Prussian blue-based sodium-ion battery cathode material, preparation method and application thereof

Abstract

The application discloses a Prussian blue type sodium ion battery positive electrode material and a preparation method and application thereof, the positive electrode material is prepared by substituting transition metal elements in Mn elements in a manganese-based Prussian blue structure; wherein the transition metal element is any one of iron, cobalt, nickel and copper, and the concentration gradient of the substitution element changes from high to low from the core of the positive electrode material particles to the middle part of the particles. The preparation method is as follows: sodium ferrocyanide, a divalent soluble salt of a substitution element, a divalent manganese salt and sodium citrate are mixed with deionized water respectively to obtain precursor solutions; the positive electrode material is prepared through a coprecipitation reaction, aging, centrifugation and drying. The application is applied to a sodium ion battery system and used as a positive electrode material. The core structure rich in substitution elements can inhibit the volume change of the material in the charging and discharging process and the Jahn-Teller effect of Mn, the shell rich in substitution elements can further inhibit the dissolution of Mn 2+ , so that the material has excellent cycle stability.

Description

Technical Field

[0001] This invention relates to a cathode material, its preparation method and application, and more particularly to a Prussian blue sodium-ion battery cathode material, its preparation method and application, belonging to the field of sodium-ion battery material technology. Background Technology

[0002] With the gradual depletion of non-renewable resources such as coal, oil, and natural gas, the development of sustainable and renewable clean energy has become an inevitable trend. However, clean energy mainly consists of intermittent and regionally limited energy types such as wind, solar, and tidal power. Therefore, advanced energy storage equipment is needed to ensure the full utilization of these energy sources. Low-cost, recyclable, and highly efficient secondary batteries provide a good solution for large-scale energy storage. Compared with lithium-ion batteries, sodium-ion batteries have advantages such as abundant raw material resources, low cost, and wide distribution, making them more suitable for large-scale energy storage.

[0003] The performance and cost of sodium-ion batteries are primarily limited by their cathode materials. Therefore, developing high-performance cathode materials is crucial for the commercial application of sodium-ion batteries. A wide variety of cathode materials exist for sodium-ion batteries, including layered oxides, polyanionic materials, and Prussian blue-based materials. Among these, Prussian blue-based cathode materials, with their large ion transport channels and simple synthesis methods, show great promise. In particular, manganese-based Prussian blue cathode materials, due to their high operating voltage and theoretical capacity, as well as low raw material and preparation costs, have become one of the most commercially promising cathode materials for sodium-ion batteries. However, during the sodium ion insertion / extraction process, manganese-based Prussian blue materials undergo structural changes and volume expansion / contraction due to phase transitions, leading to the Jahn-Teller effect of Mn and the Mn... 2+ The dissolution phenomenon occurs, ultimately leading to the collapse of the material structure and rapid degradation of its performance.

[0004] To address the aforementioned issues, researchers have proposed numerous strategies in recent years to stabilize and optimize the structure of manganese-based Prussian blue materials. For example, (CN112645354B) constructs a coating layer on the material surface via ion exchange to improve cycle stability; (CN117263210A) uses cesium to replace potassium on the material surface, thereby forming a coating and improving material performance; or (CN106920964B) uses transition metal elements to replace potassium from the grain interior to the surface in a concentration gradient, forming a unidirectional concentration gradient structure. While these strategies have achieved relatively good results, relying solely on surface coating layers or unidirectional concentration gradient structures is insufficient to fully stabilize the bulk structure and structural phase transition processes during charge and discharge. Therefore, developing a manganese-based Prussian blue cathode material that balances bulk and surface structural stability with excellent electrochemical performance is of great significance for the commercial application of sodium ions. Summary of the Invention

[0005] To address the shortcomings of the aforementioned technologies, this invention provides a Prussian blue sodium-ion battery cathode material, its preparation method, and its application.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: a Prussian blue sodium-ion battery cathode material, which is prepared by gradient substitution of Mn element in manganese-based Prussian blue structure by transition metal elements;

[0007] The transition metal element is any one of iron, cobalt, nickel, and copper, and the substitution element varies in concentration from high to low from the core and surface of the cathode material particle to the middle of the particle.

