Preparation and application of sodium cobalt hexacyanoferrate composite material based on three-dimensional honeycomb MXene

Abstract

The invention provides a preparation method and application of a sodium cobalt hexacyanoferrate composite material based on three-dimensional honeycomb MXene. The composite material is prepared through the following steps that 1, a Ti3AlC2 MAX phase reacts with a mixed solution of lithium fluoride (LiF) and concentrated hydrochloric acid (HCl), an Al atomic layer is removed through etching, and layered MXene is obtained; 2, mixing the layered MXene with a polymethyl methacrylate (PMMA) template, centrifuging, washing, drying and calcining to prepare three-dimensional honeycomb MXene; and 3, dispersing the three-dimensional honeycomb MXene in deionized water, sequentially adding a cobalt source and a sodium ferrocyanide (Na4Fe (CN) 6) solution, and carrying out an in-situ coprecipitation reaction to obtain the composite material. In the composite material, the sodium cobalt hexacyanoferrate is of a cubic structure and is uniformly embedded into a three-dimensional honeycomb MXene framework. The composite material disclosed by the invention is applied to a water-based sodium-ion battery positive electrode material, and has high conductivity, excellent rate capability and cycling stability.

Description

Technical Field

[0001] The present invention relates to the technical field of positive electrode materials for aqueous sodium-ion batteries, and particularly relates to a sodium cobalt hexacyanoferrate composite based on three-dimensional honeycomb MXene, a preparation method thereof, and an application of the composite as a positive electrode material for aqueous sodium-ion batteries. Background Art

[0002] With the continuous growth of global energy demand and the increasingly severe environmental problems, the development of efficient, low-cost and environmentally friendly energy storage technologies has become the current research focus. Aqueous sodium-ion batteries (ASIBs) are regarded as one of the ideal candidates for large-scale energy storage systems due to their rich resources, low cost and similar electrochemical properties to lithium-ion batteries. However, the practical application of aqueous sodium-ion batteries still faces many challenges, especially in the aspect of positive electrode materials, where there are key problems such as low specific capacity, poor cycling performance and insufficient conductivity.

[0003] Sodium cobalt hexacyanoferrate, as a kind of Prussian blue analogues (PBAs), has become a research hotspot for positive electrode materials of sodium-ion batteries due to its open framework structure, wide voltage window and low cost. However, sodium cobalt hexacyanoferrate material itself has the problem of poor conductivity, and it is prone to structural collapse during charge and discharge, resulting in capacity attenuation and decline in cycling performance. To address these problems, researchers usually adopt strategies such as carbon material composite, nanostructure design or introduction of conductive skeletons to improve its electrochemical performance.

[0004] MXene, as a new two-dimensional material, has high conductivity, excellent mechanical properties and rich surface chemical characteristics, showing broad application prospects in the energy storage field. Combining MXene with sodium cobalt hexacyanoferrate can not only significantly improve the conductivity of the material, but also provide stable support for sodium cobalt hexacyanoferrate through the unique flexibility of MXene, effectively suppressing its volume change during charge and discharge. However, the traditional two-dimensional MXene sheet structure is prone to stacking, which limits its full contact with active materials and ion transport efficiency, thus affecting the overall performance of the composite.

[0005] Based on the above analysis, there is still a lack of a positive electrode material for aqueous sodium-ion batteries with high specific capacity, excellent rate performance and long cycle stability in the prior art. Therefore, developing a new type of MXene-based composite material and optimizing the material structure and interface characteristics are expected to solve the above problems and promote the practical application of aqueous sodium-ion batteries. Summary of the Invention

[0006] To address the deficiencies in the prior art, the present invention provides a method for preparing a sodium hexacyanoferrate cobalt composite based on three-dimensional honeycomb MXene and an aqueous sodium-ion battery. The composite material has a high specific surface area, good electrical conductivity, and strong structural stability. The aqueous sodium-ion battery assembled from it has a high energy density and excellent cycle stability, meeting the requirements of practical applications.

