Sodium vanadium fluorophosphate positive electrode material and preparation method therefor, positive electrode sheet and sodium-ion battery

Abstract

The present application belongs to the technical field of sodium-ion batteries, and provides a sodium vanadium fluorophosphate positive electrode material, a positive electrode sheet and a sodium-ion battery. The sodium vanadium fluorophosphate positive electrode material comprises sodium vanadium fluorophosphate particles and a nitrogen-doped carbon coating layer on the surface of the sodium vanadium fluorophosphate particles, wherein the ratio of Ig to Id of the nitrogen-doped carbon coating layer in a Raman spectrum is 1-1.9. The sodium vanadium fluorophosphate positive electrode material has a porosity of 12-33% and a powder resistivity of 27,000-73,000 Ω·cm. The present application is conducive to improving the electrochemical performance, especially the cycle performance and rate performance, of the sodium vanadium fluorophosphate positive electrode material.

Description

Sodium vanadium fluorophosphate cathode material and its preparation method, cathode sheet and sodium-ion battery Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a sodium vanadium fluorophosphate cathode material and its preparation method, cathode sheet, and sodium-ion battery. Background Technology

[0002] With the rapid development of energy storage technology, lithium-ion batteries have become the preferred energy storage device in portable electronic devices, new energy vehicles, and other products. However, due to the limited global lithium resources and rising prices, researchers have begun to look for alternatives to maintain sustainable energy. Sodium-ion batteries have attracted widespread attention because of their similar working principle to lithium-ion batteries, abundant sodium resources, and lower cost.

[0003] Among numerous sodium-ion battery cathode materials, polyanionic compounds are considered to possess good cycle stability due to their stable structure, especially NaVPO4F and Na3V2(PO4)2F3 (NVPF), which have been frequently reported. Although NaVPO4F exhibits a high theoretical specific capacity, its discharge specific capacity and cycle stability are not ideal in practical applications. In contrast, NVPF, due to its high electronegativity, […]. - The strong inductive effect weakens the vanadium-oxygen covalent bond, thereby increasing the redox potential and achieving an average operating voltage of approximately 3.9V, while also exhibiting a higher theoretical specific capacity of 128 mA·h·g. -1 However, NVPF has extremely low electronic conductivity, which limits its reversible capacity at low rates, thus affecting its electrochemical performance and its practical application prospects in sodium-ion batteries.

[0004] To overcome these challenges, researchers have adopted a variety of strategies to improve the performance of NVPF, including ion doping, carbon coating, and constructing special morphologies to enhance the conductivity, improve reaction kinetics, and improve cycle stability of the material. However, the uniformity and stability of sodium vanadium fluorophosphate composites prepared by the above methods are poor, and the conductivity still needs to be improved.

[0005] Therefore, further research and development of cathode materials with better conductivity and rate performance are still needed to meet the increasing market demand. Summary of the Invention

[0006] In view of the technical problems existing in the background art, this application provides a sodium vanadium fluorophosphate cathode material and its preparation method, cathode sheet and sodium-ion battery, aiming to solve the technical problems of insufficient conductivity and rate performance of cathode materials in sodium-ion batteries in the prior art.

[0007] In a first aspect, embodiments of this application provide a sodium vanadium fluorophosphate cathode material, comprising sodium vanadium fluorophosphate particles and a nitrogen-doped carbon coating layer on the surface of the sodium vanadium fluorophosphate particles. The ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating layer is 1 to 1.9. The porosity of the sodium vanadium fluorophosphate cathode material is 12% to 33%, and the powder resistivity of the sodium vanadium fluorophosphate cathode material is 27000 Ω·cm to 73000 Ω·cm.

[0008] In the technical solution of this application embodiment, the sodium vanadium fluorophosphate cathode material contains a nitrogen-doped carbon coating layer. The ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating layer is controlled within the aforementioned range, and the porosity of the sodium vanadium fluorophosphate cathode material is also controlled within the aforementioned range. This results in a relatively uniform porous network structure in the nitrogen-doped carbon coating layer, which is beneficial for effectively controlling the volume change and agglomeration of sodium vanadium fluorophosphate particles during charge and discharge, thereby enhancing the structural stability of the sodium vanadium fluorophosphate cathode material. Simultaneously, the porous amorphous carbon coating structure and the introduction of nitrogen atoms in the nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material of this application help control the powder resistivity of the sodium vanadium fluorophosphate cathode material within the aforementioned range, contributing to enhanced conductivity and ion diffusion of the sodium vanadium fluorophosphate cathode material. This results in excellent electrochemical performance in terms of cycle performance and rate capability.

