A Prussian blue-based cathode material, its preparation method and application

Abstract

The present invention discloses a Prussian blue-based cathode material, its preparation method and application, mainly relating to the technical field of sodium-ion batteries and their cathode materials, and comprising the following steps: (1) Dissolve a transition metal salt, a complexing agent and an alkaline substance in deionized water to obtain solution A, and control the pH value of solution A to be 7.0-14.0; (2) Dissolve sodium ferrocyanide in deionized water to obtain solution B; (3) Under stirring conditions, mix solution A and solution B, with a mixing time of 5-30 min, then add an acidic substance to the mixed solution for reaction. After the reaction is completed, carry out aging, washing and drying to obtain the Prussian blue-based cathode material; The high-crystallinity Prussian blue-based cathode material prepared by the present invention, when applied to the cathode of a sodium-ion battery, can exhibit the characteristics of high capacity and high cycle stability.

Description

Technical Field

[0001] The present invention relates to a Prussian blue-based cathode material, a preparation method thereof and an application, and mainly relates to the technical field of sodium ion batteries and their cathode materials.

[0002] Background Art

[0003] Due to the abundant sodium resources, low price, and the similar principles between sodium ion batteries and lithium ion batteries as well as the compatibility of cell preparation, sodium ion batteries are considered to be one of the most promising large-scale energy storage technologies in the future. Among several sodium ion battery cathode materials, Prussian blue-based materials are considered to have great application value due to their unique three-dimensional framework structure, obvious cost advantages and safety advantages.

[0004] Among many methods for preparing Prussian blue-based materials, although the co-precipitation method has the advantages of easy industrial scale-up and low process cost. However, in this preparation method, the transition metal salt solution and the sodium ferrocyanide solution react as soon as they come into contact, resulting in many structural defects in the prepared material, thus leading to poor electrical properties of the material.

[0005] To solve this problem, there is a method of first reacting a transition metal salt with a complexing agent to prepare an insoluble transition metal organic complex, and then dispersing / dissolving the transition metal complex and sodium ferrocyanide in an aqueous solution. By adjusting the pH value of the solution, the transition metal complex is slowly dissolved to release transition metal ions and react with sodium ferrocyanide to prepare a Prussian blue-based material with high crystallinity (CN 114768780A). However, this method does not mix the reaction raw materials evenly at the molecular level before the reaction occurs. Instead, the insoluble transition metal complex first dissolves and releases transition metal ions, and then diffuses into the solution to mix and react with sodium ferrocyanide. There is a situation of uneven mixing of transition metal ions and ferrocyanide ions throughout the process. There will be a situation where one part of transition metal ions is mixed and contacted with multiple equivalents of sodium ferrocyanide, resulting in uncontrollable structural defects in the material after the reaction, thus affecting the lattice integrity of the prepared material.

[0006] There is also a method of dissolving a complexing agent and sodium ferrocyanide in an aqueous solution, and through processes such as adjusting the pH value of the reaction solution by adding acid and heating the reaction solution, sodium ferrocyanide is decomposed to release Fe 2+ and react with itself (CN 114853032 A). This method does not have the problem of uneven mixing of reaction raw materials compared with patent CN114768780A, and the lattice integrity and electrical properties of the prepared material are relatively good. However, in this method, the decomposition of sodium ferrocyanide under acidic conditions will produce toxic hydrogen cyanide, which is not conducive to industrial scale-up.

[0007] There is also a method of cooling two reaction solutions into ice, then crushing and mixing the ice, and then heating to melt the ice to initiate the reaction (NanoLett. 2022, 22, 1302 - 1310). The crystal form of the material prepared by this method is relatively good, but the reaction raw materials are only macroscopically mixed evenly at the physical level. Therefore, there are still structural defect problems in the prepared material. Summary of the Invention

[0008] The first aspect of the present invention is to provide a preparation method of a Prussian blue-based cathode material, which can make the reaction raw material solutions be mixed evenly first and then react, so that the prepared Prussian blue-based cathode material has high crystallinity, specific capacity and cycle stability.

[0009] The technical solution adopted by the present invention is as follows:

[0010] A preparation method of a Prussian blue-based cathode material, characterized by comprising the following steps:

[0011] (1) Dissolve a transition metal salt, a complexing agent and an alkaline substance in deionized water to obtain solution A, and control the pH value of solution A to be 7.0 - 14.0;

[0012] (2) Dissolve sodium ferrocyanide in deionized water to obtain solution B;

[0013] (3) Under stirring conditions, mix solution A and solution B for 5 - 30 min, then add an acidic substance to the mixed solution. The solution starts to change from clear to turbid. After the reaction is completed, stop adding the acidic substance, age to obtain a suspension, separate the suspension, wash and dry the obtained solid substance to obtain the Prussian blue-based cathode material.

