A modified Prussian blue-like material, its preparation method and application

Abstract

This disclosure provides a modified Prussian blue-based material, its preparation method, and its applications, relating to the field of Prussian blue-based materials. The modified Prussian blue-based material provided in this disclosure includes a Prussian blue-based material as a core layer and sodium iron phosphate and optional sodium M phosphate as an in-situ coating layer. This disclosure allows for the in-situ generation of sodium iron phosphate and optional sodium M phosphate on the surface of the Prussian blue-based material, resulting in a tighter coating and providing a certain degree of water resistance. This effectively isolates the Prussian blue-based material from direct contact with the electrolyte, reducing side reactions and the dissolution of metal ions, thereby improving its cycle stability. The coating method used in this disclosure is simple and easy to operate, and the prepared modified Prussian blue-based material exhibits good cycle stability and can be widely used in the preparation of cathode materials for sodium-ion batteries.

Description

Technical Field

[0001] This disclosure relates to the field of Prussian blue-based materials, and more specifically, to a modified Prussian blue-based material, its preparation method, and its application. Background Technology

[0002] In recent years, the scarcity, uneven distribution, and difficulties in the development and utilization of lithium resources have become increasingly serious problems. In contrast, sodium-ion batteries use sodium salts as their electrode material, which are more abundant and cheaper than lithium resources. Because sodium ions are larger than lithium ions, sodium-ion batteries are an economical and efficient alternative when weight and energy density requirements are not high, and are one of the important choices for future energy storage grids.

[0003] The performance of sodium-ion batteries depends on the structure and properties of the internal materials, and the performance and cost of the cathode material directly affect the market competitiveness of the entire sodium-ion battery system. Prussian blue materials, as one type of sodium-ion battery material, possess open three-dimensional channels, allowing sodium ions to migrate rapidly within the tunnels, thus exhibiting good structural stability and excellent rate performance. However, due to issues such as crystal structure vacancies, the presence of water of crystallization, surface side reactions with the electrolyte, and the dissolution of metal ions during long-term cycling, they typically exhibit poor conductivity and cycle performance, which severely restricts the development of Prussian blue materials. Therefore, coating modification of Prussian blue materials is currently a widely used approach.

[0004] Related technologies typically employ solid-phase coating methods to surface-coat Prussian blue-like materials. For example, CN116053441A describes a Prussian blue-like material comprising a core and a coating layer. The core comprises materials with the chemical formula Na... x M[Fe(CN)6] y The compound, with a coating layer comprising the chemical formula Na3V 2-Z Mg Z (PO4)3 compounds. The Prussian blue-based cathode materials prepared by this method have relatively high charge-discharge capacity and cycle performance to a certain extent. However, due to the special structure of Prussian blue-based materials, it is difficult to achieve complete coating by using a solid-phase coating method with coating liquid and material, and the effect on performance modification is limited.

[0005] In view of this, this disclosure is hereby made. Summary of the Invention

[0006] The purpose of this disclosure includes, for example, providing a modified Prussian blue material that achieves in-situ coating of Prussian blue materials through the surface of the material itself, thereby enhancing the conductivity and cycling stability of Prussian blue materials.

[0007] The purpose of this disclosure includes, for example, providing a method for preparing modified Prussian blue materials, which is simple to prepare and produces modified Prussian blue materials with excellent electrical conductivity and cycling stability.

[0008] The purpose of this disclosure includes, for example, providing the application of modified Prussian blue-like materials in the preparation of cathode materials for sodium-ion batteries.

[0009] The purpose of this disclosure includes, for example, providing a sodium-ion battery.

[0010] The embodiments of this disclosure can be implemented as follows:

[0011] In a first aspect, this disclosure provides a modified Prussian blue-based material, comprising a core layer and an in-situ coated layer formed on the surface of the core layer, wherein the core layer is a Prussian blue-based material with the structural formula Na. x M[Fe(CN)6] y Wherein, 0≤x≤2, 0≤y≤1, and M is at least one of Fe, Mn, Co, Ni, Cu and Zn. When M is Fe, the in-situ coating layer is sodium iron phosphate. When M is at least one of Mn, Co, Ni, Cu and Zn, the in-situ coating layer is a complex of sodium iron phosphate and sodium M phosphate.

[0012] In some embodiments of this disclosure, the mass ratio of the core layer to the in-situ coating layer is 1:0.05-0.1.

[0013] In some embodiments of this disclosure, the particle size of the modified Prussian blue material is 0.2-2 μm.

[0014] In some embodiments of this disclosure, the thickness of the in-situ coating layer is 10-30 nm.

[0015] Secondly, this disclosure provides a method for preparing modified Prussian blue-based materials, comprising:

[0016] Prussian blue-like materials are brought into contact with and reacted with phosphoric acid gas to form Prussian blue-like materials with a dense surface coating in situ.

[0017] In some embodiments of this disclosure, the reaction temperature of the Prussian blue material with the phosphoric acid gas is 130-160°C, and the reaction time is 2-4 hours.