[0009] In some embodiments, the mass of carbon in the nitrogen-doped carbon coating layer accounts for 1% to 4% of the mass of sodium vanadium fluorophosphate cathode material; the mass of nitrogen in the nitrogen-doped carbon coating layer accounts for 0.2% to 1.5% of the mass of sodium vanadium fluorophosphate cathode material.

[0010] In this embodiment, controlling the mass fractions of carbon and nitrogen in the nitrogen-doped carbon coating layer within the aforementioned ranges is beneficial for the formation of a porous network structure in the nitrogen-doped carbon coating layer, which more effectively improves the structural stability of the sodium vanadium fluorophosphate cathode material. Combined with appropriate doping of nitrogen, the conductivity and ion diffusion between sodium vanadium fluorophosphate particles are enhanced, thereby further improving the electrochemical performance of the sodium vanadium fluorophosphate cathode material.

[0011] In some embodiments, the D50 particle size of the sodium vanadium fluorophosphate cathode material is 0.8 μm to 14 μm, and the D100 particle size is less than 65 μm; the specific surface area of ​​the sodium vanadium fluorophosphate cathode material is 12 m². 2 / g~62m 2 / g; the average pore size of the nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material is 1.9nm~6nm; the powder resistivity of the sodium vanadium fluorophosphate cathode material is 27500Ω·cm~62000Ω·cm.

[0012] In this embodiment, by controlling the particle size, specific surface area, powder resistivity, and average pore size of the nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material, it is beneficial to further improve the electrochemical performance of the sodium vanadium fluorophosphate cathode material.

[0013] Secondly, embodiments of this application provide a method for preparing sodium vanadium fluorophosphate cathode material. The preparation method includes: mixing sodium vanadium fluorophosphate, an organic carbon source, an organic nitrogen source and a crosslinking agent, and performing a gel treatment to obtain a gel slurry; and performing a drying treatment and a carbonization treatment on the gel slurry to obtain sodium vanadium fluorophosphate cathode material, wherein the drying treatment includes spray freeze drying.

[0014] In the technical solution of this application embodiment, the preparation method uses organic carbon source and organic nitrogen source as raw materials, combined with freeze spray drying, and successfully prepares the above-mentioned sodium vanadium fluorophosphate cathode material through sol-gel method and carbonization treatment. This preparation method can not only form a nitrogen-doped carbon coating layer with a uniform porous network structure in the sodium vanadium fluorophosphate cathode material, thereby making the porosity of the sodium vanadium fluorophosphate cathode material in the range of 12% to 33%, and the ratio of Ig to Id in the Raman spectrum of its nitrogen-doped carbon coating layer in the range of 1 to 1.9; but also, since the gel slurry obtained after gel treatment is acidic, fluoride ions are easily hydrolyzed to HF under acidic conditions. The method of using freeze spray drying to dry the gel can also avoid the loss of F element caused by the volatilization of HF during the heating drying process, thereby avoiding the impurity phases such as sodium vanadium phosphate generated by the lack of F element, and thus avoiding the increase in powder resistivity and decrease in conductivity of sodium vanadium fluorophosphate cathode material caused by the presence of impurity phases. Therefore, the sodium vanadium fluorophosphate cathode material prepared by the above method can maintain excellent performance during cyclic charge-discharge, and especially exhibits excellent electrochemical performance under high-rate charge-discharge conditions.

[0015] In some embodiments, the organic carbon source is one or more of carboxymethyl cellulose, starch, sucrose and cyclodextrin; the organic nitrogen source is one or more of chitosan and gelatin; the mass ratio of the organic carbon source to the organic nitrogen source is (1:1)-(3:1); and the sum of the masses of the organic carbon source and the organic nitrogen source is 6%-18% of the mass of sodium vanadium fluorophosphate.

