Ferric sodium pyrophosphate positive electrode material, preparation method thereof, positive electrode sheet and secondary battery

Abstract

This application provides a sodium iron phosphate pyrophosphate cathode material, its preparation method, cathode sheet, and secondary battery, belonging to the field of battery materials. The sodium iron phosphate pyrophosphate cathode material includes a core and a carbon coating layer covering at least a portion of the core's surface. The core comprises sodium iron phosphate pyrophosphate (Na₄Fe₃(PO₄)₂P₂O₇). The compaction density of the sodium iron phosphate pyrophosphate cathode material is 2.4 g / mL to 2.9 g / mL, and its specific surface area is 12.5 m² / mL. 2 / g~17.3 m 2 / g, and the mass fraction of sodium iron phosphate in the sodium iron phosphate pyrophosphate cathode material is less than 1.0%. This application adopts a process of first preparing sodium iron oxide composite and then reacting it with pyrophosphate, phosphorous acid, and carbon materials. This process can significantly improve the compaction density of the prepared sodium iron phosphate pyrophosphate cathode material, and at the same time, it does not require conversion to achieve the reaction of phosphate and pyrophosphate, thereby significantly reducing the sodium iron phosphate impurity phase and significantly improving the specific capacity and uniformity of the sodium iron phosphate pyrophosphate cathode material.

Description

Technical Field

[0001] This application relates to the field of battery materials technology, specifically to a sodium iron phosphate pyrophosphate cathode material and its preparation method, cathode sheet, and secondary battery. Background Technology

[0002] Currently, the global emphasis on energy conservation, emission reduction, and environmental protection has led to explosive growth in the demand for new energy vehicles, directly driving the large-scale expansion of the lithium-ion battery industry. Lithium, as a key raw material for lithium-ion batteries, has relatively limited and unevenly distributed resources, making the supply of raw materials for lithium-ion batteries risky and uncertain. This results in significant fluctuations in lithium prices, affecting the production costs of battery companies and the stability of market supply.

[0003] Sodium-ion batteries benefit from abundant raw materials. Sodium is plentiful and widely distributed in the Earth's crust, resulting in lower raw material costs. Furthermore, the manufacturing process for sodium-ion batteries is similar to that of lithium-ion batteries, eliminating the need for large-scale reinvestment in equipment and offering a cost advantage. Replacing lithium-ion batteries with sodium-ion batteries effectively reduces dependence on lithium resources, and sodium batteries do not require lithium salts in their electrolyte. The negative electrode current collector is aluminum foil instead of copper foil, further reducing costs. Currently, the main positive electrode materials for sodium batteries include layered oxides, polyanionic salts, and Prussian blue-based materials. Among polyanionic salts, sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) has attracted widespread attention due to its structural stability, low price, and the elimination of the need for precious metals.

[0004] In existing processes for preparing sodium iron phosphate pyrophosphate, phosphorus, iron, and sodium sources are typically mixed and calcined to obtain sodium iron phosphate pyrophosphate. This process involves the conversion of phosphate to pyrophosphate, requiring the calcination temperature to be maintained at around 550℃. However, at this temperature, the compacted density of the prepared sodium iron phosphate pyrophosphate product is very low, and any deviation in temperature control during the conversion process can easily lead to incomplete phosphate conversion and the formation of a certain amount of sodium iron phosphate impurity phase. Sodium iron phosphate is electrochemically inactive, and the formation of sodium iron phosphate impurity phase leads to a decrease in the capacity of sodium iron phosphate pyrophosphate.

[0005] Therefore, there is an urgent need to provide a new sodium iron phosphate pyrophosphate cathode material and its preparation method, cathode sheet and secondary battery, to solve the problem that the existing sodium iron phosphate pyrophosphate preparation process easily produces sodium iron phosphate impurities, and the high content of sodium iron phosphate impurities in the sodium iron phosphate pyrophosphate cathode material makes it difficult to improve electrochemical performance such as charging specific capacity and discharging specific capacity. Summary of the Invention

[0006] In view of the technical problems existing in the background art, this application provides a sodium iron phosphate pyrophosphate cathode material and its preparation method, cathode sheet and secondary battery, aiming to solve the problem that the existing sodium iron phosphate pyrophosphate preparation process is prone to producing sodium iron phosphate impurity phase, and the high content of sodium iron phosphate impurity phase in the sodium iron phosphate pyrophosphate cathode material makes it difficult to improve the electrochemical performance such as charging specific capacity and discharging specific capacity.

[0007] In a first aspect, embodiments of this application provide a sodium iron phosphate pyrophosphate cathode material, comprising a core and a carbon coating layer covering at least a portion of the surface of the core, wherein the core comprises sodium iron phosphate pyrophosphate; the compaction density of the sodium iron phosphate pyrophosphate cathode material is 2.25 g / mL to 2.51 g / mL, and the specific surface area is 11.5 m². 2 / g~20 m 2 / g, and the mass fraction of sodium iron phosphate in the sodium iron phosphate pyrophosphate cathode material is less than 3.0%.

[0008] In the technical solution of this application embodiment, the compaction density of the sodium iron phosphate pyrophosphate cathode material is significantly improved, which is beneficial to improving the volumetric energy density of the sodium iron phosphate pyrophosphate cathode material. At the same time, the content of sodium iron phosphate in the sodium iron phosphate pyrophosphate cathode material is significantly reduced, which is beneficial to improving the electrochemical performance of the sodium iron phosphate pyrophosphate cathode material, such as the charge specific capacity and discharge specific capacity.

[0009] In some embodiments, the carbon element mass fraction in the sodium iron phosphate pyrophosphate cathode material is 1.0% to 5%.

[0010] In the above embodiments, controlling the carbon content in the sodium iron phosphate pyrophosphate cathode material to form a uniform carbon coating layer on the core surface is not only beneficial to improving the rate performance and charge-discharge rate of the sodium iron phosphate pyrophosphate cathode material, but also beneficial to improving the structural stability of the sodium iron phosphate pyrophosphate cathode material and extending its cycle life.

[0011] In some embodiments, the mass fraction of sodium iron phosphate in the sodium iron phosphate pyrophosphate cathode material is less than or equal to 1%.

[0012] In the above embodiments, further reducing the content of sodium iron phosphate in the sodium iron phosphate pyrophosphate cathode material is beneficial to further improve the electrochemical performance of the sodium iron phosphate pyrophosphate cathode material, such as its charge specific capacity and discharge specific capacity.

[0013] In some embodiments, the D50 particle size of the sodium iron phosphate pyrophosphate cathode material is 1 μm to 5 μm.

[0014] In the above embodiments, controlling the particle size of the sodium iron phosphate pyrophosphate cathode material is beneficial to improving the structural stability of the sodium iron phosphate pyrophosphate cathode material, shortening the diffusion path of sodium ions inside the sodium iron phosphate pyrophosphate cathode material, and improving the rate performance and cycle stability of the sodium iron phosphate pyrophosphate cathode material.

[0015] Secondly, embodiments of this application provide a method for preparing a sodium iron phosphate pyrophosphate cathode material. This method includes the following steps: mixing a sodium source and an iron source, followed by oxygen-containing calcination to obtain a sodium iron oxide composite; mixing the sodium iron oxide composite, phosphorous acid, pyrophosphate, and carbon materials, followed by oxygen-free calcination to obtain the sodium iron phosphate pyrophosphate cathode material; wherein the chemical formula of the sodium iron oxide composite is Na. x Fe3O z , 4.00≤x≤4.20, 6.49≤z≤6.60.

