A spheroidal manganese-iron-based prussian white material, a preparation method and applications thereof

Abstract

The application discloses a spherical manganese-iron-based Prussian white material and a preparation method and application thereof, and the spherical manganese-iron-based Prussian white material comprises an internal core layer and an external shell layer, the internal core layer is a Prussian blue analogue containing only sodium ions, and the external shell layer is a Prussian blue analogue containing sodium and potassium ions. The sodium-containing manganese-iron-based Prussian blue analogue is prepared through a solution co-precipitation method, and then in the post-processing of the material, the method of acid treatment and ion exchange is used to realize the ordered doping of potassium ions in the manganese-iron-based Prussian blue analogue in a small amount. The potassium ion doping effectively reduces the content of crystallization water, inhibits the Jahn-Teller effect of manganese ions, improves the discharge voltage and cycle stability of the material, and reduces the lattice distortion caused by the disorderly introduction of potassium ions.

Description

Technical Field

[0001] This invention relates to the field of new energy materials technology, specifically to a spherical manganese-iron-based Prussian white material, its preparation method, and its application. Background Technology

[0002] In recent years, the demand for electrochemical energy storage, primarily based on secondary lithium-ion batteries, has increased significantly. However, limited lithium resources restrict the sustainable application of lithium-ion batteries in large-scale energy storage. Sodium-ion batteries, with their abundant, inexpensive, and widely distributed sodium resources, are considered the best alternative to lithium-ion batteries in the field of electrochemical energy storage.

[0003] Sodium-ion batteries share the same working principle and components as lithium-ion batteries, both storing and releasing electrical energy through the extraction and insertion of ions in the positive and negative electrode materials. In the development of sodium-ion battery energy storage devices, finding a suitable cathode material remains a core factor affecting the overall battery performance. Currently, transition metal oxides, polyanionic compounds, and Prussian blue analogues are considered the most commercially viable cathode materials for sodium-ion batteries. Transition metal oxides and polyanionic compounds have received widespread attention due to their high compatibility with existing lithium-ion battery production equipment. However, the larger ionic radius of sodium ions expands the interlayer spacing, allowing water molecules to more easily penetrate the material, resulting in poorer storage stability compared to ternary lithium-ion battery materials. Furthermore, both types of materials have lower specific capacity; to achieve higher specific capacity, the potential window of the battery system must be increased, leading to problems such as lattice oxygen evolution, metal dissolution, and electrolyte decomposition. Compared to the previous two materials, Prussian blue analogues have a three-dimensional open-frame structure that can provide large sodium ion diffusion channels and have a high specific capacity comparable to commercial lithium iron phosphate, thus making them highly promising for commercial applications.

[0004] However, manganese-iron-based Prussian white materials also suffer from problems such as high water of crystallization content and manganese metal dissolution. Increasing the drying temperature during post-processing is an effective way to reduce water of crystallization; however, excessively high drying temperatures can lead to decreased material structural stability and even decomposition, releasing the toxic gas cyanide (CN)₂. Simultaneously, during charge and discharge processes, the high-spin Mn... 3+ The structural distortion caused by the Jan Taylor effect can also affect the cycling stability of materials. Therefore, it is necessary to propose a simple preparation method to improve the thermal stability of manganese-iron-based Prussian white materials, reduce the Jan Taylor effect during sodium ion insertion / extraction, and improve the cycling stability of the materials. Summary of the Invention

[0005] To address the shortcomings of poor thermal stability and structural collapse caused by the Jan Taylor effect in manganese-iron-based Prussian white materials, this invention aims to provide a spherical manganese-iron-based Prussian white material, its preparation method, and its applications. This invention first prepares a sodium-containing Prussian blue analogue via solution co-precipitation. In the post-processing of the material, acid treatment and ion exchange are used to achieve a small amount of ordered potassium ion doping in the Prussian blue analogue. Potassium ion doping effectively reduces the water of crystallization content, suppresses the Jan Taylor effect of manganese ions, improves the discharge voltage and cycle stability of the sodium ion cathode material of manganese-iron-based Prussian white, and reduces the lattice distortion caused by the disordered introduction of potassium ions.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] The first objective of this invention is to provide a method for preparing spherical manganese-iron-based Prussian white material, comprising the following steps:

[0008] (1) Dissolve a manganese-based transition metal salt and a complexing agent in deionized water to obtain precursor solution A; preferably, the manganese-based transition metal salt is one or more of manganese sulfate, hydrochloride, nitrate and acetate; the complexing agent is sodium citrate, potassium citrate, sodium ethylenediaminetetraacetate or polyvinylpyrrolidone.