[0018] In some embodiments of this disclosure, the method for preparing the modified Prussian blue material further includes preparing phosphoric acid gas: phosphorus trichloride, oxygen and water vapor are mixed and reacted in a gas phase reactor, wherein the phosphorus trichloride is oxidized and hydrolyzed to produce phosphoric acid gas.

[0019] In some embodiments of this disclosure, the molar ratio of the Prussian blue material, the phosphorus trichloride, the oxygen, and the water vapor is 7.5-15:2:1-2:6-7.

[0020] In some embodiments of this disclosure, the preparation of phosphoric acid gas further includes absorbing and removing HCl gas generated during the preparation of the phosphoric acid gas using a saturated phosphoric acid solution.

[0021] In some embodiments of this disclosure, the step of contacting and reacting Prussian blue-based materials with phosphoric acid gas includes: continuously introducing the phosphoric acid gas, prepared on-site, into the Prussian blue-based materials to carry out the reaction. In some embodiments of this disclosure, the reaction process between the Prussian blue-based materials and the phosphoric acid gas further includes: adsorbing the generated HCN gas using activated carbon and silica gel.

[0022] In some embodiments of this disclosure, when the Prussian blue material comes into contact with the phosphoric acid gas, the in-situ coating layer and the intermediate sodium phosphate are first formed on the surface of the Prussian blue material; before obtaining the Prussian blue material coated with the in-situ coating layer, the sodium phosphate in the intermediate is removed.

[0023] In some embodiments of this disclosure, the step of removing the sodium phosphate from the intermediate includes washing and drying the intermediate.

[0024] In some embodiments of this disclosure, the intermediate after washing is dried at a temperature of 80-100°C for a time of 2-3 hours.

[0025] In some embodiments of this disclosure, the preparation method of the modified Prussian blue material further includes preparing the Prussian blue material by dissolving sodium ferrocyanide and an inorganic sodium salt in water to obtain a first solution, dissolving a soluble metal salt and a complexing agent in water to obtain a second solution, adding the second solution to the first solution, precipitating the reaction, aging the solution, filtering it, and drying the filter residue.

[0026] In some embodiments of this disclosure, the step of preparing the Prussian blue-like material includes at least one of features (1)-features (10):

[0027] Feature (1): The raw material concentration of sodium ferrocyanide is 0.02mol / L-2mol / L, the raw material concentration of inorganic sodium salt is 0.05mol / L-0.15mol / L, and the molar ratio of sodium ferrocyanide to inorganic sodium salt is 1:20-40;

[0028] Feature (2): The inorganic sodium salt includes at least one of sodium chloride, sodium sulfate, sodium oxalate, sodium acetate, sodium nitrate and sodium citrate;

[0029] Feature (3): The molar ratio of the soluble metal salt to the complexing agent is 1:2-5;

[0030] Feature (4): The metal in the soluble metal salt includes at least one of Fe, Mn, Co, Cu and Zn;

[0031] Feature (5): The complexing agent includes at least one of sodium citrate, disodium ethylenediaminetetraacetate, and sodium pyrophosphate;

[0032] Feature (6): The rate at which the second solution is added to the first solution is 0.5 mL / min to 5 mL / min;

[0033] Feature (7): The precipitation reaction includes precipitation at 40-60℃ for 4-12 hours;

[0034] Feature (8): The aging time is 12h-24h;

[0035] Feature (9): The filtration method includes centrifugation, vacuum filtration or pressure filtration;

[0036] Feature (10): The drying temperature for drying the filter residue is 80-200℃ and the drying time is 12-48h.

[0037] Thirdly, this disclosure provides the application of modified Prussian blue materials as described in any of the foregoing embodiments in the preparation of cathode materials for sodium-ion batteries.

[0038] Fourthly, this disclosure provides a sodium-ion battery comprising a positive electrode material and a negative electrode material, wherein the positive electrode material comprises a modified Prussian blue material according to any of the foregoing embodiments.

[0039] Compared with the prior art, the beneficial effects of this disclosure include:

[0040] The modified Prussian blue material disclosed herein involves in-situ modification of the surface of a Prussian blue material to form sodium iron phosphate and optionally sodium M phosphate as an in-situ coating layer. Compared to an external coating layer, the modified Prussian blue material provided in this disclosure provides a tighter coating and a certain degree of waterproofing, effectively isolating the Prussian blue material from direct contact with the electrolyte, reducing side reactions with the electrolyte and the dissolution of metal ions, thereby improving its cycle stability. The coating method used in this disclosure is simple and easy to operate, and the prepared modified Prussian blue material exhibits good cycle stability and can be widely used in the preparation of cathode materials for sodium-ion batteries. Sodium-ion batteries prepared using the modified Prussian blue material provided in this disclosure as the cathode material exhibit excellent cycle stability. Attached Figure Description

[0041] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this disclosure and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 SEM images of the modified Prussian blue materials provided in this disclosure. Detailed Implementation

[0043] The embodiments of this disclosure will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of this disclosure. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0044] The endpoints and any values ​​of the ranges disclosed in this disclosure are not limited to the precise ranges or values, and such ranges or values ​​should be understood to include values ​​close to such ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed herein.

