Preparation method and application of high-conductivity Prussian blue cathode material

Abstract

The present invention discloses a preparation method and application of a high-conductivity Prussian blue-based positive electrode material. The method comprises the following steps: firstly synthesizing the Prussian blue-based positive electrode material, then dehydrating the material, placing the dehydrated Prussian blue-based positive electrode material in a conductive modification liquid for stirring and reacting, and drying the obtained solid to obtain the high-conductivity Prussian blue-based positive electrode material. The method comprises the following steps: dehydrating the Prussian blue-based positive electrode material to remove most of the crystal water, and then treating the Prussian blue-based positive electrode material with a conductive modification liquid. The boron-containing ions or molecules (such as BO3 3‑ , BF3) can form coordinated water with transition metals or replace coordinated water bound to transition metals, preventing the Prussian blue cathode material from absorbing water from the environment and generating new coordinated water and crystallization water. Furthermore, the reduction of coordinated water can further reduce the amount of crystallization water. This method can also uniformly dope boron into the Prussian blue cathode material, thereby improving its conductivity.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion battery positive electrode materials, and particularly relates to a preparation method and application of a highly conductive Prussian blue positive electrode material. Background Art

[0002] Secondary batteries, also known as rechargeable batteries, are batteries that can be charged and discharged repeatedly and used multiple times. Currently, the main secondary battery technologies include lead-acid batteries, nickel-chromium batteries, nickel-metal hydride batteries, and lithium-ion batteries. Lithium-ion batteries are widely used in the energy storage field due to their high energy density and long life. However, the low natural abundance and uneven distribution of lithium have led to a continuous increase in the cost of lithium-ion batteries. Sodium, located just below lithium in the periodic table, has chemical properties most similar to those of metallic lithium and is thousands of times more abundant in the Earth's crust. Therefore, sodium-ion batteries are expected to become a new generation of high-performance, low-cost energy storage technology. Sodium-ion batteries include transition metal oxide systems, Prussian systems, and polyanion systems. Among them, the Prussian system has obvious economic and safety advantages and has great application value, but there are still some technical problems that need to be solved.

[0003] The preparation methods for Prussian-type cathode materials for sodium-ion batteries primarily include coprecipitation, hydrothermal, and mechanical mixing. In coprecipitation and hydrothermal methods, the relatively low ksp of Na2M1[M2(CN)6] leads to rapid precipitation, which easily produces a large amount of Prussian-type cathode materials containing [M2(CN)6] vacancy defects. The exposed M2 sites readily combine with water in the reaction system to form coordinated water, which can further bind to crystallization water, occupying sodium ion storage sites and reducing specific capacity. Furthermore, during battery cycling, crystallization water and interstitial water can dissociate from the cathode material and further react with the electrolyte, resulting in reduced battery cycling performance. Therefore, reducing [M2(CN)6] vacancies is a key challenge facing Prussian-type sodium-ion batteries.

[0004] Patent CN109065883B discloses a method for modifying Prussian blue and its analogs. The method uses organic matter to modify the surface of dehydrated Prussian blue and its analogs, allowing organic molecules to occupy the position of crystallization water. While improving the sodium storage electrochemical performance of Prussian blue and its analogs, the storage stability of Prussian blue and its analogs at room temperature and pressure is also greatly improved, thereby solving the problem that dehydrated Prussian blue easily absorbs water again when stored in air, causing the electrochemical performance to decay rapidly. The organic molecules in the above scheme are connected to the coordinated water by hydrogen bonds, which can prevent the crystallization water from occupying the vacant position. However, the binding strength of the hydrogen bond is not high, and the drying and dehydration process can only remove the crystallization water, and the coordinated water cannot be removed. Therefore, the above scheme cannot solve the problem of coordinated water. At the same time, these organic molecules generally have poor conductivity, so using the above method will also lead to a decrease in the conductivity of the Prussian-type positive electrode material. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the present invention provides a preparation method of a highly conductive Prussian blue-based positive electrode material and its application.

[0006] According to one aspect of the present invention, a method for preparing a highly conductive Prussian blue-based cathode material is provided, comprising the following steps:

[0007] S1: first synthesizing a Prussian blue-based cathode material, and then dehydrating the Prussian blue-based cathode material;

[0008] S2: placing the dehydrated Prussian blue cathode material in a conductive modification liquid for stirring and reacting, separating the solid and the liquid, and drying the obtained solid to obtain the highly conductive Prussian blue cathode material; wherein the conductive modification liquid is a soluble boron-containing compound solution.