[0009] (2) Dissolve sodium ferrocyanide and sodium salt in a buffer solution to obtain precursor solution B; preferably, the molar amount of sodium ferrocyanide is 0.8-1.2 times the molar amount of manganese ions in precursor solution A; the sodium salt is one or more of sodium chloride, sodium sulfate and sodium nitrate; the buffer solution is one of citric acid-sodium citrate buffer, acetate-sodium acetate buffer, sodium dihydrogen phosphate-citric acid, and the pH buffer range of the buffer solution is 2-5.

[0010] (3) Add precursor liquid A to precursor liquid B at a rate of 1 / 5 of the volume of precursor liquid A added per hour. Stir for 3-6 hours and then let stand for 12-24 hours. Separate the precipitate and wash and dry it to obtain cubic blocky manganese iron-based Prussian white, labeled as Na-MnHCF. Preferably, the drying temperature is 80-120℃.

[0011] (4) Under a protective atmosphere such as nitrogen or argon, Na-MnHCF is immersed in an acidic solution at a temperature of 60℃-90℃ for 6-10 hours. Then, a solution containing potassium salt and reducing agent is added to the acidic solution. After ion exchange, the resulting precipitate is washed and dried to obtain spherical manganese-iron-based Prussian white material, labeled as NaK-MnHCF. Preferably, the hydrogen ion concentration in the acidic solution is 0.1mol / L to 1mol / L. The role of the acidic solution is to directionally modify the highly active sites such as the edges and vertices of the cubic manganese-iron-based Prussian white, ultimately transforming the product from a cube to a spherical shape, and opening ion channels for the uniform insertion of potassium ions. More preferably, the acidic solution is hydrochloric acid, citric acid, ethylenediaminetetraacetic acid (EDTA), polymaleic acid (PMA), or polyacrylic acid (PAA).

[0012] In a further embodiment, in step (4), the potassium salt is at least one of potassium chloride and potassium sulfate; the reducing agent is one of ascorbic acid, sodium thiosulfate, and sodium borohydride; the reducing agent mainly inhibits [Fe(CN)6]. 4- The oxidation process maintains the system's valence balance and promotes the exchange of potassium ions with sodium ions within the framework structure. The solvent in the solution containing potassium salt and reducing agent is one of the following: water, a mixture of water and ethanol (volume ratio = 1:1), a mixture of water and isopropanol (volume ratio = 1:1), or a mixture of water and DMF (volume ratio = 1:1). Further, the washing process involves washing the precipitate three times each with deionized water and anhydrous ethanol; the drying process involves vacuum drying the precipitate at 150-180°C for 12-24 hours.

[0013] The second objective of this invention is to provide a spherical manganese-iron-based Prussian white material, prepared by the method described in the first objective above, comprising an inner core layer and an outer shell layer, wherein the inner core layer is a Prussian blue analog containing only sodium ions, and the outer shell layer is a Prussian blue analog containing both sodium and potassium ions; its general structural formula is as follows: Where 0 < x < 2, 0 < y < 0.5, 0 ≤ z ≤ 1, n ≥ 0, Represents [Fe(CN)6] 4- Vacancies exist; this spherical manganese-iron-based Prussian white material has a monoclinic phase structure and a spherical morphology.

[0014] A third objective of this invention is to provide the aforementioned spherical manganese-iron-based Prussian white material as a positive electrode material for use in sodium-ion batteries.