[0016] In the technical solution of this application embodiment, a sodium iron oxide composite is prepared in advance. The preparation process of the sodium iron oxide composite does not involve carbon materials, thus avoiding the blocking effect of carbon and improving the compaction density of the sodium iron oxide composite. The sodium iron oxide composite with high compaction density reacts with pyrophosphate, phosphorous acid, and carbon materials, generating very little waste gas. This is beneficial to significantly improving the compaction density of the sodium iron phosphate pyrophosphate cathode material, while also facilitating production process control, thereby improving the product consistency of the prepared sodium iron phosphate pyrophosphate cathode material. At the same time, the preparation process of this sodium iron phosphate pyrophosphate cathode material does not require the conversion of phosphate to pyrophosphate, significantly reducing the content of sodium iron phosphate impurities, which is beneficial to significantly improving the electrochemical performance of the sodium iron phosphate pyrophosphate cathode material, such as the charge specific capacity and discharge specific capacity. The preparation method of this sodium iron phosphate pyrophosphate cathode material is simple, low-cost, easy to industrialize, and generates no wastewater, making it highly environmentally friendly.

[0017] In some embodiments, the sodium source includes any one of sodium carbonate, sodium oxalate, sodium acetate, sodium bicarbonate, sodium hydroxide, sodium nitrate, and sodium oxide; the iron source includes at least one of ferrous oxalate, ferric hydroxide, ferric oxyhydroxide, iron oxide red, ferrous acetate, and ferric nitrate; and the carbon material includes at least one of nanographite, conductive carbon black, carbon nanotubes, graphene, and Ketjen black.

[0018] In the above embodiments, the sodium source, iron source, and carbon material are all widely available, facilitating large-scale production.

[0019] In some embodiments, the D50 particle size of the sodium source is 1 μm to 10 μm, and the D50 particle size of the iron source is 1 μm to 10 μm; the molar ratio of sodium in the sodium source, iron in the iron source, phosphorous acid and pyrophosphate is (4.02-4.1):3:(1.9~1.95):(1.02~1.1).

[0020] In the above embodiments, controlling the particle size range of the sodium and iron sources and using sodium and iron sources with similar particle sizes is beneficial for obtaining a sodium iron oxide composite with suitable particle size and concentrated distribution and suitable specific surface area after the sodium and iron sources react fully. This is beneficial for obtaining a sodium iron oxide composite with high compaction density and suitable reactivity, thereby facilitating the acquisition of a sodium iron phosphate pyrophosphate cathode material with improved compaction density. Controlling the ratio of sodium, iron, and phosphorus elements participating in the reaction is beneficial for the full reaction between these elements, resulting in a sodium iron phosphate pyrophosphate cathode material with high compaction density and low content of sodium iron phosphate impurities.

[0021] In some embodiments, the compacted density of the sodium-iron oxide composite is 2.85 g / mL to 3.15 g / mL, the tapped density of the sodium-iron oxide composite is 1.9 g / mL to 2.3 g / mL, and the specific surface area of ​​the sodium-iron oxide composite is 2.3 m². 2 / g~3.9 m 2 / g.

[0022] In the above embodiments, controlling the compaction density, tap density, and specific surface area of ​​the prepared sodium iron oxide composite is beneficial for the subsequent preparation of high-compaction-density, high-capacity sodium iron phosphate pyrophosphate cathode materials.

[0023] In some embodiments, the oxygen-containing calcination treatment step includes: maintaining the oxygen volume percentage in the calcination atmosphere at 15%~20%, the calcination temperature at 700℃~850℃, and the holding time at 4 h~8 h, to obtain a sodium iron oxide composite. The oxygen-free calcination treatment step includes: calcining at 400℃~600℃ under a protective atmosphere, and the holding time at 6 h~10 h, wherein the oxygen volume fraction in the oxygen-free calcination treatment is less than 3 ppm.

[0024] In the above embodiments, controlling the atmosphere and calcination conditions of oxygen-containing calcination is beneficial to the full reaction of sodium and iron sources to obtain sodium iron oxide composite with high density and suitable reactivity; controlling the atmosphere and calcination conditions of oxygen-free calcination is beneficial to the full reaction of sodium iron oxide composite, phosphorous acid, pyrophosphate and carbon materials, thereby facilitating the obtaining of sodium iron phosphate cathode material with high density and low content of sodium iron phosphate impurities.

[0025] In some embodiments, the oxygen-free calcination treatment is further included by a primary pulverization process, which includes: mixing sodium iron oxide complex, phosphorous acid, pyrophosphate and carbon materials to obtain a mixture; pulverizing the mixture until the D50 particle size of the mixture is 1 μm to 1.5 μm.

[0026] In the above embodiments, a pulverization process is performed before the oxygen-free calcination treatment. Controlling the particle size distribution of the mixture after the first pulverization process is beneficial to controlling the particle size of the obtained sodium iron phosphate pyrophosphate cathode material, improving the structural stability of the sodium iron phosphate pyrophosphate cathode material, and enhancing the rate performance and cycle stability of the sodium iron phosphate pyrophosphate cathode material.

[0027] Thirdly, embodiments of this application provide a positive electrode sheet, which includes the sodium iron phosphate pyrophosphate positive electrode material of the first aspect of this application or the sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method of the sodium iron phosphate pyrophosphate positive electrode material of the second aspect of this application.

[0028] The positive electrode sheet of this application is prepared from the aforementioned positive electrode material. The compaction density of the aforementioned sodium iron phosphate pyrophosphate positive electrode material is significantly improved, which is beneficial to improving the electrochemical performance of the sodium iron phosphate pyrophosphate positive electrode material, such as volumetric energy density, charge specific capacity and discharge specific capacity. Therefore, the prepared positive electrode sheet has the advantages of high energy density, high charge specific capacity and high discharge specific capacity.

[0029] Fourthly, embodiments of this application provide a secondary battery, wherein the positive electrode of the secondary battery includes the positive electrode provided in the third aspect of this application.

[0030] The secondary battery of this application includes the aforementioned positive electrode sheet, and therefore has advantages such as high energy density, high charging specific capacity and high discharging specific capacity.

[0031] Compared with the prior art, the beneficial effects of this application include:

[0032] This application employs a method for first preparing a sodium iron oxide composite. The preparation process of the sodium iron oxide composite involves no carbon materials, avoiding the blocking effect of carbon and increasing the compaction density of the sodium iron oxide composite. The sodium iron oxide composite with high compaction density reacts with pyrophosphate, phosphorous acid, and carbon materials, generating very little waste gas. This significantly improves the compaction density of the sodium iron phosphate pyrophosphate cathode material while also facilitating production process control, thereby improving the consistency of the prepared sodium iron phosphate pyrophosphate cathode material. Furthermore, the preparation process of this sodium iron phosphate pyrophosphate cathode material does not require the conversion of phosphate to pyrophosphate, significantly reducing the content of sodium iron phosphate impurities and thus significantly improving the charge and discharge specific capacities of the sodium iron phosphate pyrophosphate cathode material. This preparation method for the sodium iron phosphate pyrophosphate cathode material is simple, low-cost, easy to industrialize, and generates no wastewater, making it highly environmentally friendly.

[0033] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

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

[0035] Figure 1 This is a process flow diagram of one embodiment of the preparation method of sodium iron phosphate pyrophosphate cathode material of this application;

[0036] Figure 2 This is a SEM image of the sodium iron oxide complex prepared in Example 1 of this application;

[0037] Figure 3 This is a SEM image of the sodium iron phosphate pyrophosphate cathode material prepared in Example 1 of this application;

[0038] Figure 4 The image shows the XRD pattern of the sodium iron phosphate pyrophosphate cathode material prepared in Example 1 of this application.

[0039] Figure 5 This is a particle size distribution diagram of the sodium iron phosphate pyrophosphate cathode material prepared in Example 1 of this application;

[0040] Figure 6 The specific capacity test results are shown for the sodium iron phosphate pyrophosphate cathode materials prepared in Examples 1 to 3 of this application. Detailed Implementation

[0041] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0043] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0044] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0045] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0046] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0047] In the description of the embodiments of this application, unless otherwise specified, "ppm" means the mass of the tested element, molecule or ion in parts per million of the sample mass.

[0048] In the description of the embodiments of this application, unless otherwise specified, "content" means the mass percentage of the tested element, molecule or ion in the sample.