[0045] This disclosure provides a method for preparing modified Prussian blue-based materials, comprising the following steps:

[0046] S1. Preparation of Prussian blue-like materials.

[0047] The Prussian blue-like materials (Na) disclosed herein x M[Fe(CN)6]y Wherein, 0≤x≤2, 0≤y≤1, and M is at least one of Fe, Mn, Co, Ni, Cu, and Zn, it can be prepared independently or purchased commercially. This disclosure describes a typical but non-limiting method for preparing Prussian blue-like materials, which specifically includes the following steps:

[0048] (1) Dissolve sodium ferrocyanide and inorganic sodium salt in water to obtain the first solution.

[0049] In this disclosure, the raw material concentration of sodium ferrocyanide is 0.02 mol / L-2 mol / L, the raw material concentration of inorganic sodium salt is 0.05 mol / L-0.15 mol / L, and the molar ratio of sodium ferrocyanide to inorganic sodium salt is 1:20-40; the inorganic sodium salt includes at least one of sodium chloride, sodium sulfate, sodium oxalate, sodium acetate, sodium nitrate, and sodium citrate.

[0050] In some embodiments, the concentration of sodium ferrocyanide can be, for example, any one or a range between any two of the following: 0.02 mol / L, 0.05 mol / L, 0.08 mol / L, 0.1 mol / L, 0.2 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.5 mol / L, and 2 mol / L. The concentration of the inorganic sodium salt can, for example, be any one or a range between any two of the following: 0.05 mol / L, 0.08 mol / L, 0.1 mol / L, 0.12 mol / L, 0.14 mol / L, and 0.15 mol / L. The molar ratio of sodium ferrocyanide to the inorganic sodium salt can, for example, be any one or a range between any two of the following: 1:20, 1:22, 1:24, 1:25, 1:28, 1:30, 1:32, 1:35, 1:37, 1:39, and 1:40.

[0051] (2) Dissolve the soluble metal salt and complexing agent in water to obtain a second solution.

[0052] In this disclosure, the molar ratio of the soluble metal salt to the complexing agent is 1:2-5; in some embodiments, the molar ratio of the soluble metal salt to the complexing agent may be, for example, any one of 1:2, 1:3, 1:4, 1:5, or a range between any two. The metal in the soluble metal salt includes at least one of Fe, Mn, Co, Cu, and Zn; the complexing agent includes at least one of sodium citrate, disodium ethylenediaminetetraacetate, and sodium pyrophosphate.

[0053] (3) Add the second solution to the first solution, allow the precipitate to react, then age it, filter it, and dry the filter residue.

[0054] In this disclosure, the second solution is added to the first solution at a rate of 0.5 mL / min to 5 mL / min; after addition, a precipitation reaction is carried out at 40-60℃ for 4-12 h; then aged for 12-24 h; then filtered, the filtration method including but not limited to centrifugation, vacuum filtration or pressure filtration; then the filter residue is dried at 80℃-200℃ for 12-48 h to obtain Prussian blue-like materials.

[0055] In some embodiments, the rate at which the second solution is added to the first solution can be, for example, any one or a range between any two of the following: 0.5 mL / min, 1 mL / min, 1.5 mL / min, 2 mL / min, 2.5 mL / min, 3 mL / min, 3.5 mL / min, 4 mL / min, 4.5 mL / min, and 5 mL / min. The temperature of the precipitation reaction can, for example, be any one or a range between any two of the following: 40°C, 42°C, 45°C, 48°C, 50°C, 52°C, 55°C, 57°C, 59°C, and 60°C. The time of the precipitation reaction can, for example, be any one or a range between any two of the following: 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, and 12 h. The aging time can be, for example, any one or a range between 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, and 24h. The drying temperature for drying the filter residue can be, for example, any one or a range between 80℃, 100℃, 120℃, 140℃, 150℃, 160℃, 180℃, and 200℃. The drying time can be, for example, any one or a range between 12h, 14h, 16h, 18h, 20h, 22h, 24h, 30h, 36h, 40h, and 48h. S2, Preparation of phosphoric acid gas.

[0056] In this disclosure, phosphorus trichloride, oxygen, and water vapor are mixed and reacted in a gas-phase reactor, where phosphorus trichloride is oxidized and hydrolyzed to produce phosphoric acid gas. The molar ratio of Prussian blue-like material, phosphorus trichloride, oxygen, and water vapor is 7.5-15:2:1-2:6-7, the reaction temperature in the gas-phase reactor is 180-200℃, and the reaction time is 2-4 hours.

[0057] The reaction principle is: 2PCl3 + O2 + 6H2O === 2H3PO4(g) + 6HCl. In this disclosure, phosphoric acid gas is prepared using a gas-phase reactor. Gaseous phosphorus trichloride first reacts with oxygen to oxidize into gaseous phosphorus oxychloride, which then undergoes hydrolysis with water vapor to generate phosphoric acid gas. The entire reaction occurs in the gas-phase reactor. During the reaction, HCl gas is also produced. To remove both H3PO4 and HCl gas, this disclosure removes the HCl gas by passing the mixed gas through a saturated phosphoric acid solution, and the remaining H3PO4 gas is collected.