[0015] Compared with existing technologies, the beneficial effects of this invention are reflected in:

[0016] 1. This invention employs a solution co-precipitation method, using a buffer solution as the solvent in precursor solution B. The buffer solution ensures pH stability during the reaction process, and a complexing agent is used to assist in the synthesis of the Prussian blue analog Na-MnHCF. The preparation process is simple, low-cost, and environmentally friendly. The specific working principle of the buffer solution is as follows: Because the solubility product of the Prussian blue analog is relatively small, in order to control the crystal nucleation rate, the transition metal ions coordinated by the complexing agent need to dissociate at a constant rate. The purpose of adding the buffer solvent is to maintain a relatively stable solvent environment, ensuring a constant reaction rate throughout the preparation process and increasing the Na... + Mn 2 + [Fe(CN)6] 4- The collision opportunities between ions ensure uniform nucleation and reduce the generation of defects in the crystallization process.

[0017] 2. Based on the preparation of pure-phase Prussian white, this invention acid-treats the edges and corners with large lattice defects generated during the crystal growth stage of the material. Through micro-surface engineering, the water content and vacancy defects of the material are further reduced, while providing channels for the uniform diffusion of potassium ions.

[0018] 3. This invention uses ion exchange to achieve the exchange of potassium ions in the solution with sodium ions in the Na-MnHCF bulk phase. The potassium ion concentration gradient in the solution is used as the driving force to keep the exchange of sodium ions and potassium ions under control. This does not change the crystal structure of the material, reduces the water content of the material, and improves the thermal stability of the material, so that the material will not decompose when dried at high temperatures.

[0019] 4. Potassium ion doping improves the Jan Taylor effect of the material, making the material exhibit two distinct discharge plateaus, increasing the median voltage of the material, and thus improving the mass energy density of the material. At the same time, the doped potassium ions can also achieve reversible insertion and extraction in sodium ion battery systems, ensuring the stability of the material's discharge capacity.

[0020] 5. The intermediate cubic blocky manganese-iron-based Prussian white N-aMnHCF prepared by this invention has consistent particle size and morphology. The final product NaK-MnHCF after potassium ion doping also has consistent particle size and morphology. This invention provides a systematic solution to the problem of product consistency, which is beneficial to the commercial application of Prussian blue compounds. Attached Figure Description

[0021] Figure 1 These are SEM images of the manganese-iron-based Prussian white material obtained in Example 3 of the present invention, wherein Figure (a) is the SEM image of the Na-MnHCF-3 material in Example 3, and Figure (b) is the SEM image of the NaK-MnHCF material in Example 3.

[0022] Figure 2 These are XRD patterns of the manganese-iron-based Prussian white materials obtained in Examples 1-4 of this invention.

[0023] Figure 3 These are thermogravimetric diagrams of the manganese-iron-based Prussian white materials obtained in Examples 1-3 of this invention.

[0024] Figure 4 This is the first-cycle discharge curve of the manganese-iron-based Prussian white material obtained in Examples 1-3 of the present invention when used as a sodium-ion cathode material. Detailed Implementation

[0025] The present invention and its technical effects will be clearly and completely described below with reference to the embodiments and accompanying drawings, so as to fully understand the purpose, features and effects of the present invention. Unless otherwise specified, all raw materials are commercially available products.

[0026] Example 1:

[0027] (1) Dissolve 10 mmol manganese sulfate and 40 mmol trisodium citrate in 100 mL of deionized water to obtain precursor solution A;

[0028] (2) Dissolve 8 mmol sodium ferrocyanide and 17.5 g sodium chloride in 200 mL citrate-sodium citrate buffer (pH=3) to obtain precursor solution B;

[0029] (3) Add precursor solution A dropwise to precursor solution B. After the addition is complete, stir for 6 hours and let stand for 24 hours. Wash the resulting precipitate three times with deionized water and anhydrous ethanol, and dry at 120℃ for 24 hours to obtain manganese-iron-based Prussian white. It is labeled as Na-MnHCF.

[0030] Example 2:

[0031] (1) Dissolve 10 mmol manganese sulfate and 40 mmol potassium citrate in 100 mL of deionized water to obtain precursor solution A;

[0032] (2) Dissolve 8 mmol of potassium ferrocyanide and 22.3 g of potassium chloride in 200 mL of citrate-sodium citrate buffer (pH=3) to obtain precursor solution B;

[0033] (3) Add precursor solution A dropwise to precursor solution B. After the addition is complete, stir for 6 hours and let stand for 24 hours. Wash the resulting precipitate three times with deionized water and anhydrous ethanol, and dry at 120℃ for 24 hours to obtain manganese-iron-based Prussian white. It is labeled as K-MnHCF.