[0049] In the existing technology, the molecular formula of sodium iron pyrophosphate is Na4Fe3(PO4)2P2O7. Its preparation process usually involves mixing phosphorus, iron and sodium sources and then calcining them to obtain sodium iron pyrophosphate. This process involves the conversion of phosphate to pyrophosphate, and the calcination temperature usually needs to be maintained at about 550℃. At this temperature, the compaction density of the prepared sodium iron pyrophosphate product is low. Moreover, if the temperature control is deviated during the conversion process, it is easy to cause incomplete conversion of phosphate and generate a certain amount of sodium iron phosphate impurity phase. Since sodium iron phosphate has no electrochemical activity, the generation of sodium iron phosphate impurity phase will lead to a decrease in the specific capacity of sodium iron pyrophosphate product.

[0050] To address the problem that existing sodium iron phosphate pyrophosphate preparation processes easily generate sodium iron phosphate impurities, and that the high content of sodium iron phosphate impurities in sodium iron phosphate pyrophosphate cathode materials leads to difficulties in improving electrochemical performance such as charge specific capacity and discharge specific capacity, this application provides a sodium iron phosphate pyrophosphate cathode material, its preparation method, and a secondary battery.

[0051] This application employs a method for first preparing a sodium iron oxide composite. The preparation process of the sodium iron oxide composite involves no carbon materials, avoiding the blocking effect of carbon and increasing the compaction density of the sodium iron oxide composite. The sodium iron oxide composite with high compaction density reacts with pyrophosphate, phosphorous acid, and carbon materials, generating very little waste gas. This significantly improves the compaction density of the sodium iron phosphate pyrophosphate cathode material while also facilitating production process control, thereby improving the consistency of the prepared sodium iron phosphate pyrophosphate cathode material. Furthermore, the preparation process of this sodium iron phosphate pyrophosphate cathode material does not require the conversion of phosphate to pyrophosphate, significantly reducing the content of sodium iron phosphate impurities and thus significantly improving the charge and discharge specific capacities of the sodium iron phosphate pyrophosphate cathode material. This preparation method for the sodium iron phosphate pyrophosphate cathode material is simple, low-cost, easy to industrialize, and generates no wastewater, making it highly environmentally friendly.

[0052] In a first aspect, embodiments of this application provide a sodium iron phosphate pyrophosphate cathode material, comprising a core and a carbon coating layer covering at least a portion of the surface of the core, wherein the core comprises sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2P2O7); the compaction density of the sodium iron phosphate pyrophosphate cathode material is 2.25 g / mL to 2.51 g / mL, and the specific surface area is 11.5 m². 2 / g~20 m 2 / g, and the mass fraction of sodium iron phosphate in the sodium iron phosphate pyrophosphate cathode material is less than 3.0%.

[0053] In the technical solution of this application embodiment, a sodium iron oxide composite is prepared in advance, and then reacted with pyrophosphate, phosphorous acid, and carbon materials. The compaction density of sodium iron phosphate pyrophosphate products prepared by existing processes is generally only 2.1 g / mL, while the compaction density of sodium iron phosphate pyrophosphate cathode material prepared in this application can reach more than 2.25 g / mL, and the compaction density is effectively improved. At the same time, the impurity phase of sodium iron phosphate is significantly reduced during the preparation process, which is beneficial to improving the charging specific capacity and discharging specific capacity of sodium iron phosphate pyrophosphate cathode material.

[0054] In some embodiments, the compaction density of the sodium iron phosphate pyrophosphate cathode material is 2.3 g / mL to 2.51 g / mL; further, the compaction density of the sodium iron phosphate pyrophosphate cathode material is 2.4 g / mL to 2.51 g / mL.

[0055] In some embodiments, the specific surface area of ​​the sodium iron phosphate pyrophosphate cathode material is 12 m². 2 / g~17.5 m 2 / g; Furthermore, the specific surface area of ​​the sodium iron phosphate pyrophosphate cathode material is 14 m². 2 / g~15.5 m 2 / g.

[0056] In some embodiments, the mass fraction of sodium iron phosphate contained in the sodium iron phosphate pyrophosphate cathode material is less than or equal to 1%; further, the mass fraction of sodium iron phosphate contained in the sodium iron phosphate pyrophosphate cathode material is less than or equal to 0.8%; even further, the mass fraction of sodium iron phosphate contained in the sodium iron phosphate pyrophosphate cathode material is greater than or equal to 0.5% and less than or equal to 0.78%.

[0057] In some embodiments, the carbon content in the sodium iron phosphate pyrophosphate cathode material is 1.0% to 5% by mass. Controlling the carbon content in the sodium iron phosphate pyrophosphate cathode material to form a uniform carbon coating layer on the core surface is beneficial not only for improving the rate performance and charge-discharge rate of the sodium iron phosphate pyrophosphate cathode material, but also for improving the structural stability and extending the cycle life of the sodium iron phosphate pyrophosphate cathode material. If the carbon content is too high, the carbon coating layer will be too thick, which will hinder the insertion and extraction of sodium ions, thus limiting the improvement of electrochemical performance such as charge specific capacity and discharge specific capacity of the sodium iron phosphate pyrophosphate cathode material. If the carbon content is too low, the number of favorable channels and sites for sodium ion migration will decrease, affecting the diffusion and insertion / extraction of sodium ions, reducing the electrochemical reactivity of the sodium iron phosphate pyrophosphate cathode material, and thus limiting the improvement of electrochemical performance such as charge specific capacity, discharge specific capacity and rate performance of the sodium iron phosphate pyrophosphate cathode material.

[0058] In some embodiments, the carbon content in the sodium iron phosphate pyrophosphate cathode material is 1.5% to 3% by mass. Further, the carbon content in the sodium iron phosphate pyrophosphate cathode material is 1.5% to 2% by mass.

[0059] In some embodiments, the D50 particle size of the sodium iron phosphate pyrophosphate cathode material is 1 μm to 5.1 μm. Further, the D50 particle size of the sodium iron phosphate pyrophosphate cathode material is 1 μm to 1.8 μm.

[0060] Furthermore, in some embodiments, the D10 particle size of the sodium iron phosphate pyrophosphate cathode material is 1.0 μm to 1.23 μm, the D50 particle size is 1.41 μm to 1.75 μm, and the D90 particle size is 2.21 μm to 2.6 μm.

[0061] Controlling the particle size of sodium iron phosphate pyrophosphate cathode material is beneficial to improving the structural stability of sodium iron phosphate pyrophosphate cathode material, shortening the diffusion path of sodium ions inside the material, reducing the time and energy loss of sodium ion diffusion, and improving the rate performance and cycle stability of sodium iron phosphate pyrophosphate cathode material.

[0062] In some embodiments, the mass fraction of sodium in the sodium iron phosphate pyrophosphate cathode material is 11.5% to 12.1%, and further, the mass fraction of sodium in the sodium iron phosphate pyrophosphate cathode material is 11.5% to 11.95%.

[0063] In some embodiments, the mass fraction of iron in the sodium iron phosphate pyrophosphate cathode material is 23.2% to 23.7%, and further, the mass fraction of iron in the sodium iron phosphate pyrophosphate cathode material is 23.3% to 23.6%.

[0064] In some embodiments, the mass fraction of phosphorus in the sodium iron phosphate pyrophosphate cathode material is 18.8% to 20.4%, and further, the mass fraction of phosphorus in the sodium iron phosphate pyrophosphate cathode material is 19% to 20.4%.

[0065] By controlling the content of sodium, iron, and phosphorus in sodium iron phosphate pyrophosphate cathode materials, it is beneficial to obtain sodium iron phosphate pyrophosphate cathode materials with suitable chemical composition, improve the purity of sodium iron phosphate pyrophosphate cathode materials, and thus improve the electrochemical performance of sodium iron phosphate pyrophosphate cathode materials.

[0066] In some embodiments, the water content in the sodium iron phosphate pyrophosphate cathode material is 200ppm to 1000ppm, and further, the water content in the sodium iron phosphate pyrophosphate cathode material is 200ppm to 600ppm.