[0014] The reaction principle of the present invention is:

[0015] The present invention first regulates the pH value of solution A. At a high pH value, the solubility product constant of the complexing agent and the transition metal ions in solution A increases. When solution A and solution B are mixed, the transition metal ions and ferrocyanide ions are mixed evenly at the chemical molecular level but do not react, and the mixed solution is in a clear state. Then, by adding an acidic solution or introducing an acidic gas, the solubility product constant of the complexing agent and the transition metal salt is reduced, and the reaction occurs at a low pH. The reaction solution changes from clear to turbid, thereby generating a Prussian blue-based material with high crystallinity.

[0016] The further setting of the present invention is:

[0017] In step (1):

[0018] The transition metal salt is selected from one or more of iron salts, manganese salts, nickel salts or other transition metal salts, and the concentration of the transition metal salt is 0.01 - 1.0 M.

[0019] Preferably, the transition metal salt is ferrous sulfate with a concentration of 0.5 M.

[0020] The complexing agent is selected from one or more of the complexing agents whose complexing ability varies with the pH value of the solution. Preferably, it is one or more of complexing agents such as sodium citrate, citric acid, ethylenediaminetetraacetic acid, sodium ethylenediaminetetraacetate, bipyridine, 8-hydroxyquinoline-5-sulfonic acid, phytic acid, tannic acid, alginic acid, polyacrylic acid, sodium carboxymethyl cellulose, etc.

[0021] The molar ratio of the transition metal salt to the complexing agent is 0.1 - 10.0.

[0022] Preferably, the transition metal salt is ferrous sulfate, the complexing agent is sodium citrate, and the molar ratio of the transition metal salt to the complexing agent is 1:1.

[0023] The basic substance is used to adjust the pH value of solution A to 7.0 - 14.0, and the basic substance is selected from one or more of inorganic bases and organic bases.

[0024] The basic substance is selected from one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, and ammonia water; particularly preferably sodium carbonate, and the pH value adjusts the pH value of solution A to 7.5.

[0025] In step (3):

[0026] The molar ratio of the transition metal salt to sodium ferrocyanide is 0.1 - 10.0, and the preferred molar ratio is 1.0.

[0027] The acidic substance is an inorganic acid, an organic acid, or an acidic gas, and the amount of the acidic substance used is to adjust the pH value of the reaction solution in step (3) to be less than the pH value of solution A in step (1).

[0028] The pH value adjustment range of the reaction solution in step (3) is 1.0 - 10.0, and the preferred pH value is 4.5.

[0029] When the acidic substance is a liquid, the initiation reaction is a "liquid - liquid" reaction.

[0030] When the acidic substance is a gas, the initiation reaction is a "gas - liquid" reaction.

[0031] Preferably, the acidic substance is selected from one or more of sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, formic acid, acetic acid, citric acid, or acidic gases such as sulfur trioxide, carbon dioxide, sulfur dioxide, etc. that are acidic when dissolved in water.

[0032] The aging time is 1 - 24 h, the drying temperature is 80 - 350 °C, and the drying time is 6 - 48 h. Preferably, the aging time is 2 h, the drying temperature is 120 °C, and the drying time is 12 h.

[0033] The Prussian blue-based cathode material prepared by the above method in the present invention has a structural formula of: Na x M[Fe(CN)6] y ·nH2O, where: x = 1.4 - 2, y > 0.9, and M is one or more of transition metal elements.

[0034] The second object of the present invention is to provide an application of the Prussian blue-based cathode material prepared above in the preparation of sodium-ion batteries. Through experiments, it is confirmed that the high-crystallinity Prussian blue-based cathode material prepared in the present invention, when applied to the cathode of sodium-ion batteries, can exhibit the characteristics of high capacity and high cycle stability.

[0035] The beneficial effects of the present invention are as follows:

[0036] (1) Based on the fact that the solubility product constants of the same complexing agent with the same transition metal salt ions are different at different pH values, the present invention proposes a method for preparing high-crystallinity Prussian blue-based materials. The materials prepared by this method, when applied to the cathode of sodium-ion batteries, can exhibit the characteristics of high capacity and high cycle stability.

[0037] (2) According to this reaction principle, further, the "gas-liquid" reaction can be realized to prepare Prussian blue. The "gas-liquid" process has more uniform reactivity and obvious advantages in industrial scale-up.

[0038] (3) The material of the present invention has a high capacitance and good cycle performance when applied as the cathode material of sodium-ion batteries.

