Modified prussian blue material, preparation method therefor, and use thereof

Abstract

The present disclosure relates to the field of Prussian blue materials, and provides a modified Prussian blue material, a preparation method therefor, and a use thereof. The modified Prussian blue material provided by the present disclosure comprises a Prussian blue material serving as a core layer, and sodium iron phosphate and optional sodium M phosphate which serve as an in-situ coating layer. According to the present disclosure, the sodium iron phosphate and the optional sodium M phosphate can be generated on the surface of the Prussian blue material in situ, such that the coating is tighter, the material can achieve a certain waterproof effect, the Prussian blue material can be effectively isolated from an electrolyte to avoid direct contact therebetween, side effects with the electrolyte and the dissolution of metal ions are reduced, and therefore the cycle stability of the Prussian blue material is improved. The coating means used in the present disclosure is simple and easy to operate, and the prepared modified Prussian blue material has good cycle stability and can be widely applied to the preparation of positive electrode materials in sodium ion batteries.

Description

A modified Prussian blue material and its preparation method and application Technical Field

[0001] The present disclosure relates to the field of Prussian blue materials, and in particular to a modified Prussian blue material and a preparation method and application thereof. Background Art

[0002] In recent years, lithium resource scarcity, uneven distribution, and difficulties in development and utilization have become increasingly serious. In contrast, sodium-ion batteries, whose electrode materials are primarily sodium salts, are more abundant and cheaper than lithium. Because sodium ions are larger than lithium ions, sodium-ion batteries offer a cost-effective alternative when weight and energy density are less critical, making them a key option for future energy storage grids.

[0003] The performance of sodium-ion batteries depends on the structure and performance of the materials inside the battery, and the performance and cost of the positive electrode materials are related to the market competitiveness of the entire sodium-ion battery system. As one of the sodium-ion batteries, Prussian blue materials have open three-dimensional channels, which allow sodium ions to migrate rapidly in the tunnels, so they have good structural stability and excellent rate performance. However, due to problems such as vacancies in the crystal structure, the presence of crystal water, surface side reactions with the electrolyte, and the dissolution of metal ions during long-term cycles, they usually exhibit poor conductivity and cycle performance, which seriously restrict the development of Prussian blue materials. Therefore, coating and modification of Prussian blue materials is currently a more widely used method.

[0004] Related technologies usually use solid phase coating method to coat the surface of Prussian blue materials, such as a Prussian blue material described in CN116053441A, which includes a core and a coating layer. The core includes a chemical formula of Na x M[Fe(CN)6] y The coating layer includes a compound having a chemical formula of Na3V 2-Z Mg Z (PO4)3 compounds. The Prussian blue-based cathode materials prepared by this method have, to a certain extent, high charge-discharge capacity and cycle performance. However, due to the special structure of Prussian blue-based materials, this solid-phase coating method using coating liquid and materials is difficult to achieve complete coating, and the effect of performance modification is limited.

[0005] In view of this, the present disclosure is proposed.

[0006] Summary of the Invention

[0007] The objectives of the present disclosure include, for example, providing a modified Prussian blue material that achieves an in-situ coating effect on the Prussian blue material through the surface layer of the material itself, thereby enhancing the conductivity and cycle stability of the Prussian blue material.

[0008] The objectives of the present disclosure include, for example, providing a method for preparing a modified Prussian blue material, which is simple and has excellent electrical conductivity and cycle stability.

[0009] The objectives of the present disclosure include, for example, providing the use of modified Prussian blue materials in preparing positive electrode materials for sodium ion batteries.

[0010] Objects of the present disclosure include, for example, providing a sodium ion battery.

[0011] The embodiments of the present disclosure may be implemented as follows:

[0012] In a first aspect, the present disclosure provides a modified Prussian blue material, comprising a core layer and an in-situ coating layer in-situ generated on the surface of the core layer, wherein the core layer is a Prussian blue material having a structural formula of 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 the M is Fe, the in-situ coating layer is sodium iron phosphate, when the 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.

[0013] In some embodiments of the present disclosure, the mass ratio of the core layer to the in-situ coating layer is 1:0.05-0.1.

[0014] In some embodiments of the present disclosure, the particle size of the modified Prussian blue-based material is 0.2-2 μm.

[0015] In some embodiments of the present disclosure, the thickness of the in-situ coating layer is 10-30 nm.

[0016] In a second aspect, the present disclosure provides a method for preparing a modified Prussian blue-based material, comprising:

[0017] The Prussian blue material is brought into contact with phosphoric acid gas and reacted to form a Prussian blue material with an in-situ coating layer densely coated on the surface.