[0067] In some embodiments, the free sodium content in the sodium iron phosphate pyrophosphate cathode material is 500ppm to 1100ppm, and further, the free sodium content in the sodium iron phosphate pyrophosphate cathode material is 500ppm to 1000ppm.

[0068] Controlling the moisture content and free sodium content in sodium iron phosphate pyrophosphate cathode materials helps reduce side reactions when these materials are used in secondary batteries, thereby improving their electrochemical performance.

[0069] In some embodiments, the pH value of the sodium iron phosphate pyrophosphate cathode material is 10.0~11.0, and further, the pH value of the sodium iron phosphate pyrophosphate cathode material is 10.1~10.9. Controlling the pH value of the sodium iron phosphate pyrophosphate cathode material is beneficial to improving the processing performance when preparing cathode slurries containing the sodium iron phosphate pyrophosphate cathode material, and reducing side reactions generated when the sodium iron phosphate pyrophosphate cathode material is applied to secondary batteries, thereby improving the electrochemical performance of the sodium iron phosphate pyrophosphate cathode material when applied to secondary batteries.

[0070] In some embodiments, the powder resistivity of the sodium iron phosphate pyrophosphate cathode material is 20 Ω·cm to 90 Ω·cm, and further, the powder resistivity of the sodium iron phosphate pyrophosphate cathode material is 20 Ω·cm to 40 Ω·cm. Controlling the powder resistivity of the sodium iron phosphate pyrophosphate cathode material is beneficial to improving the electrochemical performance of the sodium iron phosphate pyrophosphate cathode material, such as its charge specific capacity, discharge specific capacity, and cycle life.

[0071] In some embodiments, when the sodium iron phosphate pyrophosphate cathode material is applied to a secondary battery at 25°C, the initial charge specific capacity at a 0.2C rate is 110.2 mAh / g to 112.7 mAh / g. Further, when the sodium iron phosphate pyrophosphate cathode material is applied to a secondary battery at 25°C, the initial charge specific capacity at a 0.2C rate is 111 mAh / g to 112.7 mAh / g.

[0072] In some embodiments, when the sodium iron phosphate pyrophosphate cathode material is applied to a secondary battery at 25°C, the initial discharge specific capacity at a 0.2C rate is 99.5 mAh / g to 104 mAh / g. Further, when the sodium iron phosphate pyrophosphate cathode material is applied to a secondary battery at 25°C, the initial discharge specific capacity at a 0.2C rate is 101.5 mAh / g to 104 mAh / g.

[0073] In some embodiments, when the sodium iron phosphate pyrophosphate cathode material is applied to a secondary battery at 25°C, the initial coulombic efficiency at a 0.2C rate is 90.2%~92.45%. Further, when the sodium iron phosphate pyrophosphate cathode material is applied to a secondary battery at 25°C, the initial coulombic efficiency at a 0.2C rate is 91%~92.45%.

[0074] In some embodiments, when the sodium iron phosphate pyrophosphate cathode material is applied to a secondary battery at 25°C, the capacity retention rate after 1000 charge-discharge cycles at a 0.5C rate is 95.1% to 96.8%. Furthermore, when the sodium iron phosphate pyrophosphate cathode material is applied to a secondary battery at 25°C, the capacity retention rate after 1000 charge-discharge cycles at a 0.5C rate is 96% to 96.8%.

[0075] Secondly, embodiments of this application provide a method for preparing a sodium iron phosphate pyrophosphate cathode material. This method includes the following steps: mixing a sodium source and an iron source, followed by oxygen-containing calcination to obtain a sodium iron oxide composite; mixing the sodium iron oxide composite, phosphorous acid, pyrophosphate, and carbon materials, followed by oxygen-free calcination to obtain the sodium iron phosphate pyrophosphate cathode material; wherein the chemical formula of the sodium iron oxide composite is Na. x Fe y O z , 4.00≤x≤4.20, 6.49≤z≤6.60.

[0076] In the technical solution of this application embodiment, a sodium iron oxide composite is prepared in advance. The preparation process of the sodium iron oxide composite does not involve carbon materials, thus avoiding the blocking effect of carbon and improving the compaction density of the sodium iron oxide composite. The sodium iron oxide composite with high compaction density reacts with pyrophosphate, phosphorous acid, and carbon materials, generating very little waste gas. This is beneficial to significantly improving the compaction density of the sodium iron phosphate pyrophosphate cathode material, while also facilitating production process control and improving the product consistency of the prepared sodium iron phosphate pyrophosphate cathode material. At the same time, the preparation process of this sodium iron phosphate pyrophosphate cathode material does not require the conversion of phosphate to pyrophosphate, significantly reducing the content of sodium iron phosphate impurities, which is beneficial to significantly improving the charging specific capacity and discharging specific capacity of the sodium iron phosphate pyrophosphate cathode material. The preparation method of this sodium iron phosphate pyrophosphate cathode material is simple, low-cost, easy to industrialize, and generates no wastewater, making it highly environmentally friendly.

[0077] In some embodiments, the method for preparing sodium iron phosphate pyrophosphate cathode material provided in this application is used to prepare the sodium iron phosphate pyrophosphate cathode material as described above.

[0078] In some embodiments, 4.02 ≤ x ≤ 4.15; further, 4.02 ≤ x ≤ 4.10.

[0079] In some embodiments, 6.50 ≤ z ≤ 6.60; further, 6.51 ≤ z ≤ 6.58.

[0080] In some embodiments, the sodium content in the sodium-iron oxide complex is 25.4% to 26.2% by mass, and further, the sodium content in the sodium-iron oxide complex is 25.5% to 26.1% by mass.

[0081] In some embodiments, the mass fraction of iron in the sodium iron oxide complex is 45.8% to 46.4%, and further, the mass fraction of iron in the sodium iron oxide complex is 45.85% to 46.2%.

[0082] Specifically, the sodium source includes any one of sodium carbonate, sodium oxalate, sodium acetate, sodium bicarbonate, sodium hydroxide, sodium nitrate, and sodium oxide; the iron source includes at least one of ferrous oxalate, ferric hydroxide, ferric oxyhydroxide, iron oxide red, ferrous acetate, and ferric nitrate; and the carbon material includes at least one of nano-graphite, conductive carbon black, carbon nanotubes, graphene, and Ketjen black.

[0083] In the above embodiments, the sodium source, iron source, and carbon material are all widely available, facilitating large-scale production.

[0084] In some embodiments, the particle size of the sodium source is 1 μm to 10 μm, and the particle size of the iron source is 1 μm to 10 μm. The particle size of the sodium source includes, but is not limited to, any value among 1 μm, 2 μm, 5 μm, 7 μm, and 10 μm, or a range between any two; the particle size of the iron source includes, but is not limited to, any value among 1 μm, 2 μm, 5 μm, 7 μm, and 10 μm, or a range between any two. Controlling the particle size range of the sodium and iron sources and using sodium and iron sources with similar particle sizes is beneficial for obtaining a sodium-iron oxide composite with suitable particle size and concentrated distribution, and suitable specific surface area after sufficient reaction between the sodium and iron sources. This is beneficial for obtaining a sodium-iron oxide composite with high compaction density and suitable reactivity, thereby facilitating the acquisition of a sodium iron pyrophosphate cathode material with improved compaction density.

[0085] In some embodiments, the molar ratio of sodium in the sodium source, iron in the iron source, phosphorous acid, and pyrophosphate is (4.02~4.1):3:(1.9~1.95):(1.02~1.1). This molar ratio includes, but is not limited to, any one of the following values ​​or a range between any two: 4.02:3:1.9:1.02, 4.1:3:1.9:1.02, 4.02:3:1.95:1.02, 4.02:3:1.9:1.1, and 4.1:3:1.95:1.1. Controlling the proportions of sodium, iron, and phosphorus participating in the reaction is beneficial for the full reaction between these elements, resulting in a sodium iron phosphate pyrophosphate cathode material with high compaction density and low content of sodium iron phosphate impurities.