[0007] The present invention discloses a method for preparing a sodium hexacyanoferrate cobalt composite based on three-dimensional honeycomb MXene, comprising the following steps:

[0008] S1. Prepare layered MXene: Add Ti 3 AlC 2 MAX phase powder into a pre-prepared LiF / HCl mixed solution, control the reaction temperature within the range of 25 - 35 °C, and continuously stir for 12 - 24 hours. After the reaction is completed, obtain the precipitate by centrifugal separation, and repeatedly wash it with deionized water until neutral to finally obtain a layered MXene dispersion. In this step, the molar ratio of LiF to HCl is controlled at 1:4 to ensure the complete etching of the Al atomic layer while avoiding the damage to the MXene structure caused by over-etching.

[0009] S2. Prepare three-dimensional honeycomb MXene: First, prepare a PMMA template: Dissolve methyl methacrylate (MMA) monomer, initiator azobisisobutyronitrile (AIBN), and dispersant polyvinylpyrrolidone (PVP) in methanol, and react at 55 °C for 24 hours under nitrogen protection to obtain PMMA microspheres with a diameter of 2 - 3 µm. Subsequently, mix the layered MXene and the PMMA template at a mass ratio of 1:(1 - 5), after centrifugal washing 3 - 5 times, dry it at 60 °C for 6 - 12 hours. Finally, calcine it at 450 °C for 1 hour under nitrogen protection to remove the PMMA template and obtain MXene with a three-dimensional honeycomb structure.

[0010] S3. Prepare the composite material: Disperse the three-dimensional honeycomb MXene in deionized water, add a cobalt source solution, and adsorb the cobalt source onto the three-dimensional honeycomb MXene framework through stirring and ultrasonic treatment. Subsequently, add a sodium ferrocyanide (Na 4 Fe(CN) 6 ) solution under continuous stirring to form a composite material through an in-situ coprecipitation reaction. Among them, the cobalt source is one of cobalt chloride hexahydrate, cobalt nitrate hexahydrate, and cobalt acetate. The concentration of the cobalt source solution is 0.1 - 1 mol L -1 , the concentration of the sodium ferrocyanide solution is 0.1 - 1 mol L -1 , and the dispersion concentration of the three-dimensional honeycomb MXene in deionized water is 1 - 10 mg mL -1The molar ratio of the cobalt source, sodium ferrocyanide, and three-dimensional honeycomb MXene is 1:(0.5 - 3.5):(0.5 - 4.5).

[0011] In step S1, the molar ratio of LiF to HCl in the mixed solution is 1:4, the etching reaction temperature is 25 - 35 °C, and the reaction time is 12 - 24 hours.

[0012] In step S2, the diameter of the PMMA microspheres is 2 - 3 µm, the mass ratio of layered MXene to the PMMA template is 1:(1 - 5), centrifugal washing is performed 3 - 5 times, and the drying temperature is 60 °C for 6 - 12 hours.

[0013] In step S3, the in-situ coprecipitation reaction temperature is 20 - 30 °C, and the reaction time is 12 - 24 hours. In the composite material, sodium cobalt hexacyanoferrate has a cubic structure with a side length of 200 - 500 nm and is uniformly embedded in the three-dimensional honeycomb MXene. The composite material includes a three-dimensional honeycomb MXene skeleton and uniformly distributed sodium cobalt hexacyanoferrate nanoparticles.

[0014] The present invention also provides an application of the composite material, characterized in that the material is used as a positive electrode material for aqueous sodium-ion batteries.

[0015] The positive progressive effects of the present invention are as follows:

[0016] (1) High conductivity and excellent ion transport performance: By combining sodium cobalt hexacyanoferrate with three-dimensional honeycomb MXene, the present invention utilizes the high conductivity and unique three-dimensional structure of three-dimensional honeycomb MXene to significantly improve the electron conduction ability and ion transport efficiency of the composite material. Compared with the traditional two-dimensional MXene sheet structure, the three-dimensional honeycomb MXene effectively avoids the material stacking problem, provides a larger specific surface area and more active sites, thereby improving the electrochemistry reaction kinetics.

[0017] (2) Enhanced structural stability: As a supporting skeleton, the three-dimensional honeycomb MXene can effectively relieve the volume expansion and structural collapse of sodium cobalt hexacyanoferrate during charge and discharge, significantly improving the cycle stability of the material. Experiments show that the capacity retention rate of the composite material prepared by the present invention is not less than 80% after 10,000 cycles, far superior to the traditional sodium cobalt hexacyanoferrate material.

