Core-shell structure ferric sodium pyrophosphate positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of sodium ion battery positive electrode materials, and discloses a core-shell structure ferric sodium pyrophosphate positive electrode material and a preparation method and application thereof. The core-shell structure ferric sodium pyrophosphate positive electrode material comprises a core material and a bicontinuous phase coating layer generated on the surface of the core material in situ, the material of the inner core is Na4Fe3 (PO4) 2 (P2O7); the bicontinuous phase coating layer is (FexP) y / C1-y, x is greater than or equal to 1 and less than or equal to 2, and y is greater than or equal to 0 and less than or equal to 1. The continuous (FexP) y / C1-y conductive network is formed on the surfaces of the positive electrode particles, so that the conductivity of the ferric sodium pyrophosphate positive electrode material is greatly improved, the electrode polarization is reduced, and the rate capability of the material is improved; and the (FexP) y / C1-y is of a rigid structure, so that the volume strain in the charge-discharge process is relieved, and the cycle life of the battery is prolonged. On the basis of not increasing the cost of the raw materials, the electrochemical performance of the mixed phosphate positive electrode material Na4Fe3 (PO4) 2 (P2O7) is greatly improved.

Description

Technical Field

[0001] This application relates to the field of sodium-ion battery cathode materials technology, specifically to a core-shell structured sodium iron pyrophosphate cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries for energy storage are playing an increasingly important role in the new energy field due to their advantages such as low cost, wide temperature range application, and environmental friendliness. They are currently a research hotspot in new energy technology and have good industrialization prospects.

[0003] Among the many types of cathode materials for sodium-ion batteries used in energy storage, polyanion cathode materials have the advantages of high stability and high operating voltage; among them, mixed phosphate cathode materials, such as sodium iron pyrophosphate, are particularly noteworthy due to their open three-dimensional Na+... + The channel has a theoretical capacity of 129 mAh·g at relatively high charge / discharge voltages (~3.0V). -1 Furthermore, during charge and discharge, the crystal volume change of its positive electrode material is less than 4%, exhibiting excellent cycle performance and demonstrating great application potential in the energy storage field. However, in practical applications, the intrinsic conductivity of Na4Fe3(PO4)2(P2O7) decreases due to the phosphate-induced effect, limiting its application in large-scale energy storage. Therefore, improving the intrinsic conductivity of the mixed phosphate positive electrode material Na4Fe3(PO4)2(P2O7) is of great significance for its large-scale production and application.

[0004] To improve the intrinsic conductivity of mixed phosphate cathode materials, numerous researchers have conducted extensive studies. For example, patent application CN116750741A discloses a method for preparing titanium-doped carbon-coated sodium iron pyrophosphate material and its application. By precisely controlling the amount of titanium source added, a portion of titanium dioxide is coated on the surface of the material, while the remaining titanium dioxide is doped into the sodium iron pyrophosphate lattice as titanium ions. Combined with carbon coating, this method improves the electronic conductivity and ion diffusion rate of the sodium iron pyrophosphate composite material, thereby increasing its discharge capacity at high rates. Patent CN116565165B discloses a sodium iron pyrophosphate cathode material that uses a bicontinuous phase coating layer formed by uniformly mixing the metal oxide TiNb2O7 and a carbon layer to completely encapsulate the sodium iron pyrophosphate cathode material, improving its conductivity, ion diffusion capacity, and low-temperature cycle life. However, these methods require the introduction of transition metal elements, which not only increases costs but also complicates the preparation process. Summary of the Invention

[0005] This application provides a core-shell structured sodium iron pyrophosphate cathode material, its preparation method, and its application. Without introducing transition metal elements, the electrochemical performance of the mixed phosphate cathode material Na4Fe3(PO4)2(P2O7) is significantly improved.

[0006] To achieve the above objectives, the present application adopts the following technical solution.

[0007] In a first aspect, this application provides a core-shell structured sodium iron pyrophosphate cathode material, comprising a core material and a bicontinuous phase coating layer formed in situ on the surface of the core material.

[0008] The core material is Na4Fe3(PO4)2(P2O7);

[0009] The dual continuous phase coating layer is (Fe) x P) y / C 1-y , where 1≤x≤2, 0≤y≤1.

[0010] Preferably, the content of the dual continuous phase coating layer is 1 to 20 wt% of the mass of the sodium iron pyrophosphate cathode material.