[0086] Specifically, in some embodiments, the compacted density of the sodium-iron oxide complex is 2.85 g / mL to 3.15 g / mL; further, the compacted density of the sodium-iron oxide complex is 2.87 g / mL to 3.15 g / mL; the tapped density of the sodium-iron oxide complex is 1.9 g / mL to 2.3 g / mL; and the specific surface area of ​​the sodium-iron oxide complex is 2.3 m². 2 / g~3.9 m 2 / g. The compacted density of the sodium-iron oxide complex includes, but is not limited to, any one of the following values ​​or a range between any two: 2.85 g / mL, 2.87 g / mL, 2.90 g / mL, 3.00 g / mL, 3.10 g / mL, 3.14 g / mL, and 3.15 g / mL. The tapped density of the sodium-iron oxide complex includes, but is not limited to, any one of the following values ​​or a range between any two: 1.9 g / mL, 2.0 g / mL, 2.1 g / mL, 2.2 g / mL, and 2.3 g / mL. The tapped density of the sodium-iron oxide complex includes, but is not limited to, 2.3 m... 2 / g, 3.5 m 2 / g, 3.0 m 2 / g, 3.5 m 2 / g、3.9 m 2 Any single value in / g or any range of values ​​between the two.

[0087] In the above embodiments, controlling the compaction density, tap density, and specific surface area of ​​the prepared sodium iron oxide composite is beneficial for the subsequent preparation of high-compaction-density, high-capacity sodium iron phosphate pyrophosphate cathode materials.

[0088] In some embodiments, the free sodium content in the sodium-iron oxide complex is 800 ppm to 2000 ppm, and further, the free sodium content in the sodium-iron oxide complex is 800 ppm to 1700 ppm.

[0089] In some embodiments, the sulfate content in the sodium ferric oxide complex is 200 ppm to 300 ppm.

[0090] In some embodiments, the chloride ion content in the sodium iron oxide complex is 30 ppm to 50 ppm.

[0091] By controlling the content of free sodium, sulfate, and chloride ions in the sodium iron oxide composite, the purity of the sodium iron oxide composite can be improved, thereby reducing the occurrence of side reactions during oxygen-free calcination treatment, and thus improving the purity of the obtained sodium iron phosphate pyrophosphate cathode material. This, in turn, enhances the overall stability and electrochemical performance of the sodium iron phosphate pyrophosphate cathode material.

[0092] In some embodiments, the pH value of the sodium iron oxide complex is 10.5-13, and further, the pH value of the sodium iron oxide complex is 10.5-12. Controlling the pH value of the sodium iron oxide complex is beneficial to the thorough mixing and dispersion of the sodium iron oxide complex with phosphorous acid, pyrophosphate, and carbon materials, thereby facilitating the full reaction during oxygen-free calcination, improving the purity of the sodium iron phosphate pyrophosphate cathode material, and enhancing the overall stability and electrochemical performance of the sodium iron phosphate pyrophosphate cathode material.

[0093] In some embodiments, the oxygen-containing calcination treatment step includes: maintaining the oxygen volume percentage in the calcination atmosphere at 15%~20%, the calcination temperature at 700℃~850℃, the holding time at 4h~8h, and cooling after calcination to obtain a sodium iron oxide composite. The oxygen volume percentage in the calcination atmosphere includes, but is not limited to, any one of 15%, 16%, 17%, 18%, and 20%, or a range between any two; the calcination temperature includes, but is not limited to, any one of 700℃, 750℃, 800℃, 820℃, and 850℃, or a range between any two; and the holding time in the oxygen-containing calcination treatment includes, but is not limited to, any one of 4h, 5h, 6h, 7h, and 8h, or a range between any two.

[0094] In some embodiments, the heating rate of the oxygen-containing calcination treatment is 150 °C / h to 250 °C / h. The heating rate of the oxygen-containing calcination treatment includes, but is not limited to, any one of 150 °C / h, 160 °C / h, 180 °C / h, 200 °C / h, 220 °C / h, 250 °C / h, or a range between any two.

[0095] In the above embodiments, controlling the atmosphere and calcination conditions of the oxygen-containing calcination treatment is beneficial to the full reaction of the sodium source and the iron source to obtain a sodium oxide iron complex with high density and suitable reactivity.

[0096] In some embodiments, the oxygen-free calcination treatment step includes: calcining at a temperature of 400℃ to 600℃ under a protective atmosphere for a holding time of 6 h to 10 h, wherein the volume fraction of oxygen in the oxygen-free calcination treatment is less than 3 ppm. Further, the calcination temperature is 450℃ to 550℃. The calcination temperature for the oxygen-free calcination treatment includes, but is not limited to, any one of 400℃, 450℃, 480℃, 500℃, 520℃, 550℃, and 600℃, or a range between any two; the holding time for the oxygen-free calcination treatment includes, but is not limited to, any one of 6 h, 7 h, 8 h, 9 h, and 10 h, or a range between any two.

[0097] In some embodiments, the heating rate in the oxygen-free calcination treatment is 2 °C / min to 4 °C / min. The heating rate of the oxygen-free calcination treatment includes, but is not limited to, any one of 2 °C / min, 2.5 °C / min, 3 °C / min, 3.5 °C / min, 4 °C / min, or any range between two of these values.

[0098] Controlling the atmosphere and calcination conditions during oxygen-free calcination treatment is beneficial to the full reaction of sodium iron oxide complex, phosphorous acid, pyrophosphate and carbon materials, thereby facilitating the acquisition of sodium iron phosphate pyrophosphate cathode materials with high pressure density and low content of sodium iron phosphate impurities.

[0099] In some embodiments, the oxygen-free calcination treatment further includes a primary pulverization process, which includes: mixing sodium iron oxide complex, phosphorous acid, pyrophosphate, and carbon materials to obtain a mixture; pulverizing the mixture until the D50 particle size of the mixture is 0.5 μm to 1.5 μm. Further, the mixture is pulverized until the D50 particle size of the mixture is 1 μm to 1.5 μm.

[0100] In the above embodiments, a pulverization process is performed before the oxygen-free calcination treatment to control the particle size distribution of the mixture after the first pulverization process. This not only helps to control the particle size of the obtained sodium iron phosphate pyrophosphate cathode material to be more uniform and improve the structural stability of the sodium iron phosphate pyrophosphate cathode material, but also helps to increase the specific surface area of ​​the sodium iron phosphate pyrophosphate cathode material.

[0101] In some embodiments, after the oxygen-free calcination treatment, the process further includes a secondary crushing treatment, sieving, and iron removal, wherein the D50 particle size after the secondary crushing treatment is 1 μm to 5 μm.

[0102] Controlling the particle size distribution of the mixture after secondary crushing is beneficial for the prepared sodium iron phosphate pyrophosphate cathode material to have a suitable particle size distribution and specific surface area, so that the electrode can fully contact the electrolyte and improve the specific capacity of the sodium iron phosphate pyrophosphate cathode material.

[0103] Thirdly, embodiments of this application provide a positive electrode sheet, which includes the sodium iron phosphate pyrophosphate positive electrode material of the first aspect of this application or the sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method of the sodium iron phosphate pyrophosphate positive electrode material of the second aspect of this application.

[0104] The positive electrode sheet of this application is prepared from the aforementioned positive electrode material. The compaction density of the aforementioned sodium iron phosphate pyrophosphate positive electrode material is significantly improved, which is beneficial to improving the volumetric energy density, charge specific capacity and discharge specific capacity of the sodium iron phosphate pyrophosphate positive electrode material. Therefore, the prepared positive electrode sheet has the advantages of high energy density, high charge specific capacity and high discharge specific capacity.

[0105] Fourthly, embodiments of this application provide a secondary battery, wherein the positive electrode of the secondary battery includes the positive electrode provided in the third aspect of this application.

[0106] The secondary battery of this application includes the aforementioned positive electrode, and therefore has the advantages of high energy density, high charge specific capacity and high discharge specific capacity.

