Method for improving crystallinity of prussian blue, prussian blue, positive electrode material and battery

Abstract

The application relates to the technical field of positive electrode materials, and particularly discloses a method for improving the crystallinity of Prussian blue, Prussian blue, a positive electrode material and a battery. The method for improving the crystallinity of Prussian blue comprises the following steps: mixing a low-crystallinity Prussian blue and its analogues, an aqueous solution of an alcohol compound, hexacyanoferrate (II) and a reducing substance to obtain a reaction solution; reacting the reaction solution at a temperature of 40-80 DEG C to prepare solid precipitates; and washing and drying the prepared solid precipitates to obtain high-crystallinity Prussian blue. The method utilizes a reducing environment, an aqueous solution of an alcohol compound and high-concentration Fe(CN)6 4‑ to promote the separation of crystal water in the material structure from the crystal coordination, Fe(CN)6 4‑ to enter the structure and combine with M (M=Fe, Mn, etc.) in the Prussian blue and its analogues, thereby reducing the vacancy and crystal water content in the structure. The crystallinity of the Prussian blue and its analogues is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of positive electrode materials, in particular to a method for improving the crystallinity of Prussian blue, Prussian blue, a positive electrode material and a battery. BACKGROUND

[0002] Environmental deterioration and energy consumption intensify the demand for green energy, but most green energy has the characteristics of intermittency and regionality, which is difficult to meet the actual application, while secondary batteries can well avoid this shortcoming. Lithium-ion batteries have the advantages of long cycle life and high energy density, and have been widely studied and applied, playing an important role in the power market and energy storage market. However, lithium resources are scarce and unevenly distributed, and as the demand for lithium-ion batteries grows, the price of lithium is rising rapidly, making it difficult to meet the low-cost requirements of the energy storage market. Sodium ions and lithium ions are in the same main group and have similar physicochemical properties. The secondary battery system based on sodium ions can reduce the cost by 30-40% compared with lithium-ion batteries, and has received attention in recent years.

[0003] Prussian blue, as a positive electrode material for sodium-ion batteries, has the advantages of low cost, high structural adjustability and good cycle performance, and is one of the potential positive electrode materials for the commercial application of sodium-ion batteries in the future. 4- Structural defects such as vacancies are the main factors affecting the sodium storage performance of Prussian blue positive electrode materials. At present, the crystal structure of Prussian blue is mainly controlled by adjusting the reaction conditions.

[0004] Therefore, it is necessary to provide a method for improving the crystallinity of Prussian blue to at least partially solve the above problems. SUMMARY

[0005] A series of simplified concepts are introduced in the summary section, which will be further described in detail in the specific embodiment section. The summary section of the present application does not mean to attempt to limit the key features and necessary technical features of the claimed technical solution, nor to attempt to determine the protection scope of the claimed technical solution.

[0006] To at least partially solve the above problems, the first aspect of the present application provides a method for improving the crystallinity of Prussian blue, comprising the following steps:

[0007] S1, mixing a low-crystallinity Prussian blue and its analogues, an aqueous solution of an alcohol compound, hexacyanoferrate(II) and a reducing substance to obtain a reaction solution;

[0008] S2, reacting the reaction solution in S1 at a temperature of 40-80℃ to prepare a solid precipitate;

[0009] S3, washing and drying the solid precipitate prepared in S2 to obtain high-crystallinity Prussian blue.

[0010] According to the method for improving the crystallinity of Prussian blue provided in the first aspect of the application, the low-crystallinity Prussian blue and the like are subjected to an optimized treatment to reduce the vacancy and crystalline water defects. The purpose of using the aqueous solution of the alcohol compound is to use water to dissolve the reactants, and to use the alcohol compound molecules to promote the water molecules in the structure of the Prussian blue and the like to be separated from the crystal structure, so as to help the hexacyanoferrate ions to enter the structure of the Prussian blue and the like, thereby reducing the vacancy content. The preparation method provided in the application has low requirements on conditions, does not produce toxic gas in the synthesis process, and has low difficulty in large-scale production.

[0011] In addition, the method for improving the crystallinity of Prussian blue according to the above-mentioned embodiments of the application can further have the following additional technical features:

[0012] In some embodiments of the application, the mass ratio of the alcohol compound to water in the aqueous solution of the alcohol compound is 1:3-6:1; and / or the concentration of the hexacyanoferrate (II) salt is 1-10 mol / L.

[0013] In some embodiments of the application, the weight ratio of the low-crystallinity Prussian blue and the like, the aqueous solution of the alcohol compound, the hexacyanoferrate (II) salt, and the reducing substance is 10-30:90-110:140-160:1-10.