[0058] Step S2 in this disclosure is merely a typical but not limiting example of providing phosphoric acid gas. It should be understood that step S2 of this disclosure may be omitted if other methods are used to prepare or commercially available phosphoric acid gas can be provided.

[0059] S3. Prepare Prussian blue-like materials coated with in-situ coating layers.

[0060] (1) Prussian blue materials are brought into contact with phosphoric acid gas and reacted to form an intermediate in which the surface of the Prussian blue materials is coated with sodium iron phosphate (and optional sodium M phosphate) and sodium phosphate.

[0061] There are various ways to bring Prussian blue materials into contact with phosphoric acid gas, including but not limited to placing the Prussian blue materials in a container filled with phosphoric acid gas (e.g., a stirred tank), or injecting phosphoric acid gas using a nozzle or other gas injection device to directly mix it with the Prussian blue materials. For example, this disclosure provides a typical but non-limiting example: using phosphoric acid gas to pass through a container containing Prussian blue materials for the reaction, which yields better reaction results. The phosphoric acid gas can be introduced into the container all at once or continuously; this disclosure does not limit this, as long as the reaction volume and reaction time are ensured. To ensure mixing effectiveness, a vibration device or stirring device can also be used to mix the Prussian blue materials with the phosphoric acid gas, avoiding the accumulation of Prussian blue materials and resulting in uneven mixing with phosphoric acid gas in certain areas.

[0062] The reaction temperature of Prussian blue-based materials with phosphoric acid gas is 130-160℃, and the reaction time is 2-4 hours. In this disclosure, by controlling the amount of Prussian blue-based materials and phosphoric acid gas, as well as the temperature and reaction time during the reaction process, in-situ coating of the surface of Prussian blue-based materials can be effectively achieved. Specifically, the amount of phosphoric acid gas is controlled by adjusting the molar ratio between the raw materials (Prussian blue-based materials, phosphorus trichloride, oxygen, and water vapor) to 7.5-15:2:1-2:6-7. All the phosphoric acid gas generated from the aforementioned amounts of phosphorus trichloride, oxygen, and water vapor is introduced into the container holding the Prussian blue-based materials for the reaction.

[0063] The reaction temperature of Prussian blue materials with phosphoric acid gas can be, for example, any one or a range of two of 130℃, 135℃, 140℃, 145℃, 150℃, 155℃, and 160℃, and the reaction time can be, for example, any one or a range of two of 2h, 2.5h, 3h, 3.5h, and 4h.

[0064] The principle of the reaction between Prussian blue materials and phosphoric acid gas is as follows (using Na) x M[Fe(CN)6] y (For example, where x is 2 and y is 1):

[0065] 3Na2Fe[Fe(CN)6] + 6H3PO4(g) = 6NaFePO4 + 18HCN;

[0066] 3Na2M[Fe(CN)6] + 6H3PO4(g) ==3NaFePO4 + 3NaMPO4 + 18HCN (where M is not Fe). Based on the above principle, this disclosure also includes the adsorption treatment of the generated HCN gas by activated carbon and silica gel during the reaction of Prussian blue materials with phosphoric acid gas.

[0067] (2) Remove the intermediate sodium phosphate to form a Prussian blue material with a dense surface coating of sodium iron phosphate (and optional sodium M phosphate).

[0068] The steps for removing sodium phosphate from the intermediate include: washing the intermediate with water, followed by drying the washed intermediate at 80-100°C for 2-3 hours. It should be understood that sodium phosphate is produced when sodium is in excess in Prussian blue-based materials. If sodium is not in excess in the Prussian blue-based materials, sodium phosphate may not be produced, and therefore, in such cases, it is not necessary to remove sodium phosphate from the intermediate.

[0069] In this disclosure, sodium phosphate can be removed by simple water washing, and the final product is a Prussian blue material with a dense surface coating of sodium iron phosphate (and optional sodium M phosphate). In this application, sodium iron phosphate (and optional sodium M phosphate) is coated in situ onto the surface of the Prussian blue material. The bonding performance between sodium iron phosphate (and optional sodium M phosphate) and the Prussian blue material is excellent, and the coating is dense. This enables the modified Prussian blue material to have a certain waterproof effect, effectively isolates the core layer of the Prussian blue material from direct contact with the electrolyte, reduces side reactions with the electrolyte and dissolution of metal ions, thereby improving its cycle stability.

[0070] The modified Prussian blue material obtained by the above preparation method includes a Prussian blue material as the core layer and sodium iron phosphate (and optionally sodium M phosphate, where M is not Fe) as the in-situ coating layer, with a particle size of 0.2-2 μm and a mass ratio of the core layer to the in-situ coating layer of 1:0.05-0.1. The above-mentioned modified Prussian blue material can be widely used in the preparation of positive electrode materials for sodium-ion batteries. Specifically, this disclosure also provides a sodium-ion battery comprising a positive electrode material and a negative electrode material, wherein the positive electrode material comprises the above-mentioned modified Prussian blue material. Sodium-ion batteries prepared using the modified Prussian blue material provided in this disclosure as the positive electrode material exhibit excellent cycle stability.

