Cobalt-iron-sodium cyanide positive electrode material and application thereof

Abstract

The application provides a cobalt-iron sodium cyanide positive electrode material and application thereof. The positive electrode material is prepared by the following steps: 1, carboxylating carbon nanotubes; 2, in an ethanol solvent, carrying out a solvothermal reaction on cobalt acetate tetrahydrate, urea and the carboxylated carbon nanotubes according to a mass ratio of 1:(0.5-5):(0.005-0.05) to obtain a cobalt source precursor; 3, carrying out a coprecipitation reaction on a sodium source, the cobalt source precursor, sodium ferrocyanide and a surfactant according to a mass ratio of (1-10):1:(0.5-5):(0.5-4), and aging the reaction solution to obtain the cobalt-iron sodium cyanide positive electrode material. The cobalt-iron sodium cyanide positive electrode material of the application has excellent conductivity, excellent rate performance and cycle stability when applied to a water-based sodium ion battery.

Description

Technical Field

[0001] This invention relates to a sodium cobalt ferrocyanide cathode material and its application, particularly to a sodium cobalt ferrocyanide cathode material and its application in aqueous sodium-ion batteries, belonging to the field of battery electrode preparation. Background Technology

[0002] Lithium-ion batteries, with their high energy density and long cycle life, are widely used in all aspects of society. However, lithium resources are unevenly distributed globally and their total amount is relatively small, leading to a continuous increase in the cost of lithium-ion batteries. Sodium resources, on the other hand, have reserves thousands of times greater than lithium resources, and sodium-ion batteries are compatible with existing lithium-ion battery production equipment, significantly reducing manufacturing costs. Notably, this cost advantage will be further amplified in large-scale energy storage and some low-speed transportation applications. However, compared to lithium ions, sodium ions have a larger radius, making their insertion and extraction processes between electrode materials relatively difficult, thus limiting the energy density of the battery.

[0003] In 2022, M. Zain Bin Amjad synthesized a composite material of perovskite-structured NaNiF3 and multi-walled carbon nanotubes (MWCNTs) and successfully applied it to aqueous sodium-ion batteries. (Amjad, MZB, Iqbal, N., Ali, G. et al. Synthesis of NaNiF3 and its composite with multi-walled carbon nanotubes as cathode materials for aqueous sodium-ion battery. J Mater Sci: Mater Electron 33, 16987–17000 (2022).). Although the introduction of carbon nanotubes improved the electrochemical performance of the composite material, the actual specific capacity was still not ideal due to the single-electron redox reaction of NaNiF3. Furthermore, the relatively large particle size of NaNiF3 in the composite material also reduced its rate performance.

[0004] Aqueous sodium-ion batteries (ASIBs) have attracted widespread research and attention due to their advantages such as safety, environmental friendliness, and low cost. Currently, the cathode materials for ASIBs mainly fall into three categories: layered transition metal oxides, polyanionic compounds, and Prussian blue analogues. Among them, sodium cobalt iron cyanide (SOC) has long been considered one of the most promising cathode materials for ASIBs due to its open three-dimensional framework structure, which provides rapid ion diffusion channels. However, the presence of numerous vacancies and poor electronic conductivity in the crystal severely affects the electrochemical performance of SOC. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a sodium cobalt iron cyanide cathode material, its preparation method, and an aqueous sodium-ion battery. This sodium cobalt iron cyanide cathode material exhibits high specific surface area, good conductivity, and strong structural stability. The aqueous sodium-ion battery assembled from this material demonstrates excellent rate performance and cycle stability, meeting the requirements of practical applications.

[0006] The present invention solves the above-mentioned technical problems through the following technical solutions.

[0007] A sodium cobalt ferrocyanide cathode material, characterized in that the cathode material comprises carbon nanotubes and hollow sodium cobalt ferrocyanide prisms, wherein the carbon nanotubes are intercalated within the hollow sodium cobalt ferrocyanide prisms, and the mass ratio of the carbon nanotubes to the hollow sodium cobalt ferrocyanide prisms is 1:(1-100); the cathode material is prepared by the following steps:

[0008] S1. Carboxylation treatment of carbon nanotubes: according to Prepare a mixed solution of HNO3 and H2SO4 by volume ratio, add CNTs to it, reflux and condense, wash with deionized water, centrifuge, and freeze dry under vacuum to obtain carboxylated CNTs;