[0008] Preferably, the general formula of the positive electrode material is Na. n M x Mn 1-x [Fe(CN)6] z ·mH2O, where M is a transition metal element, 1<n≤2, 0.1<x≤0.5, 0.8<z≤1, 0<m≤4.

[0009] A method for preparing Prussian blue-based sodium-ion battery cathode materials includes the following steps:

[0010] Step 1: Mix soluble divalent manganese salt, sodium citrate, and deionized water to obtain solution A;

[0011] Step 2: Mix sodium ferrocyanide with deionized water to obtain solution B;

[0012] Step 3: Mix the divalent soluble salt of the substituted element, sodium citrate, and deionized water to obtain solution C. Divide solution C into solution C1 and solution C2 according to volume ratio.

[0013] Step 4: Mix sodium citrate with deionized water to obtain solution D;

[0014] Step 5: Add solutions B and C1 simultaneously to solution D using a peristaltic pump to carry out a co-precipitation reaction and obtain particle cores rich in substituted elements.

[0015] Step 6: When a certain amount of reaction solution C1 is consumed, add solution A dropwise to solution C1 using a peristaltic pump. Then, add solution C1 containing manganese and solution B dropwise to solution D simultaneously using a peristaltic pump to carry out a coprecipitation reaction.

[0016] Step 7: When a certain amount of reaction solution A is consumed, solution C2 is added dropwise to solution A using a peristaltic pump. Then, solution C1 containing the substitution element and manganese element and solution B are added dropwise to solution D simultaneously using a peristaltic pump to carry out a co-precipitation reaction and obtain a suspension.

[0017] Step 8: After stirring the suspension, age it, then rinse it several times with deionized water. Separate the suspension using a centrifuge, and place the separated material in an oven to dry until completely dry. This yields a Prussian blue sodium-ion battery cathode material with a bidirectional substitution element concentration gradient structure.

[0018] Preferably, in step one, the soluble divalent manganese salt in solution A is at least one of nitrate, chloride, acetate, and sulfate, and its concentration is 0.1 to 1 mol / L;

[0019] In step two, the concentration of sodium ferrocyanide in solution B is 0.1–1 mol / L.

[0020] Preferably, in step three, the divalent soluble salt of the substituted element in solution C is a nitrate, chloride, acetate, or sulfate of any one of the elements selected from iron, cobalt, nickel, and copper, with a concentration of 0.1–1 mol / L, and the volume ratio of solution C1 to solution C2 is 1:9–9:1.

[0021] The concentration of sodium citrate in solutions A, C, and D is 0.5–4 mol / L.

[0022] Preferably, the ratio of the sum of the molar amounts of divalent manganese in solution A and the molar amounts of divalent ions of substituted elements in solution C to the molar amounts of sodium ferrocyanide in solution B is 1:1; the ratio of the molar amounts of soluble divalent manganese salts to the molar amounts of soluble salts of divalent substituted elements in solution C is 9:1 to 1:1.

[0023] Preferably, the volume ratio of solution A to solution C is 2:1 to 10:1; the total volume ratio of solution A and solution C to the volume ratio of solution B and solution D is 1:1:1; and the relationship between the feeding rates of solutions A, B, C1, and C2 via the peristaltic pump satisfies the requirement that solutions A, C1, C2, and B are added simultaneously.

[0024] Preferably, the ratio of the amount of solution A consumed to the original volume of solution A is 2:5 to 7:10; the ratio of the amount of solution C1 consumed to the original volume of solution C1 is 0.2:10 to 1:2.

[0025] Preferably, in step eight, the centrifuge speed is 5000 rpm to 8000 rpm, the centrifugation time is 5 min to 15 min, the drying temperature is 60℃ to 90℃, and the drying time is 10 h to 15 h.

[0026] Throughout the entire preparation process, the coprecipitation reaction temperature was 25℃~40℃; the stirring time of the suspension was 0.5h~2h; the aging temperature was 25℃~90℃; and the aging time was 2h~24h.