[0034] Example 3:

[0035] (1) Dissolve 10 mmol manganese sulfate and 40 mmol trisodium citrate in 100 mL of deionized water to obtain precursor solution A;

[0036] (2) Dissolve 8 mmol sodium ferrocyanide and 17.5 g sodium chloride in 200 mL citrate-sodium citrate buffer (pH=3) to obtain precursor solution B;

[0037] (3) Add precursor solution A dropwise to precursor solution B. After the addition is complete, stir for 6 hours and let stand for 24 hours. Wash the precipitate three times with deionized water and anhydrous ethanol respectively, and dry at 120℃ for 24 hours to obtain manganese iron-based Prussian white, labeled as Na-MnHCF-3.

[0038] (4) The product Na-MnHCF-3 was immersed in a 1 mol / L hydrochloric acid solution while a protective nitrogen gas was introduced, and the mixture was allowed to stand at 80°C for 6 h. A 0.5 mol / L potassium chloride solution containing 0.8 g ascorbic acid was added to the hydrochloric acid solution, and the mixture was allowed to stand at 60°C for 6 h to complete the ion exchange. The resulting product was washed three times with deionized water and anhydrous ethanol, and dried at 120°C for 24 h to obtain spherical manganese-iron-based Prussian white material. It is labeled as NaK-MnHCF.

[0039] Example 4:

[0040] (1) Dissolve 10 mmol manganese sulfate and 40 mmol trisodium citrate in 100 mL of deionized water to obtain precursor solution A;

[0041] (2) Dissolve 8 mmol sodium ferrocyanide, 8.5 g sodium chloride and 8.5 g potassium chloride in 200 mL citrate-sodium citrate buffer (pH=3) to obtain precursor solution B;

[0042] (3) Add precursor solution A dropwise to precursor solution B. After the addition is complete, stir for 6 hours and let stand for 24 hours. Wash the resulting precipitate three times with deionized water and anhydrous ethanol, and dry at 120℃ for 24 hours to obtain manganese-iron-based Prussian white. It is labeled as NaK-MnHCF-4.

[0043] Comparative Example 1

[0044] Compared with Example 3, the difference in this comparative example is that hydrochloric acid solution treatment is not performed; all other processes are the same. The specific experimental methods are as follows:

[0045] (1) Dissolve 10 mmol manganese sulfate and 40 mmol trisodium citrate in 100 mL of deionized water to obtain precursor solution A;

[0046] (2) Dissolve 8 mmol sodium ferrocyanide and 17.5 g sodium chloride in 200 mL citrate-sodium citrate buffer (pH=3) to obtain precursor solution B;

[0047] (3) Add precursor solution A dropwise to precursor solution B. After the addition is complete, stir for 6 hours and let stand for 24 hours. Wash the precipitate three times with deionized water and anhydrous ethanol respectively, and dry at 120℃ for 24 hours to obtain manganese iron-based Prussian white, labeled as MnHCF-1.

[0048] (4) MnHCF-1 was soaked in a 0.5 mol / L, 100 mL potassium chloride solution containing 0.8 g ascorbic acid and allowed to stand at 60 °C for 6 h to complete ion exchange. The ion-exchange treated MnHCF-1 was washed three times with deionized water and anhydrous ethanol, and dried at 120 °C for 24 h to obtain the final product, manganese-iron-based Prussian white.

[0049] Comparative Example 2

[0050] Compared with Example 3, the difference in this comparative example is that water was used to replace the buffer solution in step 2, while all other processes are the same. The specific experimental methods are as follows:

[0051] (1) Dissolve 10 mmol manganese sulfate and 40 mmol trisodium citrate in 100 mL of deionized water to obtain precursor solution A;

[0052] (2) Dissolve 8 mmol of sodium ferrocyanide and 17.5 g of sodium chloride in 200 mL of water to obtain precursor solution B;

[0053] (3) Add precursor solution A dropwise to precursor solution B. After the addition is complete, stir for 6 hours and let stand for 24 hours. Wash the resulting precipitate three times with deionized water and anhydrous ethanol respectively, and dry at 120℃ for 24 hours to obtain manganese iron-based Prussian white, labeled as MnHCF-2.