[0016] In this embodiment, the use of specific types of organic carbon and organic nitrogen sources allows for better control of the structure and morphology of the prepared sodium vanadium fluorophosphate cathode material, further improving its uniformity and stability. This results in a more significant improvement in the conductivity and electron transport rate of the sodium vanadium fluorophosphate cathode material. Furthermore, by controlling the amount of organic carbon and organic nitrogen sources, the prepared sodium vanadium fluorophosphate cathode material can have suitable carbon and nitrogen content, which is beneficial for the formation of a porous network structure in the nitrogen-doped carbon coating layer. This more effectively enhances the structural stability of the sodium vanadium fluorophosphate cathode material. Combined with appropriate nitrogen doping, the conductivity and ion diffusion between sodium vanadium fluorophosphate particles are enhanced, thereby further improving the electrochemical performance of the sodium vanadium fluorophosphate cathode material.

[0017] In some embodiments, the crosslinking agent is one or more of citric acid, tannic acid, tannic acid, and phytic acid; the mass of the crosslinking agent is 15%-25% of the mass of sodium fluorophosphate.

[0018] In this embodiment, the crosslinking agent plays a role in crosslinking and complexing ions during the gel treatment process. Controlling the amount of crosslinking agent added within an appropriate range is beneficial to controlling the carbon content in the prepared sodium vanadium fluorophosphate cathode material, thus avoiding excessively high or low carbon content in the final product.

[0019] In some embodiments, the gel treatment temperature is 30°C-50°C, and the gel treatment time is 2h-6h.

[0020] In this embodiment, by controlling the process conditions such as temperature and time of gel treatment, the organic carbon source, organic nitrogen source and crosslinking agent can undergo a more efficient gelation reaction with sodium vanadium fluorophosphate, thereby improving the performance of the final sodium vanadium fluorophosphate cathode material.

[0021] In some embodiments, the atomization pressure of the spray freeze-drying is 0.1 MPa-0.4 MPa; the carbonization treatment includes a segmented carbonization treatment, which sequentially includes a first carbonization treatment, a second carbonization treatment, and a third carbonization treatment. The temperature of the first carbonization treatment is 100℃-120℃, and the holding time of the first carbonization treatment is 2h-4h. The temperature of the second carbonization treatment is 300℃-350℃, and the holding time of the second carbonization treatment is 1h-3h. The temperature of the third carbonization treatment is 600℃-750℃, and the holding time of the third carbonization treatment is 5-8h.

[0022] In this embodiment, by controlling the atomization pressure of the spray freeze-drying within the above-mentioned range, the drying efficiency is improved, the loss of F element during the drying process is further reduced, and the impurity phases such as sodium vanadium phosphate generated due to the loss of F element are reduced, thereby improving the performance of the obtained sodium vanadium fluorophosphate cathode material; the segmented carbonization treatment is beneficial to forming a nitrogen-doped carbon coating layer with a more uniform structure.

[0023] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector and an active material layer disposed on the surface of the current collector. The active material layer includes any of the sodium vanadium fluorophosphate positive electrode materials described above or sodium vanadium fluorophosphate positive electrode materials prepared by any of the above-described methods.

[0024] In this embodiment, because the sodium vanadium fluorophosphate cathode material contains a nitrogen-doped carbon coating layer, and the ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating layer is controlled within the range of 1 to 1.9, and the porosity of the sodium vanadium fluorophosphate cathode material is controlled within the range of 12% to 33%, and it has the aforementioned porous amorphous carbon coating structure and the introduction of nitrogen atoms, it is beneficial to control the powder resistivity of the sodium vanadium fluorophosphate cathode material within the range of 27000 Ω·cm to 73000 Ω·cm. Therefore, the sodium vanadium fluorophosphate cathode material has good structural stability and excellent electrochemical performance in terms of cycle performance and rate performance. Therefore, the cathode sheet of this application also has good stability and excellent electrochemical performance such as cycle performance and rate performance.

[0025] Fourthly, embodiments of this application provide a sodium-ion battery containing the aforementioned positive electrode.

[0026] In this embodiment, since the sodium-ion battery includes the aforementioned positive electrode, its electrochemical performance, such as cycle performance and rate performance, is significantly improved, especially under high-rate charge-discharge conditions.

[0027] 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

[0028] 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.