[0018] In some embodiments of the present disclosure, the reaction temperature of the Prussian blue material and the phosphoric acid gas is 130-160° C., and the reaction time is 2-4 hours.

[0019] In some embodiments of the present disclosure, the method for preparing the modified Prussian blue material further comprises preparing phosphoric acid gas: phosphorus trichloride, oxygen and water vapor are mixed and reacted in a gas phase reactor, and the phosphorus trichloride is oxidized and hydrolyzed to produce phosphoric acid gas.

[0020] In some embodiments of the present 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.

[0021] In some embodiments of the present disclosure, the preparing phosphoric acid gas further comprises removing HCl gas generated during the preparation of the phosphoric acid gas by absorbing it with a saturated phosphoric acid solution.

[0022] In some embodiments of the present disclosure, the step of contacting and reacting a Prussian blue-based material with phosphoric acid gas includes continuously passing the phosphoric acid gas prepared on-site into the Prussian blue-based material for reaction. In some embodiments of the present disclosure, the step of contacting and reacting the Prussian blue-based material with the phosphoric acid gas also includes adsorbing the generated HCN gas using activated carbon and silica gel.

[0023] In some embodiments of the present disclosure, when the Prussian blue material contacts the phosphoric acid gas, an intermediate of 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 in the intermediate is removed.

[0024] In some embodiments of the present disclosure, the step of removing the sodium phosphate from the intermediate comprises: washing the intermediate with water and drying the intermediate.

[0025] In some embodiments of the present disclosure, the intermediate after washing is dried at a temperature of 80-100° C. and a drying time of 2-3 hours.

[0026] In some embodiments of the present disclosure, the method for preparing the modified Prussian blue material further comprises preparing the Prussian blue material: 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, aging after precipitation reaction, filtering, and drying the filter residue.

[0027] In some embodiments of the present disclosure, the step of preparing the Prussian blue-based material includes at least one of features (1) to (10):

[0028] Feature (1): The raw material concentration of the sodium ferrocyanide is 0.02 mol / L-2 mol / L, the raw material concentration of the inorganic sodium salt is 0.05 mol / L-0.15 mol / L, and the molar ratio of the sodium ferrocyanide to the inorganic sodium salt is 1:20-40;

[0029] Feature (2): The inorganic sodium salt includes at least one of sodium chloride, sodium sulfate, sodium oxalate, sodium acetate, sodium nitrate and sodium citrate;

[0030] Feature (3): The molar ratio of the soluble metal salt to the complexing agent is 1:2-5;

[0031] Feature (4): The metal in the soluble metal salt includes at least one of Fe, Mn, Co, Cu and Zn;

[0032] Feature (5): The complexing agent comprises at least one of sodium citrate, disodium edetate and sodium pyrophosphate;

[0033] Feature (6): The rate at which the second solution is added to the first solution is 0.5 mL / min-5 mL / min;

[0034] Feature (7): The precipitation reaction comprises precipitation at 40-60°C for 4h-12h;

[0035] Feature (8): The aging time is 12h-24h;

[0036] Feature (9): The filtration method includes centrifugation, suction filtration or pressure filtration;

[0037] Feature (10): The drying temperature for drying the filter residue is 80-200° C., and the drying time is 12-48 hours.

[0038] In a third aspect, the present disclosure provides the use of the modified Prussian blue material as described in any of the aforementioned embodiments in preparing a positive electrode material for a sodium ion battery.

[0039] In a fourth aspect, the present disclosure provides a sodium ion battery comprising a positive electrode material and a negative electrode material, wherein the positive electrode material comprises the modified Prussian blue material according to any one of the aforementioned embodiments.

[0040] Compared with the prior art, the advantages of the present invention include:

[0041] The modified Prussian blue material provided by the present invention is formed by in-situ modification of the surface of the Prussian blue material to form sodium iron phosphate and optional sodium M phosphate as an in-situ coating layer. Compared with the external coating layer, the modified Prussian blue material provided by the present invention is more tightly coated and can make the material have a certain waterproof effect, effectively isolating the direct contact between the Prussian blue material and the electrolyte, reducing the side reaction with the electrolyte and the dissolution of metal ions, thereby improving its cycle stability. The coating method adopted in the present invention is simple and easy to operate. The prepared modified Prussian blue material has good cycle stability and can be widely used in the preparation of positive electrode materials in sodium ion batteries. The sodium ion battery prepared using the modified Prussian blue material provided by the present invention as the positive electrode material has good cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0043] FIG1 is a SEM image of the modified Prussian blue-based material provided by the present disclosure. DETAILED DESCRIPTION

[0044] The embodiments of the present disclosure will be described in detail below with reference to the examples. However, those skilled in the art will appreciate that the following examples are intended only to illustrate the present disclosure and should not be construed as limiting the scope of the present disclosure. Where specific conditions are not specified in the examples, the experiments were performed under conventional conditions or the conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, all are commercially available conventional products.