[0009] S2. Preparation of cobalt source precursor: Cobalt acetate tetrahydrate, urea, and carboxylated carbon nanotubes are reacted with an ethanol solvent under solvothermal conditions, washed with anhydrous ethanol, and centrifuged and dried to obtain the cobalt source precursor; wherein, the mass ratio of cobalt acetate tetrahydrate, urea, and carboxylated carbon nanotubes is 1:(0.5-5):(0.005-0.05), and the concentration of carbon nanotubes in the ethanol solvent is 0.01-1 g / L;

[0010] S3. Preparation of sodium cobalt ferrocyanide cathode material: The solid obtained by co-precipitation reaction of sodium source, cobalt source precursor, sodium ferrocyanide, surfactant, ethanol and water is aged, washed with deionized water and anhydrous ethanol, and centrifuged and dried to obtain sodium cobalt ferrocyanide cathode material; wherein, the mass ratio of sodium source, cobalt source precursor, sodium ferrocyanide and surfactant is (1-10):1:(0.5-5):(0.5-4), the volume ratio of ethanol and water is 1:(0.5-10), and the concentration of cobalt source precursor in ethanol solvent is 5-50 g / L, and the concentration of sodium source, sodium ferrocyanide and surfactant in aqueous solvent is 5-50 g / L.

[0011] In step S1, the carbon nanotube has a diameter of 3-15 nm and a length of 15-30 μm.

[0012] In step S3, the sodium source is one or more selected from sodium chloride, sodium sulfate, sodium carbonate, sodium bicarbonate and sodium dihydrogen phosphate; the surfactant is one or more selected from polyvinylpyrrolidone, sodium dodecylbenzene sulfonate and sodium dodecyl sulfate.

[0013] In step S2, the temperature of the solvothermal reaction is preferably 40-100℃, the reaction time is 6-48h, the drying temperature is preferably 60-80℃, and the drying time is 6-12h.

[0014] In step S3, the mass ratio of the sodium source, cobalt source precursor, sodium ferrocyanide, and surfactant is (1-9):1:(0.5-4.5):(0.5-3.5), the coprecipitation reaction temperature is 0-40℃, the coprecipitation reaction aging time is 6-48h, the drying temperature is 60-80℃, and the drying time is 6-12h.

[0015] In the aforementioned sodium cobalt iron cyanide cathode material, the term "penetration" refers to the carbon nanotubes passing through one or more faces of a hollow sodium cobalt iron cyanide prism, the carbon nanotubes passing through at least two hollow sodium cobalt iron cyanide prisms, and each hollow sodium cobalt iron cyanide prism having at least one carbon nanotube.

[0016] The diameter of the sodium cobalt ferrocyanide hollow prism is preferably 200-500 nm, and the length of the sodium cobalt ferrocyanide hollow prism is preferably 0.5-2 μm.

[0017] The present invention also provides an application of sodium cobalt iron cyanide cathode material, characterized in that the material is used as a cathode material for aqueous sodium-ion batteries.

[0018] The positive and progressive effects of this invention are as follows:

[0019] In the prior art, sodium cobalt ferrocyanide is mainly prepared by rapid co-precipitation. Due to the excessively fast reaction rate, its morphology is difficult to control, resulting in poor crystal structure stability. In this invention, cobalt ions are first converted into hollow prismatic cobalt precursors and then combined with carbon nanotubes in an appropriate ratio. The precursors are then converted in a mixed solvent of ethanol and water to obtain sodium cobalt ferrocyanide with regular morphology, stable structure, and good conductivity.

[0020] The sodium cobalt iron cyanide cathode material of this invention has advantages such as large specific surface area, high structural stability, and good conductivity. Aqueous sodium-ion batteries prepared using the sodium cobalt iron cyanide cathode material provided by this invention exhibit excellent rate performance and cycle stability, and have certain application potential. Attached Figure Description

[0021] Figure 1 This is a scanning electron microscope image of the sodium cobalt iron cyanide cathode material prepared in Example 3;

[0022] Figure 2 Scanning electron microscope image of the sodium cobalt iron cyanide cathode material prepared in Comparative Example 3.

[0023] Figure 3 This is a scanning electron microscope image of the cobalt source precursor prepared in Example 3;

[0024] Figure 4 This is a transmission electron microscope (TEM) image of the sodium cobalt iron cyanide cathode material prepared in Example 3;

[0025] Figure 5 The cyclic voltammetry curves of the sodium cobalt iron cyanide cathode material prepared in Example 3 are shown below.