[0018] (3) High specific capacity and excellent rate performance: Due to the introduction of three-dimensional honeycomb MXene, the composite material has a higher specific surface area and a shorter ion diffusion path, thus showing a higher specific capacity and excellent rate performance.

[0019] (4) Simple preparation process and environmentally friendly: The preparation method of the present invention adopts a template-assisted method and an in-situ coprecipitation method, with a simple process, mild conditions, and is easy to scale up production. At the same time, the raw materials used are low-cost and environmentally friendly, meeting the development concept of green chemistry. Description of the Drawings

[0020] Figure 1 It is a scanning electron microscope image of the sodium cobalt hexacyanoferrate composite material based on three-dimensional honeycomb MXene prepared in Example 3.

[0021] Figure 2 It is a comparison chart of the impedance of the cathode material before and after introducing three-dimensional honeycomb MXene in Example 4.

[0022] Figure 3 It is a comparison chart of the cyclic voltammograms of the composite cathode material at different scanning rates in Example 4.

[0023] Figure 4 It is a comparison chart of the rate performance of the three-electrode battery before and after introducing three-dimensional honeycomb MXene in Example 4.

[0024] Figure 5 It is a comparison chart of the cycling performance of the three-electrode battery before and after introducing three-dimensional honeycomb MXene in Example 4.

[0025] Figure 6 It is a comparison chart of the cycling performance of the full battery before and after introducing three-dimensional honeycomb MXene in Example 5. Detailed Embodiments

[0026] The present invention provides a sodium cobalt hexacyanoferrate (NaCoHCF) composite material based on three-dimensional honeycomb MXene, its preparation method, and the application of the composite material as a cathode material for aqueous sodium-ion batteries. To make the purpose, technical solution, and implementation effect of the present invention clearer, the present invention will be further described in detail in combination with the drawings and embodiments.

[0027] Example 1 - Layered Ti 3 T 2 C x Preparation of MXene

[0028] The Ti 3 T 2 C x MXene used in the present invention is prepared by a method of etching with fluoride salts combined with freeze-thaw treatment, and the specific operation steps are as follows:

[0029] S1. Preparation of the precursor solution: Dissolve solid powder of lithium fluoride (LiF) in concentrated hydrochloric acid (HCl, mass fraction 36 - 38%) at a molar ratio of 1:4, and stir with a magnetic stirrer at a speed of 500 - 800 rpm for 10 - 15 minutes until a uniform and transparent mixed solution is formed; Subsequently, under continuous stirring, add Ti 3 AlC 2 MAX phase powder into the above mixed solution at a rate of 0.5 - 1 g min -1 . Control the temperature of the reaction system within the range of 25 - 35 °C, and maintain magnetic stirring for 12 - 24 hours for the etching reaction; Among them, the mass - to - volume ratio of Ti 3 AlC 2 powder to the mixed solution is 1 g:10 - 15 mL.

[0030] S2. Separation and purification of the product: Transfer the reaction mixture obtained in step S1 to a centrifuge tube, centrifuge at a speed of 8000 - 10000 rpm for 5 - 10 minutes, and discard the supernatant; Add deionized water to the precipitate, after ultrasonic dispersion for 5 - 10 minutes, centrifuge and separate again, repeat the above washing process until the pH value of the supernatant reaches 6.0 ± 0.2; After the last centrifugation, retain the precipitate, and add an appropriate amount of deionized water to prepare a suspension with a concentration of 5 - 10 mg mL -1 .

[0031] S3. Freezing - thawing treatment: Transfer the suspension obtained in step S2 to a low - temperature resistant container, place it in a - 80 °C ultra - low temperature refrigerator and freeze for 12 hours, then thaw in a 25 °C water bath for 4 hours, which is a complete freezing - thawing cycle; Repeat the above freezing - thawing process a total of 4 times, and observe the dispersion state of the suspension after each cycle; After completing 4 cycles, transfer the suspension to a sealed container filled with nitrogen, centrifuge at a speed of 10000 rpm for 8 minutes under nitrogen protection, and collect the supernatant to obtain a stable Ti 3 C 2 T x - MXene dispersion; The concentration of the dispersion is 2 - 5 mg mL -1 .