[0011] A second aspect of this application provides a method for preparing the above-mentioned core-shell structured sodium iron pyrophosphate cathode material, comprising:

[0012] S1, sodium source, first iron source, phosphorus source and soluble carbon source are dispersed in water to obtain precursor solution; the precursor solution is dried to obtain precursor, which is calcined under a reducing atmosphere to obtain Na4Fe3(PO4)2(P2O7)@C cathode material;

[0013] S2, Na4Fe3(PO4)2(P2O7)@C cathode material and surfactant are uniformly dispersed in ethanol, and then a second iron source and phosphorus source are added to carry out a co-precipitation reaction. The solid phase is collected, dried and the precursor is obtained.

[0014] S3. In a reducing atmosphere, the precursor is selectively coated in situ by microwave calcination to obtain a core-shell structured sodium iron pyrophosphate cathode material.

[0015] Preferably, the sodium source includes at least one of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium acetate, or sodium pyrophosphate;

[0016] The first iron source includes at least one of ferric nitrate, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, iron(II) oxide, ferric oxide, ferrous oxide, ferric oxalate, ferric phosphate, or ferric pyrophosphate.

[0017] The phosphorus source includes at least one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, triammonium phosphate, sodium phosphate, phosphoric acid, pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.

[0018] The soluble carbon source includes at least one of oxalic acid, citric acid, soluble starch, sucrose, or glucose.

[0019] The surfactant includes at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, or polymaleic acid.

[0020] The second iron source includes at least one of ferric chloride, ferrous chloride, ferrous sulfate, ferric nitrate, or ferric sulfate.

[0021] Preferably, the molar ratio of sodium source, iron source, and phosphorus source in S1 is 4:3:4; the amount of soluble carbon source used accounts for 5-30% of the total mass of sodium source, iron source, and phosphorus source.

[0022] The molar ratio of iron source to phosphorus source in S2 is 1~2:1; the amount of surfactant is 0.1~10wt% of the mass of Na4Fe3(PO4)2(P2O7)@C cathode material.

[0023] Preferably, the drying described in S1 is spray drying;

[0024] The calcination process specifically involves: first heating to 300 ℃ at a heating rate of 2 ℃ / min and holding at that temperature for 3 h; then heating to 400-650 ℃ at a heating rate of 1-5 ℃ / min and holding at that temperature for 10-25 h.

[0025] In S1 and S3, the reducing atmosphere is a mixture of argon and hydrogen, wherein the volume fraction of hydrogen is 1-10%.

[0026] Preferably, in the microwave calcination described in S3, the microwave reactor power is 400-600W, and the holding time is 10-180min.

[0027] A third aspect of this application provides the application of the above-described core-shell structured sodium iron pyrophosphate cathode material or the core-shell structured sodium iron pyrophosphate cathode material prepared by the above-described preparation method in sodium-ion batteries.

[0028] A fourth aspect of this application provides a positive electrode sheet for a sodium-ion battery, comprising a current collector and an active material coated on the surface of the current collector;

[0029] The active material includes the above-mentioned core-shell structured sodium iron pyrophosphate cathode material or the core-shell structured sodium iron pyrophosphate cathode material prepared by the above preparation method.

[0030] A fifth aspect of this application provides a sodium-ion battery, wherein the positive electrode is the positive electrode sheet of the aforementioned sodium-ion battery.

[0031] Compared with the prior art, the beneficial effects of this application are as follows:

[0032] The core-shell structured sodium iron pyrophosphate cathode material of this application utilizes the principle that different raw materials have different dielectric loss factors. During microwave calcination, the surface endothermic degree is greater than that of the internal main phase region, thereby generating highly conductive (Fe) ions in situ on the cathode material surface. x P) y / C 1-y (1≤x≤2, 0≤y≤1) compounds, and the internal main phase region will not decompose pyrophosphate due to excessive temperature. This application achieves this by forming continuous (Fe) on the surface of the positive electrode particles. x P) y / C 1-y The conductive network significantly improves the conductivity of the sodium iron pyrophosphate cathode material, reduces electrode polarization, and enhances the material's rate performance; furthermore, (Fe... x P) y / C 1-y Its rigid structure alleviates volumetric strain during charging and discharging, thus extending the battery's cycle life. This application achieves a significant improvement in the electrochemical performance of the mixed phosphate cathode material Na4Fe3(PO4)2(P2O7) without increasing raw material costs.

[0033] This application improves the thermal efficiency of the production process by combining microwave heating with spray drying, while ensuring that the material is heated evenly as a whole, avoiding local overheating or burning, and improving the consistency of the quality and performance of the cathode material particles. Attached Figure Description

[0034] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 The XRD patterns are of the sodium iron pyrophosphate cathode materials prepared in Examples 1-4 and Comparative Example 1.

[0036] Figure 2 The rate performance test results of the sodium iron pyrophosphate cathode materials prepared in Examples 1-5 and Comparative Examples 1-4 are shown in the voltage range of 1.7-4.3V.