[0009] In some embodiments of the present invention, the chemical formula of the highly conductive Prussian blue cathode material is Na x M1 y [M2(CN)6] 1-z □ z D a (6z-a)H2O, where M1 and M2 are transition metals, □ represents a vacancy, D is a boron compound, H2O represents the coordinated water content, 0<x≤2, 0<y≤1, 0.05<a≤6z, 0.01<z≤0.2.

[0010] In some embodiments of the present invention, in step S1, the dehydration method is vacuum drying, the vacuum drying temperature is 80-150° C., and the drying time is 8-24 hours.

[0011] In some embodiments of the present invention, in step S1, the Prussian blue cathode material is prepared by the following coprecipitation method: (1) sodium ferrocyanide and an inorganic sodium salt are dissolved in water to prepare a mixed solution A, and a transition metal salt and sodium citrate are dissolved in water to prepare a mixed solution B; (2) under heating conditions, the mixed solution B is added dropwise to the mixed solution A to react, and after the reaction is completed, the mixture is aged, and a solid-liquid separation is performed to obtain a precipitate, and the precipitate is washed to obtain the Prussian blue cathode material.

[0012] In some embodiments of the present invention, in step S2, the soluble boron-containing compound in the conductive modification solution is at least one of boric acid, borate, metaboric acid, metaborate, tetrafluoroboric acid, tetrafluoroborate, tetrahydroborate or boron trifluoride.

[0013] In some embodiments of the present invention, in step S2, the pH of the conductive modification solution is controlled to be 6.5-9.5. Prussian blue materials have the lowest solubility under neutral conditions and are soluble in weak acids and weak bases. Strong acids and strong bases will cause the positive electrode material to decompose.

[0014] In some embodiments of the present invention, when the soluble boron-containing compound is boron trifluoride, the temperature of the conductive modification solution is controlled at 20-35°C. Boron trifluoride has a low boiling point. If the temperature is too high, it will easily volatilize. If the temperature is too low, the binding efficiency of boron trifluoride to the Prussian blue-based cathode material will be too low. Once boron trifluoride forms a stable coordination bond with the Prussian blue-based cathode material, it will not fall off during normal use.

[0015] In some embodiments of the present invention, in step S2, the concentration of the soluble boron-containing compound in the conductive modification solution is 0.1-5 mol / L. The concentration of the boron-containing compound can be adjusted as needed to ensure that the doping amount of the boron-containing compound is within an appropriate range.

[0016] In some embodiments of the present invention, in step S2, the solid-to-liquid ratio of the dehydrated Prussian blue-based cathode material to the conductive modification liquid is 1:(1-3) g / ml.

[0017] In some embodiments of the present invention, in step S2, the stirring reaction time is 8-16 hours.

[0018] The present invention also provides application of the preparation method in preparing sodium ion batteries.

[0019] According to a preferred embodiment of the present invention, there are at least the following beneficial effects:

[0020] The present invention dehydrates the Prussian blue cathode material to remove most of the crystal water, and then uses a conductive modification liquid to treat the Prussian blue cathode material. The boron ions or molecules (such as BO3 3- BF3) can form coordinated water with transition metals or replace the coordinated water already bound to the transition metals, preventing the Prussian blue cathode material from absorbing water from the environment and generating new coordinated water and crystallization water. Furthermore, the reduction of coordinated water can further reduce the amount of crystallization water. This method also allows for the uniform doping of boron into the Prussian blue cathode material, thereby improving its electrical conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0022] Figure 1 This is a SEM image of the highly conductive Prussian blue-based cathode material prepared in Example 1 of the present invention;

[0023] Figure 2 This is a charge and discharge diagram of the highly conductive Prussian blue-based positive electrode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0024] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0025] Example 1

[0026] This example prepares a highly conductive Prussian blue cathode material. The specific process is as follows:

[0027] (1) preparing a mixed solution A of 1 mol / L sodium ferrocyanide and 2 mol / L sodium chloride, and a mixed solution B of 1 mol / L manganese sulfate and 1 mol / L sodium citrate, adding 1 L of the mixed solution A to the reactor, and then adding 600 mL of the mixed solution B dropwise to the reactor at 1 ml / min through a peristaltic pump while stirring continuously, controlling the reaction temperature at 60° C. and the pH at 6.5-9.5, generating a precipitate after the reaction, aging for 6 hours, filtering the precipitate, washing it twice with deionized water, and then washing it twice with acetonitrile to obtain a Prussian blue cathode material;

[0028] (2) transferring the Prussian blue cathode material to a vacuum drying oven at 120° C. and drying for 12 h to obtain a dehydrated Prussian blue cathode material;

[0029] (3) The dehydrated Prussian blue cathode material was placed in a stirrer, and a 1 mol / L boric acid aqueous solution (temperature 20°C) was added according to a solid-liquid ratio of 1:1 g / ml, and the pH of the boric acid aqueous solution was adjusted to 6.5 with sodium hydroxide. After stirring for 12 hours, the precipitate was obtained by centrifugation and transferred to a vacuum drying oven at 120°C for drying for 12 hours to obtain a highly conductive Prussian blue cathode material.