[0045] The endpoints of the ranges and any values ​​disclosed in this disclosure are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0046] The present disclosure provides a method for preparing a modified Prussian blue material, comprising the following steps:

[0047] S1. Prepare Prussian blue materials.

[0048] The Prussian blue material (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) can be independently prepared or commercially purchased. The present disclosure lists a typical but non-limiting method for preparing a Prussian blue-based material, which specifically includes the following steps:

[0049] (1) Sodium ferrocyanide and an inorganic sodium salt are dissolved in water to obtain a first solution.

[0050] In the present 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.

[0051] In certain embodiments, the raw material concentration of sodium ferrocyanide can be, for example, any one of 0.02mol / L, 0.05mol / L, 0.08mol / L, 0.1mol / L, 0.2mol / L, 0.5mol / L, 0.8mol / L, 1mol / L, 1.5mol / L, 2mol / L, or a range value between any two thereof. The raw material concentration of the inorganic sodium salt can be, for example, any one of 0.05mol / L, 0.08mol / L, 0.1mol / L, 0.12mol / L, 0.14mol / L, 0.15mol / L, or a range value between any two thereof. The molar ratio of sodium ferrocyanide to the inorganic sodium salt can be, for example, any one of 1:20, 1:22, 1:24, 1:25, 1:28, 1:30, 1:32, 1:35, 1:37, 1:39, 1:40, or a range value between any two thereof.

[0052] (2) Dissolving the soluble metal salt and the complexing agent in water to obtain a second solution.

[0053] In the present disclosure, the molar ratio of the soluble metal salt to the complexing agent is 1:2-5. In certain embodiments, the molar ratio of the soluble metal salt to the complexing agent can be, for example, any one of 1:2, 1:3, 1:4, 1:5, or a range between any two thereof. The metal in the soluble metal salt includes at least one of Fe, Mn, Co, Cu, and Zn; and the complexing agent includes at least one of sodium citrate, disodium EDTA, and sodium pyrophosphate.

[0054] (3) Add the second solution to the first solution, allow to age after precipitation reaction, filter, and dry the filter residue.

[0055] In the present disclosure, the second solution is added to the first solution at a rate of 0.5 mL / min-5 mL / min; after the addition is completed, a precipitation reaction is carried out at 40-60° C. for 4 h-12 h; then aged for 12 h-24 h; then filtered, and the filtering method includes but is not limited to centrifugation, suction filtration or pressure filtration; then the filter residue is dried at 80° C.-200° C. for 12-48 h to obtain a Prussian blue material.

[0056] In certain embodiments, the rate at which the second solution is added to the first solution can be, for example, 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, 5 mL / min, or any range between two thereof. The temperature of the precipitation reaction can be, for example, 40°C, 42°C, 45°C, 48°C, 50°C, 52°C, 55°C, 57°C, 59°C, or any range between two thereof. The time for the precipitation reaction can be, for example, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, or any range between two thereof. The aging time can be, for example, any one of 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, and 24 hours, or a range between any two of them. The drying temperature for drying the filter residue can be, for example, any one of 80° C., 100° C., 120° C., 140° C., 150° C., 160° C., 180° C., and 200° C., or a range between any two of them. The drying time can be, for example, any one of 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, 24 hours, 30 hours, 36 hours, 40 hours, and 48 hours, or a range between any two of them. S2. Preparing phosphoric acid gas.

[0057] In the present disclosure, phosphorus trichloride, oxygen, and water vapor are mixed and reacted in a gas-phase reactor, where the phosphorus trichloride is oxidized and hydrolyzed to produce phosphoric acid gas. The molar ratio of the Prussian blue-based 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°C, and the reaction time is 2-4 hours.

[0058] The reaction principle is: 2PCl³ + O² + 6H²O === 2H³PO⁴ (gas) + 6HCl. In the present disclosure, phosphoric acid gas is produced using a gas-phase reactor. Gaseous phosphorus trichloride first reacts with oxygen to oxidize into gaseous phosphorus oxychloride, which then undergoes a hydrolysis reaction with water vapor to generate phosphoric acid gas. The entire reaction occurs within the gas-phase reactor. HCl gas is also produced during the reaction. To address both H³PO⁴ and HCl gas, the present disclosure removes the HCl gas by passing the mixed gas through a saturated phosphoric acid solution to absorb it. The remaining H³PO⁴ gas is then collected.