[0054] (4) The product Na-MnHCF-2 was immersed in a 1 mol / L hydrochloric acid solution while purging with nitrogen gas as a protective gas, and allowed to stand at 80°C for 6 h. A 0.5 mol / L potassium chloride solution containing 0.8 g ascorbic acid and 100 mL of potassium chloride solution was added to the hydrochloric acid solution while purging with nitrogen gas as a protective gas, and allowed to stand at 60°C for 6 h to complete the ion exchange. The ion-exchange treated MnHCF-2 was washed three times with deionized water and anhydrous ethanol, and dried at 120°C for 24 h to obtain the final product manganese-iron-based Prussian white.

[0055] Structural characterization

[0056] Figure 1The image shows a SEM image of the manganese-iron-based Prussian white material obtained in Example 3. It can be seen that the manganese-iron-based Prussian white prepared by the co-precipitation method exhibits a uniform cubic shape with a size of approximately 500 nm. After acid treatment and potassium ion doping, the material becomes spherical. Spherical particles can reduce the charge resistance of ion transfer, increase the diffusion rate of sodium ions, and also have better fluidity and dispersibility, which is beneficial for the preparation and coating of cathode material slurry. From a crystallographic perspective, the edges and vertices of the cubic material disrupt the periodic arrangement of atoms, representing the most severe lattice distortion. The high density of crystal defects is also the main area for water enrichment. Etching away the areas with high defect density through acid treatment can significantly reduce the water content of the material and also facilitate the formation of spherical morphology, allowing potassium ions to be uniformly embedded within the crystal.

[0057] Figure 2 The XRD patterns of the materials prepared in Examples 1-4 show that when the alkali metal ions in the material consist only of sodium ions (Na-MnHCF), the material exhibits a monoclinic phase; when a small amount of sodium ions are replaced by potassium ions (NaK-MnHCF) through ion exchange in post-processing, the material still exhibits a monoclinic phase; when potassium ion doping (NaK-MnHCF-4) is performed during the synthesis process, the potassium ions are in a disordered state in the crystal lattice, and the crystal symmetry decreases; when all the alkali metal ions in the material are potassium ions, the material exhibits an orthorhombic phase.

[0058] Figure 3 The thermogravimetric diagrams of the materials prepared in Examples 1-3 show that during the material synthesis process, the larger radius of potassium ions crowds out the positions of zeolite water in the interstitial spaces of the crystal lattice, greatly reducing the crystal water content of the product. The potassium ions introduced through ion exchange can also significantly reduce the crystal water content of the material, while raising the lower limit of the temperature at which the material decomposes to produce cyanide (CN)2, thus improving the thermal stability of the material.

[0059] Electrochemical performance testing

[0060] Using the materials prepared in the above embodiments and comparative examples as active materials, electrochemical performance tests were conducted according to the following method: The active materials, acetylene black, and polyvinylidene fluoride were mixed and ground evenly at a mass ratio of 7:2:1 (wt%). Then, N-methylpyrrolidone was added and ground into a slurry, which was then coated onto carbon-coated aluminum foil and dried in a vacuum drying oven at 100°C for 12 hours to obtain the positive electrode sheet for sodium-ion batteries. The electrolyte used was 1M NaClO4 / (DEC:EC = 1:1, v:v), with 10vt% fluoroethylene carbonate added. A sodium sheet was used as the counter electrode for the half-cell of the sodium-ion battery. The battery assembly was carried out in a glove box protected by high-purity argon. The electrochemical test results are shown in Table 1.

[0061] Table 1:

[0062]

[0063] As shown in Table 1, compared with the examples, the comparative examples contain less sodium ions. Furthermore, due to the influence of crystal water and the "dissolution" of manganese metal, the material gradually exhibits only a single electron transfer during cycling, resulting in a faster decrease in discharge capacity. Comparative Example 2 did not use a buffer solution, which is detrimental to pH control of the solution system and cannot guarantee the stable solution environment required for the reaction. This affects the dissociation rate of the complexing agent and metal ions, accelerating crystal growth and heterogeneous nucleation, leading to the formation of more vacancy defects and crystal water, resulting in poor material performance. In Examples 2 and 4, the high potassium ion content and its disordered state reduced the structural stability of the material, and the discharge capacity also decreased significantly. Example 3, by introducing potassium ions during the material post-treatment process through ion exchange, effectively improves the cycling stability and rate performance of the material.