[0037] Figure 3This is a SEM image of the core-shell structure sodium iron pyrophosphate cathode material of Example 5. Detailed Implementation

[0038] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0039] In the following description of this embodiment, the terms "including", "comprising", "having", and "containing" are all open-ended terms, meaning that they include but are not limited to.

[0040] In the following description of this embodiment, the term "and / or" is used to describe the association relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, B existing alone, and A and B existing simultaneously. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0041] In the following description of this embodiment, the term "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0042] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms "a" and "the" as used in the embodiments of this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0043] Those skilled in the art should understand that, in the following description of the embodiments of this application, the sequence of numbers does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0044] Those skilled in the art will understand that the numerical ranges in the embodiments of this application should be understood as each intermediate value between the upper and lower limits of the specifically disclosed range. Each smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this application. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0045] Unless otherwise stated, the technical / scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. While this application describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this application. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0046] In a first aspect, this application provides a core-shell structured sodium iron pyrophosphate cathode material, which includes a core material and a bicontinuous phase coating layer generated in situ on the surface of the core material; wherein the content of the bicontinuous phase coating layer is 1 to 20 wt% of the mass of the sodium iron pyrophosphate cathode material.

[0047] The core material is Na4Fe3(PO4)2(P2O7);

[0048] The dual continuous phase coating layer is (Fe) x P) y / C 1-y , where 1≤x≤2, 0≤y≤1.

[0049] In this application, the coating layer (Fe) is selectively generated in situ on the surface of the core material Na4Fe3(PO4)2(P2O7). x P) y / C 1-y With high electrical conductivity, the continuous conductive network it forms significantly improves the electrical conductivity of the sodium iron pyrophosphate cathode material, reduces electrode polarization, and enhances the material's rate performance; furthermore, the coating layer (Fe... x P) y / C 1-y Its rigid structure alleviates volumetric strain during charging and discharging, extending the battery's cycle life. The cathode material of this application exhibits high conductivity and high specific capacity, resulting in a significant improvement in the electrochemical performance of the mixed phosphate cathode material Na4Fe3(PO4)2(P2O7) without increasing raw material costs.

[0050] Secondly, this application provides a method for preparing the above-mentioned core-shell structured sodium iron pyrophosphate cathode material, including:

[0051] S1, sodium source, first iron source, phosphorus source and soluble carbon source are dispersed in water to obtain precursor solution; the precursor solution is spray dried to obtain precursor, which is calcined under a reducing atmosphere to obtain Na4Fe3(PO4)2(P2O7)@C cathode material;

[0052] In this application, the sodium source is selected from at least one of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium acetate, or sodium pyrophosphate.

[0053] The first iron source is selected from at least one of ferric nitrate, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, iron(II) oxide, ferric oxide, ferrous oxide, ferric oxalate, ferric phosphate, or ferric pyrophosphate.

[0054] The phosphorus source is selected from at least one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, triammonium phosphate, sodium phosphate, phosphoric acid, pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.

[0055] The soluble carbon source is selected from at least one of oxalic acid, citric acid, soluble starch, sucrose, or glucose.

[0056] The molar ratio of sodium, iron, and phosphorus sources is 4:3:4; the amount of soluble carbon source used accounts for 5-30% of the total mass of sodium, iron, and phosphorus sources.

[0057] The reducing atmosphere is a mixture of argon and hydrogen, wherein the volume fraction of hydrogen is 1-10%; the calcination is specifically performed as follows: first, the temperature is raised to 300 ℃ at a heating rate of 2 ℃ / min and held for 3 h; then, the temperature is raised to 400-650 ℃ at a heating rate of 1-5 ℃ / min and held for 10-25 h.

[0058] S2, Na4Fe3(PO4)2(P2O7)@C cathode material and surfactant are uniformly dispersed in ethanol, and then a second iron source and phosphorus source are added to carry out a co-precipitation reaction. The solid phase is collected, dried and the precursor is obtained.

[0059] In this application, the second iron source is a soluble iron salt, preferably at least one of ferric chloride, ferrous chloride, ferrous sulfate, ferric nitrate, or ferric sulfate. In the coprecipitation reaction, a suitable molar ratio of iron to phosphorus is only achieved within a specific range. If the molar ratio of iron to phosphorus is too high, unstable iron-phosphorus compounds are formed, leading to a decrease in electrochemical performance. The preferred molar ratio of iron source to phosphorus source is 1 to 2:1.

[0060] The surfactant includes at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, or polymaleic acid, wherein the amount of surfactant used is 0.1 to 10 wt% of the mass of the Na4Fe3(PO4)2(P2O7)@C cathode material.