[0107] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0108] I. Preparation Method

[0109] Example 1

[0110] The specific steps for preparing the sodium iron phosphate pyrophosphate cathode material in this embodiment are as follows:

[0111] (1) Sodium carbonate (sodium source) and iron oxide red (iron source) were mixed using a high-speed mixer. The D50 particle size of sodium carbonate was 5.5 μm, and the D50 particle size of iron oxide red was 4.8 μm. The molar ratio of sodium in sodium carbonate to iron in iron oxide red was 4.06:3. After mixing, sodium carbonate and iron oxide red were subjected to oxygen-containing calcination treatment. The heating rate was 200℃ / h, the calcination temperature was 800℃, and the calcination time was 6h. During the oxygen-containing calcination treatment, air was introduced and the oxygen content (by volume fraction) in the furnace was maintained at 17%. The sodium oxide iron complex was obtained, and its morphology results are as follows. Figure 2 As shown in Table 1, samples of the sodium oxide-iron complex were taken and tested.

[0112] Table 1

[0113]

[0114] (2) The above sodium iron oxide composite was mixed with phosphorous acid and pyrophosphate. The molar ratio of sodium in the sodium source, iron in the iron source, phosphorous acid, and pyrophosphate was 4.06:3:1.93:1.07. Ketjen black (carbon material) was then added. After mixing the above materials, they were pulverized once until the D50 particle size of the mixture was 1.23 μm. Then, the mixture was placed in a furnace and subjected to oxygen-free calcination under an inert atmosphere. Nitrogen gas was introduced during the oxygen-free calcination process to maintain the oxygen content in the furnace below 3 ppm. The temperature was increased to 530℃ at a heating rate of 3℃ / min and held at this temperature for 8 h. The material was then cooled until the temperature of the material was ≤100℃ before being discharged. Then, it was pulverized a second time until the D50 particle size was 1.7 μm. Finally, it was sieved, iron removed, and packaged in a constant temperature and humidity chamber with a humidity of 8% to obtain sodium iron phosphate pyrophosphate cathode material. The mass fraction of carbon in the prepared sodium iron phosphate pyrophosphate cathode material was 1.95%. The morphology results of the sodium iron phosphate pyrophosphate cathode material are as follows: Figure 3 As shown, the XRD results are as follows Figure 4As shown (the standard spectral line below is 89-0579 (Sodium Cobalt Phosphate-Na4Co3(PO4)2(P2O7) standard spectrum), the particle size distribution results such as D50 particle size are as follows. Figure 4 As shown.

[0115] Example 2

[0116] The specific steps for preparing the sodium iron phosphate pyrophosphate cathode material in this embodiment are as follows:

[0117] (1) Sodium oxalate (sodium source) and ferrous oxalate (iron source) were mixed using a high-speed mixer. The D50 particle size of sodium oxalate was 1 μm, and the D50 particle size of ferrous oxalate was 1 μm. The molar ratio of sodium in sodium oxalate to iron in ferrous oxalate was 4.06:3. After mixing, sodium oxalate and ferrous oxalate were subjected to oxygen-containing calcination treatment. The heating rate was 150℃ / h, the calcination temperature was 700℃, and the calcination time was 4h. During the oxygen-containing calcination treatment, air was introduced and the oxygen content (by volume fraction) in the furnace was maintained at 15%, resulting in sodium ferric oxide composite. The sodium ferric oxide composite was sampled and tested, and the results are shown in Table 2.

[0118] Table 2

[0119]

[0120] (2) The above sodium iron oxide complex was mixed with phosphorous acid and pyrophosphate. The molar ratio of sodium element in the sodium source, iron element in the iron source, phosphorous acid and pyrophosphate was 4.06:3:1.93:1.07. Conductive carbon black (carbon material) was added. The above materials were mixed and then pulverized once until the D50 particle size of the mixture was 1 μm. Then it was put into a furnace and calcined under an inert atmosphere. Nitrogen was introduced during the calcination process to maintain the oxygen content in the furnace below 3 ppm. The temperature was raised to 450℃ at a heating rate of 2℃ / min and held at this temperature for 6 h. The material was then cooled until the temperature of the material was ≤100℃ and discharged. Then it was pulverized a second time until the D50 particle size was 1 μm. Then it was screened, iron removed and packaged in a constant temperature and humidity room with a humidity of 5% to obtain sodium iron phosphate pyrophosphate cathode material. The mass fraction of carbon element in the prepared sodium iron phosphate pyrophosphate cathode material was 1.95%.

[0121] Example 3

[0122] The specific steps for preparing the sodium iron phosphate pyrophosphate cathode material in this embodiment are as follows:

[0123] (1) Sodium acetate (sodium source) and ferrous acetate (iron source) were mixed using a high-speed mixer. The D50 particle size of sodium acetate was 10 μm, and the D50 particle size of ferrous acetate was 10 μm. The molar ratio of sodium in sodium acetate to iron in ferrous acetate was 4.06:3. After mixing, sodium acetate and ferrous acetate were subjected to oxygen-containing calcination treatment. The heating rate was 250℃ / h, the calcination temperature was 850℃, and the calcination time was 8h. During the oxygen-containing calcination treatment, air was introduced and the oxygen content (by volume fraction) in the furnace was maintained at 20%, resulting in sodium oxide ferric oxide composite. The sodium oxide ferric oxide composite was sampled and tested, and the results are shown in Table 3.

[0124] Table 3

[0125]

[0126] (2) The above sodium iron oxide composite was mixed with phosphorous acid and pyrophosphate. The molar ratio of sodium in the sodium source, iron in the iron source, phosphorous acid and pyrophosphate was 4.06:3:1.93:1.07. Graphene (carbon material) was added. The above materials were mixed and then pulverized once until the D50 particle size of the mixture was 1.5 μm. Then it was put into a furnace and calcined under an inert atmosphere. Nitrogen was introduced during the calcination process to maintain the oxygen content in the furnace below 3 ppm. The temperature was raised to 550℃ at a heating rate of 4℃ / min and held at this temperature for 10h. The material was then cooled until the temperature of the material was ≤100℃ and discharged. Then it was pulverized a second time until the D50 particle size was 5 μm. Then it was screened, iron removed and packaged in a constant temperature and humidity room with a humidity of 5% to obtain sodium iron phosphate pyrophosphate cathode material. The mass fraction of carbon in the prepared sodium iron phosphate pyrophosphate cathode material was 1.95%.

[0127] Example 4

[0128] The difference between this embodiment and Embodiment 1 is that:

[0129] (1) Sodium bicarbonate (sodium source) and iron oxide red (iron source) were mixed using a high-speed mixer. The D50 particle size of sodium bicarbonate was 5.5 μm, and the D50 particle size of iron oxide red was 4.8 μm. The molar ratio of sodium in sodium bicarbonate to iron in iron oxide red was 4.02:3. After mixing, sodium bicarbonate and iron oxide red were subjected to oxygen-containing calcination treatment. The heating rate was 200℃ / h, the calcination temperature was 800℃, and the calcination time was 6h. During the oxygen-containing calcination treatment, air was introduced and the oxygen content (by volume fraction) in the furnace was maintained at 17%, resulting in sodium oxide iron complex. The sodium oxide iron complex was sampled and tested, and the results are shown in Table 4.

[0130] Table 4

[0131]

[0132] Example 5

[0133] The difference between this embodiment and Embodiment 1 is that:

[0134] (1) Sodium hydroxide (sodium source) and ferric hydroxide (iron source) were mixed using a high-speed mixer. The D50 particle size of sodium hydroxide was 5.5 μm, and the D50 particle size of ferric hydroxide was 4.8 μm. The molar ratio of sodium in sodium hydroxide to iron in ferric hydroxide was 4.1:3. After mixing sodium hydroxide and iron oxide red, oxygen-containing calcination was performed. The heating rate was 200℃ / h, the calcination temperature was 800℃, and the calcination time was 6h. During the oxygen-containing calcination process, air was introduced and the oxygen content (by volume fraction) in the furnace was maintained at 17%, resulting in sodium oxide ferric oxide composite. The sodium oxide ferric oxide composite was sampled and tested, and the results are shown in Table 5.