[0039] (4) The operation of the present invention is simple. The reaction process and end point are both related to the pH value of the reaction solution, which is convenient for identification and control. Description of the Drawings

[0040] Figure 1 It is the reaction solution diagram corresponding to different time points when adding an acidic solution in Example 1 of the present invention;

[0041] Figure 2 It is the SEM diagram of the Prussian blue-based material prepared in Example 1 of the present invention;

[0042] Figure 3 It is the charge-discharge curve diagram of the Prussian blue-based material prepared in Example 1 of the present invention;

[0043] Figure 4 It is the cycle stability curve diagram of the Prussian blue-based material prepared in Example 1 of the present invention;

[0044] Figure 5 It is the XRD comparison diagram of the Prussian blue-based materials prepared in Example 1, Example 2 and Comparative Example 4 of the present invention. Detailed Embodiments

[0045] The following further elaborates on the substantial features and advantages of the present invention through the description of embodiments.

[0046] Example 1

[0047] (1) Solution A: At 25 °C, 0.05 mol of ferrous sulfate and 0.05 mol of sodium citrate were dissolved in 100 mL of deionized water, and a certain amount of sodium carbonate was added to make the pH value of the solution greater than 7.5.

[0048] (2) Solution B: At 25 °C, 0.05 mol of sodium ferrocyanide was dissolved in 100 mL of deionized water.

[0049] (3) Solution A was quickly poured into Solution B while stirring. The stirring speed was 1200 rpm. After stirring for 5 min after addition, a 2.5 mol / L sulfuric acid solution was added to the mixed solution at a rate of 10 mL / min. When the pH value of the solution reached 4.5, the addition of acid was stopped, and stirring was continued while maintaining the temperature for 2 h. The reaction solution was centrifuged, washed 3 times each with deionized water and ethanol, and dried in a vacuum drying oven at 120 °C for 12 h to obtain the Prussian blue-based material, and the dry yield of the material was 99.2%.

[0050] Product confirmation:

[0051] First, in combination with Figure 1 shown, a reaction control diagram at different time points of adding an acidic solution is shown. In the Figure 1 leftmost view, it is the view after mixing Solution A and Solution B. It can be seen from this figure that before adding the acidic solution, the mixed solution has always been a dark green transparent solution, and no product is formed. In the Figure 1 second picture from the left in the figure, it is the state when the acidic solution was first added to the mixed solution for 5 min. It can be seen from the figure that after adding the acidic solution, the reaction was triggered, and the color of the solution became lighter and began to become turbid. From the 3rd and 4th pictures, it can be seen that as the acidic solution continued to be added, the reaction continued, and the color of the solution slowly turned grayish white. The 5th picture is the state of the reaction solution after adding the quantitative acidic solution. At this time, the reaction solution is white and turbid, and the white substance inside is the Prussian blue-based material.

[0052] Based on the above changes, the inventor analyzed the reasons as follows: In Solution A involved in the present invention, by regulating the pH value of the solution, the solubility product constant of the complexing agent and the transition metal ions is increased. At high pH values, the PKsp of Fe 2+ and citrate is 15.5. After mixing with Solution B, the transition metal ions and ferrocyanide ions are mixed uniformly at the chemical molecular level, but no reaction occurs, and the mixed solution is in a clear state. By adding an acidic solution or introducing an acidic gas, the solubility product constant of the complexing agent and the transition metal salt is reduced. At low pH values, Fe 2+The PKsp of citrate is 3.08, which causes the reaction to occur, the reaction solution to become turbid, and a Prussian blue-like material with high crystallinity to be formed.

[0053] Figure 2 The SEM image of the Prussian blue-like material prepared in Example 1 is shown. The crystal particles of the material are about 2 - 4 μm, the surface of the material is smooth, the particles are complete, and the crystallinity is high.

[0054] Performance test:

[0055] Test method: The Prussian blue-like material prepared in Example 1, SP, and PVDF are mixed according to a mass ratio of 8:1:1, added with NMP and stirred into a slurry, coated on aluminum foil, and made into a positive electrode sheet of a sodium-ion battery through drying, die-cutting, and laminating. Using metallic sodium as the negative electrode, glass fiber (brand Whatman GF / D) as the separator, and a solution of NaPF6 in propylene carbonate (PC) / ethyl methyl carbonate (EMC) as the electrolyte, a CR2025 button battery is assembled in a glove box filled with argon for charge and discharge testing.

[0056] Test standard:

[0057] Under the conditions of a current density of 100 mAh / g and a voltage range of 2.0 - 4.0 V, the capacity and cycle performance of the battery are tested at 1C respectively.

[0058] Test results: Under the condition of 1C, the initial discharge capacity of the battery is 142.6 mAh / g, and after 350 cycles, the capacity retention rate is 80%.