[0027] An application of a Prussian blue-based sodium-ion battery cathode material: In a sodium-ion battery system, the prepared Prussian blue-based sodium-ion battery cathode material is used as the cathode material.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] 1) The Prussian blue sodium-ion battery cathode material of the present invention has a bidirectional substitution element concentration gradient structure. The core structure rich in substitution elements suppresses the material volume change and the Jahn-Teller effect of Mn during charging and discharging; the outer shell rich in substitution elements further suppresses the Mn... 2+ The dissolution of the Prussian blue cathode material gives it both excellent bulk structure and surface structure stability, thus resulting in excellent cycle performance.

[0030] 2) The preparation method disclosed in this invention uses different transition metal elements (iron, cobalt, nickel, copper) to replace manganese in the material. Through optimization of the preparation process and substitution elements, gradient structure and core and surface design rich in substitution elements, the product has excellent structural stability and electrochemical performance. This preparation method has the advantages of low cost and simple process, and is suitable for large-scale industrial production. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of the Prussian blue sodium-ion battery cathode material prepared in Example 1 of the present invention.

[0032] Figure 2This is a scanning electron microscope (SEM) image of the Prussian blue sodium-ion battery cathode material prepared in Example 1 of the present invention.

[0033] Figure 3 The X-ray diffraction (XRD) patterns are shown for the Prussian blue sodium-ion battery cathode materials prepared in Examples 1, 1, and 2 of this invention.

[0034] Figure 4 The cycling performance of the Prussian blue sodium-ion battery cathode materials prepared in Examples 1, 1, and 2 of this invention at 0.1C is shown. Detailed Implementation

[0035] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0036] Example 1

[0037] 1) Mix 0.024 mol manganese chloride, 0.08 mol sodium citrate, and deionized water to obtain 34 ml of solution A. The concentration of manganese chloride is 0.6 mol / L, and the concentration of sodium citrate is 2 mol / L.

[0038] 2) Mix 0.03 mol of sodium ferrocyanide with deionized water to obtain 50 ml of solution B. The concentration of sodium ferrocyanide in this solution is 0.6 mol / L.

[0039] 3) Mix 0.006 mol nickel chloride, 0.02 mol sodium citrate, and deionized water to obtain 16 ml of solution C, which is then divided into C1 and C2 with a volume ratio of 1:1. The concentration of nickel chloride is 0.6 mol / L, and the concentration of sodium citrate is 2 mol / L.

[0040] 4) Mix 0.1 mol of sodium citrate with deionized water to obtain 50 ml of solution D. The concentration of sodium citrate is 2 mol / L.

[0041] 5) Solution B and solution C1 were simultaneously added to solution D via a peristaltic pump to carry out a co-precipitation reaction, resulting in particle cores rich in substituted elements; the dropping rates of the two solutions were the same, and were 1 ml / min.

[0042] 6) When 2 ml of reaction solution C1 is consumed, solution A is added dropwise to solution C1 using a peristaltic pump. Then, solution C1 containing nickel and manganese elements and solution B are simultaneously added dropwise to solution D using a peristaltic pump to carry out a coprecipitation reaction. The dropping rate of solutions A, B and C1 is the same: 1 ml / min.

[0043] 7) When 25 ml of reaction solution A is consumed, solution C2 is added dropwise to solution A using a peristaltic pump. Then, solution C1 containing nickel and manganese and solution B are added dropwise to solution D simultaneously using a peristaltic pump to carry out a co-precipitation reaction and obtain a suspension. The dropping rates of solutions A, B, C1 and C2 are the same, all of which are 1 ml / min.

[0044] 8) After stirring the suspension at 25℃ for 2 hours, it was aged at 80℃ for 20 hours, rinsed several times with deionized water, and then centrifuged at 7000 rpm for 10 minutes. The centrifuged material was then dried in an oven at 80℃ for 12 hours to obtain a manganese-based Prussian blue sodium-ion battery cathode material with a bidirectional nickel ion concentration gradient. 1.65 Ni 0.18 Mn 0.82 [Fe(CN)6] 0.91 ·2.3H2O.

[0045] See the structural diagram of this material. Figure 1 The material consists of cubic particles with a nickel-rich core and surface. Inside the particles, the nickel concentration gradually decreases from the core and surface towards the center, while the manganese concentration gradually increases towards the center.