[0032] Example 2 - Preparation of three - dimensional honeycomb MXene (HMX)

[0033] S1. Preparation of PMMA template: Dissolve 0.1 g of free radical initiator azobisisobutyronitrile (AIBN) in 100 mL of methanol, add 4 g of polyvinylpyrrolidone (PVP), and stir at 500 rpm for 30 minutes at 25 °C until completely dissolved; add 20 mL of methyl methacrylate (MMA) monomer to the above solution, and purge with high-purity nitrogen (purity ≥99.999%) at a flow rate of 200 mL min -1 for 30 minutes to ensure an oxygen-free environment in the system; transfer the reaction system to a 55 °C constant temperature water bath, and continuously stir at 600 rpm under nitrogen protection for 24 hours to carry out a free radical polymerization reaction; after the reaction is completed, centrifuge the product at 10000 rpm for 10 minutes, discard the supernatant, add an equal volume of methanol to resuspend the precipitate, and repeat centrifugation and washing 3 times; transfer the finally obtained precipitate to a freeze dryer and freeze-dry at -50 °C and 0.1 mbar for 24 hours to obtain a white powdery PMMA template material.

[0034] S2. Preparation of MXene / PMMA composite material: Disperse the PMMA template prepared in step S1 in 100 mL of deionized water to prepare a dispersion with a concentration of 10 mg mL -1 ; take 40 mg of layered MXene and disperse it in 10 mL of deionized water, and ultrasonically treat it for 30 minutes to obtain a uniformly dispersed MXene dispersion; slowly add the MXene dispersion to the PMMA dispersion, and stir at 800 rpm at 25 °C for 30 minutes to uniformly coat the MXene nanosheets on the surface of the PMMA microspheres; centrifuge the mixed dispersion at 8000 rpm for 5 minutes, discard the supernatant, add 50 mL of deionized water to resuspend the precipitate, and repeat centrifugation and washing until the supernatant is clear and transparent.

[0035] S3. Preparation of three-dimensional honeycomb-like MXene: Transfer the precipitate obtained in step S2 to a vacuum drying oven and dry it at 60 °C for 12 hours; place the dried sample in a tubular furnace, and heat it to 450 °C at a heating rate of 5 °C min -1 under a nitrogen atmosphere, and keep it at this temperature for 1 hour for pyrolysis treatment to remove the PMMA template; after natural cooling to room temperature, a MXene material with a three-dimensional honeycomb-like structure is obtained.

[0036] Preparation of Example 3 - NaCoHCF / HMX Composite Material

[0037] S1. Preparation and loading of precursor solution: Disperse three-dimensional honeycomb-like MXene (HMX) in deionized water to prepare a dispersion with a concentration of 1 - 10 mg mL -1The dispersion was ultrasonically treated for 30 minutes to obtain a uniformly dispersed HMX suspension; a cobalt source solution with a concentration of 0.1 - 1 mol / L was prepared and filtered through a 0.22 μm filter membrane for standby; the cobalt source solution was slowly added to the HMX dispersion at a rate of 1 mL min -1 The rate was slowly added to the HMX dispersion, and the molar ratio of the cobalt source to HMX was controlled to be 1:(0.5 - 4.5); at 25 °C, it was stirred at 800 rpm for 2 hours, and at the same time, intermittent ultrasonic treatment (ultrasonic for 5 minutes every 15 minutes) was supplemented to make Co 2 ⁺ was fully adsorbed on the surface of the three-dimensional framework of HMX and in the internal pores.

[0038] S2. In-situ coprecipitation reaction: Prepare an aqueous solution of sodium ferrocyanide (Na -1 Fe(CN) 4 ) with a concentration of 0.1 - 1 mol L 6 , filter it through a 0.22 μm filter membrane for standby; slowly add the sodium ferrocyanide solution to the mixture obtained in step S1 at a rate of 0.5 mL min -1 , and control the molar ratio of Na 4 Fe(CN) 6 to the cobalt source to be (0.5 - 3.5):1; let it stand at 20 - 30 °C for 12 - 24 hours for in-situ coprecipitation reaction; after the reaction, centrifuge the product at 8000 rpm for 10 minutes to collect the precipitate; wash the precipitate 3 times with deionized water, and centrifuge at 8000 rpm for 10 minutes after each washing; dry the final product in a vacuum drying oven at 60 °C for 12 hours to obtain the NaCoHCF / HMX composite material. As Figure 1 shown, this is the scanning electron micrograph of the NaCoHCF / HMX composite material prepared in Example 3, indicating the successful synthesis of the composite material.