[0059] Step S2 in the present disclosure is merely a typical but non-limiting example for providing phosphoric acid gas. It should be understood that step S2 of the present disclosure may be omitted as long as phosphoric acid gas can be provided by using other methods for preparation or commercial purchase.

[0060] S3. Prepare a Prussian blue material coated with an in-situ coating layer.

[0061] (1) A Prussian blue material is contacted with phosphoric acid gas and reacted to form an intermediate in which the surface of the Prussian blue material is coated with sodium iron phosphate (and optionally sodium M phosphate) and sodium phosphate.

[0062] There are many ways to contact Prussian blue materials with phosphoric acid gas, including but not limited to placing the Prussian blue material in a container filled with phosphoric acid gas (such as a stirred tank), or injecting phosphoric acid gas using a nozzle or other gas injection equipment to directly mix it with the Prussian blue material. For example, the present disclosure provides a typical but non-limiting example: phosphoric acid gas is introduced into a container with Prussian blue material for reaction, which has a better reaction effect. Phosphoric acid gas can be introduced into the container at one time or continuously. The present disclosure does not limit this. It is only necessary to ensure the reaction amount and reaction time. In order to ensure the mixing effect, a vibration device or a stirring device can also be provided to achieve the mixing of the Prussian blue material and the phosphoric acid gas, so as to avoid the accumulation of Prussian blue material and the uneven mixing of the local contact with the phosphoric acid gas.

[0063] The reaction temperature of the Prussian blue material and phosphoric acid gas is 130-160°C, and the reaction time is 2-4 hours. In the present disclosure, by controlling the amounts of the Prussian blue material and phosphoric acid gas, as well as the temperature and reaction time during the reaction, the surface of the Prussian blue material can be effectively coated in situ. The amount of phosphoric acid gas is controlled by adjusting the molar ratio of the raw materials (Prussian blue material, phosphorus trichloride, oxygen, and water vapor) to 7.5-15:2:1-2:6-7. The phosphoric acid gas generated by the aforementioned amounts of phosphorus trichloride, oxygen, and water vapor is introduced into a container containing the Prussian blue material for reaction.

[0064] The reaction temperature of the Prussian blue material and phosphoric acid gas can be, for example, any one of 130°C, 135°C, 140°C, 145°C, 150°C, 155°C, 160°C, or a range between any two of them, and the reaction time can be, for example, any one of 2h, 2.5h, 3h, 3.5h, 4h, or a range between any two of them.

[0065] The principle of the reaction between Prussian blue materials and phosphoric acid gas is as follows (with Na x M[Fe(CN)6] y (For example, where x is 2 and y is 1): 3Na2Fe[Fe(CN)6]+6H3PO4(gas)==6NaFePO4+18HCN; 3Na2M[Fe(CN)6]+6H3PO4(gas)==3NaFePO4+3NaMPO4+18HCN (in this case, M is not Fe). Based on the above principle, the present disclosure further includes using activated carbon and silica gel to adsorb the generated HCN gas during the contact reaction between the Prussian blue material and phosphoric acid gas.

[0066] (2) removing the sodium phosphate intermediate to form a Prussian blue material whose surface is densely coated with sodium iron phosphate (and optionally sodium M phosphate).

[0067] The step of removing sodium phosphate from the intermediate comprises washing the intermediate with water, and then drying the washed intermediate at 80-100° C. for 2-3 hours. It should be understood that sodium phosphate is produced when there is an excess of sodium in the Prussian blue-based material. If there is not an excess of sodium in the Prussian blue-based material, sodium phosphate may not be produced. Therefore, in such a case, it is not necessary to remove the sodium phosphate from the intermediate.

[0068] In the present disclosure, sodium phosphate can be removed by simple water washing, and the final product obtained is a Prussian blue material whose surface is densely coated with sodium iron phosphate (and optional sodium M phosphate). Since in the present application, sodium iron phosphate (and optional sodium M phosphate) is in situ coated on the surface of the Prussian blue material, sodium iron phosphate (and optional sodium M phosphate) has good binding performance with the Prussian blue material and good coating density, the modified Prussian blue material can have a certain waterproof effect, can effectively isolate the Prussian blue material of the core layer from direct contact with the electrolyte, reduce side reactions with the electrolyte and the dissolution of metal ions, thereby improving its cycle stability.