[0059] Alternative Example 1: Influence of the selection of different acid concentrations and acid addition rates on the product performance

[0060] The preparation method is the same as that in Example 1, except that: the concentration and addition rate of sulfuric acid in step (3) are adjusted, and their influence on the product performance is detected, as shown in Table 1.

[0061] Table 1

[0062]

[0063] As shown in Table 1, the concentration and addition rate of the acid in step (3) have a great influence on the cycle stability in the electrical performance of the prepared Prussian blue-like material. The applicant analyzed through experiments and research:

[0064] When the sulfuric acid concentration is determined, the slower the acid addition rate, the better the cycle stability of the prepared material. This is because a slower acid addition rate can trigger the reaction slowly, resulting in better crystal form integrity of the generated material. However, when the acid addition rate is reduced to less than 10 mL / min, the increase in the cycle stability of the material is not obvious. Therefore, an acid addition rate of 10 mL / min is preferably selected.

[0065] When the acid addition rate is determined, the lower the concentration of the acid, the better the cycle stability of the prepared material. This is the same principle as determining the acid concentration to reduce the acid addition rate. A lower acid concentration also initiates the reaction slowly, resulting in a better crystal form integrity of the generated material. However, when the acid concentration is reduced below 2.5 mol / L, the increase in the cycle stability of the material is not obvious. Therefore, a sulfuric acid concentration of 2.5 mol / L is preferably selected.

[0066] In summary, the speed of the reaction initiated by the initiator sulfuric acid is comprehensively affected by the acid concentration and the acid addition rate. For sulfuric acid, when the concentration is 2.5 mol / L and the acid addition rate is 10 mL / min, the electrical properties of the prepared material are better. Further reducing the acid concentration or the acid addition rate is of little significance. Therefore, 2.5 mol / L sulfuric acid and an acid addition rate of 10 mL / min are preferably selected.

[0067] Replacement Example 2: Influence of the selection of different acidic liquids on the product performance

[0068] The preparation method is the same as that of Example 1, except that: the type of acid selected in step (3) is adjusted, and its influence on the product performance is detected, as shown in Table 2.

[0069] Table 2

[0070] Serial number Acidic solution Capacitance (mAh / g) Number of cycles to maintain 80% capacity Example 1 Sulfuric acid 142.6 350 Replacement Example 2-1 Hydrochloric acid 143.5 357 Replacement Example 2-2 Phosphoric acid 135.2 280 Replacement Example 2-3 Nitric acid 145.2 355 Replacement Example 2-4 Acetic acid 143.5 361 Replacement Example 2-5 Ascorbic acid 142.9 365 。

[0071] As shown in Table 2, the replacement of the acid in step (3) has a great influence on the cycle stability in the electrical properties of the prepared Prussian blue-based material. Through experiments and research analysis by the applicant:

[0072] When the acid concentration and the acid addition rate are determined, the material prepared with a monobasic acid as the selected acid has better cycle stability than that prepared with a dibasic acid or a polybasic acid. This is because the monobasic acid initiates the reaction more slowly than the polybasic acid, which is more conducive to the formation of Prussian blue-based materials with good crystal forms and excellent electrical properties. Similarly, the material prepared with a weak acid as the selected acid has slightly better cycle stability than that prepared with a strong acid. However, considering the raw material cost and equipment requirements comprehensively, it is more beneficial to select sulfuric acid for future industrial promotion. Therefore, sulfuric acid is preferably selected as the acidic substance for this process.

[0073] Replacement Example 3: Influence of different acid addition cut-off pH values on the product performance

[0074] The preparation method is the same as that of Example 1, except that: the acid addition cut-off pH value in step (3) is adjusted, and its influence on the product yield and performance is detected, as shown in Table 3.

[0075] Table 3

[0076] Serial number Cut-off pH value Yield Capacitance (mAh / g) Number of cycles to maintain 80% capacity Example 1 4.5 99.2％ 142.6 350 Replacement Example 3-1 7.0 80％ 143.1 348 Replacement Example 3-2 6.0 89％ 142.9 351 Replacement Example 3-3 5.0 95％ 143.2 354 Replacement Example 3-4 4.0 99％ 142.7 353 。

[0077] As shown in Table 3, different acid addition cut-off pH values in step (3) have little effect on the electrical properties of the prepared material, but have a greater impact on the yield of the prepared material. Through experiments and research analysis, the applicant found that:

[0078] When the complexing agent is fixed, the lower the pH value of the material, the poorer the complexing ability of the complexing agent. When it is lower than a certain value, the transition metal salt complexed by the complexing agent completely dissociates and is all converted into Prussian blue-based materials, so that the yield of the material reaches the maximum. However, for the same complexing agent, since the complexing ability is the same, the electrical properties of the prepared materials are almost the same. Therefore, for the complexing agent sodium citrate, the preferred acid addition cut-off pH value is 4.5.