[0014] In some embodiments of the application, in the step S1, the hexacyanoferrate (II) salt, the reducing substance, and the low-crystallinity Prussian blue and the like are sequentially added to the aqueous solution of the alcohol compound.

[0015] In some embodiments of the application, the hexacyanoferrate (II) salt is one or more of sodium ferrocyanide decahydrate, anhydrous sodium ferrocyanide, and potassium ferrocyanide; and / or

[0016] The alcohol compound is one or more of ethanol, methanol, and ethylene glycol; and / or

[0017] The reducing substance is one or more of ascorbic acid, sodium ascorbate, and sodium iodide.

[0018] The reaction is carried out under one of stirring, standing, and ultrasonic; and / or

[0019] The reaction time of the reaction is 5-20 h; and / or

[0020] The reaction is carried out under a sealed condition.

[0021] In some embodiments of the present application, the step S3 satisfies any one or several of the following conditions:

[0022] (1) Filtration is performed before the washing;

[0023] (2) The washing specifically refers to washing three times with deionized water and anhydrous ethanol respectively;

[0024] (3) Filtration is performed after the washing;

[0025] (4) The drying specifically refers to drying in a vacuum oven for 20-30h.

[0026] The second aspect of the present application provides a Prussian blue prepared by the above method.

[0027] The Prussian blue provided by the second aspect of the present application has high crystallinity.

[0028] The third aspect of the present application provides a positive electrode material comprising the above Prussian blue.

[0029] The positive electrode material provided by the third aspect of the present application has the characteristics of high capacity, long cycle life and good cycle stability.

[0030] The fourth aspect of the present application provides a battery comprising the above positive electrode material.

[0031] The battery provided by the fourth aspect of the present application is a sodium ion battery, and the Prussian blue in the sodium ion battery positive electrode material is reversibly inserted / deintercalated into the Prussian blue crystal structure by sodium ions to realize an electrochemical reaction.

[0032] Additional aspects and advantages of the present application will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0033] The above and / or additional aspects and advantages of the present application will become apparent and be readily appreciated from the following description, including the accompanying drawings in which:

[0034] Figure 1 is a schematic diagram of the principle of the method for improving the crystallinity of Prussian blue according to the present application. DETAILED DESCRIPTION

[0035] Embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings, in which the same or similar reference numerals refer to the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by reference to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as limiting the present application.

[0036] The application provides a method for improving the crystallinity of Prussian blue, comprising the following steps:

[0037] S1, mixing a low-crystallinity Prussian blue and its analogues, an alcohol compound aqueous solution, hexacyanoferrate (II) and a reducing material to obtain a reaction solution;

[0038] S2, reacting the reaction solution in S1 at a temperature of 40-80 DEG C to prepare a solid precipitate;

[0039] S3, washing and drying the solid precipitate prepared in S2 to obtain high-crystallinity Prussian blue.

[0040] According to the embodiment of the application, the reducing environment, the alcohol compound aqueous solution and the medium-high concentration Fe(CN)6 4- facilitate the separation of the crystal water from the crystal coordination in the material structure, the medium-high concentration Fe(CN)6 4- is more easily introduced into the structure of Prussian blue and its analogues, the low concentration Fe(CN)6 4- cannot achieve the purpose of replacing the crystal water in the structure. Figure 1 As shown in the formula, Fe(CN)6 4- is introduced into the structure and combined with M (M = Fe, Mn, etc.) in Prussian blue and its analogues, thereby reducing the vacancy and crystal water content in the structure, improving the crystallinity of Prussian blue and its analogues. The reaction temperature should be controlled at 40-80 DEG C to ensure that the reaction is fully carried out and the Prussian blue crystal structure is not damaged. At the same time, if the reaction is carried out in the form of stirring, the stirring speed also needs to be properly controlled to ensure that the particle size is uniform.

[0041] Further, the mass ratio of the alcohol compound to water in the alcohol compound aqueous solution is 1:3 to 6:1. The alcohol compound is one or more of ethanol, methanol and ethylene glycol. Preferably, the alcohol compound is ethanol. According to the above ratio, when the ethanol capacity is too high, the metal salt and the reducing material required for the reaction cannot be effectively dissolved, and when the aqueous solution ratio is too high, the water molecules in the structure of Prussian blue and its analogues cannot be fully separated from the structure, and it is difficult to greatly improve the crystallinity of Prussian blue and its analogues.

[0042] The concentration of the hexacyanoferrate (II) is 1-10 mol / L.

