Sodium ferric phosphate pyrophosphate positive electrode material and preparation method thereof

Abstract

The invention provides a ferric sodium pyrophosphate positive electrode material and a preparation method thereof. The preparation method comprises the following steps: preparing materials according to the chemical formula of the positive electrode material, dissolving a sodium source, an iron source, a phosphorus source and a chelating agent in deionized water, and uniformly mixing; carrying out spray drying on the uniformly mixed solution to obtain precursor powder; dissolving the precursor powder in absolute ethyl alcohol, uniformly mixing and drying to obtain a dried precursor; and pre-sintering the dried precursor, and sintering at the temperature of between 480 and 550 DEG C for 8 to 10 hours to obtain the Na4Fe2.8 (PO4) 2P2O7 positive electrode material. By controlling the atomic number ratio of iron to phosphorus in the positive electrode material, the lattice parameter and the unit cell volume of the positive electrode material are reduced, the ferric sodium phosphate pyrophosphate positive electrode material with relatively high phase purity can be prepared, the electrochemical performance and the ion transport dynamics performance of the ferric sodium phosphate pyrophosphate material are improved, and the application prospect is wide. And the material has relatively good reversible specific capacity and rate capability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of positive electrode material preparation, and more particularly to a sodium iron phosphate pyrophosphate positive electrode material and a preparation method thereof. BACKGROUND

[0002] The sodium iron phosphate pyrophosphate positive electrode material (Na4Fe3(PO4)2P2O7) is a polyanion positive electrode material, belongs to the orthorhombic Pn21a space group, has a theoretical specific capacity of 129 mAh / g, and has an average working voltage of 3.2 V. Its stable crystal structure and 3D sodium ion diffusion channel make it an extremely stable positive electrode material in the charging and discharging process. The volume change rate in the charging and discharging process is less than 4%, and the material preparation process has little pollution, is non-toxic, has low cost, and has excellent electrochemical performance, so it has attracted widespread attention. However, the intrinsic electronic conductivity of this type of positive electrode material is low, and NaFePO4 and Na2FeP2O7 impurity phases are easily generated during the preparation process, which reduces the actual specific capacity of the material and the kinetic performance in the charging and discharging process. Therefore, a method for improving the phase purity and kinetic performance of the material needs to be found.

[0003] In the prior art, common methods for improving the phase purity of the positive electrode material include adjusting the temperature, sintering atmosphere, raw material ratio, and optimizing the preparation process, and improving the electronic conductivity and kinetic performance of the material by means such as carbon coating or doping. However, the adjustment of the temperature and sintering atmosphere is easily affected by the raw material system and environmental conditions, resulting in insufficient consistency of the results. In addition, overly complex preparation methods will limit their industrial application. SUMMARY

[0004] In view of the deficiencies in the prior art, one of the purposes of the present application is to solve one or more problems in the prior art. For example, one of the purposes of the present application is to provide a method for preparing a sodium iron phosphate pyrophosphate positive electrode material with high phase purity by adjusting the atomic ratio of iron to phosphorus in the positive electrode material.

[0005] In one aspect, the present application provides a preparation method of a sodium iron phosphate pyrophosphate positive electrode material, which can include the following steps: according to the chemical formula of the positive electrode material, dissolving a sodium source, an iron source, a phosphorus source, and a chelating agent in deionized water and mixing uniformly; spray drying the mixed solution to obtain a precursor powder; dissolving the precursor powder in anhydrous ethanol, mixing uniformly, and drying to obtain a dried precursor; pre-sintering the dried precursor, and then sintering it at 480-550 DEG C for 8-10 h to obtain a Na4Fe 2.8 (PO4)2P2O7 positive electrode material.

[0006] Further, the pre-sintering temperature can be 300-350 DEG C, and the pre-sintering time can be 3-5 h.

[0007] Further, the molar ratio of the chelating agent to the sodium source can be (0.3-0.5):1; the chelating agent can be at least one of citric acid, ethylenediaminetetraacetic acid, ethylenediaminedisuccinic acid, gluconic acid, and tartaric acid.