[0135] Table 5

[0136]

[0137] (2) The above sodium iron oxide composite was mixed with phosphorous acid and pyrophosphate. The molar ratio of sodium in the sodium source, iron in the iron source, phosphorous acid, and pyrophosphate was 4.10:3:1.93:1.07. Conductive carbon black (carbon material) was then added. After mixing the above materials, they were pulverized once until the D50 particle size of the mixture was 1.23 μm. Then, the mixture was placed in a furnace and subjected to oxygen-free calcination under an inert atmosphere. Nitrogen gas was introduced during the oxygen-free calcination process to maintain the oxygen content in the furnace below 3 ppm. The temperature was increased to 530℃ at a heating rate of 3℃ / min and held at this temperature for 8 h. The material was then cooled until the temperature of the material was ≤100℃ before being discharged. Then, it was pulverized a second time until the D50 particle size was 1.7 μm. Finally, it was sieved, iron removed, and packaged in a constant temperature and humidity chamber with a humidity of 8% to obtain sodium iron phosphate pyrophosphate cathode material. The mass fraction of carbon in the prepared sodium iron phosphate pyrophosphate cathode material was 1.95%. %.

[0138] Example 6

[0139] The difference between this embodiment and Embodiment 1 is that:

[0140] (1) Sodium nitrate (sodium source) and ferric nitrate (iron source) were mixed using a high-speed mixer. The D50 particle size of sodium nitrate was 5.5 μm, and the D50 particle size of ferric nitrate was 4.8 μm. The molar ratio of sodium in sodium nitrate to iron in ferric nitrate was 4.0:3. After mixing, sodium nitrate and ferric nitrate were subjected to oxygen-containing calcination treatment. The heating rate was 200℃ / h, the calcination temperature was 800℃, and the calcination time was 6h. During the oxygen-containing calcination treatment, air was introduced and the oxygen content (by volume fraction) in the furnace was maintained at 17%, resulting in sodium oxide ferric oxide composite. The sodium oxide ferric oxide composite was sampled and tested, and the results are shown in Table 6.

[0141] Table 6

[0142]

[0143] Example 7

[0144] The difference between this embodiment and Embodiment 1 is that:

[0145] (1) Sodium oxide (sodium source) and ferric hydroxide (iron source) were mixed using a high-speed mixer. The D50 particle size of sodium oxide was 5.5 μm, and the D50 particle size of ferric hydroxide was 4.8 μm. The molar ratio of sodium in sodium oxide to iron in ferric hydroxide was 4.12:3.0. After mixing, sodium oxide and ferric hydroxide were subjected to oxygen-containing calcination treatment. The heating rate was 200℃ / h, the calcination temperature was 800℃, and the calcination time was 6h. During the oxygen-containing calcination treatment, air was introduced and the oxygen content (by volume fraction) in the furnace was maintained at 17%. Sodium oxide-iron composite was obtained. The chemical formula of sodium oxide-iron composite is: Na x Fe y O z The sodium oxide-iron complex was sampled and tested, and the results are shown in Table 7:

[0146] Table 7

[0147]

[0148] Example 8

[0149] Compared with Example 1, the difference is that in step (2), the above sodium iron oxide complex is mixed with phosphorous acid and pyrophosphate, and the molar ratio of sodium element in sodium source, iron element in iron source, phosphorous acid and pyrophosphate is 4.07:3:1.9:1.02.

[0150] Example 9

[0151] Compared with Example 1, the difference is that in step (2), the above sodium iron oxide complex is mixed with phosphorous acid and pyrophosphate, and the molar ratio of sodium element in sodium source, iron element in iron source, phosphorous acid and pyrophosphate is 4.07:3:1.95:1.1.

[0152] Example 10

[0153] Compared with Example 1, the difference is that in step (2), the above sodium iron oxide complex is mixed with phosphorous acid and pyrophosphate, and the molar ratio of sodium element in sodium source, iron element in iron source, phosphorous acid and pyrophosphate is 4.07:3:1.85:1.0.

[0154] Example 11

[0155] Compared with Example 1, the difference is that in step (2), the above sodium iron oxide complex is mixed with phosphorous acid and pyrophosphate, and the molar ratio of sodium element in sodium source, iron element in iron source, phosphorous acid and pyrophosphate is 4.07:3:2.0:1.12.

[0156] Example 12

[0157] Compared with Example 1, the difference is that in step (2), the mass fraction of carbon in the prepared sodium iron phosphate pyrophosphate cathode material is 1.5%.

[0158] Example 13

[0159] Compared with Example 1, the difference is that in step (2), the mass fraction of carbon in the prepared sodium iron phosphate pyrophosphate cathode material is 3%.

[0160] Example 14

[0161] Compared with Example 1, the difference is that in step (2), the mass fraction of carbon in the prepared sodium iron phosphate pyrophosphate cathode material is 1.0%.

[0162] Example 15

[0163] Compared with Example 1, the difference is that in step (2), the mass fraction of carbon in the prepared sodium iron phosphate pyrophosphate cathode material is 5.0%.

[0164] Example 16

[0165] The difference compared to Example 1 is that the oxygen-free calcination temperature is 400°C.

[0166] Example 17

[0167] The difference compared to Example 1 is that the oxygen-free calcination temperature is 600°C.

[0168] Comparative Example 1

[0169] Comparative Example 1 uses a traditional solid-state method to prepare sodium iron phosphate pyrophosphate cathode material. The specific steps are as follows: First, iron phosphate is synthesized by adding ferrous sulfate, ammonium monohydrogen phosphate, and hydrogen peroxide in a molar ratio of 1:1.08:0.8 to precipitate and react to obtain iron phosphate. Then, sodium phosphate and sodium acetate are added in a molar ratio of 1:1.02:1.05, followed by pure water to form a slurry. Next, Ketjen black (a carbon material) is added in the same amount as in Example 1, resulting in a solid content of 30%. After grinding, the slurry particle size is reduced to 200 nm. The slurry is then spray-dried, and the resulting dried material is placed in a furnace. Nitrogen gas is introduced during calcination to maintain an oxygen content below 3 ppm. The temperature is increased to 530°C at a rate of 3°C / min and held at this temperature for 8 hours. The material is then cooled to a temperature ≤100°C before being discharged. A secondary pulverization process is then performed using a pulverizer to reduce the D50 particle size to 1.7. The sample was then sieved, iron removed, and packaged in a constant temperature and humidity chamber with a humidity of 8% to obtain sodium iron phosphate pyrophosphate cathode material.

[0170] II. Testing Methods

[0171] 1. Morphological test:

[0172] The sodium iron phosphate pyrophosphate cathode material prepared in Example 1 was tested using a scanning electron microscope.

[0173] 2. XRD test:

[0174] The sodium iron phosphate pyrophosphate cathode material prepared in Example 1 was tested using X-ray diffraction. The mass fraction of sodium iron phosphate was defined as the ratio of the main peak height of sodium iron phosphate to that of sodium iron phosphate pyrophosphate.

[0175] 3. Particle size test:

[0176] The sodium iron phosphate pyrophosphate cathode material prepared in Example 1 was tested using a laser particle size analyzer.

[0177] 4. Determination of specific surface area of ​​solid substances by gas adsorption BET method.

[0178] 5. Compacted density: GB / T 24533, Powder compacted density determination, pressure is 3T.

[0179] 6. Powder resistivity: The four-probe method was used to test the resistivity at a pressure of 10 MPa.

[0180] 7. Na, Fe, and P element content: tested using inductively coupled plasma atomic emission spectrometry;

[0181] 8. Mass fraction of carbon: measured using an infrared C / S analyzer;

[0182] 9. Free water content: Tested using the Karl Fischer (KF) method;

[0183] 10. Free Sodium content: The free sodium content was determined by potentiometric titration.

