps: Weigh 1.5 mmol Ni(NO3)2·6H2O, 3 mmol Co(NO3)2·6H2O, 2 mmol NH4F, and 6 mmol urea and dissolve them in 40 mL of deionized water to obtain solution A. Stir solution A for 30 min and then transfer it to a reaction vessel. The reaction vessel is kept at 130℃ for 5 h. After the reaction is complete, the reactants are washed three times with deionized water and once with anhydrous ethanol by centrifugation to obtain the sample. The sample is then dried in a vacuum oven at 60℃ for 12 h to obtain the precursor M(OH)2. M(OH)2 and boric acid are mixed and ground at a mass ratio of 1:0.

5. The ground mixture is placed in a quartz boat and then placed in a muffle furnace at 5℃·min. -1 The temperature was increased at a rate and held at 400℃ for 3 h. After calcination, the product was washed three times alternately with hot water at 80℃ and anhydrous ethanol by vacuum filtration. After washing, it was placed in a vacuum oven and dried at 60℃ for 12 h to obtain boron-containing low crystallinity NiCo2O4 electrode material.

5. A boron-containing, low-crystallinity NiCo2O4 electrode material, characterized in that, It is prepared by any one of claims 1 to 4.

6. The boron-containing low-crystallinity NiCo2O4 electrode material according to claim 5, characterized in that, At a current density of 1 A·g -1 At that time, the specific capacitance is not less than 700 F·g -1 After 8,000 charge-discharge cycles, the capacitance retention rate is no less than 95%.

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