[0069] The modified Prussian blue material obtained by the above preparation method includes a Prussian blue material as a core layer and sodium iron phosphate (and optional sodium phosphate M, where M is not Fe) as an 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 in sodium ion batteries. Specifically, the present disclosure also provides a sodium ion battery, which includes a positive electrode material and a negative electrode material, and the positive electrode material includes the above-mentioned modified Prussian blue material. The sodium ion battery prepared using the modified Prussian blue material provided by the present disclosure as the positive electrode material has good cycle stability.

[0070] The technical solution of the present disclosure is further described below in conjunction with specific implementation methods.

[0071] Example 1

[0072] This embodiment provides a method for preparing a modified Prussian blue material, which comprises the following steps:

[0073] S1. Prepare Prussian blue materials.

[0074] 0.02 mol of sodium ferrocyanide (concentration of 1.00 mol / L) and 0.6 mol of sodium chloride (concentration of 0.1 mol / L) were dissolved in 100 mL of deionized water and stirred thoroughly to obtain a first solution; 0.02 mol of ferrous chloride and 0.05 mol of sodium citrate were dissolved in 100 mL of deionized water and stirred thoroughly to obtain a second solution; the second solution was added to the first solution at a rate of 1 mL / min, the solution was precipitated at 50°C for 6 h, aged for 16 h, and then filtered. The filter residue was dried at 100°C for 24 h.

[0075] S2. Prepare phosphoric acid gas by gas phase reaction.

[0076] In a gas phase reactor, phosphorus trichloride, oxygen and water vapor are added according to a molar ratio of 15:2:1:6 among Prussian blue material, phosphorus trichloride, oxygen and water vapor. 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 introduced into a saturated phosphoric acid solution, the HCl gas is absorbed, and the discharged phosphoric acid gas is collected.

[0077] S3. Prepare a Prussian blue material coated with sodium iron phosphate.

[0078] The Prussian blue material prepared in step S1 is placed in a stirred tank, and the phosphoric acid gas produced in step S2 is continuously introduced. The Prussian blue material and the phosphoric acid gas react at 150°C for 3 hours. During the reaction, the tail gas is adsorbed by activated carbon and silica gel to form an intermediate whose surface is coated with sodium iron phosphate and sodium phosphate. The surface of the intermediate is rinsed with water to remove the sodium phosphate; it is dried at 80°C for 3 hours to obtain a Prussian blue material with a surface densely coated with sodium iron phosphate (as shown in Figure 1). The particle size of the Prussian blue material with a surface densely coated with sodium iron phosphate is 100-120nm, and the mass ratio of the core layer to the in-situ coating layer is 1:0.05.

[0079] Example 2-3

[0080] Embodiment 2-3 is basically the same as embodiment 1, except that the parameters in steps S2 and S3 are different.

[0081] In Example 2:

[0082] S2. Prepare phosphoric acid gas by gas phase reaction.

[0083] In a gas phase reactor, phosphorus trichloride, oxygen and water vapor are added according to a molar ratio of 10:2:1:6 among Prussian blue material, phosphorus trichloride, oxygen and water vapor. 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 introduced into a saturated phosphoric acid solution, the HCl gas is absorbed, and the discharged phosphoric acid gas is collected.

[0084] S3. Preparation of a Prussian blue-like material coated with ferrous phosphate: The Prussian blue-like material prepared in step S1 is placed in a stirred tank and the phosphoric acid gas produced in step S2 is continuously introduced. The Prussian blue-like material and the phosphoric acid gas react at 130°C for 4 hours. During the reaction, the exhaust gas is adsorbed using activated carbon and silica gel, forming an intermediate having ferrous phosphate and sodium phosphate coated on the surface of the Prussian blue-like material. The surface of the intermediate is rinsed with water to remove the sodium phosphate and dried at 100°C for 2 hours to obtain a Prussian blue-like material densely coated with ferrous phosphate. The particle size of the Prussian blue-like material densely coated with ferrous phosphate is 100-120 nm, and the mass ratio of the core layer to the in-situ coating layer is 1:0.075.

[0085] In Example 3:

[0086] S2. Prepare phosphoric acid gas by gas phase reaction.

[0087] In a gas phase reactor, phosphorus trichloride, oxygen and water vapor are added according to a molar ratio of 7.5:2:1:6 among a Prussian blue material, phosphorus trichloride, oxygen and water vapor. 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 introduced into a saturated phosphoric acid solution, the HCl gas is absorbed, and the discharged phosphoric acid gas is collected.