[0071] The technical solution of this disclosure will be further described below with reference to specific embodiments.

[0072] Example 1

[0073] This embodiment provides a method for preparing modified Prussian blue-based materials, which includes the following steps:

[0074] S1. Preparation of Prussian blue-like materials.

[0075] 0.02 mol sodium ferrocyanide (concentration 1.00 mol / L) and 0.6 mol sodium chloride (concentration 0.1 mol / L) were dissolved in 100 mL of deionized water and stirred thoroughly to obtain the first solution; 0.02 mol ferrous chloride and 0.05 mol sodium citrate were dissolved in 100 mL of deionized water and stirred thoroughly to obtain the second solution; the second solution was added to the first solution at a rate of 1 mL / min, and the precipitation reaction was carried out at 50 °C for 6 h, then aged for 16 h, filtered, and the filter residue was dried at 100 °C for 24 h.

[0076] S2. Phosphoric acid gas is prepared by gas-phase reaction.

[0077] In a gas-phase reactor, phosphorus trichloride, oxygen, and water vapor are added in a molar ratio of Prussian blue material, phosphorus trichloride, oxygen, and water vapor of 15:2:1:6. The phosphorus trichloride is oxidized and hydrolyzed at 190°C for 3 hours to produce phosphoric acid gas and HCl gas. The phosphoric acid gas and HCl gas are passed together into a saturated phosphoric acid solution to absorb the HCl gas, and the discharged phosphoric acid gas is collected.

[0078] S3. Preparation of Prussian blue-like materials coated with sodium iron phosphate.

[0079] The Prussian blue material obtained in step S1 was placed in a stirred tank, and phosphoric acid gas generated in step S2 was continuously introduced. The Prussian blue material and phosphoric acid gas reacted at 150°C for 3 hours. During the reaction, activated carbon and silica gel were used to adsorb the tail gas, forming an intermediate with sodium iron phosphate and sodium phosphate coated on the surface of the Prussian blue material. The surface of the intermediate was rinsed with water to remove sodium phosphate; it was then dried at 80°C for 3 hours to obtain a Prussian blue material with a dense surface coating of sodium iron phosphate (e.g., Prussian blue). Figure 1 (As shown). The Prussian blue-like material with a dense surface coating of sodium iron phosphate has a particle size of 100-120 nm, and the mass percentage of the core layer and the in-situ coating layer is 1:0.05.

[0080] Example 2-3

[0081] Examples 2-3 are basically the same as Example 1, except that the parameters in steps S2 and S3 are different.

[0082] In Example 2:

[0083] S2. Phosphoric acid gas is prepared by gas-phase reaction.

[0084] In a gas-phase reactor, phosphorus trichloride, oxygen, and water vapor are added in a molar ratio of Prussian blue material, phosphorus trichloride, oxygen, and water vapor of 10:2:1:6. The phosphorus trichloride is oxidized and hydrolyzed at 190°C for 3 hours to produce phosphoric acid gas and HCl gas. The phosphoric acid gas and HCl gas are passed together into a saturated phosphoric acid solution to absorb the HCl gas, and the discharged phosphoric acid gas is collected.

[0085] S3. Preparation of Prussian blue-like materials coated with ferrous phosphate: The Prussian blue-like material obtained in step S1 was placed in a stirred tank, and phosphoric acid gas generated in step S2 was continuously introduced. The Prussian blue-like material and phosphoric acid gas reacted at 130℃ for 4 hours. During the reaction, activated carbon and silica gel were used to adsorb the tail gas, forming an intermediate with ferrous phosphate and sodium phosphate coated on the surface of the Prussian blue-like material. The surface of the intermediate was rinsed with water to remove sodium phosphate; it was then dried at 100℃ for 2 hours to obtain a Prussian blue-like material with a dense surface coating of ferrous phosphate. The particle size of this Prussian blue-like material with a dense surface coating of ferrous phosphate was 100-120 nm, and the mass ratio of the core layer to the in-situ coating layer was 1:0.075.

[0086] In Example 3:

[0087] S2. Phosphoric acid gas is prepared by gas-phase reaction.

[0088] In a gas-phase reactor, phosphorus trichloride, oxygen, and water vapor are added in a molar ratio of Prussian blue material, phosphorus trichloride, oxygen, and water vapor of 7.5:2:1:6. The phosphorus trichloride is oxidized and hydrolyzed at 190°C for 3 hours to produce phosphoric acid gas and HCl gas. The phosphoric acid gas and HCl gas are passed together into a saturated phosphoric acid solution to absorb the HCl gas, and the discharged phosphoric acid gas is collected.

[0089] S3. Preparation of Prussian blue-like materials coated with ferrous phosphate: The Prussian blue-like material obtained in step S1 was placed in a stirred tank, and phosphoric acid gas generated in step S2 was continuously introduced. The Prussian blue-like material and phosphoric acid gas were reacted at 160°C for 2 hours. During the reaction, activated carbon and silica gel were used to adsorb the tail gas, forming an intermediate with ferrous phosphate and sodium phosphate coated on the surface of the Prussian blue-like material. The surface of the intermediate was rinsed with water to remove sodium phosphate; it was then dried at 90°C for 3 hours to obtain a Prussian blue-like material with a dense surface coating of ferrous phosphate. The particle size of this Prussian blue-like material with a dense surface coating of ferrous phosphate was 100-120 nm, and the mass ratio of the core layer to the in-situ coating layer was 1:0.1.