[0008] Further, the spray drying can include drying using a spray dryer, wherein the inlet temperature is controlled to be 200-230 DEG C, the outlet temperature is controlled to be 100-130 DEG C, the atomization pressure is controlled to be 0.4-0.6 Mpa, and the feeding speed is controlled to be 15-40 rpm.

[0009] Further, the anhydrous ethanol can account for 20-50% of the mass of the precursor powder.

[0010] Further, the sodium source can be at least one of sodium dihydrogen phosphate, sodium carbonate, sodium pyrophosphate, and sodium phosphate; the iron source can be at least one of ferric nitrate, ferrous oxalate, and ferrous chloride; the phosphorus source can be at least one of sodium dihydrogen phosphate and diammonium phosphate; and the carbon source can be at least one of citric acid, sucrose, starch, and ascorbic acid.

[0011] Another aspect of the present application provides a sodium iron phosphate pyrophosphate positive electrode material, wherein the atomic ratio of iron to phosphorus in the positive electrode material can be 2.8:4.

[0012] Further, the positive electrode material can be coated with a carbon layer on the surface.

[0013] Compared with the prior art, the present application has at least one of the following beneficial effects:

[0014] (1) The present application controls the atomic ratio of iron to phosphorus in the positive electrode material, so that the lattice parameter and the unit cell volume of the positive electrode material are reduced, and a sodium iron phosphate pyrophosphate positive electrode material with high phase purity can be prepared, the electrochemical performance and ion transport kinetics performance of the sodium iron phosphate pyrophosphate (NFPP) material are improved, and the material has good reversible specific capacity and rate performance.

[0015] (2) The preparation method of the present application is simple, the process is short, and large-scale industrial production can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0016] The above and other objects and features of the present application will become more apparent from the following description made with reference to the accompanying drawings, in which:

[0017] Figure 1 The XRD pattern of the sodium iron phosphate pyrophosphate positive electrode material prepared in Example 1 of the present application.

[0018] Figure 2 The XRD pattern of the sodium iron phosphate pyrophosphate positive electrode material prepared in Comparative Example 1 of the present application.

[0019] Figure 3 The image shows the XRD pattern of the sodium iron pyrophosphate cathode material prepared in Comparative Example 2 of this invention.

[0020] Figure 4 The image shows the XRD pattern of the sodium iron pyrophosphate cathode material prepared in Comparative Example 3 of this invention.

[0021] Figure 5 This is a SEM image of the sodium iron pyrophosphate cathode material prepared in Example 1 of the present invention.

[0022] Figure 6 This is a comparison chart of the charge-discharge curves of the sodium iron pyrophosphate cathode materials prepared in the embodiments of the present invention and Comparative Examples 1 to 3.

[0023] Figure 7 This is a comparison chart of the rate performance of sodium iron pyrophosphate cathode materials prepared in the embodiments of the present invention and Comparative Examples 1 to 3. Detailed Implementation

[0024] The sodium iron pyrophosphate cathode material and its preparation method according to the present invention will be described in detail below with reference to the accompanying drawings and exemplary embodiments.

[0025] This invention provides a method for preparing sodium iron pyrophosphate cathode material, which may include the following steps:

[0026] S01, according to the chemical formula of the positive electrode material, dissolve the sodium source, iron source, phosphorus source and chelating agent in deionized water and mix well;

[0027] S02, the mixed solution is spray-dried to obtain precursor powder;

[0028] S03, the precursor powder is dissolved in anhydrous ethanol, mixed and dried to obtain the dried precursor.

[0029] S04, after pre-calcining the dried precursor, is then sintered at 480℃~550℃ for 8h~10h to obtain Na4Fe. 2.8 (PO4)2P2O7 cathode material.

[0030] In some implementations, the ratio of iron to phosphorus atoms in the cathode material is 2.8:4, and the chemical formula of the cathode material is Na₄Fe. 2.8 (PO4)2P2O7. By controlling the atomic ratio of Fe to P in the cathode material, a sodium iron pyrophosphate (NFPP) cathode material with high phase purity was prepared, improving the various electrochemical properties and ion transport kinetics of the NFPP material. The prepared NFPP cathode material achieved a discharge specific capacity of 108.9 mAh / g at 0.1C and still maintained a reversible specific capacity of 84 mAh / g at 10C.