[0088] S3. Preparation of a Prussian blue-like material coated with ferrous phosphate: The Prussian blue-like material prepared in step S1 is placed in a stirred tank and the phosphoric acid gas produced in step S2 is continuously introduced. The Prussian blue-like material and the phosphoric acid gas are reacted at 160°C for 2 hours. During the reaction, the exhaust gas is adsorbed using activated carbon and silica gel to form an intermediate having ferrous phosphate and sodium phosphate coated on the surface of the Prussian blue-like material. The surface of the intermediate is rinsed with water to remove the sodium phosphate and dried at 90°C for 3 hours to obtain a Prussian blue-like material densely coated with ferrous phosphate. The particle size of the Prussian blue-like material densely coated with ferrous phosphate is 100-120 nm, and the mass ratio of the core layer to the in-situ coating layer is 1:0.1.

[0089] Example 4

[0090] Example 4 is substantially the same as Example 1, with the only difference being that the contacting manner of the Prussian blue material and the phosphoric acid gas in step S3 is different.

[0091] In this embodiment, a Prussian blue material is placed in a stirred tank and phosphoric acid gas is introduced into the stirred tank at one time. The particle size of the Prussian blue 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.

[0092] Example 5

[0093] This embodiment is basically the same as embodiment 1, and the only difference is that the parameters of step S1 are different.

[0094] S1. Prepare Prussian blue materials.

[0095] 0.02 mol of sodium ferrocyanide (concentration of 2.00 mol / L) and 0.8 mol of sodium chloride (concentration of 0.15 mol / L) were dissolved in 100 mL of deionized water and stirred thoroughly to obtain a first solution; 0.02 mol of manganese chloride and 0.1 mol of sodium citrate were dissolved in 100 mL of deionized water and stirred thoroughly to obtain a second solution; the second solution was added to the first solution at a rate of 3 mL / min, the precipitation reaction was carried out at 60° C. for 10 h, and then aged for 12 h, and then filtered, and the filter residue was dried at 160° C. for 12 h to obtain a Prussian blue material.

[0096] Example 6

[0097] 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 steps of "rinsing the surface of the intermediate with water to remove the sodium phosphate; and drying at 80°C for 3 hours" in embodiment 1 are omitted. The intermediate whose surface is coated with ferrous phosphate and sodium phosphate is directly used as the modified Prussian blue material.

[0098] Comparative Example 1

[0099] The difference between this comparative example and Example 1 is that this comparative example uses a coating liquid to coat the Prussian blue material.

[0100] S1. Prepare Prussian blue materials.

[0101] 0.02 mol of sodium ferrocyanide (concentration of 1.00 mol / L) and 0.6 mol of sodium chloride (concentration of 0.1 mol / L) were dissolved in 100 mL of deionized water and stirred thoroughly to obtain a first solution; 0.02 mol of ferrous chloride and 0.05 mol of sodium citrate were dissolved in 100 mL of deionized water and stirred thoroughly to obtain a second solution; the second solution was added to the first solution at a rate of 1 mL / min, the solution was precipitated at 50°C for 6 h, aged for 16 h, and then filtered. The filter residue was dried at 100°C for 24 h.

[0102] S2. Prepare coating solution.

[0103] 0.02 mol of citric acid was dissolved in deionized water, and 0.04 mol of ferrous phosphate powder was added and stirred to obtain a coating suspension.

[0104] S2. Prepare coating materials.

[0105] The Prussian blue core and coating solution were stirred at 50°C at a rate of 500 r / min for 3 h and then centrifuged. The filter residue was dried at 200°C under a nitrogen atmosphere for 24 h.

[0106] Comparative Example 2

[0107] The difference between this comparative example and Example 1 is that this comparative example is an uncoated Prussian blue material.

[0108] 0.02 mol of sodium ferrocyanide (concentration of 1.00 mol / L) and 0.6 mol of sodium chloride (concentration of 0.1 mol / L) were dissolved in 100 mL of deionized water and stirred thoroughly to obtain a first solution; 0.02 mol of ferrous chloride and 0.05 mol of sodium citrate were dissolved in 100 mL of deionized water and stirred thoroughly to obtain a second solution; the second solution was added to the first solution at a rate of 1 mL / min, the solution was precipitated at 50°C for 6 h, aged for 16 h, and then filtered. The filter residue was dried at 100°C for 24 h.

[0109] Comparative Example 3

[0110] The difference between this comparative example and Example 1 is that in this comparative example, the molar ratio of the Prussian blue 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 material is 1:0.2.