[0046] SEM analysis showed that the product particle size ranged from 0.5 μm to 4 μm. (See [link to relevant documentation]). Figure 2 In this embodiment, the combined effects of bidirectional nickel concentration gradient substitution and aging crystal growth process result in product particles with a near-cubic morphology and the formation of a small number of small particles.

[0047] XRD analysis revealed characteristic diffraction peaks of the monoclinic structure at 16.8°, 23.5°, 24.1°, and 34°, corresponding to (200), (2-20), (220), and (400) diffraction peaks, respectively, confirming that the material in this embodiment is a monoclinic structure. (See [link to relevant documentation]). Figure 3 . Figure 3 The XRD results of this embodiment were also compared with those of Comparative Example 1 (Prussian blue sodium-ion battery cathode material without elemental substitution) and Comparative Example 2 (Prussian blue sodium-ion battery cathode material with a unidirectional substituted element concentration gradient structure). The aforementioned materials exhibited characteristic diffraction peaks of (200), (2-20), (220), and (400) at 16.8°, 23.5°, 24.1°, and 34°, all showing a monoclinic structure. These results not only demonstrate that the crystal structures of the aforementioned materials are consistent, both being monoclinic, but also prove that the gradient substitution of nickel has no significant impact on the bulk crystal structure of the manganese-based material, indicating that the substituted element nickel has good compatibility with the original material framework elements.

[0048] Using the Prussian blue-based material prepared in this embodiment as the positive electrode and metallic sodium as the counter electrode, a coin cell was assembled in an argon-filled glove box using a 1 mol / L NaPF6 / propylene carbonate (PC):ethylene carbonate (EC):fluorinated ethylene carbonate (FEC) (45:45:10) organic electrolyte. The assembled battery underwent long-cycle testing at a voltage range of 2–4 V and a current density of 0.1 C (1 C = 170 mAh / g). At a current density of 0.1 C, the initial capacity of the material obtained in this embodiment was 140.3 mAh / g, and the capacity retention rate after 150 cycles was 82.6%. The battery after 150 cycles was disassembled, and the Mn and Fe element contents in the electrolyte and separator of the battery system were determined using inductively coupled plasma mass spectrometry (ICP-MS). The Mn and Fe element contents were very low, exceeding the instrument's detection limit, and therefore undetectable. The absence of detected Mn and Fe elements in the diaphragm and electrolyte indicates that the material's bidirectional elemental gradient structure provides a more stable bulk and surface structure, suppressing the Jahn-Teller effect and Mn content. 2+ The dissolution phenomenon occurs; thanks to the good structural stability of the material, the cathode material prepared in this embodiment has excellent cycle stability.

[0049] Example 2

[0050] The difference from Example 1 is that nickel chloride in solution C is replaced with an equimolar amount of cobalt chloride; thus, a Prussian blue sodium-ion battery cathode material with a bidirectional cobalt concentration gradient is obtained.

[0051] Example 3

[0052] The difference from Example 1 is that nickel chloride in solution C is replaced with an equimolar amount of ferric chloride; thus, a Prussian blue sodium-ion battery cathode material with a bidirectional iron concentration gradient is obtained.

[0053] Example 4

[0054] The difference from Example 1 is that nickel chloride in solution C is replaced with an equimolar amount of copper chloride; thus, a Prussian blue sodium-ion battery cathode material with a bidirectional copper element concentration gradient is obtained.

[0055] Comparative Example 1:

[0056] 1) Mix 0.03 mol manganese chloride, 0.1 mol sodium citrate, and deionized water to obtain 50 ml of solution A. The concentration of manganese chloride is 0.6 mol / L, and the concentration of sodium citrate is 2 mol / L.

[0057] 2) Mix 0.03 mol of sodium ferrocyanide with deionized water to obtain 50 ml of solution B. The concentration of sodium ferrocyanide in solution B is 0.6 mol / L.

[0058] 3) Mix 0.1 mol of sodium citrate with deionized water to obtain 50 ml of solution C. The concentration of sodium citrate in C is 2 mol / L.