[0031] In some embodiments, the sodium source may be at least one of sodium dihydrogen phosphate, sodium carbonate, sodium pyrophosphate, and sodium phosphate. The iron source may be at least one of ferric nitrate, ferrous oxalate, and ferrous chloride. The phosphorus source may be at least one of sodium dihydrogen phosphate and ammonium dihydrogen phosphate. The carbon source may be at least one of citric acid, sucrose, starch, and ascorbic acid.

[0032] In some implementations, dissolving the sodium source, iron source, phosphorus source, and chelating agent in deionized water may include first mixing the iron source and chelating agent, and then adding the sodium source and phosphorus source.

[0033] In some embodiments, the chelating agent can be at least one selected from citric acid, ethylenediaminetetraacetic acid (EDTA), ethylenediaminedisuccinic acid, gluconic acid, and tartaric acid. Adding a chelating agent to chelate with the metal element allows iron to coordinate with the polydentate ligands in the chelating agent, preventing the reaction with phosphate ions to form iron phosphate precipitate, reducing the formation of impurity phases in subsequent preparations. Furthermore, the chelating agent, after decomposition during sintering, can form a carbon coating layer on the surface of the cathode material. The amount of chelating agent used can be in a molar ratio of (0.3–0.5):1 with the sodium source. For example, the molar ratio of chelating agent to sodium source can be 0.4:1.

[0034] In some implementations, spray drying can be performed in a spray dryer. Parameters controlling the spray drying process may include: an inlet air temperature of 200°C to 230°C, an outlet temperature of 100°C to 130°C, an atomization pressure of 0.4 MPa to 0.6 MPa, and a feed rate of 15 rpm to 40 rpm. For example, the inlet air temperature could be 220°C, the outlet temperature 120°C, the atomization pressure 0.5 MPa, and the feed rate 25 rpm.

[0035] In some implementations, anhydrous ethanol can account for 20%-50% of the precursor powder mass. Adding anhydrous ethanol can further remove the water of crystallization from the precursor and dissolve nitrate ions, preventing material oxidation and the generation of nitrogen oxides and other gases during pre-calcination, which could lead to material splashing and affect pre-calcination. For example, anhydrous ethanol can account for 25%-45% of the precursor powder mass. As another example, anhydrous ethanol can account for 30% of the precursor powder mass.

[0036] In some implementations, the pre-firing temperature can be 300℃ to 350℃, and the pre-firing time can be 3h to 5h. For example, the pre-firing temperature can be 330℃, and the pre-firing time can be 4h.

[0037] In some implementations, the precursor, after pre-calcination, can be sintered at 480℃~550℃ for 8h~10h to obtain Na4Fe. 2.8(PO4)2P2O7 cathode material. For example, it can be sintered at 500°C for 9 hours.

[0038] In some implementations, a magnetic stirrer can be used to mix the materials in step S01. The stirring speed of the magnetic stirrer can be set to 1000 rpm to 2000 rpm, the stirring temperature can be set to 60°C, and the stirring time can be 5 hours.

[0039] In some embodiments, a magnetic stirrer can be used for mixing in step S03. The stirrer speed can be set to 3000 rpm to 5000 rpm, the stirring temperature can be set to 20°C to 40°C, and the stirring time can be 1 hour to 2 hours. In some embodiments, drying can be performed using a forced-air drying oven, the temperature of which can be set to 70°C to 80°C, and the drying time can be 5 hours to 10 hours.

[0040] In some implementations, pre-firing and sintering can be carried out in a tube furnace. The pre-firing and sintering atmosphere can be set to high-purity Ar or an Ar-H2 mixture, wherein the volume percentage of H2 can be 5%.

[0041] Another aspect of the present invention provides a sodium iron pyrophosphate cathode material, wherein the ratio of iron to phosphorus atoms in the cathode material can be 2.8:4, and the chemical formula of the cathode material is Na₄Fe₂O₃. 2.8 (PO4)2P2O7.

[0042] In some implementations, the surface of the cathode material may be coated with a carbon layer.

[0043] To better understand the present invention, specific examples are provided below to further illustrate the content of the present invention, but the content of the present invention is not limited to the examples below.