[0111] Comparative Example 4

[0112] The difference between this comparative example and Example 1 is that in this comparative example, the reaction temperature of the Prussian blue material and the phosphoric acid gas is 200° C., the reaction time is 2 h, and the reaction is carried out under normal pressure.

[0113] Experimental example

[0114] The performance of the Prussian blue cathode materials prepared in the examples and comparative examples was tested, and the results are shown in Table 1.

[0115] Test method: Assembly of button sodium ion battery: The Prussian blue positive electrode material, acetylene black and polyvinylidene fluoride (PVDF) prepared in the embodiment are mixed in a mass ratio of 7:2:1, and the mixed slurry is coated on aluminum foil, dried and cut into discs as the positive electrode; the metallic sodium sheet is used as the negative electrode, and Whatman glass fiber (GF / D) is used as the separator; the organic electrolyte is prepared by NaClO4, EC (ethylene carbonate), DEC (diethyl carbonate) and FEC (fluoroethylene carbonate), the NaClO4 concentration is 1.0 mol / L, the volume ratio of EC to DEC is 1:1, and the mass fraction of FEC in the electrolyte is 5%; the sodium ion battery is assembled in an argon glove box;

[0116] Cycling performance was tested at 25°C (1C / 1C 2.0-4.3V). For water absorption testing, samples in both the examples and comparative examples were dried at 120°C for 12 hours. The dried samples were then divided into two groups (Group 1 and Group 2). The first group was immediately tested for water content using a Karl Fischer titrator, while the second group was exposed to air for 1 hour and then tested for water content using a Karl Fischer titrator. The cutoff temperature for the water content test was 170°C.

[0117] The results are shown in the following table:

[0118] As can be seen from the above table, the cycle retention rates of the modified Prussian blue materials provided in Examples 1-6 of the present disclosure are significantly higher than those in Comparative Examples 1-4, and the water content is also significantly lower than that in Comparative Examples 1-4, which fully proves that the modified Prussian blue materials provided in Examples 1-6 of the present disclosure generate a denser in-situ coating layer, which can make the material have a certain waterproof effect, can effectively isolate the direct contact of the Prussian blue material with the electrolyte, reduce the side reaction with the electrolyte and the dissolution of metal ions, thereby improving its cycle stability. Further, it can be seen from the data of Comparative Example 1 that it uses a coating liquid to protect the Prussian blue material, and the coating density is significantly worse than that of Example 1. Comparative Example 2 is not coated, and its cycle retention rate and water content are the worst. In Comparative Example 3, the amount of phosphoric acid gas introduced is increased, and a dense in-situ coating layer can also be generated, so its water content is not much different from that of Example 1, but due to excessive phosphoric acid gas, the consumption of the Prussian blue material is greater than that of Example 1, thereby reducing the cycle retention rate. In Comparative Example 4, the reaction temperature of the Prussian blue material and phosphoric acid gas increased, resulting in a slight decrease in the retention rate and a slight increase in the water absorption rate, which fully proves that the increase in temperature will lead to a decrease in the density of the coating layer.

[0119] It should be understood that the steps for preparing Prussian blue-like materials in the present 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 the present disclosure.

[0120] In summary, the modified Prussian blue material provided by the present disclosure is formed by in-situ modification of the surface of the Prussian blue material to form sodium iron phosphate and optional sodium phosphate M as an in-situ coating layer. Compared with the external coating layer, the modified Prussian blue material provided by the present disclosure is more tightly coated, and can make the material have a certain waterproof effect, and can effectively isolate the direct contact between the Prussian blue material and the electrolyte, reduce the side reaction with the electrolyte and the dissolution of metal ions, thereby improving its cycle stability. The coating method adopted in the present disclosure is simple and easy to operate, and the prepared modified Prussian blue material has good cycle stability, which can be widely used in the preparation of positive electrode materials in sodium ion batteries. The sodium ion battery prepared using the modified Prussian blue material provided by the present disclosure as the positive electrode material has good cycle stability.

[0121] The above describes in detail the optional embodiments of the present disclosure, but the present disclosure is not limited thereto. Within the technical concept of the present disclosure, various simple variations of the technical solution of the present disclosure can be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as the contents disclosed by the present disclosure and fall within the scope of protection of the present disclosure. Industrial Applicability

[0122] The modified Prussian blue material provided by the present invention is formed by in-situ modification of the surface of the Prussian blue material to form sodium iron phosphate (and optional sodium phosphate M, where M is not Fe) as an in-situ coating layer. Compared with the external coating layer, the modified Prussian blue material provided by the present invention is more tightly coated and can make the material have a certain waterproof effect, effectively isolate the direct contact between the Prussian blue material and the electrolyte, reduce the side reaction with the electrolyte and the dissolution of metal ions, thereby improving its cycle stability. The coating method adopted in the present invention is simple and easy to operate, and the prepared modified Prussian blue material has good cycle stability, which can be widely used in the preparation of positive electrode materials in sodium ion batteries. The sodium ion battery prepared using the modified Prussian blue material provided by the present invention as the positive electrode material has good cycle stability.