[0059] 4) Solution A and solution B are simultaneously added to solution C using a peristaltic pump to carry out a coprecipitation reaction, wherein the dropping rate of the two solutions is the same and is 1 ml / min.

[0060] 5) After stirring the suspension at 25°C for 2 hours, it was aged at 80°C for 20 hours, rinsed several times with deionized water, and then centrifuged at 7000 rpm for 10 minutes. The centrifuged material was then placed in an oven and dried at 80°C for 12 hours to obtain the material of Comparative Example 1, which is a manganese-based Prussian blue material without elemental substitution.

[0061] XRD analysis revealed characteristic diffraction peaks of the monoclinic structure at 16.8°, 23.5°, 24.1°, and 34°, corresponding to (200), (2-20), (220), and (400) diffraction peaks, respectively, confirming that the product corresponding to Comparative Example 1 is a monoclinic structure. (See [link to relevant documentation]). Figure 3 This result is consistent with the result measured in Example 1, proving that the bidirectional nickel concentration gradient substitution in Example 1 does not have a significant impact on the bulk structure of the material.

[0062] Using the Prussian blue-based material prepared in this comparative example as the positive electrode and metallic sodium as the counter electrode, a coin cell was assembled in an argon-filled glove box using a 1 mol / L NaPF6 / propylene carbonate (PC):ethylene carbonate (EC):fluorinated ethylene carbonate (FEC) (45:45:10) organic electrolyte. The assembled cells were subjected to long-term cycling tests at a voltage range of 2–4 V and a current density of 0.1 C (1C = 170 mAh / g). At a current density of 0.1 C, the initial capacity of the material obtained in this example was 166.4 mAh / g, and the capacity retention after 150 cycles was 36.1%. The battery corresponding to Comparative Example 1 after 150 cycles was disassembled, and the Mn and Fe element contents in the electrolyte and separator of the battery system were determined by inductively coupled plasma mass spectrometry (ICP-MS). The Mn and Fe element contents were 0.57 mg / L and 0.12 mg / L, respectively. The small amount of Fe was due to a small amount of Fe[(CN)6] released after structural collapse. 4- This is caused by the entry into the electrolyte. The elemental analysis results indicate that, compared to the Prussian blue material with a two-way nickel concentration gradient in Example 1, this comparative example exhibits poor bulk and surface structure stability, leading to increased Jahn-Teller effect and reduced Mn content.2+ The occurrence of dissolution ultimately leads to the collapse of the material structure and rapid degradation of its performance.

[0063] Comparative Example 2:

[0064] 1) Mix 0.024 mol manganese chloride, 0.08 mol sodium citrate, and deionized water to obtain 34 ml of solution A. The concentration of manganese chloride is 0.6 mol / L, and the concentration of sodium citrate is 2 mol / L.

[0065] 2) Mix 0.03 mol of sodium ferrocyanide with deionized water to obtain 50 ml of solution B. The concentration of sodium ferrocyanide in solution B is 0.6 mol / L.

[0066] 3) Mix 0.006 mol nickel chloride, 0.02 mol sodium citrate, and deionized water to obtain 16 ml of solution C. The concentration of nickel chloride is 0.6 mol / L, and the concentration of sodium citrate is 2 mol / L.

[0067] 4) Mix 0.1 mol of sodium citrate with deionized water to obtain 50 ml of solution D. The concentration of sodium citrate in this solution is 2 mol / L.

[0068] 5) Solution C is added dropwise to solution A using a peristaltic pump. Then, solutions A and B, which contain nickel and manganese, are simultaneously added dropwise to solution D using a peristaltic pump to carry out a co-precipitation reaction. The dropping rate of solutions A, B, and C is the same: 1 ml / min.

[0069] 6) After stirring the suspension at 25°C for 2 hours, it was aged at 80°C for 20 hours, rinsed several times with deionized water, and then centrifuged at 7000 rpm for 10 minutes. The centrifuged material was then dried in an oven at 80°C for 12 hours to obtain the material of Comparative Example 1, which is a Prussian blue sodium-ion battery cathode material with a unidirectional nickel concentration gradient. The surface of the material is a nickel-rich layer, and the nickel concentration decreases gradually towards the interior of the material particles.