[0026] Figure 6 The graph shows a comparison of the rate performance of the samples prepared in Examples 1, 2, and 3 and Comparative Examples 1, 2, and 3.

[0027] Figure 7 A comparison chart of the cycling performance of samples prepared in Example 3 and Comparative Example 1. Detailed Implementation

[0028] The present invention is further illustrated below by way of examples, but these examples do not limit the invention to the scope of the embodiments described. Experimental methods not specifically described in the following examples were performed according to conventional methods and conditions, or as selected according to the product instructions. All reagents and raw materials used in this invention are commercially available.

[0029] Example 1

[0030] This embodiment prepares the sodium cobalt ferrocyanide cathode material according to the following steps:

[0031] Step 1 according to Prepare a 160 mL mixed solution of HNO3 and H2SO4 by volume ratio, and then accurately weigh 1 g of CNTs into it. The solution is then transferred to a flask and refluxed at 80 °C for 3 h. After the suspension has cooled to room temperature, it is washed with deionized water and centrifuged until the pH of the supernatant in the centrifuge tube is 7. Finally, it is freeze-dried under vacuum for 12 h to obtain the processed CNTs.

[0032] Step 2: Accurately weigh 25 mg of carboxylated CNTs and add them to 100 mL of ethanol, then sonicate for 30 min. Next, add 1 g of cobalt acetate tetrahydrate (C4H6CoO4·4H2O) and continue sonicating for another 30 min. Then, add 1.49 g of urea (CH4N2O), stir until completely dissolved, and continue sonicating for another 30 min. Finally, transfer the solution to a flask and stir in an oil bath at 65 °C for 24 h. After the solution cools naturally to room temperature, wash with anhydrous ethanol and centrifuge several times. Finally, transfer the obtained solid to a vacuum oven at 60 °C and dry overnight to obtain the cobalt source precursor.

[0033] Step 3: Dissolve 60 mg of cobalt source precursor in 60 mL of ethanol and sonicate for 30 min until uniformly dispersed, labeling this solution A. Dissolve 60 mg of Na₄[Fe(CN)₆], 60 mg of NaCl, and 60 mg of polyvinylpyrrolidone (PVP) in 60 mL of deionized water, labeling this solution B. Quickly pour solution A into solution B, stir continuously for 6 h, and then age at 25 °C for 24 h. After the reaction is complete, discard the supernatant, and wash several times with water and anhydrous ethanol by centrifugation. Transfer the solid to an 80 °C vacuum oven and dry continuously for 12 h to obtain sodium cobalt iron cyanide cathode material.

[0034] Example 2

[0035] In the preparation method of sodium cobalt iron cyanide cathode material provided in this embodiment, in step 2, the mixture is stirred at a constant temperature in an oil bath at 65°C for 4 hours. Other steps and conditions are the same as in Example 1.

[0036] Example 3

[0037] In the preparation method of sodium cobalt ferrocyanide cathode material provided in this embodiment, 10 mg of carboxylated CNTs are added in step 2, and the other steps and conditions are the same as in Example 1.

[0038] Step 4 uses sodium cobalt iron cyanide as the positive electrode, carbon-coated sodium titanium phosphate as the negative electrode, 1M Na2SO4 as the electrolyte, and a CR2016 button cell casing to assemble an aqueous sodium-ion full cell.

[0039] Comparative Example 1

[0040] In the preparation method of sodium cobalt ferrocyanide cathode material provided in this comparative example, carboxylated CNTs are not added in step 2, and the other steps and conditions are the same as in Example 1.

[0041] Comparative Example 2

[0042] In the preparation method of sodium cobalt ferrocyanide cathode material provided in this comparative example, the standing temperature in step 3 is 0℃, and the other steps and conditions are the same as those in comparative example 1.

[0043] Comparative Example 3

[0044] In the preparation method of sodium cobalt iron cyanide cathode material provided in this embodiment, 50 mg of uncarboxylated CNTs are added in step 2, and the other steps and conditions are the same as in Example 1.