[0061] S3. In a reducing atmosphere, the precursor is selectively coated in situ by microwave calcination to obtain a core-shell structured sodium iron pyrophosphate cathode material.

[0062] In this application, the reducing atmosphere is a mixture of argon and hydrogen, wherein the volume fraction of hydrogen is 1-10%.

[0063] In this application, microwave calcination has a significant impact on the electrochemical performance of the core-shell structured sodium iron pyrophosphate cathode material. Both excessively high and low microwave calcination power will reduce the electrochemical performance of the cathode material. In this application, a microwave reactor power of 400-600W and a holding time of 10-180 min are preferred.

[0064] This application improves the thermal efficiency of the production process by combining microwave heating with spray drying, while ensuring that the material is heated evenly as a whole, avoiding local overheating or burning, and improving the consistency of the quality and performance of the cathode material particles.

[0065] This application utilizes the principle that different raw materials have different dielectric loss factors to achieve a greater degree of surface heat absorption than the internal main phase region during microwave calcination, thereby generating highly conductive (Fe) phase in situ on the surface of the cathode material. x P) y / C 1-y (1≤x≤2, 0≤y≤1) compounds, and the internal main phase region will not decompose pyrophosphate due to excessive temperature. The continuous (Fe) surface of the positive electrode particles... x P) y / C 1-y The conductive network not only greatly improves the conductivity of sodium iron pyrophosphate cathode material, reduces electrode polarization, and improves the rate performance of the material, but also alleviates volume strain during charging and discharging due to its rigid structure, thus extending the cycle life of the battery.

[0066] The core-shell structured sodium iron pyrophosphate cathode material of this application can be used as a cathode active material in sodium-ion batteries.

[0067] This application also provides a sodium-ion battery positive electrode sheet, which includes a current collector and an active material coated on the surface of the current collector. The active material includes the core-shell structure sodium iron pyrophosphate positive electrode material of this application. The core-shell structure sodium iron pyrophosphate positive electrode material of this application is used as the positive electrode active material, and a slurry is prepared by uniformly mixing it with a conductive agent and a binder. The slurry is then uniformly coated onto the current collector and dried to obtain the sodium-ion battery positive electrode sheet. Notably, the sodium-ion battery positive electrode sheet of this application has no special requirements for the conductive agent and binder; commonly used conductive agents and binders for sodium-ion batteries can be used.

[0068] This application also provides a sodium-ion battery, wherein the positive electrode is the sodium-ion battery positive electrode sheet described in this application. By using the aforementioned sodium-ion battery positive electrode sheet as the positive electrode and a sodium sheet as the negative electrode, and assembling it with an electrolyte and a separator, a sodium-ion battery is obtained. The sodium-ion battery of this application has no special requirements for the electrolyte or separator; commonly used electrolytes and separators for sodium-ion batteries can be used.

[0069] The present application will be specifically described below through examples.

[0070] Example 1

[0071] This embodiment provides a method for preparing a core-shell structured sodium iron pyrophosphate cathode material with the chemical formula Na4Fe3(PO4)2(P2O7)@(FeP). 0.5 / C 0.5 Specifically, it includes:

[0072] S1, 2.3996g sodium dihydrogen phosphate, 6.06g ferric nitrate nonahydrate, and 1.02g citric acid were dissolved in deionized water and stirred evenly to obtain a precursor solution; the precursor solution was dried at 165℃ using a spray dryer to obtain precursor powder; the precursor powder was heated to 300℃ at a heating rate of 2℃ / min in an argon-hydrogen mixed gas atmosphere (hydrogen volume content of 5%), held at that temperature for 3h, and then heated to 550℃ at a heating rate of 2℃ / min, held at that temperature for 10h, and then sintered in the furnace and cooled to room temperature to obtain sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@C.

[0073] S2, the above Na4Fe3(PO4)2(P2O7)@C was dispersed in an ethanol solution, 0.1041 g of polyethylene glycol (3% by mass) was added, and the mixture was stirred at 500 rpm with a magnetic stirrer. Then, 0.6377 g of ferric nitrate and 0.1815 g of ammonium dihydrogen phosphate were added until a white precipitate appeared in the dispersion. After stirring for 1 h, the mixture was centrifuged at 6000 rpm and washed. The resulting solid was transferred to a vacuum drying oven and dried overnight at 120 °C to obtain the precursor.

[0074] S3. In an argon-hydrogen mixed gas atmosphere (hydrogen volume content of 5%), the precursor was transferred to a microwave reactor and heated at 500W microwave power for 30 min, then cooled to room temperature to obtain the core-shell structured sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@(FeP). 0.5 / C 0.5 .