[0079] Replacement Example 4: Influence of the selection of different complexing agents on the product performance

[0080] The preparation method is the same as that of Example 1, except that: the selection of the complexing agent in step (1) is adjusted, and its influence on the product performance is detected, as shown in Table 4.

[0081] Table 4

[0082]

[0083]

[0084] As shown in Table 4, the selection of different complexing agents in step (1) has a greater impact on the electrical properties of the prepared material. Through experiments and research analysis, the applicant found that: at the same pH value, there are differences in the complexing ability of different complexing agents for transition metal salt ions. Therefore, after adjusting the pH value, the crystal forms of the prepared materials are quite different, resulting in different electrical properties of the materials. Citric acid and sodium citrate exist in the form of citrate ions in the solution, which can be said to be the same complexing agent, so the electrical properties of the prepared materials are almost the same. Therefore, to compare the electrical properties of these materials, sodium citrate is preferably used as the complexing agent.

[0085] Replacement Example 5: Influence of the selection of the molar ratio of different complexing agents and transition metal salts on the product performance

[0086] The preparation method is the same as that of Example 1, except that: the molar ratio of the complexing agent and transition metal ions in step (1) is adjusted, and its influence on the product performance is detected, as shown in Table 5.

[0087] Table 5

[0088] Serial number Ratio Capacitance (mAh / g) Number of cycles to maintain 80% capacity Example 1 1.0 142.6 350 Replacement Example 5-1 0.5 143.5 102 Replacement Example 5-2 0.9 140.9 271 Replacement Example 5-3 0.95 141.6 307 Replacement Example 5-4 1.05 142.7 352 Replacement Example 5-5 1.1 142.9 351 Replacement Example 5-6 1.5 143.1 349 。

[0089] As shown in Table 5, the selection of different molar ratios of complexing agents and transition metal ions in step (1) has a great influence on the electrical properties of the prepared materials. Through experiments and research analysis, the applicant found that when using a complexing agent, it is necessary to ensure that all the transition metal ions in the reaction solution are completely complexed. Otherwise, the prepared materials will have many defects and poor electrical properties. When the amount of the complexing agent is small, it cannot completely complex the transition metal ions. When the amount of the complexing agent is higher than a certain value, the complexing agent is in abundance, resulting in waste of the complexing agent and unable to further improve the electrical properties of the prepared materials. In summary, when the molar ratio of the complexing agent to the transition metal ion is 1.0, the electrical properties of the prepared materials are relatively excellent and there is no waste of the complexing agent. Therefore, it is preferred that the molar ratio of the complexing agent to the transition metal ion is 1.0.

[0090] Example 2: Adding acidic gas

[0091] (1) Solution A: At 25°C, dissolve 0.05 mol of ferrous sulfate and 0.05 mol of sodium citrate in 100 mL of deionized water, and add a certain amount of sodium carbonate to make the pH value of the solution greater than 7.5.

[0092] (2) Solution B: At 25°C, dissolve 0.05 mol of sodium ferrocyanide in 100 mL of deionized water.

[0093] (3) Quickly pour Solution A into Solution B, stir while adding, with a stirring speed of 1200 rpm. After adding and stirring for 5 min, introduce SO3 gas into the mixture at a speed of 10 mL / min. Stop introducing SO3 gas when the pH value of the solution reaches 4.5, and continue to keep warm and stir for 2 h. Centrifuge the reaction solution, wash it 3 times with deionized water and ethanol respectively, and dry it in a vacuum drying oven at 120°C for 12 h to obtain the Prussian blue-based material.

[0094] Performance test:

[0095] Test method: Mix the Prussian blue-based material prepared in Example 2 with SP and PVDF according to a mass ratio of 8:1:1, add NMP and stir into a slurry, coat it on an aluminum foil, and make a positive electrode sheet of a sodium-ion battery through drying, die-cutting and laminating. Using metallic sodium as the negative electrode, glass fiber (brand Whatman GF / D) as the separator, and a solution of NaPF6 in propylene carbonate (PC) / ethyl methyl carbonate (EMC) as the electrolyte, assemble a CR2025 coin cell in a glove box filled with argon for charge-discharge testing.

[0096] Test standard:

[0097] Under the conditions of a current density of 100 mAh / g and a voltage range of 2.0 - 4.0 V, test the capacity and cycle performance of the battery at 1C respectively.

[0098] Test results: Under the condition of 1C, the initial discharge capacity of the battery is 145.9 mAh / g, and after 479 cycles, the capacity retention rate is 80%.