[0070] XRD analysis revealed characteristic diffraction peaks of the monoclinic structure at 16.8°, 23.5°, 24.1°, and 34°, corresponding to (200), (2-20), (220), and (400) Å, respectively, confirming that the comparison sample is a monoclinic structure. (See [link to relevant documentation]). Figure 3 The XRD results indicate that unidirectional elemental gradient substitution has no significant effect on the crystal structure of the material.

[0071] Using the Prussian blue-based material prepared in this comparative example as the positive electrode and metallic sodium as the counter electrode, a coin cell was assembled in an argon-filled glove box using a 1 mol / L NaPF6 / propylene carbonate (PC):ethylene carbonate (EC):fluorinated ethylene carbonate (FEC) (45:45:10) organic electrolyte. The assembled cells underwent long-term cycling tests at a voltage range of 2–4 V and a current density of 0.1 C (1C = 170 mAh / g). At a current density of 0.1 C, the initial capacity of the material obtained in this example was 138.8 mAh / g, and the capacity retention rate after 150 cycles was 67.5%. Compared with Comparative Example 1, the electrochemical performance of Comparative Example 2 was significantly improved; however, compared with the Prussian blue material with a two-way nickel concentration gradient in Example 1, the cycling stability of the material was still poor.

[0072] The battery corresponding to Comparative Example 2, after 150 cycles, was disassembled, and the Mn and Fe element contents in the electrolyte and separator of the battery system were determined using inductively coupled plasma mass spectrometry (ICP-MS). The Mn and Fe element contents were 0.12 mg / L and 0.05 mg / L, respectively. The small amount of Fe was due to a small amount of Fe[(CN)6] released after structural collapse. 4- This is caused by the entry of Mn into the electrolyte. Compared with Comparative Example 1, the content of Mn and Fe elements in the separator and electrolyte of the battery system corresponding to Comparative Example 2 is significantly reduced; however, compared with Example 1, the material corresponding to Comparative Example 2 still exhibits the Jahn-Teller effect of Mn and Mn. 2+ Dissolution phenomenon. This result demonstrates that Comparative Example 2, which only possesses a unidirectional nickel concentration gradient structure, still suffers from insufficient bulk structural stability, resulting in the continued occurrence of the Jahn-Teller effect and Mn dissolution. 2+ This phenomenon ultimately leads to the collapse of the material structure and the deterioration of its electrochemical performance.

[0073] The present invention provides Prussian blue sodium-ion battery cathode materials with a bidirectional substitution element concentration gradient structure prepared through the above embodiments, and Prussian blue sodium-ion battery cathode materials with no elemental substitution and with a unidirectional substitution element concentration gradient structure prepared by comparison. Table 1 below shows the results. Figure 4 The electrochemical test results of the materials corresponding to Example 1, Comparative Example 1, and Comparative Example 2 are statistically presented. The test voltage was 2-4V and the current density was 0.1C. Table 2 below shows the elemental contents of Mn and Fe in the separator and electrolyte of the battery system after 150 cycles, as measured by inductively coupled plasma mass spectrometry (ICP-MS).

[0074] Table 1 Electrochemical performance data of the examples and comparative examples

[0075] Example 1 Comparative Example 1 Comparative Example 2 Initial capacity / mAh / g 140.3 166.4 138.8 Capacity after 150 cycles / mAh / g 115.9 60.8 93.8 Capacity retention rate / % 82.6 36.1 67.5

[0076] Table 2 shows the Mn and Fe element content (mg / L) in the electrolyte and diaphragm of the examples and comparative examples after 150 cycles, as determined by ICP-MS.