[0043] Further, the weight ratio of the low-crystallinity Prussian blue and its analogues, the alcohol compound aqueous solution, the hexacyanoferrate (II) and the reducing material is 10-30:90-110:140-160:1-10. This combination of formula fully utilizes the characteristics of various components, improves the reaction rate and the crystallization quality.

[0044] It should be noted that the weight ratio of the above-mentioned raw materials is the optimal implementation range, and in the present application, the progress of the reaction is not limited by the composition ratio, that is, the reaction can still occur beyond the range. In practical applications, the component ratio can be adjusted according to specific needs and conditions.

[0045] Further, the hexacyanoferrate (II) salt is one or more of sodium ferrocyanide decahydrate, anhydrous sodium ferrocyanide, and potassium ferrocyanide. The hexacyanoferrate (II) salt is easily soluble and can provide sufficient ferrous ions (Fe 2+ ) in the solvent. This enables the generation of Prussian blue faster during the reaction, improving the reaction rate. Preferably, the hexacyanoferrate (II) salt is sodium ferrocyanide salt, avoiding the entry of K ions into the structure, affecting the electrochemical performance.

[0046] On the basis of the above-mentioned embodiments, the reducing substance is one or more of ascorbic acid, sodium ascorbate, and sodium iodide. By adjusting the reduction potential of the reaction environment, the formation and crystallization of Prussian blue are affected. Ascorbic acid, sodium ascorbate, and sodium iodide can provide electrons to ferrous ions (Fe 3+ ), reducing them to ferrous ions (Fe 2+ ), thereby promoting the formation of Prussian blue.

[0047] Further, the reaction is preferably carried out under stirring conditions. By stirring the reaction solution, the components and temperature in the reaction solution can be uniformly distributed, ensuring that the reaction proceeds under the same conditions. Preferably, the reaction is carried out under slow stirring conditions. During the formation and crystallization of Prussian blue, excessive shear force can cause the crystalline particles to break, thereby affecting the crystallinity and quality of the product. Slow stirring can reduce the shear force and reduce the risk of crystalline damage. Slow stirring allows the crystalline particles to grow in a relatively calm environment, which is conducive to the formation of larger and more complete crystals, and helps to improve the crystallinity and quality of Prussian blue.

[0048] Further, the reaction time of the reaction is 5-20h. Sufficient reaction time is conducive to the progress of the reaction as expected, reduces the possibility of side reactions, and helps to improve the purity and crystallinity of Prussian blue.

[0049] Further, the reaction is carried out under sealed conditions. In air, Prussian blue is easily oxidized, resulting in reduced crystallinity. Carrying out the reaction under sealed conditions can effectively isolate air and prevent Prussian blue from being oxidized, thereby improving crystallinity. In the process of preparing Prussian blue, water plays an important role in the formation and growth of crystals, and carrying out the reaction under sealed conditions can effectively reduce water loss and provide a good environment for the crystallization of Prussian blue. Under sealed conditions, the concentration and temperature of the reaction solution can be more easily controlled within a suitable range, thereby improving the reaction rate and facilitating the formation and growth of Prussian blue.

[0050] Further, in the step S3, any one or several of the following conditions is met:

[0051] (1) Filtration is performed before the washing;

[0052] (2) The washing specifically refers to washing three times with deionized water and anhydrous ethanol respectively;

[0053] (3) Filtration is performed after the washing;

[0054] (4) The drying specifically refers to drying in a vacuum oven for 20-30 h.

[0055] Through the treatment of these steps, the purity of the sample can be significantly improved.

[0056] In the present application, the transition metal elements selected for the synthesis of Prussian blue with low crystallinity and its analogues include but are not limited to ferrous sulfate heptahydrate, manganese sulfate monohydrate, nickel sulfate heptahydrate, and mixtures with different proportions.

[0057] The second aspect of the present application provides a Prussian blue prepared by the above method. By simply treating Prussian blue with low crystallinity, Prussian blue with high crystallinity can be obtained.

[0058] The third aspect of the present application provides a positive electrode material, which comprises a current collector and an active layer arranged on at least one surface of the current collector, and the active layer comprises Prussian blue prepared by the above method, a conductive additive, a binder, etc. The Prussian blue, the conductive additive, etc. are adhered together and attached to the current collector by the binder. Preferably, by improving the crystallinity of Prussian blue, the sodium storage performance thereof is improved, and the positive electrode material is used in a sodium ion battery, which helps to form an electrode with high capacity, long cycle life and good cycle stability.

[0059] The fourth aspect of the present application provides a battery comprising the above electrode. The Prussian blue in the positive electrode material of the sodium ion battery is reversibly inserted / deintercalated by sodium ions into the Prussian blue crystal structure, realizing an electrochemical reaction.