[0099] The applicant analyzed through experiments and research:

[0100] Using gaseous SO3 as the acidic substance, when it is introduced into the reaction solution, it first undergoes a process of reacting with water to form H2SO4, and then H2SO4 dissociates into H + , free H + and then acts on the complexing agent to dissociate the transition metal ions to initiate the reaction. Compared with directly adding an H2SO4 solution to release H + , using SO3 as the acidic substance causes H + to be released in two steps, and the initiation of the reaction is relatively milder, slowing down the growth rate of the Prussian blue-based material, so that the prepared material has fewer defects and higher crystallinity. As Figure 5 shown, for the comparison of the XRD patterns of the materials prepared in Example 1 and Example 2, the XRD peak pattern of the material prepared by introducing gaseous SO3 is sharper than that of the material prepared by directly adding an H2SO4 solution, and the crystallinity of the material is higher. Therefore, the electrical performance is also better.

[0101] Comparative Example 1:

[0102] Dissolve 1 g of citric acid in 0.1 mol / L NaOH aqueous solution, stir well for 30 min to obtain a sodium citrate aqueous solution, then add 1 mol / L ferrous sulfate solution to the sodium citrate aqueous solution, continue stirring for 4 h, filter out the precipitate, wash it thoroughly with water, and dry it at 60 °C to obtain ferrous citrate precipitate. Mix ferrous citrate and sodium ferrocyanide in a molar ratio of 1:1, add sulfuric acid to adjust the pH value to 5.0, react in an aqueous solution for 24 h, then centrifuge the reaction solution, wash it 3 times each with deionized water and ethanol, and dry it in a vacuum drying oven at 120 °C for 12 h to obtain the Prussian blue-based material.

[0103] Performance test:

[0104] Test method: Mix the Prussian blue-based material prepared in Comparative Example 1 with SP and PVDF in a mass ratio of 8:1:1, add NMP and stir to form a slurry, coat it on an aluminum foil, and make a positive electrode sheet of a sodium-ion battery through drying, die-cutting and laminating. Using metallic sodium as the negative electrode, glass fiber (brand Whatman GF / D) as the separator, and a solution of NaPF6 in propylene carbonate (PC) / ethyl methyl carbonate (EMC) as the electrolyte, assemble a CR2025 button battery in a glove box filled with argon for charge and discharge tests.

[0105] Test standard:

[0106] Under the conditions of a current density of 100 mAh / g and a voltage range of 2.0 - 4.0 V, the capacity and cycling performance of the battery were tested at 1C respectively.

[0107] Test results: Under the condition of 1C, the initial discharge capacity of the battery was 138.9 mAh / g, and after 209 cycles, the capacity retention rate was 80%.

[0108] Comparative Example 2:

[0109] Dissolve 10 mg of sodium polyacrylate in 100 ml of deionized water; weigh 0.1 g of sodium ferrocyanide and dissolve it evenly in the above solution, adjust the pH value of the solution to 1 with sulfuric acid, and stir for 30 min at room temperature to obtain a clear precursor solution. Transfer the above precursor solution to a water bath at 40 °C and react for 1 h. After standing and cooling to room temperature, collect the precipitate. Dry the obtained precipitate in an oven at 80 °C for 1 h to obtain the cathode material for a high-rate sodium-ion battery based on Prussian blue.

[0110] Performance test:

[0111] Test method: Mix the Prussian blue-based material prepared in Comparative Example 2 with SP and PVDF according to a mass ratio of 8:1:1, add NMP and stir into a slurry, coat it on aluminum foil, and make a cathode material electrode sheet for a sodium-ion battery through drying, punching, and laminating. Using metallic sodium as the anode, glass fiber (grade Whatman GF / D) as the separator, and a solution of NaPF6 in propylene carbonate (PC) / ethyl methyl carbonate (EMC) as the electrolyte, assemble a CR2025 coin cell in a glove box filled with argon for charge-discharge testing.

[0112] Test standard:

[0113] Under the conditions of a current density of 100 mAh / g and a voltage range of 2.0 - 4.0 V, the capacity and cycling performance of the battery were tested at 1C respectively.

[0114] Test results: Under the condition of 1C, the initial discharge capacity of the battery was 143.5 mAh / g, and after 348 cycles, the capacity retention rate was 80%.

[0115] Comparative Example 3:

[0116] (1) Solution A: Dissolve 0.05 mol of ferrous sulfate in 100 mL of deionized water, cool it to -10 °C to freeze the solution into ice, and crush it for standby.

[0117] (2) Solution B: Dissolve 0.05 mol of sodium ferrocyanide in 100 mL of deionized water, cool it to -10 °C to freeze the solution into ice, and crush it for standby.