[0029] Figure 1 is a schematic diagram of the process flow for preparing the sodium vanadium fluorophosphate cathode material of this application;

[0030] Figure 2 is a scanning electron microscope image of the sodium vanadium fluorophosphate cathode material of Example 1 of this application;

[0031] Figure 3 is a scanning electron microscope image of the sodium vanadium fluorophosphate cathode material of Comparative Example 1 of this application;

[0032] Figure 4 is a scanning electron microscope image of the sodium vanadium fluorophosphate cathode material of Comparative Example 2 of this application. Detailed Implementation

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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).

[0039] As analyzed in the background section of this application, there are technical problems with insufficient conductivity and rate performance of the cathode material in sodium-ion batteries. In order to solve this problem, this application provides a sodium vanadium fluorophosphate cathode material, its preparation method, cathode sheet, and sodium-ion battery.

[0040] In a first aspect, embodiments of this application provide a sodium vanadium fluorophosphate cathode material, comprising sodium vanadium fluorophosphate particles and a nitrogen-doped carbon coating layer on the surface of the sodium vanadium fluorophosphate particles. The ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating layer is 1 to 1.9. The porosity of the sodium vanadium fluorophosphate cathode material is 12% to 33%, and the powder resistivity of the sodium vanadium fluorophosphate cathode material is 27000 Ω·cm to 73000 Ω·cm.

[0041] In the technical solution of this application embodiment, the sodium vanadium fluorophosphate cathode material contains a nitrogen-doped carbon coating layer. The ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating layer is controlled within the aforementioned range, and the porosity of the sodium vanadium fluorophosphate cathode material is also controlled within the aforementioned range. This results in a relatively uniform porous network structure in the nitrogen-doped carbon coating layer, which is beneficial for effectively controlling the volume change and agglomeration of sodium vanadium fluorophosphate particles during charge and discharge, thereby enhancing the structural stability of the sodium vanadium fluorophosphate cathode material. Simultaneously, the porous amorphous carbon coating structure and the introduction of nitrogen atoms in the nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material of this application help control the powder resistivity of the sodium vanadium fluorophosphate cathode material within the aforementioned range, contributing to enhanced conductivity and ion diffusion of the sodium vanadium fluorophosphate cathode material. This results in excellent electrochemical performance in terms of cycle performance and rate capability.

[0042] In Raman spectroscopy, the g peak typically refers to the peak located at 1580 cm⁻¹. -1 Nearby peaks, for example, located at 1500cm -1 -1700cm -1 The peaks within the range are used to characterize the degree of graphitization of carbon materials, that is, to characterize the degree of graphitization of nitrogen-doped carbon coatings; while the d peak usually refers to the peak located at 1350 cm⁻¹ in the Raman spectrum. -1 Nearby peaks, for example, located at 1200cm -1 -1400cm -1The peaks within the specified range are used to characterize defects in carbon materials, specifically defects in nitrogen-doped carbon coatings. The ratio of Ig to Id represents the intensity ratio of the g peak and the d peak in the Raman spectrum (the value of Ig / Id), and is used to evaluate the integrity and quality of the graphite structure in the nitrogen-doped carbon coating. In this application, the conductivity and rate performance of the cathode material are improved by controlling the ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating on the surface of the sodium vanadium fluorophosphate cathode material, combined with the porosity and powder resistivity of the sodium vanadium fluorophosphate cathode material. Specifically, the ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating is 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, etc., but is not limited thereto. Preferably, the ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating is 1 to 1.7, more preferably 1 to 1.4.

[0043] Specifically, the porosity of the aforementioned sodium vanadium fluorophosphate cathode material is 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, etc., or other values ​​within the aforementioned range. Preferably, the porosity of the aforementioned sodium vanadium fluorophosphate cathode material is 15% to 26%, more preferably 16% to 24%.

[0044] Specifically, the resistivity of the above-mentioned sodium vanadium fluorophosphate cathode material is 28000 Ω·cm, 30000 Ω·cm, 32000 Ω·cm, 34000 Ω·cm, 36000 Ω·cm, 38000 Ω·cm, 40000 Ω·cm, 42000 Ω·cm, 44000 Ω·cm, 46000 Ω·cm, 48000 Ω·cm, 50000 Ω·cm, 52000 Ω·cm, 54000 Ω·cm, 56000 Ω·cm, 58000 Ω·cm, 60000 Ω·cm, 65000 Ω·cm, 70000 Ω·cm, etc., but is not limited to these. Preferably, the resistivity of the above-mentioned sodium vanadium fluorophosphate cathode material is 27500 Ω·cm to 62000 Ω·cm, more preferably 27900 Ω·cm to 45000 Ω·cm.