[0060] The present application will be described with reference to specific examples, which are illustrative in nature and are not intended to limit the application in any way.

[0061] Example 1

[0062] In 100 g of an aqueous ethanol solution (ethanol: water = 1:1), 150 g of sodium ferrocyanide decahydrate, 5 g of ascorbic acid, and 20 g of low-crystallinity Prussian blue and its analogues were sequentially added, and then heated in a slow stirring manner in an oil bath at 40°C for 12 h, with the container being sealed during the stirring. Thereafter, the resulting solution was filtered under vacuum using a circulating water vacuum pump, washed three times with deionized water and anhydrous ethanol, and then the filtered product was dried in a vacuum oven for 24 h, to obtain high-crystallinity Prussian blue and its analogues.

[0063] Example 2

[0064] In 100 g of an aqueous ethanol solution (ethanol: water = 1:3), 150 g of sodium ferrocyanide decahydrate, 5 g of sodium ascorbate, and 20 g of low-crystallinity Prussian blue and its analogues were sequentially added, and then heated in a slow stirring manner in an oil bath at 60°C for 20 h, with the container being sealed during the stirring. Thereafter, the resulting solution was filtered under vacuum using a circulating water vacuum pump, washed three times with deionized water and anhydrous ethanol, and then the filtered product was dried in a vacuum oven for 20 h, to obtain high-crystallinity Prussian blue and its analogues.

[0065] Example 3

[0066] In 100 g of an aqueous ethanol solution (ethanol: water = 6:1), 150 g of sodium ferrocyanide decahydrate, 5 g of sodium iodide, and 20 g of low-crystallinity Prussian blue and its analogues were sequentially added, and then heated in a slow stirring manner in an oil bath at 80°C for 5 h, with the container being sealed during the stirring. Thereafter, the resulting solution was filtered under vacuum using a circulating water vacuum pump, washed three times with deionized water and anhydrous ethanol, and then the filtered product was dried in a vacuum oven for 30 h, to obtain high-crystallinity Prussian blue and its analogues.

[0067] Comparative Example 1

[0068] 48.4 g of sodium ferrocyanide was dissolved in 100 ml of water to form solution A, 1 mol of a transition metal salt was dissolved in 100 ml of water to form solution B, solution A was added dropwise to solution B, and then heated in a slow stirring manner in an oil bath at 60°C for 12 h. Thereafter, the resulting solution was filtered under vacuum using a circulating water vacuum pump, washed three times with deionized water and anhydrous ethanol, and then the filtered product was dried in a vacuum oven for 24 h, to obtain low-crystallinity Prussian blue and its analogues.

[0069] In the present comparative example, the transition metal salt is one or more of ferrous sulfate heptahydrate, manganese sulfate monohydrate, and nickel sulfate heptahydrate.

[0070] Alternatively, the low crystallinity Prussian blue and analogues thereof prepared in the present comparative example can be used to prepare the high crystallinity Prussian blue and analogues thereof in Examples 1-3.

[0071] Comparative Example 2

[0072] In 100 g of water, 150 g of sodium ferrocyanide decahydrate, 5 g of ascorbic acid, and 20 g of low crystallinity Prussian blue and analogues thereof were sequentially weighed and placed in an oil bath at 60 °C for slow stirring for 12 h, with the container being sealed during the stirring process. The resulting solution was then filtered under vacuum using a circulating water vacuum pump, washed three times with deionized water and anhydrous ethanol, and finally the product obtained by filtration was dried in a vacuum oven for 24 h to obtain the modified Prussian blue and analogues thereof.

[0073] Performance evaluation of Examples 1-3 and Comparative Examples 1-2: Examples 1-3 all improved the crystallinity of Prussian blue and analogues thereof to some extent, so that the specific discharge capacity and cycle stability of Prussian blue and analogues thereof were higher than those of Comparative Example 1 and Comparative Example 2.

[0074] Stripping test: the Prussian blue positive electrodes prepared using Examples 1-3 and Comparative Examples 1-3 were respectively used as active materials for electrochemical performance test and analysis, and the results are shown in Table 1. The specific steps are as follows:

[0075] S1: Preparation of positive electrode sheet. The active material, Ketjen black (carbon black), and PVDF were mixed in a weight ratio of 8:1:1, dispersed in an organic solvent NMP to form a uniform slurry. Then the slurry was cast (to form an active layer) on a carbon-coated aluminum foil (current collector) by a doctor blade method, and vacuum dried at 120 °C for 12 h, and then cut into positive electrode sheets with a diameter of 12 mm.