[0075] Example 2

[0076] This embodiment provides a method for preparing a core-shell structured sodium iron pyrophosphate cathode material with the chemical formula Na4Fe3(PO4)2(P2O7)@(Fe2P). 0.5 / C 0.5 Specifically, it includes:

[0077] S1, 2.3996g sodium dihydrogen phosphate, 6.06g ferric nitrate nonahydrate, and 1.02g citric acid were dissolved in deionized water and stirred evenly to obtain a precursor solution; the precursor solution was dried at 165℃ using a spray dryer to obtain precursor powder; the precursor powder was heated to 300℃ at a heating rate of 2℃ / min in an argon-hydrogen mixed gas atmosphere (hydrogen volume content of 5%), held at that temperature for 3h, and then heated to 550℃ at a heating rate of 2℃ / min, held at that temperature for 10h, and then sintered in the furnace and cooled to room temperature to obtain sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@C.

[0078] S2, the above Na4Fe3(PO4)2(P2O7)@C was dispersed in an ethanol solution, 0.1041 g of polyethylene glycol (3% by mass) was added, and the mixture was stirred at 500 rpm with a magnetic stirrer. Then, 0.3881 g of ferric nitrate, 0.1105 g of ammonium dihydrogen phosphate, and 0.2699 g of ferric sulfate were added until a light yellow precipitate appeared in the dispersion. After stirring for 1 h, the mixture was centrifuged at 6000 rpm and washed. The resulting solid was transferred to a vacuum drying oven and dried overnight at 120 °C to obtain the precursor.

[0079] S3. In an atmosphere of argon-hydrogen mixed gas (hydrogen volume content of 5%), the precursor was transferred to a microwave reactor and heated at 500W microwave power for 30 min, then cooled to room temperature to obtain the core-shell structured sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@(Fe2P). 0.5 / C 0.5 .

[0080] Example 3

[0081] This embodiment provides a method for preparing a core-shell structured sodium iron pyrophosphate cathode material with the chemical formula Na4Fe3(PO4)2(P2O7)@(Fe2P).0.75 / C 0.25 Specifically, it includes:

[0082] S1, 2.3996g sodium dihydrogen phosphate, 6.06g ferric nitrate nonahydrate, and 0.38g citric acid were dissolved in deionized water and stirred evenly to obtain a precursor solution; the precursor solution was dried at 165℃ using a spray dryer to obtain precursor powder; the precursor powder was heated to 300℃ at a heating rate of 2℃ / min in an argon-hydrogen mixed gas atmosphere (hydrogen volume content of 5%), held at that temperature for 3h, and then heated to 550℃ at a heating rate of 2℃ / min, held at that temperature for 10h, and then sintered in the furnace and cooled to room temperature to obtain sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@C.

[0083] S2, the above Na4Fe3(PO4)2(P2O7)@C was dispersed in an ethanol solution, 0.0975 g of polyethylene glycol (3% by mass) was added, and the mixture was stirred at 500 rpm with a magnetic stirrer. Then, 0.5821 g of ferric nitrate, 0.1657 g of ammonium dihydrogen phosphate, and 0.4048 g of ferric sulfate were added until a light yellow precipitate appeared in the dispersion. After stirring for 1 h, the mixture was centrifuged at 6000 rpm and washed. The resulting solid was transferred to a vacuum drying oven and dried overnight at 120 °C to obtain the precursor.

[0084] S3. In an argon-hydrogen mixed gas atmosphere (hydrogen volume content of 5%), the precursor was transferred to a microwave reactor and heated at 500W microwave power for 30 min, then cooled to room temperature to obtain the core-shell structured sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@(Fe2P). 0.75 / C 0.25 .

[0085] Example 4

[0086] This embodiment provides a method for preparing a core-shell structured sodium iron pyrophosphate cathode material with the chemical formula Na4Fe3(PO4)2(P2O7)@(Fe2P). 0.25 / C 0.75 Specifically, it includes:

[0087] S1, 2.3996g sodium dihydrogen phosphate, 6.06g ferric nitrate nonahydrate, and 1.65g citric acid were dissolved in deionized water and stirred evenly to obtain a precursor solution; the precursor solution was dried at 165℃ using a spray dryer to obtain precursor powder; the precursor powder was heated to 300℃ at a heating rate of 2℃ / min in an argon-hydrogen mixed gas atmosphere (hydrogen volume content of 5%), held at that temperature for 3h, and then heated to 550℃ at a heating rate of 2℃ / min, held at that temperature for 10h, and then sintered in the furnace and cooled to room temperature to obtain sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@C.