[0030] The results of ICP and TG tests showed that the molecular formula of the sample was Na 1.61 Mn[Fe(CN)6] 0.92 □ 0.08 (BO3) 0.42 0.06H2O.

[0031] Example 2

[0032] This example prepares a highly conductive Prussian blue cathode material. The specific process is as follows:

[0033] (1) preparing a mixed solution A of 1 mol / L sodium ferrocyanide and 2 mol / L sodium chloride, and a mixed solution B of 1 mol / L manganese sulfate and 1 mol / L sodium citrate, adding 1 L of the mixed solution A to the reactor, and then adding 600 mL of the mixed solution B dropwise to the reactor at 1 ml / min through a peristaltic pump while stirring continuously, controlling the reaction temperature at 60° C. and the pH at 6.5-9.5, generating a precipitate after the reaction, aging for 6 hours, filtering the precipitate, washing it twice with deionized water, and then washing it twice with acetonitrile to obtain a Prussian blue cathode material;

[0034] (2) transferring the Prussian blue cathode material to a vacuum drying oven at 120° C. and drying for 12 h to obtain a dehydrated Prussian blue cathode material;

[0035] (3) The dehydrated Prussian blue cathode material was placed in a stirrer, and a 2 mol / L BF3 anhydrous ethanol solution (temperature 20°C) was added according to a solid-liquid ratio of 1:1 g / ml. After stirring for 12 hours, the precipitate was obtained by centrifugation and transferred to a vacuum drying oven at 120°C for drying for 12 hours to obtain a highly conductive Prussian blue cathode material.

[0036] The results of ICP and TG tests showed that the molecular formula of the sample was Na 1.63 Mn[Fe(CN)6] 0.93 □ 0.07 (BF3) 0.31 0.11H2O.

[0037] Example 3

[0038] This example prepares a highly conductive Prussian blue cathode material. The specific process is as follows:

[0039] (1) preparing a mixed solution A of 1 mol / L sodium ferrocyanide and 2 mol / L sodium chloride, and a mixed solution B of 1 mol / L manganese sulfate and 1 mol / L sodium citrate, adding 1 L of the mixed solution A to the reactor, and then adding 600 mL of the mixed solution B dropwise to the reactor at 1 ml / min through a peristaltic pump while stirring continuously, controlling the reaction temperature at 60° C. and the pH at 6.5-9.5, generating a precipitate after the reaction, aging for 6 hours, filtering the precipitate, washing it twice with deionized water, and then washing it twice with acetonitrile to obtain a Prussian blue cathode material;

[0040] (2) transferring the Prussian blue cathode material to a vacuum drying oven at 120° C. and drying for 12 h to obtain a dehydrated Prussian blue cathode material;

[0041] (3) The dehydrated Prussian blue cathode material was placed in a stirrer, and a 2 mol / L sodium borohydride aqueous solution (temperature 20°C, pH = 9.5) was added at a solid-liquid ratio of 1:1 g / ml. After stirring for 12 hours, the precipitate was obtained by centrifugation and transferred to a vacuum drying oven at 120°C for 12 hours to obtain a highly conductive Prussian blue cathode material.

[0042] The results of ICP and TG tests showed that the molecular formula of the sample was Na 1.70 Mn[Fe(CN)6] 0.92 □ 0.08 (BH3) 0.35 0.13H2O.

[0043] Comparative Example 1

[0044] This comparative example is the dehydrated Prussian blue cathode material obtained in step (2) of Example 1. The molecular formula of the product is Na 1.61 Mn[Fe(CN)6] 0.92 □ 0.08 0.48H2O.

[0045] Comparative Example 2

[0046] This comparative example prepared a Prussian blue cathode material, and the specific process was as follows:

[0047] (1) preparing a mixed solution A of 1 mol / L sodium ferrocyanide and 2 mol / L sodium chloride, and a mixed solution B of 1 mol / L manganese sulfate and 1 mol / L sodium citrate, adding 1 L of the mixed solution A to the reactor, and then adding 600 mL of the mixed solution B dropwise to the reactor at 1 ml / min through a peristaltic pump while stirring continuously, controlling the reaction temperature at 60° C. and the pH at 6.5-9.5, generating a precipitate after the reaction, aging for 6 hours, filtering the precipitate, washing it twice with deionized water, and then washing it twice with acetonitrile to obtain a Prussian blue cathode material;

[0048] (2) transferring the Prussian blue cathode material to a vacuum drying oven at 120° C. and drying for 12 h to obtain a dehydrated Prussian blue cathode material;

[0049] (3) The dehydrated Prussian blue cathode material was placed in a stirrer, and isopropanol was added at a solid-liquid ratio of 1:1 g / ml. After stirring for 12 hours, the precipitate was obtained by centrifugation and transferred to a vacuum drying oven at 120°C for 12 hours to obtain the Prussian blue cathode material.