[0039] Example 4 - Assembly and performance testing of a three-electrode test system

[0040] This example provides a method for preparing pure NaCoHCF material, which is characterized by not adding three-dimensional honeycomb MXene, and the other preparation steps are the same as those in Example 3.

[0041] The assembly steps of the three-electrode test system are as follows:

[0042] S1. Preparation of the working electrode: First, accurately weigh 70 mg of the active material, 20 mg of Super-P, and 10 mg of polyvinylidene fluoride (PVDF) to ensure a total mass of 100 mg. Transfer the weighed mixture to an agate mortar, and add 2 mL of N-methylpyrrolidone (NMP) as a dispersant. Grind the mixture thoroughly with the mortar for 30 minutes until a homogeneous slurry is formed, and control the slurry viscosity within the range of 500 - 800 mPa·s. Uniformly coat the prepared slurry on the pretreated nickel foam current collector, and strictly control the coating thickness to be 100 ± 10 μm. Finally, place the coated electrode in a vacuum drying oven at 80 °C and dry it for 12 hours to ensure that the moisture content is less than 0.1%.

[0043] S2. Preparation of the electrolyte: Weigh 14.2 g of anhydrous sodium sulfate (Na 2 SO 4 ) and dissolve it in 100 mL of ultrapure water; filter the solution through a 0.22 μm filter membrane to obtain 1 mol / L Na -1 Na 2 SO 4 electrolyte.

[0044] S3. Assembly of the three-electrode system: Fix the working electrode, a platinum sheet counter electrode (1 cm × 1 cm), and an Ag / AgCl reference electrode in the electrolytic cell. Inject 50 mL of the deoxygenated 1 mol / L Na 2 SO 4 electrolyte. Place the electrolytic cell in a 25 °C constant temperature water bath.

[0045] Perform AC impedance tests on the NaCoHCF / HMX composite material and pure NaCoHCF using an electrochemical workstation. The charge transfer resistance at the electrode / electrolyte interface of the three-electrode cell system composed of pure NaCoHCF is 10.6 Ω. While the charge transfer resistance at the interface of the composite material is only 3.4 Ω, and the test results are as Figure 2 shown.

[0046] Perform cyclic voltammetry tests on the NaCoHCF / HMX composite material using an electrochemical workstation. In the potential window of 0 to 1 V (vs. Ag / AgCl), perform tests at a scanning rate of 0.2 - 1 mV s -1 . The shapes of the CV curves at different scanning rates are similar, proving its excellent electrochemical stability, and the test results are as Figure 3 shown.

[0047] Perform rate performance tests on the NaCoHCF / HMX composite material and pure NaCoHCF using a Wuhan Blue Electric CT2001A battery test system. The test voltage range is 0 - 1 V, and the test results are as Figure 4as shown

[0048] The cyclic performance of the NaCoHCF / HMX composite material and pure NaCoHCF was tested using a Wuhan Blue Electric CT2001A battery test system at a current density of 2 A g -1 The test was carried out at a current density, and the test voltage range was 0 - 1 V. The test results are as Figure 5 shown

[0049] Example 5 - Assembly and Performance Test of the Full Battery

[0050] S1. Electrode preparation: For the preparation of the positive electrode, the current collector in Example 4 was changed to a stainless - steel foil; for the preparation of the negative electrode, the active material was changed to NTP@C, the current collector was changed to a stainless - steel foil, and other steps and conditions remained unchanged.

[0051] S2. Full - battery assembly: The assembly was carried out in an air atmosphere. The battery positive - electrode case, spring piece, and gasket were placed in sequence. The negative - electrode sheet (diameter 14 mm) was put in, and 50 μL of electrolyte was dropped. A glass - fiber separator (diameter 16 mm) was placed, and 50 μL of electrolyte was dropped; the positive - electrode sheet (diameter 14 mm) was put in, and 50 μL of electrolyte was dropped; the gasket and positive - electrode case were placed in sequence, and the battery was encapsulated under a pressure of 10 MPa using a button - cell encapsulation machine.