[0077] Example 1 Comparative Example 1 Comparative Example 2 Mn 0 0.57 0.15 Fe 0 0.12 0.05

[0078] Table 1 above shows the electrochemical test results of the examples and comparative examples. Comparative Example 1, with its unsubstituted Prussian blue sodium-ion battery cathode material, exhibits a high initial capacity (166.4 mAh / g), which can be attributed to the absence of electrochemically inactive nickel in the material structure; after 150 cycles at 0.1C, it has only a reversible capacity of 60.8 mAh / g, with a capacity retention of 36.1%. Comparative Example 2, with its unidirectionally substituted Prussian blue sodium-ion battery cathode material, has an initial capacity of 138.8 mAh / g, and a capacity of 93.8 mAh / g after 150 cycles at 0.1C, with a capacity retention of 67.5%. Compared to Comparative Examples 1 and 2, Example 1 demonstrates excellent cycle stability, retaining a capacity of 115.9 mAh / g after 150 cycles at 0.1C, with a capacity retention of 82.6%. The above electrochemical test structure comparison proves that the Prussian blue sodium-ion battery cathode material with a two-way substituted element concentration gradient structure has excellent electrochemical performance.

[0079] Table 2 shows the Mn and Fe element contents in the separator and electrolyte of the battery systems corresponding to the above materials, as determined by ICP-MS. The battery system corresponding to the material in Comparative Example 1 showed relatively high Mn and Fe element contents (0.57 mg / L and 0.12 mg / L, respectively), indicating that the manganese-based Prussian blue sodium-ion battery cathode material underwent the Jahn-Teller effect of Mn and Mn during charge-discharge. 2+ Dissolution occurs, leading to material structural collapse and rapid capacity decay. In Comparative Example 2, the Mn and Fe content in the corresponding battery system was measured to be 0.15 mg / L and 0.05 mg / L, respectively. Compared to Comparative Example 1, manganese dissolution was somewhat suppressed, but due to the still poor stability of the bulk structure, the Jahn-Teller effect of Mn and Mn dissolution still occur during the electrochemical process. 2+ Dissolution phenomenon. Comparison with the results measured in the battery system corresponding to the above materials, Mn and Fe elements were measured in Example 1; this result demonstrates that the Prussian blue sodium-ion battery cathode material with a bidirectional substitution element concentration gradient structure has a more stable bulk phase and surface structure, effectively suppressing the Jahn-Teller effect of Mn and Mn... 2+ The dissolution phenomenon enables the material to maintain excellent structural stability and cycling performance during long cycles.

[0080] As can be seen from the above, this invention achieves stability of the bulk phase and surface structure of the material by constructing a bidirectional substitution element concentration gradient structure, with the core and shell of the cathode material being rich in substitution elements, and the substitution elements changing in a gradient from the core and shell towards the center of the material. This suppresses the Jahn-Teller effect of Mn and Mn... 2+ The dissolution phenomenon occurs, thereby obtaining a Prussian blue-based sodium-ion battery cathode material with good electrochemical cycling stability. This method has advantages such as simple preparation process, low raw material cost, and short production cycle. Furthermore, when the Prussian blue-based cathode material provided in this embodiment of the invention is applied to the cathode of a sodium-ion battery, the sodium-ion battery exhibits excellent cycle stability, retaining a capacity of 115.9 mAh / g after 150 cycles at 0.1C, with a capacity retention rate of 82.6%. Based on the Prussian blue-based sodium-ion battery cathode material with excellent cycle stability provided in this embodiment of the invention, it has good commercial application prospects in the field of sodium-ion batteries.

[0081] The above embodiments are not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the technical solution of the present invention are also within the protection scope of the present invention.

Claims

1. A Prussian blue-based sodium-ion battery cathode material, characterized in that: This cathode material is prepared by gradient substitution of Mn elements in a manganese-based Prussian blue structure with transition metal elements. The transition metal element is any one of iron, cobalt, nickel, and copper, and the substitution element varies in concentration from high to low from the core and surface of the cathode material particle to the middle of the particle.

2. The Prussian blue sodium-ion battery cathode material according to claim 1, characterized in that: The general formula of the cathode material is Na. n M x Mn 1-x [Fe(CN)6] z ·mH2O, where M is a transition metal element, 1<n≤2, 0.1<x≤0.5, 0.8<z≤1, 0<m≤4.