[0050] The results of ICP and TG tests showed that the molecular formula of the sample was Na 1.63 Mn[Fe(CN)6] 0.92 □ 0.08 0.48H2O (IPA) 0.28 .

[0051] Test example

[0052] In order to verify the performance of the Prussian blue cathode material prepared by the present invention, the products prepared by the methods of each embodiment were used as the cathode, metallic sodium was used as the anode, glass fiber was used as the separator, and EC / DEC solution of sodium hexafluorophosphate was used as the electrolyte. A sodium ion half-cell was assembled in a glove box, and charge and discharge tests were performed at an operating voltage of 2 to 4 V and different current densities. At the same time, the products of Comparative Examples 1-2 were used as control samples and the same tests were performed. The results are shown in Table 1, wherein the charge and discharge curves of the assembled battery test of the product obtained in Example 1 are shown in Table 1. Figure 2 shown.

[0053] Table 1

[0054]

[0055]

[0056] As can be seen from Table 1, the first charge and discharge specific capacity of the Prussian blue cathode material prepared by the present invention is increased by more than 10 mAh / g relative to that of Comparative Example 1, and has better rate performance and cycle performance. The gram capacity of Example 2 is higher than that of Example 1 and Example 3. This is because the molecular weight of the boron compound of Examples 1 and 3 is larger, which has a certain impact on the gram capacity. Although the cycle retention rate of Comparative Example 2 is improved to a certain extent relative to Comparative Example 1, the gram capacity and rate performance of Comparative Example 2 are significantly reduced relative to Examples 1-3 and even Comparative Example 1. This is because Comparative Example 2 uses non-conductive isopropyl alcohol to treat the Prussian blue cathode material, thereby reducing the conductivity of the cathode material. At the same time, it can only compete with crystal water for coordinated water, but cannot replace coordinated water, so it still has a high coordinated water content.

[0057] While the embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to the embodiments described above. Various modifications may be made within the scope of knowledge possessed by a person skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof may be combined with one another unless there is a conflict.

Claims

1. A method for preparing a highly conductive Prussian blue cathode material, characterized in that: The following steps are involved: S1: first synthesizing a Prussian blue-based cathode material, and then dehydrating the Prussian blue-based cathode material; S2: placing the dehydrated Prussian blue-based cathode material in a conductive modification liquid for stirring and reacting, separating the solid and the liquid, and drying the resulting solid to obtain the highly conductive Prussian blue-based cathode material; wherein the conductive modification liquid is a soluble boron-containing compound solution; In step S2, the soluble boron-containing compound in the conductive modification liquid is at least one of boric acid, borate, metaboric acid, metaborate, tetrafluoroboric acid, tetrafluoroborate, tetrahydroborate or boron trifluoride; the concentration of the soluble boron-containing compound in the conductive modification liquid is 0.1-5 mol / L.

2. The preparation method according to claim 1, characterized in that The chemical formula of the highly conductive Prussian blue cathode material is Na x M1 y [M2(CN)6] 1-z □ z D a (6z-a)H2O, where M1 and M2 are transition metals, □ represents a vacancy, D is a boron compound, H2O represents the coordinated water content, 0<x≤2, 0<y≤1, 0.05<a≤6z, 0.01<z≤0.

2.

3. The preparation method according to claim 1, characterized in that In step S1, the dehydration method is vacuum drying, the vacuum drying temperature is 80-150° C., and the drying time is 8-24 hours.

4. The preparation method according to claim 1, characterized in that In step S2, the pH of the conductive modification solution is controlled to be 6.5-9.

5.

5. The preparation method according to claim 1, characterized in that When the soluble boron-containing compound is boron trifluoride, the temperature of the conductive modification liquid is controlled at 20-35°C.

6. The preparation method according to claim 1, characterized in that In step S2, the solid-to-liquid ratio of the dehydrated Prussian blue positive electrode material to the conductive modification liquid is 1:(1-3) g / ml.

7. The preparation method according to claim 1, characterized in that In step S2, the stirring reaction time is 8-16 hours.

8. Use of the preparation method according to any one of claims 1 to 7 in the preparation of sodium ion batteries.

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