[0076] S2: Assemble 2016 type stripping batteries in an inert gas-filled glove box, in which the contents of H2O and O2 are both less than 0.1 ppm. Use a piece of metallic sodium as the counter electrode, use 1M NaClO4 (EC:DEC = 1:1 vol%) + 5% FEC as the electrolyte, and use a commercial glass fiber separator. Use a LAND CT2001A battery test system to perform constant current charging / discharging tests at room temperature, and the test procedure is as follows: 1C charging to 4.2V, 1C discharging to 2V, and cycle test. The results are shown in the following table:

[0077] Discharge capacity (mAh / g) 300 cycle capacity retention (%) Example 1 115 80 Example 2 122 87 Example 3 117 75 Comparative Example 1 98 70 Comparative Example 2 105 73

[0078] The crystallinity of Prussian blue and its analogues is improved in Examples 1-3 to some extent, and the specific discharge capacity and cycle stability of Prussian blue and its analogues are higher than those of Comparative Examples 1 and 2. In Example 2, the reaction temperature and the proportion of ethanol are more conducive to accelerating the structure of Prussian blue and its analogues to high crystallinity, and the reaction time is longer, the reaction is more thorough, the crystallinity of Prussian blue and its analogues is improved more obviously, and the specific discharge capacity and cycle stability of Prussian blue and its analogues are more excellent. Although the discharge capacity of Example 3 is improved more obviously than that of the comparative example, the cycle stability is poor due to the destruction of the structure caused by the excessively high reaction temperature. Compared with Comparative Example 1, although it is difficult for water molecules in the structure of Prussian blue and its analogues to separate from the crystal structure in the pure water solvent reaction system, the vacancy content of Prussian blue and its analogues will still decrease under the high Fe(CN)6 4- concentration and reducing reaction environment, and the electrochemical performance is slightly improved.

[0079] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, different embodiments or examples described in the present specification and the features of different embodiments or examples can be combined and combined by those skilled in the art without contradiction.

[0080] Although the embodiments of the present application have been shown and described above, it should be understood that the above-described embodiments are exemplary and should not be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

Claims

1. A method for improving the crystallinity of Prussian blue, characterized in that, Includes the following steps: S1. A reaction solution is prepared by mixing low-crystallinity Prussian blue and its analogues, aqueous solutions of alcohols, ferric hexacyanate (II), and reducing agents. S2. The reaction solution in S1 is reacted at a temperature of 40-80℃ to prepare a solid precipitate; S3. Wash and dry the solid precipitate obtained in S2 to obtain highly crystalline Prussian blue; The mass ratio of the alcohol compound to water in the aqueous solution of the alcohol compound is 1:3 to 6:1; and / or the concentration of the hexacyanoferric(II) salt is 1-10 mol / L.

2. The method for improving the crystallinity of Prussian blue according to claim 1, characterized in that, The weight ratio of the low-crystallinity Prussian blue and its analogues, aqueous solutions of alcohols, ferric(II) hexacyanate, and reducing agents is 10–30: 90–110: 140–160: 1–10.

3. The method for improving the crystallinity of Prussian blue according to claim 1, characterized in that, In step S1, the hexacyanoferrate(II) salt, the reducing agent, the low-crystallinity Prussian blue and its analogues are sequentially added to an aqueous solution of an alcohol compound.

4. The method for improving the crystallinity of Prussian blue according to claim 1, characterized in that, The hexacyanoferrate(II) salt is one or more of sodium ferrocyanide decahydrate, anhydrous sodium ferrocyanide, and potassium ferrocyanide; and / or The alcohol compound is one or more selected from ethanol, methanol, and ethylene glycol; and / or The reducing substance is one or more of ascorbic acid, sodium ascorbate, and sodium iodide.

5. The method for improving the crystallinity of Prussian blue according to claim 1, characterized in that, The reaction is carried out under one of the following conditions: stirring, standing, or sonication; and / or The reaction time is 5-20 hours; and / or The reaction is carried out under sealed conditions.

6. The method for improving the crystallinity of Prussian blue according to claim 1, characterized in that, In step S3, one or more of the following conditions must be met: (1) The washing process involves vacuum filtration; (2) The washing process specifically involves rinsing three times each with deionized water and anhydrous ethanol; (3) After washing, the material is filtered. (4) The drying process specifically involves drying in a vacuum oven for 20-30 hours.

7. A Prussian blue, characterized in that, Prepared using the method described in any one of claims 1-6.

8. A positive electrode material, characterized in that, The cathode material includes Prussian blue according to claim 7.

9. A battery, characterized in that, Including the cathode material according to claim 8.

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