[0090] Example 4

[0091] Example 4 is basically the same as Example 1, except that the contact method between the Prussian blue material and the phosphoric acid gas in step S3 is different.

[0092] In this embodiment, Prussian blue-like material is placed in a stirred tank, and phosphoric acid gas is introduced into the stirred tank in one go. The particle size of the Prussian blue-like material with dense ferrous phosphate coating is 100-120 nm, and the mass ratio of the core layer to the in-situ coating layer is 1:0.05.

[0093] Example 5

[0094] This embodiment is basically the same as Embodiment 1, except that the parameters of step S1 are different.

[0095] S1. Preparation of Prussian blue-like materials.

[0096] 0.02 mol sodium ferrocyanide (concentration 2.00 mol / L) and 0.8 mol sodium chloride (concentration 0.15 mol / L) were dissolved in 100 mL of deionized water and stirred thoroughly to obtain the first solution; 0.02 mol manganese chloride and 0.1 mol sodium citrate were dissolved in 100 mL of deionized water and stirred thoroughly to obtain the second solution; the second solution was added to the first solution at a rate of 3 mL / min, and the precipitation reaction was carried out at 60 °C for 10 h, followed by aging for 12 h, and then filtered. The filter residue was dried at 160 °C for 12 h to obtain Prussian blue-like materials.

[0097] Example 6

[0098] The difference between this embodiment and Embodiment 1 is that the sodium phosphate on the intermediate is not removed in this embodiment, that is, the "rinsing the surface of the intermediate with water to remove sodium phosphate and drying at 80°C for 3 hours" in Embodiment 1 is omitted. Instead, the intermediate with ferrous phosphate and sodium phosphate coated on the surface of the Prussian blue material is directly used as the modified Prussian blue material.

[0099] Comparative Example 1

[0100] The difference between this comparative example and Example 1 is that this comparative example uses a coating liquid to coat Prussian blue-like materials.

[0101] S1. Preparation of Prussian blue-like materials.

[0102] 0.02 mol sodium ferrocyanide (concentration 1.00 mol / L) and 0.6 mol sodium chloride (concentration 0.1 mol / L) were dissolved in 100 mL of deionized water and stirred thoroughly to obtain the first solution; 0.02 mol ferrous chloride and 0.05 mol sodium citrate were dissolved in 100 mL of deionized water and stirred thoroughly to obtain the second solution; the second solution was added to the first solution at a rate of 1 mL / min, and the precipitation reaction was carried out at 50 °C for 6 h, then aged for 16 h, filtered, and the filter residue was dried at 100 °C for 24 h.

[0103] S2. Prepare the coating solution.

[0104] Dissolve 0.02 mol of citric acid in deionized water, add 0.04 mol of ferrous phosphate powder and stir to obtain a coated suspension.

[0105] S2. Prepare the coating material.

[0106] The Prussian blue core and coating solution were stirred at 500 r / min for 3 h at 50 °C and then centrifuged. The filter residue was dried at 200 °C for 24 h under a nitrogen atmosphere.

[0107] Comparative Example 2

[0108] The difference between this comparative example and Example 1 is that this comparative example uses uncoated Prussian blue-based materials.

[0109] 0.02 mol sodium ferrocyanide (concentration 1.00 mol / L) and 0.6 mol sodium chloride (concentration 0.1 mol / L) were dissolved in 100 mL of deionized water and stirred thoroughly to obtain the first solution; 0.02 mol ferrous chloride and 0.05 mol sodium citrate were dissolved in 100 mL of deionized water and stirred thoroughly to obtain the second solution; the second solution was added to the first solution at a rate of 1 mL / min, and the precipitation reaction was carried out at 50 °C for 6 h, then aged for 16 h, filtered, and the filter residue was dried at 100 °C for 24 h.

[0110] Comparative Example 3

[0111] The difference between this comparative example and Example 1 is that in this comparative example, the molar ratio of Prussian blue-based material, phosphorus trichloride, oxygen, and water vapor is 3:2:1:6. The mass ratio of the core layer to the in-situ coating layer in the prepared modified Prussian blue-based material is 1:0.2.

[0112] Comparative Example 4

[0113] The difference between this comparative example and Example 1 is that in this comparative example, the reaction temperature of the Prussian blue material with phosphoric acid gas is 200°C, the reaction time is 2 hours, and the reaction is carried out under normal pressure.

[0114] Experimental Example

[0115] The performance of the Prussian blue cathode materials prepared in the test examples and comparative examples is shown in Table 1.