[0184] 11. Specific capacity testing based on sodium-ion batteries:

[0185] Sodium iron pyrophosphate prepared in each embodiment and comparative example was mixed with ultrafine carbon powder (Super P, SP) and polyvinylidene fluoride (PVDF) in a mass ratio of 92:4:4. Then, N-methylpyrrolidone (NMP) was added to form a slurry, which was then coated onto aluminum foil, dried, and pressed into a sheet. Using the sodium sheet as the negative electrode, sodium hexafluorophosphate (NaPF6) as the sodium salt, and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1 as organic solvents, the sodium salt was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1M. A button cell was then prepared, and its first charge specific capacity, first discharge specific capacity, and first-cycle coulombic efficiency at a rate of 0.2C were tested.

[0186] 12. For capacity retention testing, sodium iron phosphate pyrophosphate is assembled into a coin cell according to the specific capacity testing method of sodium-ion batteries. After cycling 1000 times at 0.5C rate and 25℃, the ratio of the remaining capacity to the initial capacity is the capacity retention rate.

[0187] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0188] I. The sodium iron phosphate pyrophosphate cathode materials prepared in Examples 1-17 and Comparative Example 1 were tested and analyzed. The results are shown in Table 8.

[0189] Table 8

[0190]

[0191] II. The electrical properties of the sodium iron phosphate pyrophosphate cathode materials prepared in Examples 1-17 and Comparative Example 1 were tested and analyzed. The results are shown in Table 9.

[0192] Table 9

[0193]

[0194] Comparative Example 1 uses the traditional solid-state method to prepare sodium iron phosphate pyrophosphate cathode material. As can be seen from the comparison of the results in Tables 1 and 8-9, the sodium iron phosphate pyrophosphate cathode material prepared in Example 1 not only has a higher compaction density and a lower sodium iron phosphate impurity phase, but also has better electrical performance than Comparative Example 1. This shows that the performance of the sodium iron phosphate pyrophosphate cathode material prepared by the method of this application is significantly better than that of the cathode material prepared by the traditional solid-state method.

[0195] Figure 6 The table shows the first charge and first discharge curves at 0.2C rate for the sodium iron phosphate pyrophosphate cathode materials obtained in Examples 1-3, respectively, when applied to secondary batteries. The first charge specific capacity and first discharge specific capacity results are consistent with the corresponding results for Examples 1-3 in Table 9. The results corresponding to Examples 1-5 and Examples 6-7 in Tables 8-9 show that controlling the molar ratio of sodium to iron in the sodium and iron sources to (4.02-4.1):3 is beneficial for obtaining a sodium-iron oxide composite with suitable chemical composition, thereby improving the electrochemical performance of the sodium iron phosphate pyrophosphate cathode material, such as compaction density, first charge specific capacity, first discharge specific capacity, and cycle performance.

[0196] The results corresponding to Examples 1, 8, 9 and Examples 10, 11 in Tables 8-9 show that controlling the molar ratio of sodium in the sodium source, iron in the iron source, phosphorous acid, and pyrophosphate to (4.02-4.1):3:(1.9-1.95):(1.02-1.1) is beneficial for the full reaction between the raw materials during oxygen-free calcination, effectively reducing the content of sodium iron phosphate in the sodium iron phosphate pyrophosphate cathode material, thereby obtaining a sodium iron phosphate pyrophosphate cathode material with further improved electrochemical performance such as initial charge specific capacity, initial discharge specific capacity, and cycle performance.

[0197] The results corresponding to Examples 1, 12, 13 and Examples 14, 15 in Tables 8-9 show that controlling the carbon element mass fraction of the sodium iron phosphate pyrophosphate cathode material to 1.5%-3% is more conducive to improving the electrochemical performance of the sodium iron phosphate pyrophosphate cathode material, such as the first charge specific capacity, the first discharge specific capacity and the cycle performance.

[0198] The results corresponding to Examples 1-3 and Examples 16 and 17 in Tables 8-9 show that controlling the temperature of the oxygen-free calcination treatment to 450℃-550℃ is more conducive to the full progress of the reaction and reduces the occurrence of side reactions. This effectively reduces the content of sodium iron phosphate in the sodium iron phosphate pyrophosphate cathode material, thereby obtaining a sodium iron phosphate pyrophosphate cathode material with further improved electrochemical performance such as initial charge specific capacity, initial discharge specific capacity, and cycle performance.

[0199] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing a sodium iron phosphate pyrophosphate positive electrode material, characterized in that, The method comprises the following steps; mixing a sodium source and an iron source, and performing oxygen-containing calcination treatment to obtain a sodium iron oxide composite; mixing the sodium iron oxide composite, phosphorous acid, pyrophosphoric acid and carbon material, and performing anaerobic calcination treatment to obtain a sodium iron pyrophosphate positive electrode material; wherein the sodium iron oxide complex has a chemical formula of Na x Fe3O z , 4.00≤x≤4.20, 6.49≤z≤6.

60.

2. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that, the sodium source comprises any one of sodium carbonate, sodium oxalate, sodium acetate, sodium bicarbonate, sodium hydroxide, sodium nitrate and sodium oxide; and / or the iron source comprises at least one of ferrous oxalate, iron hydroxide, iron oxyhydroxide, red iron oxide, ferrous acetate and iron nitrate; the carbon material comprises at least one of nano-graphite, conductive carbon black, carbon nanotube, graphene and ketjen black.

3. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that, The D50 particle size of the sodium source is 1-10 μm, and the D50 particle size of the iron source is 1-10 μm. The molar ratio of sodium in the sodium source, iron in the iron source, phosphorous acid and pyrophosphoric acid is (4.02-4.1):3:(1.9-1.95):(1.02-1.1).

4. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that, The compaction density of the sodium-iron oxide composite is 2.85 g / mL to 3.15 g / mL, the tap density of the sodium-iron oxide composite is 1.9 g / mL to 2.3 g / mL, and the specific surface area of the sodium-iron oxide composite is 2.3 m 2 / g to 3.9 m 2 / g.

5. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that, The oxygen-containing calcination treatment comprises maintaining the oxygen volume percentage in the calcination atmosphere at 15-20%, the calcination temperature at 700-850°C, and the holding time at 4-8 h to obtain the sodium iron oxide composite. The anaerobic calcination treatment comprises a protective atmosphere, a calcination temperature of 400-600°C, and a holding time of 6-10 h, and the oxygen volume percentage in the anaerobic calcination treatment is less than 3 ppm.

6. The method of claim 1, wherein the sodium iron pyrophosphate positive electrode material is prepared by the steps of: mixing iron phosphate and sodium phosphate to form a mixture; and heating the mixture to form the sodium iron pyrophosphate positive electrode material. The method further comprises a primary crushing treatment before the anaerobic calcination treatment, and the primary crushing treatment comprises the following steps: mixing the sodium iron oxide composite, phosphorous acid, pyrophosphoric acid and carbon material to obtain a mixture; crushing the mixture to a D50 particle size of 1-1.5 μm.

7. A sodium iron phosphate pyrophosphate positive electrode material, characterized in that, The sodium iron pyrophosphate positive electrode material prepared by the method of any one of claims 1-6 comprises an inner core and a carbon coating layer coated on at least part of the surface of the inner core, and the inner core comprises sodium iron pyrophosphate; The compaction density of the sodium iron pyrophosphate positive electrode material is 2.25 g / mL-2.51 g / mL, the specific surface area is 11.5 m 2 / g-20 m 2 / g, and the mass fraction of sodium iron phosphate contained in the sodium iron pyrophosphate positive electrode material is less than 3.0 %.

8. The sodium iron (III) phosphate pyrophosphate cathode material of claim 7, wherein, the mass fraction of carbon in the sodium iron pyrophosphate positive electrode material is 1.0-5%; the mass fraction of sodium iron phosphate contained in the sodium iron pyrophosphate positive electrode material is less than or equal to 1%; the D50 particle size of the sodium iron pyrophosphate positive electrode material is 1-5.1 μm.

9. A positive electrode sheet characterized by comprising: The positive electrode sheet comprises the sodium iron pyrophosphate positive electrode material of any one of claims 7-8 or the sodium iron pyrophosphate positive electrode material prepared by the method of any one of claims 1-6.

10. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode sheet of claim 9.

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