[0045] Effect Example

[0046] 1. Morphological and structural characterization

[0047] Figure 1This is a scanning electron microscope (SEM) image of the hollow sodium cobalt iron cyanide prism prepared in Example 3. Figure 1 As can be seen, the sodium cobalt ferrocyanide prepared in Example 3 is a hollow prism with a large number of regularly distributed particles on its surface. Simultaneously, carbon nanotubes pass through the hollow sodium cobalt ferrocyanide prism, and some carbon nanotubes surround the outside of the hollow sodium cobalt ferrocyanide prism, forming an interconnected conductive network. The hollow sodium cobalt ferrocyanide prism is approximately 1 μm long and 300 nm in diameter. The carbon nanotubes are approximately 20 μm long and 5 nm in diameter. In Example 3, the reaction was carried out in an oil bath for 24 hours. Meanwhile, when the mass ratio of sodium source, cobalt source, sodium ferrocyanide, and surfactant was 1:1:1:1, and the volume ratio of ethanol to water was 1:1, the solvent system had low polarity, resulting in a more complete reaction and better maintenance of a uniform morphology.

[0048] Figure 2 The image shows a scanning electron microscope (SEM) image of the sodium cobalt iron cyanide cathode material prepared in Comparative Example 3. Compared with Example 3, it can be observed that the uncarboxylated carbon nanotubes exhibit severe aggregation and have poor composite effect with the hollow sodium cobalt iron cyanide prisms.

[0049] Figure 3 This is a scanning electron microscope image of the cobalt source precursor prepared in Example 3. Figure 3 It is known that carbon nanotubes connect the cobalt source precursor prisms in series to form a highly efficient conductive network, which greatly improves the conductivity of the material.

[0050] Figure 4 The image shown is a transmission electron microscope (TEM) image of the sodium cobalt iron cyanide cathode material prepared in Example 3. Figure 4 The transmission electron microscopy (TEM) images clearly demonstrate that the sodium cobalt ferrocyanide prepared in Example 3 is a hollow prism with a large number of regularly distributed particles on its surface. Simultaneously, carbon nanotubes penetrate the hollow prisms of the sodium cobalt ferrocyanide, and some carbon nanotubes surround the outside of the hollow prisms, forming an interconnected conductive network.

[0051] 2. Specific surface area test:

[0052] The specific surface area of ​​the materials was tested using a specific surface area analyzer. The specific surface areas of the sodium cobalt iron cyanide cathode materials prepared in the above examples and comparative examples are shown in Table 1.

[0053] Table 1

[0054] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 <![CDATA[Specific surface area / (m 2 / g)]]> 57.9 50.8 59.1 26.4 27.9

[0055] The specific surface area data from the examples and comparative examples show that the addition of carboxylated carbon nanotubes increases the specific surface area of ​​the sodium cobalt iron cyanide cathode material, providing more reactive sites. This is mainly attributed to two factors: firstly, the carboxylated carbon nanotubes themselves have a large specific surface area; and secondly, the addition of carboxylated carbon nanotubes restricts crystal growth and reduces particle size.

[0056] 3. Cyclic Voltmeter-Ammeter Test

[0057] Using the sodium cobalt ferrocyanide cathode material prepared in Example 3 above as the working electrode, a platinum sheet (Pt) as the counter electrode, and an Ag / AgCl electrode as the reference electrode, cyclic voltammetry was performed on a CHI660E electrochemical workstation. The test results are as follows: Figure 5 As shown.

[0058] 4. Ratio Performance Test

[0059] Rate performance was tested using a three-electrode system with the positive electrode material as the working electrode, a Pt sheet as the counter electrode, Ag / AgCl as the reference electrode, and 1M Na2SO4 solution as the electrolyte. Tests were conducted at different current densities using a Wuhan Landian CT2001A electrochemical workstation, with a test voltage range of 0–1V. The test results are shown in Table 2. Figure 6 As shown.

[0060] Table 2

[0061]

[0062] The rate performance data from the examples and comparative examples show that the rate performance of the sodium cobalt ferrocyanide cathode with added carbon nanotubes is significantly better than that of the comparative sample at different current densities. Comparing the data from different examples reveals that the rate performance of the sodium cobalt ferrocyanide cathode is related to factors such as the carbon nanotube pretreatment process, the amount added, and the precursor reaction time. The rate performance of Examples 1 and 3 is not significantly different, but excessive addition of carboxylated carbon nanotubes reduces the reversible specific capacity. Example 2 exhibits the worst rate performance among the three examples because the precursor reaction time is too short, resulting in incomplete reaction. The low reversible capacity of Comparative Example 3 at different current densities is mainly attributed to the poor composite effect of the uncarboxylated carbon nanotubes, resulting in minimal improvement in overall conductivity.