[0044] Example 1

[0045] A method for preparing sodium iron pyrophosphate cathode material may include the following steps:

[0046] Step 1, according to Na4Fe 2.8 (PO4)2P2O7 ingredients: iron source ferric nitrate and chelating agent citric acid are dissolved in deionized water and stirred at 60°C and 2000 rpm using a magnetic stirrer for 1 hour. After the chelation reaction is fully carried out, sodium source and phosphorus source sodium dihydrogen phosphate are added and stirring is continued for 5 hours. Citric acid accounts for 30% of the molar ratio of sodium source.

[0047] Step 2: Spray dry the solution prepared in Step 1. Set the inlet air temperature to 230℃, the outlet air temperature to 120℃, the air pump pressure to 0.5 MPa, and the feed speed to 20 rpm.

[0048] Step 3: Dissolve the precursor powder obtained by spray drying in step 2 in anhydrous ethanol, with anhydrous ethanol accounting for 30 wt.% of the powder weight. Stir with a magnetic stirrer for 2 hours and vacuum dry in a vacuum drying oven at 80°C for 12 hours to obtain the dried precursor powder.

[0049] Step 4: The precursor powder obtained from drying in Step 3 is pre-calcined in a tube furnace under an Ar-H2 (5% H2) atmosphere at a pre-calcination temperature of 300℃ for 5 hours; then the temperature is further increased to 500℃ for sintering for 10 hours to obtain Na4Fe. 2.8 (PO4)2P2O7 sodium iron pyrophosphate cathode material (NFPP-2.8).

[0050] Comparative Example 1

[0051] Compared with Example 1, this comparative example follows the method of Na4Fe 2.7 (PO4)2P2O7 is prepared by adding sodium, iron, and phosphorus sources to obtain Na4Fe. 2.7 (PO4)2P2O7 sodium iron pyrophosphate cathode material (NFPP-2.7), otherwise the same as in Example 1.

[0052] Comparative Example 2

[0053] Compared with Example 1, this comparative example follows the method of Na4Fe 2.9 (PO4)2P2O7 is prepared by adding sodium, iron, and phosphorus sources to obtain Na4Fe. 2.9 (PO4)2P2O7 sodium iron pyrophosphate cathode material (NFPP-2.9), otherwise the same as in Example 1.

[0054] Comparative Example 3

[0055] Compared with Example 1, this comparative example uses Na4Fe3(PO4)2P2O7 as the sodium source, iron source and phosphorus source to obtain Na4Fe3(PO4)2P2O7 sodium iron pyrophosphate phosphate cathode material (NFPP-3), and other aspects are the same as in Example 1.

[0056] The material testing methods are as follows:

[0057] I. Sample to be tested

[0058] All materials used in the experiment were analytical grade or higher chemical materials. Sodium iron pyrophosphate cathode materials were prepared according to the methods of Example 1 and Comparative Examples 1-3. 20 mmol of each prepared cathode material was used for later use.

[0059] II. Phase Purity Testing

[0060] Detection method: XRD detection was performed on the prepared samples, and the detection results were fitted using GSAS-II software to obtain the proportion of each phase in different samples.

[0061] XRD analysis was performed on Example 1 and Comparative Examples 1-3, and the results are as follows: Figures 1-4 As shown, where Figure 1 The image shows the XRD pattern of the sodium iron pyrophosphate cathode material prepared in Example 1 of this invention. Figure 2 The image shows the XRD pattern of the sodium iron pyrophosphate cathode material prepared in Comparative Example 1 of this invention. Figure 3 The image shows the XRD pattern of the sodium iron pyrophosphate cathode material prepared in Comparative Example 2 of this invention. Figure 4 The image shows the XRD pattern of the sodium iron pyrophosphate cathode material prepared in Comparative Example 3 of this invention. Figure 5 This is a SEM image of the sodium iron pyrophosphate cathode material prepared in Example 1 of this invention. From... Figure 5 As can be seen, the sodium iron pyrophosphate cathode material of the present invention is in the form of spherical particles.