3. The method for preparing the Prussian blue sodium-ion battery cathode material according to any one of claims 1 or 2, characterized in that: Includes the following steps: Step 1: Mix soluble divalent manganese salt, sodium citrate, and deionized water to obtain solution A; Step 2: Mix sodium ferrocyanide with deionized water to obtain solution B; Step 3: Mix the divalent soluble salt of the substituted element, sodium citrate, and deionized water to obtain solution C. Divide solution C into solution C1 and solution C2 according to volume ratio. Step 4: Mix sodium citrate with deionized water to obtain solution D; Step 5: Add solutions B and C1 simultaneously to solution D using a peristaltic pump to carry out a co-precipitation reaction and obtain particle cores rich in substituted elements. Step 6: When a certain amount of reaction solution C1 is consumed, add solution A dropwise to solution C1 using a peristaltic pump. Then, add solution C1 containing manganese and solution B dropwise to solution D simultaneously using a peristaltic pump to carry out a coprecipitation reaction. Step 7: When a certain amount of reaction solution A is consumed, solution C2 is added dropwise to solution A using a peristaltic pump. Then, solution C1 containing the substitution element and manganese element and solution B are added dropwise to solution D simultaneously using a peristaltic pump to carry out a co-precipitation reaction and obtain a suspension. Step 8: After stirring the suspension, age it, then rinse it several times with deionized water. Separate the suspension using a centrifuge, and place the separated material in an oven to dry until completely dry. This yields a Prussian blue sodium-ion battery cathode material with a bidirectional substitution element concentration gradient structure.

4. The method for preparing the Prussian blue sodium-ion battery cathode material according to claim 3, characterized in that: In step one, the soluble divalent manganese salt in solution A is at least one of nitrate, chloride, acetate and sulfate, and its concentration is 0.1 to 1 mol / L; In step two, the concentration of sodium ferrocyanide in solution B is 0.1–1 mol / L.

5. The method for preparing the Prussian blue sodium-ion battery cathode material according to claim 3, characterized in that: In step three, the divalent soluble salt of the substituted element in solution C is a nitrate, chloride, acetate, or sulfate of any one of the elements selected from iron, cobalt, nickel, and copper, with a concentration of 0.1–1 mol / L, and the volume ratio of solution C1 to solution C2 is 1:9–9:

1. The concentration of sodium citrate in solutions A, C, and D is 0.5–4 mol / L.

6. The method for preparing the Prussian blue sodium-ion battery cathode material according to claim 3, characterized in that: The ratio of the sum of the molar amounts of divalent manganese in solution A and the molar amounts of divalent ions of substituted elements in solution C to the molar amounts of sodium ferrocyanide in solution B is 1:1; the ratio of the molar amounts of soluble divalent manganese salts to the molar amounts of soluble salts of divalent substituted elements in solution C is 9:1 to 1:

1.

7. The method for preparing the Prussian blue sodium-ion battery cathode material according to claim 3, characterized in that: The volume ratio of solution A to solution C is 2:1 to 10:1, and the total volume of solution A and solution C is 1:1:1 with the volume ratio of solution B and solution D. The relationship between the feeding rates of solutions A, B, C1, and C2 via the peristaltic pump satisfies the requirement that solutions A, C1, C2, and B are added simultaneously.

8. The method for preparing the Prussian blue sodium-ion battery cathode material according to claim 3, characterized in that: The ratio of the consumption of solution A to its original volume is 2:5 to 7:10; the ratio of the consumption of solution C1 to its original volume is 0.2:10 to 1:

2.

9. The method for preparing the Prussian blue sodium-ion battery cathode material according to claim 3, characterized in that: In step eight, the centrifuge speed is 5000 rpm to 8000 rpm, the centrifugation time is 5 min to 15 min; the drying temperature is 60℃ to 90℃, and the drying time is 10 h to 15 h. Throughout the entire preparation process, the coprecipitation reaction temperature was 25℃~40℃; the stirring time of the suspension was 0.5h~2h; the aging temperature was 25℃~90℃; and the aging time was 2h~24h.

10. An application of a Prussian blue-based sodium-ion battery cathode material, characterized in that: In a sodium-ion battery system, the Prussian blue sodium-ion battery cathode material prepared according to any one of claims 3-9 is used as the cathode material.

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