[0116] Test Method: Assembly of Button Sodium-ion Battery: The Prussian blue cathode material, acetylene black, and polyvinylidene fluoride (PVDF) prepared in the examples were mixed in a mass ratio of 7:2:1. The uniformly mixed slurry was coated onto aluminum foil, dried, and cut into discs as the cathode. A sodium metal sheet was used as the anode, and Whatman glass fiber (GF / D) was used as the separator. The organic electrolyte was prepared from NaClO4, EC (ethylene carbonate), DEC (diethyl carbonate), and FEC (fluoroethylene carbonate). The concentration of NaClO4 was 1.0 mol / L, the volume ratio of EC to DEC was 1:1, and the mass fraction of FEC in the electrolyte was 5%. The sodium-ion battery was assembled in an argon glove box.

[0117] Cyclic performance was tested at 25℃ (1C / 1C 2.0~4.3V); for water absorption performance testing, the samples in the examples and comparative examples were dried at 120℃ for 12h, and then divided into two groups (Group 1 and Group 2). The water content of Group 1 was immediately tested using a Karl Fischer moisture analyzer, and the water content of Group 2 was tested using a Karl Fischer moisture analyzer after being placed in air for 1h. The cutoff temperature for water content testing was 170℃.

[0118] The results are shown in the table below:

[0119]

[0120] As can be seen from the table above, the cycle retention rates of the modified Prussian blue materials provided in Examples 1-6 of this disclosure are significantly higher than those in Comparative Examples 1-4, and their water content is also significantly lower than that in Comparative Examples 1-4. This fully demonstrates that the modified Prussian blue materials provided in Examples 1-6 of this disclosure generate a denser in-situ coating layer, which enables the material to achieve a certain degree of waterproofing, effectively isolating the Prussian blue material from direct contact with the electrolyte, reducing side reactions with the electrolyte and the dissolution of metal ions, thereby improving its cycle stability. Furthermore, the data from Comparative Example 1 shows that it uses a coating solution to protect the Prussian blue material, and the coating density is significantly worse than that of Example 1. Comparative Example 2, without coating, has the worst cycle retention rate and water content. Comparative Example 3 increases the amount of phosphoric acid gas introduced, which can also generate a dense in-situ coating layer, so its water content is not much different from that of Example 1. However, due to the excessive phosphoric acid gas, the consumption of Prussian blue material is greater than that of Example 1, thus reducing the cycle retention rate. In Comparative Example 4, the reaction temperature of the Prussian blue material with phosphoric acid gas was increased, resulting in a slight decrease in retention rate and a slight increase in water absorption rate, which fully demonstrates that the increase in temperature leads to a decrease in the density of the coating layer.

[0121] It should be understood that the steps for preparing Prussian blue-like materials in this disclosure only list a typical but non-limiting preparation method and related parameters. Prussian blue-like materials can be obtained by adjusting the parameters within the scope of this disclosure.

[0122] In summary, the modified Prussian blue material provided in this disclosure, through in-situ modification of the surface of the Prussian blue material to form sodium iron phosphate and optionally sodium M phosphate as an in-situ coating layer, provides a tighter coating compared to an external coating layer. This coating also provides a certain degree of water resistance, effectively isolating the Prussian blue material from direct contact with the electrolyte, reducing side reactions and metal ion dissolution, thereby improving its cycle stability. The coating method used in this disclosure is simple and easy to operate, and the prepared modified Prussian blue material exhibits good cycle stability, making it widely applicable in the preparation of cathode materials for sodium-ion batteries. Sodium-ion batteries prepared using the modified Prussian blue material provided in this disclosure as the cathode material demonstrate excellent cycle stability.

[0123] The optional embodiments of this disclosure have been described in detail above; however, this disclosure is not limited thereto. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, including combining the various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in this disclosure and are all within the protection scope of this disclosure.

[0124] Industrial applicability

[0125] The modified Prussian blue material disclosed herein involves in-situ modification of the surface of a Prussian blue material to form sodium iron phosphate (and optionally sodium M phosphate, where M is not Fe) as an in-situ coating layer. Compared to an external coating layer, the modified Prussian blue material provided in this disclosure provides a tighter coating and a certain degree of waterproofing, effectively isolating the Prussian blue material from direct contact with the electrolyte, reducing side reactions with the electrolyte and the dissolution of metal ions, thereby improving its cycle stability. The coating method used in this disclosure is simple and easy to operate, and the prepared modified Prussian blue material exhibits good cycle stability, making it widely applicable in the preparation of cathode materials for sodium-ion batteries. Sodium-ion batteries prepared using the modified Prussian blue material provided in this disclosure as the cathode material demonstrate excellent cycle stability.

Claims

1. A modified Prussian blue-based material, characterized in that, It includes a core layer and an in-situ coating layer formed on the surface of the core layer. The core layer is a Prussian blue-like material with the structural formula Na. x M[Fe(CN)6] y Wherein, 0≤x≤2, 0≤y≤1, M is at least one of Fe, Mn, Co, Ni, Cu and Zn, when M is Fe, the in-situ coating layer is sodium iron phosphate, when M is at least one of Mn, Co, Ni, Cu and Zn, the in-situ coating layer is a composite of sodium iron phosphate and sodium M phosphate, the mass ratio of the core layer to the in-situ coating layer is 1:0.05-0.1; the preparation method of the modified Prussian blue material includes: contacting and reacting the Prussian blue material with phosphoric acid gas to form a Prussian blue material with a dense surface coated with an in-situ coating layer; the reaction temperature of the Prussian blue material with the phosphoric acid gas is 130-160℃, and the reaction time is 2-4h.