[0045] In some embodiments, to further improve the electrochemical performance of sodium vanadium fluorophosphate cathode material, the mass of carbon in the nitrogen-doped carbon coating layer accounts for 1% to 4% of the mass of the sodium vanadium fluorophosphate cathode material. This is beneficial for the formation of a porous network structure in the nitrogen-doped carbon coating layer, and more effectively improves the structural stability of the sodium vanadium fluorophosphate cathode material. When the carbon content in the carbon coating layer is too low, the coating effect of the carbon coating layer is poor, the electronic conductivity of sodium vanadium fluorophosphate is low, and the capacity is low. When the carbon content is too high, it is not conducive to ion transport, which easily leads to a low capacity of the finished product. Specifically, the mass of carbon in the nitrogen-doped carbon coating layer can account for 1%, 1.2%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, etc., of the sodium vanadium fluorophosphate cathode material, but is not limited thereto. Preferably, the mass of carbon in the nitrogen-doped carbon coating layer accounts for 1.6% to 3.2% of the mass of the sodium vanadium fluorophosphate cathode material, more preferably 2.1% to 3.2%.

[0046] The introduction of nitrogen atoms into the nitrogen-doped carbon coating alters the electronic structure of the porous network of the carbon aerogel, improving its electronic conductivity. This helps reduce the internal resistance of sodium-ion batteries using sodium vanadium fluorophosphate (CNF) cathode materials, thereby increasing the charge-discharge efficiency of the sodium-ion battery. This allows the CNF cathode material to maintain excellent performance during cyclic charge-discharge, especially exhibiting outstanding electrochemical performance under high-rate charge-discharge conditions. In some embodiments, the mass of nitrogen in the nitrogen-doped carbon coating accounts for 0.2% to 1.5% of the mass of the CNF cathode material. Controlling the mass of nitrogen in the nitrogen-doped carbon coating within this range and appropriately altering its electronic structure further improves the electronic conductivity and ion diffusion of the CNF cathode material, reduces the internal resistance of sodium-ion batteries using CNF cathode materials, and thus improves the charge-discharge efficiency of the sodium-ion battery. Specifically, the mass percentage of nitrogen in the nitrogen-doped carbon coating layer is 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, etc., of the sodium vanadium fluorophosphate cathode material, but is not limited thereto. Preferably, the mass percentage of nitrogen in the nitrogen-doped carbon coating layer is 0.4% to 0.96% of the sodium vanadium fluorophosphate cathode material, more preferably 0.5% to 0.95%.

[0047] Understandably, since sodium vanadium fluorophosphate particles contain almost no carbon or nitrogen elements, the nitrogen content or carbon content in the nitrogen-doped carbon coating layer is the same as the nitrogen content or carbon content in the sodium vanadium fluorophosphate cathode material of this application.

[0048] In some embodiments, to further improve the performance of the sodium vanadium fluorophosphate cathode material, the D50 particle size of the sodium vanadium fluorophosphate cathode material is 0.8 μm to 14 μm, which can provide a more suitable diffusion path for sodium ions and avoid the decrease in electrochemical performance caused by an increase in the diffusion path of sodium ions due to excessively large particle size. The D50 particle size is the particle size value corresponding to a volume distribution percentage of 50% when the particle sizes of the sodium vanadium fluorophosphate cathode material are sorted from smallest to largest. Specifically, the D50 particle size of the sodium vanadium fluorophosphate cathode material is 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc., but is not limited to these. Preferably, the D50 particle size of the sodium vanadium fluorophosphate cathode material is 1 μm to 10 μm, and more preferably, the D50 particle size of the sodium vanadium fluorophosphate cathode material is 2 μm to 9 μm. The D100 particle size of sodium vanadium fluorophosphate cathode material is less than 65 μm, which is beneficial for further improving the performance of sodium vanadium fluorophosphate cathode material. Preferably, the D100 particle size of sodium vanadium fluorophosphate cathode material is 21 μm to 65 μm, more preferably 21 μm to 35 μm. Here, D100 particle size is the particle size value corresponding to 100% of the volume distribution percentage when the particle sizes of sodium vanadium fluorophosphate cathode material are sorted from smallest to largest. Specifically, the D100 particle size of sodium vanadium fluorophosphate cathode material can be 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, 25 μm, 30 μm, etc., but is not limited to these.