[0062] Table 1 shows the phase purity test results of the sodium iron pyrophosphate cathode materials prepared in Example 1 and Comparative Examples 1-3, and provides a comparative analysis. Figures 1-4 Table 1 shows that the sodium iron pyrophosphate cathode material prepared in Example 1 has higher phase purity.

[0063] Table 1. Phase purity test results of sodium ferric pyrophosphate cathode material

[0064]

[0065] III. Electrochemical Performance Testing

[0066] Test method: A positive electrode sheet prepared by mixing the obtained sodium iron pyrophosphate positive electrode material (80 wt.%), acetylene black (10 wt.%), and polyvinylidene fluoride (10 wt.%) was used as the positive electrode of the sodium-ion battery, and a metallic sodium sheet was used as the negative electrode. 100 μL of electrolyte was added dropwise. The electrolyte was 1 mol NaClO4 dissolved in EC / PC (1:1V) + 5% FEC. Glass fiber was used as the separator. 2025 button batteries were assembled in a glove box with oxygen and water content both below 0.1 ppm. The reversible specific capacity of the material was then tested by constant current charge and discharge at a current density of 0.1C. The rate test was conducted by constant current charge and discharge at current densities of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C, with 5 cycles for each rate. The test conditions were a charge cutoff voltage of 4.2V and a discharge cutoff voltage of 1.5V.

[0067] The electrochemical performance test results are shown in Table 2. The rate performance comparison of sodium iron pyrophosphate cathode materials is as follows: Figure 6 As shown, the rate performance is compared to, for exampleFigure 7 As shown.

[0068] Table 2. Specific capacity and rate performance of sodium iron pyrophosphate cathode material

[0069]

[0070]

[0071] As shown in Tables 1 and 2, the sodium iron pyrophosphate cathode material prepared by the method of the present invention has high phase purity, which can reduce the content of impurity phases and improve the reversible specific capacity and rate performance of the material.

[0072] Although the present invention has been described above in conjunction with exemplary embodiments, those skilled in the art will understand that various modifications and changes can be made to the exemplary embodiments of the present invention without departing from the spirit and scope defined by the claims.

Claims

1. A method for preparing sodium iron pyrophosphate cathode material, characterized in that, Includes the following steps: According to the chemical formula of the positive electrode material, the sodium source, iron source, phosphorus source and chelating agent are dissolved in deionized water and mixed well. The mixed solution was spray-dried to obtain precursor powder. The precursor powder was dissolved in anhydrous ethanol, mixed well and dried to obtain the dried precursor. After pre-calcining the dried precursor, it was then sintered at 480℃~550℃ for 8h~10h to obtain Na4Fe. 2.8 (PO4)2P2O7 cathode material.

2. The method for preparing sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The pre-firing temperature is 300℃~350℃, and the pre-firing time is 3h~5h.

3. The method for preparing sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, The molar ratio of the chelating agent to the sodium source is (0.3–0.5):1; the chelating agent is at least one of citric acid, ethylenediaminetetraacetic acid, ethylenediaminedisuccinic acid, gluconic acid, and tartaric acid.

4. The method for preparing sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, Spray drying involves using a spray dryer to dry the material, wherein the inlet air temperature is controlled at 200℃~230℃, the outlet temperature at 100℃~130℃, the atomization pressure at 0.4Mpa~0.6Mpa, and the feed speed at 15rpm~40rpm.

5. The method for preparing sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, Anhydrous ethanol accounts for 20%-50% of the precursor powder mass.

6. The method for preparing sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, The sodium source is at least one of sodium dihydrogen phosphate, sodium carbonate, sodium pyrophosphate, and sodium phosphate; the iron source is at least one of ferric nitrate, ferrous oxalate, and ferrous chloride; the phosphorus source is at least one of sodium dihydrogen phosphate and ammonium dihydrogen phosphate; and the carbon source is at least one of citric acid, sucrose, starch, and ascorbic acid.

7. A sodium iron pyrophosphate cathode material prepared by the method for preparing sodium iron pyrophosphate cathode material according to any one of claims 1 to 6, characterized in that, The ratio of iron to phosphorus atoms in the cathode material is 2.8:

4.

8. The sodium iron pyrophosphate cathode material according to claim 7, characterized in that, The surface of the cathode material is coated with a carbon layer.

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