[0088] S2, the above Na4Fe3(PO4)2(P2O7)@C was dispersed in an ethanol solution, 0.1105 g of polyethylene glycol (3% by mass) was added, and the mixture was stirred at 500 rpm with a magnetic stirrer. Then, 0.1940 g of ferric nitrate, 0.0553 g of ammonium dihydrogen phosphate, and 0.1349 g of ferric sulfate were added until a light yellow precipitate appeared in the dispersion. After stirring for 1 h, the mixture was centrifuged at 6000 rpm and washed. The resulting solid was transferred to a vacuum drying oven and dried overnight at 120 °C to obtain the precursor.

[0089] S3. In an atmosphere of argon-hydrogen mixed gas (hydrogen volume content of 5%), the precursor was transferred to a microwave reactor and heated at 500W microwave power for 30 min, then cooled to room temperature to obtain the core-shell structured sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@(Fe2P). 0.25 / C 0.75 .

[0090] Example 5

[0091] This embodiment provides a method for preparing a core-shell structured sodium iron pyrophosphate cathode material with the chemical formula Na4Fe3(PO4)2(P2O7)@(FeP). 0.25 / C 0.75 Specifically, it includes:

[0092] S1, 2.3996g sodium dihydrogen phosphate, 6.06g ferric nitrate nonahydrate, and 1.65g citric acid were dissolved in deionized water and stirred evenly to obtain a precursor solution; the precursor solution was dried at 165℃ using a spray dryer to obtain precursor powder; the precursor powder was heated to 300℃ at a heating rate of 2℃ / min in an argon-hydrogen mixed gas atmosphere (hydrogen volume content of 5%), held at that temperature for 3h, and then heated to 550℃ at a heating rate of 2℃ / min, held at that temperature for 10h, and then sintered in the furnace and cooled to room temperature to obtain sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@C.

[0093] S2, the above Na4Fe3(PO4)2(P2O7)@C was dispersed in an ethanol solution, 0.1105 g of polyethylene glycol (3% by mass) was added, and the mixture was stirred at 500 rpm with a magnetic stirrer. Then, 0.3189 g of ferric nitrate and 0.0908 g of ammonium dihydrogen phosphate were added until a white precipitate appeared in the dispersion. After stirring for 1 h, the mixture was centrifuged at 6000 rpm and washed. The resulting solid was transferred to a vacuum drying oven and dried overnight at 120 °C to obtain the precursor.

[0094] S3. In an argon-hydrogen mixed gas atmosphere (hydrogen volume content of 5%), the precursor was transferred to a microwave reactor and heated at 500W microwave power for 30 min, then cooled to room temperature to obtain the core-shell structured sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@(FeP). 0.25 / C 0.75 .

[0095] Comparative Example 1

[0096] Comparative Example 1 provides a method for preparing a sodium iron pyrophosphate cathode material with the chemical formula Na4Fe3(PO4)2(P2O7)@C, specifically including:

[0097] 2.3996 g of sodium dihydrogen phosphate, 6.06 g of ferric nitrate nonahydrate, and 1.02 g of citric acid were dissolved in deionized water and stirred until homogeneous to obtain a precursor solution. The precursor solution was dried at 165 °C using a spray dryer to obtain a precursor powder. The precursor powder was heated to 300 °C at a heating rate of 2 °C / min in an argon-hydrogen mixed gas atmosphere (hydrogen volume content of 5%), held at that temperature for 3 h, and then heated to 550 °C at a heating rate of 2 °C / min and held at that temperature for 10 h. After sintering, the powder was cooled to room temperature in the furnace to obtain sodium iron pyrophosphate cathode material Na4Fe3(PO4)2(P2O7)@C.

[0098] Comparative Example 2

[0099] Comparative Example 2 provides a method for preparing a core-shell structured sodium iron pyrophosphate cathode material with the chemical formula Na4Fe3(PO4)2(P2O7)@(Fe3P). 0.5 / C 0.5 Specifically, it includes:

[0100] S1 is the same as in Example 1;

[0101] S2, the above Na4Fe3(PO4)2(P2O7)@C was dispersed in an ethanol solution, 0.1041 g of polyethylene glycol (3% by mass) was added, and the mixture was stirred at 500 rpm with a magnetic stirrer. Then, 0.2789 g of ferric nitrate, 0.0794 g of ammonium dihydrogen phosphate, and 0.3880 g of ferric sulfate were added until a light yellow precipitate appeared in the dispersion. After stirring for 1 h, the mixture was centrifuged at 6000 rpm and washed. The resulting solid was transferred to a vacuum drying oven and dried overnight at 120 °C to obtain the precursor.

[0102] S3 is the same as in Example 1.

[0103] Comparative Example 3

[0104] The difference between Comparative Example 3 and Example 4 is that the microwave calcination power was 300W, while all other aspects were the same as in Example 4. The chemical formula of the core-shell structured sodium iron pyrophosphate cathode material prepared in Comparative Example 3 is Na4Fe3(PO4)2(P2O7)@(Fe2P). 0.25 / C 0.75 .