[0118] (3) Mix crushed ice A and crushed ice B, and let it naturally warm up to room temperature of 25 °C. Stir while the stirring speed is 1200 rpm. After the ice completely melts, continue to keep it warm and stir for 2 h. Centrifuge the reaction solution, wash it 3 times with deionized water and ethanol respectively, and dry it in a vacuum drying oven at 120 °C for 12 h to obtain the Prussian blue-based material.

[0119] Vacuum drying for 12 h gives the Prussian blue-based material.

[0120] Performance test:

[0121] Test method: Mix the Prussian blue-based material prepared in Comparative Example 3, SP, and PVDF according to a mass ratio of 8:1:1, add NMP and stir to form a slurry, coat it on aluminum foil, and make a positive electrode sheet of a sodium-ion battery through drying, die-cutting, and laminating. Use metallic sodium as the negative electrode, glass fiber (grade Whatman GF / D) as the separator, and a solution of NaPF6 in propylene carbonate (PC) / ethyl methyl carbonate (EMC) as the electrolyte. Assemble a CR2025 button battery in a glove box filled with argon for charge-discharge testing.

[0122] Test standard:

[0123] Under the conditions of a current density of 100 mAh / g and a voltage range of 2.0 - 4.0 V, test the capacity and cycle performance of the battery at 1C respectively.

[0124] Test results: At 1C, the initial discharge capacity of the battery is 132.7 mAh / g, and after 276 cycles, the capacity retention rate is 80%.

[0125] Comparative Example 4:

[0126] (1) Solution A: At 25 °C, dissolve 0.05 mol of ferrous sulfate and 0.05 mol of sodium citrate in 100 mL of deionized water for standby.

[0127] (2) Solution B: At 25 °C, dissolve 0.05 mol of sodium ferrocyanide in 100 mL of deionized water for standby.

[0128] (3) At 25 °C, add Solution A to Solution B at a rate of 10 mL / min while stirring, with a stirring speed of 1200 rpm. After adding Solution A completely, continue to keep it warm and stir for 2 h. Centrifuge the reaction solution, wash it 3 times with deionized water and ethanol respectively, and dry it in a vacuum drying oven at 120 °C for 12 h to obtain the Prussian blue-based material.

[0129] Performance test:

[0130] Test method: The Prussian blue-based material prepared in Comparative Example 4 was mixed with SP and PVDF at a mass ratio of 8:1:1, and NMP was added and stirred into a slurry, which was then coated on aluminum foil. After drying, die-cutting, and laminating, the positive electrode sheet of the sodium-ion battery was made. Using metallic sodium as the negative electrode, glass fiber (brand Whatman GF / D) as the separator, and a solution of NaPF6 in propylene carbonate (PC) / ethyl methyl carbonate (EMC) as the electrolyte, a CR2025 button battery was assembled in a glove box filled with argon for charge and discharge tests.

[0131] Test standard:

[0132] Under the conditions of a current density of 100 mAh / g and a voltage range of 2.0 - 4.0 V, the capacity and cycle performance of the battery at 1C were respectively tested.

[0133] Test results: Under the condition of 1C, the initial discharge capacity of the battery was 123.5 mAh / g, and after 59 cycles, the capacity retention rate was 80%.

[0134] The applicant analyzed through experiments and research:

[0135] The Prussian blue-based material with high crystallinity refers to the material with few or no lattice defects. For the Fe-based Prussian blue-based materials prepared in Examples 1 - 2 and Comparative Examples 1 - 4 of the present invention, it means that during the reaction, Fe 2+ and ferrocyanide ions can come into contact and react in a 1:1 ratio, so that there are neither Fe 2+ defects nor ferrocyanide defects.

[0136] In Comparative Example 1, the poorly soluble metal complex ferrous citrate was first prepared, and then the reaction was carried out by adjusting the acidity to slowly dissolve the solid ferrous citrate in the aqueous solution of sodium ferrocyanide. In this way, one of the reaction raw materials undergoes a process of first dissolving and then dissociating, which can effectively slow down the reaction rate, thus being beneficial to the formation of high-crystallinity materials. However, this method does not first mix the reaction raw materials evenly at the molecular level and then carry out the reaction. Instead, the poorly soluble transition metal complex first dissolves, releases transition metal ions, and then diffuses into the solution to mix and react with sodium ferrocyanide. There is a situation of uneven mixing of transition metal ions and ferrocyanide ions throughout the process, and there will be a situation where one portion of transition metal ions is mixed and contacted with multiple equivalents of sodium ferrocyanide. The prepared material will have Fe 2+ defects, thereby affecting the lattice integrity of the prepared material and the electrical properties are also relatively poor.