[0063] 5. Cyclic stability test

[0064] The cycle stability test used the same three-electrode system as the rate performance test, employing a Wuhan Landian CT2001A electrochemical workstation at 200 mA g. -1 The tests were conducted at current density, with a test voltage range of 0–1 V. The cycle performance of the sodium cobalt iron cyanide cathode materials prepared in Example 3 and Comparative Example 1 is as follows: Figure 7 As shown in the figure, the sample of Example 3 had a higher specific capacity than Comparative Example 1 after 400 charge-discharge cycles, indicating that the addition of carbon nanotubes improved the cycle stability of the sodium cobalt iron cyanide cathode material.

Claims

1. A sodium cobalt iron cyanide cathode material, characterized in that, The positive electrode material comprises carbon nanotubes and hollow sodium cobalt ferrocyanide prisms, with the carbon nanotubes intercalated within the hollow sodium cobalt ferrocyanide prisms. The mass ratio of the carbon nanotubes to the hollow sodium cobalt ferrocyanide prisms is 1:(1-100). The positive electrode material is prepared through the following steps: S1. Carboxylation of carbon nanotubes: Prepare a mixed solution of HNO3 and H2SO4 according to the volume ratio of VHNO3:VH2SO4 = 3:1, add CNTs to it, after reflux and cooling, wash with deionized water, centrifuge, and freeze dry under vacuum to obtain carboxylated CNTs. S2. Preparation of cobalt source precursor: Cobalt acetate tetrahydrate, urea and carboxylated carbon nanotubes were reacted with solvothermal reaction in ethanol solvent, washed with anhydrous ethanol, and dried by centrifugation to obtain cobalt source precursor. The mass ratio of cobalt acetate tetrahydrate, urea, and carboxylated carbon nanotubes is 1:(0.5-5):(0.005-0.05), and the concentration of carbon nanotubes in ethanol solvent is 0.01-1 g / L. S3. Preparation of sodium cobalt ferrocyanide cathode material: The solid obtained by co-precipitation reaction of sodium source, cobalt source precursor, sodium ferrocyanide, surfactant, ethanol and water is aged, washed with deionized water and anhydrous ethanol, and centrifuged and dried to obtain sodium cobalt ferrocyanide cathode material; wherein, the mass ratio of sodium source, cobalt source precursor, sodium ferrocyanide and surfactant is (1-10):1:(0.5-5):(0.5-4), the volume ratio of ethanol and water is 1:(0.5-10), and the concentration of cobalt source precursor in ethanol solvent is 5-50 g / L, and the concentration of sodium source, sodium ferrocyanide and surfactant in aqueous solvent is 5-50 g / L.

2. The sodium cobalt iron cyanide cathode material as described in claim 1, characterized in that, In step S1, the carbon nanotube has a diameter of 3-15 nm and a length of 15-30 μm.

3. The sodium cobalt iron cyanide cathode material as described in claim 1, characterized in that, In step S3, the sodium source is one or more selected from sodium chloride, sodium sulfate, sodium carbonate, sodium bicarbonate and sodium dihydrogen phosphate; the surfactant is one or more selected from polyvinylpyrrolidone, sodium dodecylbenzene sulfonate and sodium dodecyl sulfate.

4. The sodium cobalt iron cyanide cathode material as described in claim 1, characterized in that, In step S2, the temperature of the solvothermal reaction is 40-100℃, the reaction time is 6-48h, the drying temperature is 60-80℃, and the drying time is 6-12h.

5. The sodium cobalt iron cyanide cathode material as described in claim 1, characterized in that, In step S3, the mass ratio of the sodium source, cobalt source precursor, sodium ferrocyanide, and surfactant is (1-9):1:(0.5-4.5):(0.5-3.5), the coprecipitation reaction temperature is 0-40℃, the coprecipitation reaction aging time is 6-48h, the drying temperature is 60-80℃, and the drying time is 6-12h.

6. The sodium cobalt iron cyanide cathode material as described in claim 1, characterized in that, The term "interpenetration" refers to the carbon nanotube passing through one or more faces of a hollow sodium cobalt iron cyanide prism, the carbon nanotube passing through at least two hollow sodium cobalt iron cyanide prisms, and each hollow sodium cobalt iron cyanide prism having at least one carbon nanotube.

7. The sodium cobalt iron cyanide cathode material as described in claim 1 or 6, characterized in that, The hollow sodium cobalt ferrocyanide prism has a diameter of 200-500 nm and a length of 0.5-2 μm.

8. An application of the sodium cobalt iron cyanide cathode material as described in claim 1, characterized in that, The material is used as a cathode material in aqueous sodium-ion batteries.

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