[0105] Comparative Example 4

[0106] The difference between Comparative Example 4 and Example 4 is that the microwave calcination power was 800W, while all other aspects were the same as in Example 4. The chemical formula of the core-shell structured sodium iron pyrophosphate cathode material prepared in Comparative Example 4 is:

[0107] Na4Fe3(PO4)2(P2O7)@(Fe2P) 0.25 / C 0.75 .

[0108] The electrochemical performance and microstructure of the core-shell structured sodium iron pyrophosphate cathode material prepared in the examples were evaluated, and the results are as follows:

[0109] 1. The sodium iron pyrophosphate cathode materials prepared in Examples 1-4 and Comparative Example 1 were characterized by XRD, and their XRD spectra are shown below. Figure 1 As shown.

[0110] from Figure 1 It can be seen that both the examples and comparative examples successfully synthesized the Na4Fe3(PO4)2(P2O7) main phase, and the obtained cathode materials exhibited good crystallinity. Diffraction peaks corresponding to FeP or Fe2P appeared in Examples 1-4, indicating that (Fe... x P) y / C 1-y Continuous phase coating layer.

[0111] Specifically, in Example 1, diffraction peaks at 56.06° and 59.6° were observed, indicating the presence of an FeP coating layer; while in Examples 2, 3, and 4, diffraction peaks at 40.28° and 44.21° were observed, indicating the presence of an Fe2P coating layer. Furthermore, the figures show that the diffraction peak intensity in Example 3 was the strongest as the Fe2P compound content increased. Comparative Example 1 synthesized a Na4Fe3(PO4)2(P2O7) main phase, which did not exhibit any diffraction peaks from the iron phosphorus compound.

[0112] In other words, the embodiments of this application have successfully prepared a bicontinuous phase coating of phosphorus iron compound and carbon, and different proportions of phosphorus iron compound are synthesized depending on the phosphorus iron ratio.

[0113] 2. Electrochemical performance testing

[0114] The positive electrode materials of Examples 1-5 and Comparative Examples 1-4 were mixed with conductive carbon black and binder in an 8:1:1 ratio. An appropriate amount of N-methylpyrrolidone was added and the mixture was thoroughly mixed to form a slurry. The slurry was then uniformly coated onto aluminum foil using a 50 μm doctor blade, dried, and cut into 12 mm diameter discs to obtain the sodium-ion battery positive electrode. This sodium-ion battery positive electrode was used as the electrode sheet, with a sodium sheet as the counter electrode. The electrolyte consisted of 1M sodium perchlorate dissolved in a 1:1 solvent of propylene carbonate and ethylene carbonate, with 5% fluoroethylene carbonate added. A sodium-ion coin cell was assembled using glass fiber as the separator.

[0115] Electrochemical tests were performed on the obtained coin cells. The test voltage range was 1.7-4.3V, and the test rate was 0.1C-50C. Cyclic testing was conducted with a voltage range of 2-4.2V and a test rate of 20C. The test results are shown in Table 1 and... Figure 2 As shown.

[0116] Table 1. Electrochemical performance test results of the cathode materials in the examples and comparative examples.

[0117]

[0118] from Figure 2 As can be seen, compared with the comparative example, the rate performance of the core-shell structured sodium iron pyrophosphate cathode material prepared in the embodiments of this application is significantly improved.

[0119] As shown in Table 1, compared with the sodium iron pyrophosphate cathode material of Comparative Example 1, the core-shell structure sodium iron pyrophosphate cathode materials of Examples 1-5, which have a dual continuous phase coating of phosphorus iron compound and carbon, show a significant increase in initial discharge specific capacity at different rates. Among them, the core-shell structure sodium iron pyrophosphate cathode material prepared in Example 5 exhibits the best electrochemical performance.

[0120] In this application, different proportions of phosphorus-iron compounds are obtained depending on the phosphorus-iron ratio. As can be seen from the test results of Examples 1-5 and Comparative Example 2, except that Fe3P is easily oxidized in air and affects the performance of the cathode material, other phosphorus-iron compounds such as FeP and Fe2P can improve the intrinsic conductivity of the cathode material.

[0121] The test results from Example 5 and Comparative Examples 3-4 show that the microwave heating power during the synthesis process affects the synthesis of the bicontinuous phase coating layer, thereby affecting the electrochemical performance of the cathode material. If the microwave heating power is too low during synthesis, the iron-phosphorus compound is difficult to form, resulting in a low-conductivity impurity phase in the surface coating layer, leading to low cathode material performance. Conversely, if the microwave heating power is too high during synthesis, the pyrophosphate ions in the main phase material inside the bulk phase decompose, destroying the crystal structure of the main phase and causing a significant decrease in the cathode material's capacity.