Claims

1. A modified Prussian blue material, characterized in that: It includes a core layer and an in-situ coating layer in-situ generated on the surface of the core layer, wherein the core layer is a Prussian blue material, and its structural formula is 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 the M is Fe, the in-situ coating layer is sodium iron phosphate, when the 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.

2. The modified Prussian blue material according to claim 1, characterized in that: The mass ratio of the core layer to the in-situ coating layer is 1:0.05-0.

1.

3. The modified Prussian blue material according to any one of claims 1 to 2, characterized in that: The particle size of the modified Prussian blue material is 0.2-2 μnm.

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

5. A method for preparing a modified Prussian blue material according to any one of claims 1 to 4, characterized in that: include: The Prussian blue material is contacted with phosphoric acid gas and reacted to form a Prussian blue material with an in-situ coating layer densely coated on the surface.

6. The method for preparing the modified Prussian blue material according to claim 5, characterized in that: The reaction temperature of the Prussian blue material and the phosphoric acid gas is 130-160° C., and the reaction time is 2-4 hours.

7. The method for preparing the modified Prussian blue material according to any one of claims 5 to 6, characterized in that: The method for preparing the modified Prussian blue material further comprises preparing phosphoric acid gas: phosphorus trichloride, oxygen and water vapor are mixed and reacted in a gas phase reactor, and the phosphorus trichloride is oxidized and hydrolyzed to generate phosphoric acid gas.

8. The method for preparing the modified Prussian blue material according to claim 7, 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.

9. The method for preparing the modified Prussian blue material according to any one of claims 7 to 8, characterized in that: The preparing phosphoric acid gas also includes removing HCl gas generated in the process of preparing the phosphoric acid gas by absorbing it with a saturated phosphoric acid solution.

10. The method for preparing the modified Prussian blue material according to any one of claims 5 to 9, characterized in that: The step of contacting and reacting the Prussian blue material with phosphoric acid gas includes: continuously passing the phosphoric acid gas prepared on site into the Prussian blue material for reaction.

11. The method for preparing the modified Prussian blue material according to claim 10, characterized in that: The process of the Prussian blue material contacting and reacting with the phosphoric acid gas also includes: using activated carbon and silica gel to adsorb the generated HCN gas.

12. The method for preparing the modified Prussian blue material according to any one of claims 5 to 11, characterized in that: When the Prussian blue material contacts 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 method also includes removing the sodium phosphate of the intermediate.

13. The method for preparing the modified Prussian blue material according to claim 12, characterized in that: The step of removing the sodium phosphate of the intermediate comprises: washing and drying the intermediate.

14. The method for preparing the modified Prussian blue material according to claim 13, characterized in that: The intermediate after washing is dried at a drying temperature of 80-100° C. and a drying time of 2-3 hours.

15. The method for preparing the modified Prussian blue material according to any one of claims 5 to 14, characterized in that: The preparation method of the modified Prussian blue material also includes preparing the Prussian blue material: 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, aging after precipitation reaction, filtering, and drying the filter residue.

16. The method for preparing the modified Prussian blue material according to claim 15, characterized in that: The step of preparing the Prussian blue material comprises at least one of features (1) to (10): Feature (1): The raw material concentration of the sodium ferrocyanide is 0.02mol / L-2mol / L, the raw material concentration of the inorganic sodium salt is 0.05mol / L-0.15mol / L, and the molar ratio of the sodium ferrocyanide to the 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-5 mL / min; Feature (7): The precipitation reaction comprises precipitation at 40-60°C for 4h-12h; Feature (8): The aging time is 12h-24h; Feature (9): The filtering method includes centrifugation, suction filtration or pressure filtration; Feature (10): The drying temperature for drying the filter residue is 80-200° C., and the drying time is 12-48 hours.

17. Use of the modified Prussian blue material according to any one of claims 1 to 4 in preparing positive electrode materials for sodium ion batteries.

18. A sodium ion battery, characterized in that: The invention comprises a positive electrode material and a negative electrode material, wherein the positive electrode material comprises the modified Prussian blue material according to any one of claims 1 to 4.

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