2. The modified Prussian blue material according to claim 1, characterized in that, The particle size of the modified Prussian blue material is 0.2-2 μm.

3. The modified Prussian blue material according to claim 1, characterized in that, The thickness of the in-situ coating layer is 10-30 nm.

4. A method for preparing a modified Prussian blue-based material as described in any one of claims 1-3, characterized in that, include: Prussian blue-based materials are brought into contact with and reacted with phosphoric acid gas to form Prussian blue-based materials with a dense surface coating of an in-situ coating layer; the reaction temperature of the Prussian blue-based materials with the phosphoric acid gas is 130-160℃, and the reaction time is 2-4h.

5. The method for preparing the modified Prussian blue-based material according to claim 4, characterized in that, The preparation method of the modified Prussian blue material also includes the preparation of phosphoric acid gas: phosphorus trichloride, oxygen and water vapor are mixed and reacted in a gas phase reactor, wherein the phosphorus trichloride is oxidized and hydrolyzed to produce phosphoric acid gas.

6. The method for preparing the modified Prussian blue-based material according to claim 5, characterized in that, The molar ratio of the Prussian blue material, the phosphorus trichloride, the oxygen, and the water vapor is 7.5-15:2:1-2:6-7.

7. The method for preparing the modified Prussian blue-based material according to claim 5, characterized in that, The preparation of phosphoric acid gas also includes absorbing and removing HCl gas generated during the preparation of phosphoric acid gas using a saturated phosphoric acid solution.

8. The method for preparing the modified Prussian blue-based material according to claim 4, characterized in that, The steps of contacting and reacting Prussian blue materials with phosphoric acid gas include: continuously introducing the phosphoric acid gas, which is prepared on-site, into the Prussian blue materials to carry out the reaction.

9. The method for preparing the modified Prussian blue-based material according to claim 8, characterized in that, The process of reacting the Prussian blue material with the phosphoric acid gas also includes: using activated carbon and silica gel to adsorb the generated HCN gas.

10. The method for preparing the modified Prussian blue-based material according to claim 4, characterized in that, When the Prussian blue material comes into contact with the phosphoric acid gas, an intermediate coated with the in-situ coating layer and sodium phosphate is first formed on the surface of the Prussian blue material; before obtaining the Prussian blue material coated with the in-situ coating layer, the sodium phosphate is removed from the intermediate.

11. The method for preparing the modified Prussian blue-based material according to claim 10, characterized in that, The step of removing the sodium phosphate from the intermediate includes washing and drying the intermediate with water.

12. The method for preparing the modified Prussian blue-based material according to claim 11, characterized in that, The intermediate after washing is dried at a temperature of 80-100℃ for 2-3 hours.

13. The method for preparing the modified Prussian blue-based material according to claim 4, characterized in that, The preparation method of the modified Prussian blue material further includes the preparation of the Prussian blue material: dissolving sodium ferrocyanide and inorganic sodium salt in water to obtain a first solution, dissolving a soluble metal salt and a complexing agent in water to obtain a second solution, adding the second solution to the first solution, precipitating the reaction, aging the solution, filtering it, and drying the filter residue.

14. The method for preparing the modified Prussian blue-based material according to claim 13, characterized in that, The steps for preparing Prussian blue-like materials include at least one of features (1) to features (10): Feature (1): The raw material concentration of sodium ferrocyanide is 0.02 mol / L-2 mol / L, the raw material concentration of inorganic sodium salt is 0.05 mol / L-0.15 mol / L, and the molar ratio of sodium ferrocyanide to inorganic sodium salt is 1:20-40; Feature (2): The inorganic sodium salt includes at least one of sodium chloride, sodium sulfate, sodium oxalate, sodium acetate, sodium nitrate and sodium citrate; Feature (3): The molar ratio of the soluble metal salt to the complexing agent is 1:2-5; Feature (4): The metal in the soluble metal salt includes at least one of Fe, Mn, Co, Cu and Zn; Feature (5): The complexing agent includes at least one of sodium citrate, disodium ethylenediaminetetraacetate, and sodium pyrophosphate; Feature (6): The rate at which the second solution is added to the first solution is 0.5 mL / min to 5 mL / min; Feature (7): The precipitation reaction includes precipitation at 40-60℃ for 4-12 hours; Feature (8): The aging time is 12h-24h; Feature (9): The filtration method includes centrifugation, vacuum filtration or pressure filtration; Feature (10): The drying temperature for drying the filter residue is 80-200℃ and the drying time is 12-48h.

15. The application of the modified Prussian blue material as described in any one of claims 1-3 in the preparation of cathode materials for sodium-ion batteries.

16. A sodium-ion battery, characterized in that, It includes a positive electrode material and a negative electrode material, wherein the positive electrode material comprises a modified Prussian blue material according to any one of claims 1-3.

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