[0052] S3. Electrochemical performance test: Cyclic stability test: It could be stably cycled 2500 times at a current density of 2 A g -1 The specific capacity was about 110 mAh g -1 , as Figure 6 .

[0053] The above - mentioned is only the preferred embodiment of the present invention. It should be noted that for those scientific research personnel in the technical field, without departing from the technology of the present invention, several improvements and modifications can still be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a sodium cobalt hexacyanoferrate composite material based on a three-dimensional honeycomb MXene, characterized in that: The following steps are involved: S1. Preparation of layered MXene: reacting the Ti3AlC2 MAX phase with a mixed solution, etching and removing the Al atomic layer, and obtaining a layered MXene; the mixed solution is a mixed solution of lithium fluoride (LiF) and concentrated hydrochloric acid (HCl), wherein the molar ratio of LiF to HCl is 1:4; S2. Preparation of three-dimensional honeycomb MXene: mixing layered MXene with a polymethyl methacrylate (PMMA) template, separating by centrifugation and washing with deionized water and ethanol to obtain a precipitate; drying the precipitate at 60 °C, and then calcining it at 450 °C for 1 hour under nitrogen protection to remove the PMMA template to obtain a three-dimensional honeycomb MXene; S3. Preparation of composite materials: Disperse the three-dimensional honeycomb MXene in deionized water, add a cobalt source solution, and adsorb the cobalt source on the three-dimensional honeycomb MXene framework by stirring and ultrasonic treatment; then add sodium ferrocyanide (Na4Fe(CN)6) solution under continuous stirring, and form the final composite material through an in situ coprecipitation reaction; the molar ratio of the cobalt source, sodium ferrocyanide and three-dimensional honeycomb MXene is 1:(0.5-3.5):(0.5-4.5).

2. The preparation method according to claim 1, characterized in that In step S1, the molar ratio of LiF to HCl in the mixed solution is 1:4, the etching reaction temperature is 25-35° C., and the reaction time is 12-24 hours.

3. The preparation method according to claim 1, characterized in that: In step S2, the preparation method of the PMMA template is: dissolving methyl methacrylate (MMA) monomer, initiator azobisisobutyronitrile (AIBN) and dispersant polyvinylpyrrolidone (PVP) in methanol, reacting at 55 °C for 24 hours under nitrogen protection to obtain PMMA microspheres; the diameter of the PMMA microspheres is 2-3µm.

4. The preparation method according to claim 1, characterized in that: In step S2, the mass ratio of the layered MXene to the PMMA template is 1:(1-5), the number of centrifugal washings is 3-5 times, the drying temperature is 60 °C, and the drying time is 6-12 hours.

5. The preparation method according to claim 1, characterized in that: In step S3, the cobalt source is one of cobalt chloride hexahydrate, cobalt nitrate hexahydrate, and cobalt acetate.

6. The preparation method according to claim 1, characterized in that: In step S3, the concentration of the cobalt source solution is 0.1-1 mol L -1 , the concentration of sodium ferrocyanide solution is 0.1-1 mol L -1 The dispersion concentration of 3D honeycomb MXene in deionized water is 1-10 mg mL -1 .

7. The preparation method according to claim 1, characterized in that: In step S3, the temperature of the in-situ coprecipitation reaction is preferably 20-30° C., and the reaction aging time is preferably 12-24 hours.

8. The preparation method according to claim 1, characterized in that: In step S3, the sodium cobalt hexacyanoferrate in the composite material has a cubic structure with a side length of 200-500 nm and is uniformly embedded in the three-dimensional honeycomb MXene skeleton.

9. A sodium cobalt hexacyanoferrate composite material based on a three-dimensional honeycomb MXene prepared by the method according to any one of claims 1 to 8, characterized in that: The composite material includes a three-dimensional honeycomb MXene skeleton and sodium cobalt hexacyanoferrate nanoparticles uniformly embedded in the surface and internal pores of the skeleton.

10. An application of the sodium cobalt hexacyanoferrate composite material based on three-dimensional honeycomb MXene as claimed in claim 1, characterized in that: The composite material is used as a positive electrode material for aqueous sodium ion batteries.

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