[0137] In Comparative Example 2, there is no situation of uneven mixing of reaction raw materials at the molecular level, and the lattice integrity and electrical properties of the prepared material are also relatively good. However, this method can only use sodium ferrocyanide as the raw material, which simultaneously provides the Fe required for preparing the Prussian blue-based material2+ and ferrocyanide ions. Sodium ferrocyanide is relatively expensive, which will inevitably increase the raw material cost. At the same time, sodium ferrocyanide decomposes under acidic conditions to produce toxic hydrocyanic acid, which is not conducive to industrial scale-up.

[0138] In Comparative Example 3, the reaction solutions A and B were frozen into ice, then pulverized and mixed, and the temperature was raised for reaction. Although this ensured the stable contact ratio of the reaction raw materials to a certain extent, which was 1:1, this was only at the macroscopic level. At the microscopic level, there was a great uncertainty in the ratio of the reaction raw materials. Therefore, although the crystallinity of the material prepared by this method was better, there were still structural defect problems.

[0139] In Comparative Example 4, the reaction solutions reacted as soon as they came into contact, and the prepared material had many crystal defects and poor electrical properties. Combining Figure 5 as shown: The peak shape of the XRD pattern of the prepared material was weak, which also indicated that the crystallinity of the material was low.

[0140] In summary, the technical solution proposed in this application makes the reaction solutions mix evenly, the reaction raw materials reach a stable and controllable ratio at the molecular level, and then the reaction proceeds through an acidic material, so that the prepared Prussian blue-based material has better crystallinity; at the same time, this application not only uses "liquid-liquid" contact to initiate the reaction, but also further proposes a "gas-liquid" reaction to initiate the reaction to prepare Prussian blue materials. In this process, an acid is first generated by the reaction of an acidic gas and water, and then the generated acid initiates the reaction, further increasing the crystallinity integrity of the prepared material. At the same time, this process also has the advantages of good synthesis uniformity, easy observation and control during the process, low raw material cost, and easy engineering scale-up.

Claims

1. A preparation method of a Prussian blue-based cathode material, characterized in that, It includes the following steps: (1) Dissolve a transition metal salt, a complexing agent, and an alkaline substance in deionized water to obtain solution A, and control the pH value of solution A to be 7.0 - 14.0; The complexing ability of the complexing agent varies with the pH value of the solution. The complexing agent is selected from one or more of sodium citrate, citric acid, ethylenediaminetetraacetic acid, sodium ethylenediaminetetraacetate, bipyridine, 8-hydroxyquinoline-5-sulfonic acid, phytic acid, tannic acid, alginic acid, polyacrylic acid, and sodium carboxymethylcellulose; (2) Dissolve sodium ferrocyanide in deionized water to obtain solution B; (3) Under stirring conditions, mix solution A and solution B for 5 - 30 min, then add an acidic substance to the mixture. The solution starts to change from clear to turbid. When the reaction is completed, stop adding the acidic substance, age to obtain a suspension, separate the suspension, wash and dry the obtained solid substance to obtain a Prussian blue-based cathode material.

2. The preparation method of a Prussian blue-based cathode material according to claim 1, wherein, In step (1): The transition metal salt is selected from one or more of iron salts, manganese salts, nickel salts, or other transition metal salts, and the concentration of the transition metal salt is 0.01 - 1.0 M.

3. The preparation method of a Prussian blue-based cathode material according to claim 1, characterized in that, In step (1): The molar ratio of the transition metal salt to the complexing agent is 0.1 - 10.

0.

4. The preparation method of a Prussian blue-based cathode material according to claim 1, characterized in that, In step (1): The alkaline substance is used to adjust the pH value of solution A to 7.0 - 14.0, and the alkaline substance is selected from one or more of inorganic bases and organic bases.

5. The preparation method of a Prussian blue-based cathode material according to claim 4, characterized in that, In step (1): The alkaline substance is sodium carbonate, and the alkaline substance adjusts the pH value of solution A to 7.

5.

6. The preparation method of a Prussian blue-based cathode material according to claim 1, characterized in that, In step (3): The acidic substance is an inorganic acid, an organic acid, or an acidic gas, and the amount of the acidic substance used is to adjust the pH value of the reaction solution in step (3) to be less than the pH value of solution A in step (1).

7. The preparation method of a Prussian blue-based cathode material according to claim 1, characterized in that, In step (3): The acidic substance is selected from one or more of sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, formic acid, acetic acid, citric acid, or one or more of the gases such as sulfur trioxide, carbon dioxide, and sulfur dioxide that are acidic when dissolved in water. The amount of the acidic substance used is to adjust the pH value of the reaction solution in step (3) to 1.0 - 10.

0.

8. A Prussian blue-based cathode material prepared by the method according to claim 1.

9. Use of a Prussian blue-based cathode material prepared by the method according to claim 1 in the preparation of a sodium-ion battery.

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