[0064] Figure 4 The 0.1C initial discharge curves for Examples 1-3 show that the Prussian white (Na-MnHCF) material containing only sodium ions exhibits only a clear plateau, while the NaK-MnHCF and K-MnHCF materials containing potassium ions show a higher Mn discharge rate because potassium ions suppress the Jamette effect. 2+ / Mn 3+ Fe 2+ / Fe 3+ The redox reaction is fully demonstrated, showing two distinct charge-discharge plateaus. At the same time, the increase in the material's charge-discharge voltage also means an increase in the material's energy density.

[0065] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for the preparation of a spheroidal manganese-iron-based prussian white material, characterized in that: The method comprises the following steps: (1) dissolving a manganese-based transition metal salt and a complexing agent in water to obtain a precursor solution A; (2) dissolving sodium ferrocyanide and a sodium salt in a buffer solution to obtain a precursor solution B; the sodium salt is one or more of sodium chloride, sodium sulfate and sodium nitrate; the buffer solution is one of a citric acid-sodium citrate buffer solution, an acetic acid-sodium acetate buffer solution and sodium dihydrogen phosphate-citric acid, and the pH buffering range of the buffer solution is 2-5; (3) adding the precursor solution A to the precursor solution B, stirring and mixing, then standing and aging, separating the obtained precipitate, and washing and drying the precipitate to obtain cubic manganese-iron-based Prussian white, marked as Na-MnHCF; (4) immersing the Na-MnHCF in an acidic solution under a protective atmosphere, then adding a solution containing a potassium salt and a reducing agent to the acidic solution, performing ion exchange, washing and drying the obtained precipitate to obtain a spherical manganese-iron-based Prussian white material; the potassium salt is at least one of potassium chloride and potassium sulfate. The spherical manganese-iron-based Prussian white material comprises an inner core layer and an outer shell layer, the inner core layer is a Prussian blue analogue containing only sodium ions, and the outer shell layer is a Prussian blue analogue containing sodium and potassium ions.

2. The method of producing a spheroidized ferromanganese-iron-based prussian blue material according to claim 1, characterized by: In step (1), the manganese-based transition metal salt is one or more of a manganese sulfate, a manganese chloride, a manganese nitrate and a manganese acetate; and the complexing agent is sodium citrate, potassium citrate, sodium ethylenediaminetetraacetate or polyvinylpyrrolidone.

3. The method of producing a spheroidized ferromanganese-iron-based prussian blue material according to claim 1, characterized by: In step (3), the standing and aging time is 12-24 h, and the drying temperature is 80-120 °C.

4. The method of producing a spheroidized ferromanganese-iron-based prussian blue material according to claim 1, characterized by: In step (4), the hydrogen ion concentration in the acidic solution is 0.1 mol / L-1 mol / L.

5. The method of producing a spheroidized ferromanganese-iron-based prussian blue material according to claim 4, characterized in that: In step (4), the acidic solution is hydrochloric acid, citric acid, ethylenediaminetetraacetic acid, polymaleic acid or polyacrylic acid.

6. The method of producing a spheroidized ferromanganese-iron-based prussian blue material according to claim 4 or 5, characterized in that: In step (4), the reducing agent is one of ascorbic acid, sodium thiosulfate and sodium borohydride.

7. The method of producing a spheroidized ferromanganese-iron-based prussian blue material according to claim 1, characterized by: In step (4), the drying temperature is 150-180 °C.

8. A spheroidized ferromanganese-iron-based prussian white material, characterized in that: The spherical manganese-iron-based Prussian white material is prepared by the preparation method of any one of claims 1-7, and comprises an inner core layer and an outer shell layer, the inner core layer is a Prussian blue analogue containing only sodium ions, and the outer shell layer is a Prussian blue analogue containing sodium and potassium ions.

9. The spherical manganese-iron-based Prussian white material of claim 8 is used as a positive electrode material in a sodium ion battery.

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