[0122] The SEM image of the core-shell structured sodium iron pyrophosphate cathode material prepared in Example 5 is shown below. Figure 3 As shown. From Figure 3 It can be seen that the surface of the cathode material has a coating layer structure, namely a dual continuous phase coating layer structure.

[0123] Although this application has been described in detail in this specification with general descriptions and specific embodiments, some modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, such modifications or improvements made without departing from the spirit of this application are all within the scope of protection claimed in this application.

Claims

1. A core-shell structured sodium iron phosphate positive electrode material, characterized in that, The core-shell structure phosphoferric sodium positive electrode material comprises a core material and a bicontinuous phase coating layer generated in situ on the surface of the core material. The core material is Na4Fe3(PO4)2(P2O7). The bicontinuous phase coating is (Fe x P) y / C 1-y where 1 < x < 2, 0 < y < 1.

2. The core-shell structured sodium iron phosphate positive electrode material according to claim 1, characterized in that, The content of the bicontinuous phase coating layer is 1-20 wt% of the mass of the phosphoferric sodium pyrophosphate positive electrode material.

3. The method of producing a core-shell structured sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that, The method comprises the following steps: S1, dispersing a sodium source, a first iron source, a phosphorus source and a soluble carbon source in water to obtain a precursor solution; drying the precursor solution to obtain a precursor, and calcining the precursor in a reducing atmosphere to obtain a Na4Fe3(PO4)2(P2O7)@C positive electrode material; S2, uniformly dispersing the Na4Fe3(PO4)2(P2O7)@C positive electrode material and a surfactant in ethanol, and then adding a second iron source and a phosphorus source to perform a co-precipitation reaction, collecting a solid phase, and drying to obtain a precursor; S3, performing selective in-situ coating on the precursor by microwave calcination in a reducing atmosphere to obtain a core-shell structure phosphoferric sodium positive electrode material.

4. The preparation method according to claim 3, characterized in that, The sodium source comprises at least one of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium acetate or sodium pyrophosphate; The first iron source comprises at least one of ferric nitrate, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, magnetite, hematite, ferrous oxide, ferric oxalate, ferric phosphate or ferric pyrophosphate; The phosphorus source comprises at least one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, triammonium phosphate, sodium phosphate, phosphoric acid, pyrophosphoric acid, sodium pyrophosphate or sodium dihydrogen pyrophosphate; The soluble carbon source comprises at least one of oxalic acid, citric acid, soluble starch, sucrose or glucose; The surfactant comprises at least one of sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, polyvinyl pyrrolidone, polyethylene glycol, polyacrylic acid or polymaleic acid; The second iron source comprises at least one of ferric chloride, ferrous chloride, ferrous sulfate, ferric nitrate or ferric sulfate.

5. The preparation method according to claim 3, characterized in that, In S1, the molar ratio of the sodium source, the iron source and the phosphorus source is 4:3:4; and the amount of the soluble carbon source accounts for 5-30% of the total mass of the sodium source, the iron source and the phosphorus source; In S2, the molar ratio of the iron source and the phosphorus source is 1-2:1; and the amount of the surfactant accounts for 0.1-10 wt% of the mass of the Na4Fe3(PO4)2(P2O7)@C positive electrode material.

6. The preparation method according to claim 3, characterized in that, In S1, the drying is spray drying; The calcination specifically comprises: first, increasing the temperature to 300 ℃ at a temperature increasing rate of 2 ℃ / min, and maintaining the temperature for 3 h; and then, increasing the temperature to 400-650 ℃ at a temperature increasing rate of 1-5 ℃ / min, and maintaining the temperature for 10-25 h; In S1 and S3, the reducing atmosphere is a mixed gas of argon and hydrogen, wherein the volume fraction of hydrogen is 1-10%.

7. The preparation method according to claim 3, characterized in that, In S3, the microwave calcination is performed in a microwave reactor with a power of 400-600 W, and the holding time is 10-180 min.

8. Use of the core-shell structure phosphoferric sodium positive electrode material of claims 1-2 or the core-shell structure phosphoferric sodium positive electrode material prepared by the preparation method of claims 3-7 in a sodium ion battery.

9. A sodium-ion battery positive electrode sheet, characterized by, The active material is coated on the surface of the current collector. The active material comprises the core-shell structure sodium iron phosphate pyrophosphate positive electrode material according to claims 1-2 or the core-shell structure sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method according to claims 3-7.

10. A sodium-ion battery, characterized in that, The positive electrode of the sodium-ion battery is the positive electrode sheet according to claim 9.

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