[0049] In some embodiments of this application, the specific surface area of ​​the above-mentioned sodium vanadium fluorophosphate cathode material is 12 m². 2 / g~62m 2 The specific surface area of ​​the sodium vanadium fluorophosphate cathode material is reduced by 12 m² / g, which helps to lower the powder resistivity and control the specific surface area within the aforementioned range. This facilitates the preparation of the cathode sheet and makes it easier to remove moisture from the sodium vanadium fluorophosphate cathode material during the drying process, thus benefiting the preparation and performance improvement of the cathode sheet. Specifically, the specific surface area of ​​the sodium vanadium fluorophosphate cathode material can be 12 m² / g. 2 / g、14m 2 / g, 16m 2 / g、18m 2 / g、20m 2 / g、22m 2 / g、24m 2 / g、26m 2 / g、28m 2 / g、29m 2 / g, etc., but not limited to this. The preferred specific surface area of ​​the above-mentioned sodium vanadium fluorophosphate cathode material is 12m². 2 / g~62m 2 / g, further preferably 12m 2 / g~30m2 / g, more preferably 19m 2 / g~30m 2 / g.

[0050] In some embodiments of this application, the average pore size of the nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material is 1.9 nm to 6 nm. The nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material with this average pore size exhibits good ion diffusion properties, which is beneficial for further improving the overall performance of the sodium vanadium fluorophosphate cathode material. Specifically, the average pore size of the nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material is 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm, etc., but is not limited to these. Preferably, the average pore size of the nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material is 3 nm to 6 nm.

[0051] Secondly, embodiments of this application provide a method for preparing a sodium vanadium fluorophosphate cathode material, as shown in Figure 1. The preparation method includes:

[0052] S1. Mix sodium vanadium fluorophosphate, organic carbon source, organic nitrogen source and crosslinking agent, and perform gel treatment to obtain gel slurry;

[0053] S2. The gel slurry is dried and carbonized to obtain sodium vanadium fluorophosphate cathode material. The drying process includes spray freeze drying.

[0054] This preparation method uses organic carbon and organic nitrogen sources as raw materials, combined with freeze-spray drying, and successfully prepares the sodium vanadium fluorophosphate cathode material as described above through sol-gel method and carbonization treatment. This method not only enables the sodium vanadium fluorophosphate cathode material to form a nitrogen-doped carbon coating layer with a uniform porous network structure, resulting in a porosity of 12%–33%, but also achieves an Ig to Id ratio of 1–1.9 in the Raman spectrum of the nitrogen-doped carbon coating layer. Furthermore, since the gel slurry obtained after gel treatment is acidic, fluoride ions are easily hydrolyzed to HF under acidic conditions. Using freeze-spray drying to dry the gel avoids the loss of fluorine (F) element due to HF volatilization during heating drying, thus avoiding the formation of impurities such as sodium vanadium phosphate due to F deficiency in the finished product. This prevents the increase in powder resistivity and decrease in conductivity of the sodium vanadium fluorophosphate cathode material caused by the presence of impurities. Therefore, the sodium vanadium fluorophosphate cathode material prepared by the above method maintains excellent performance during cyclic charge-discharge, especially exhibiting excellent electrochemical performance under high-rate charge-discharge conditions.

[0055] The aforementioned organic carbon and organic nitrogen sources can be selected from existing technologies. In some embodiments, the organic carbon source is one or more of carboxymethyl cellulose, starch, sucrose, and cyclodextrin. Using the aforementioned natural polymer materials as carbon sources, after processing, the surface of the obtained sodium vanadium fluorophosphate cathode material is coated with an amorphous carbon layer, and the primary particles are interconnected through a porous carbon network structure, which is beneficial to significantly improving the performance of the sodium vanadium fluorophosphate cathode material.

[0056] In some embodiments, the organic nitrogen source is one or more of chitosan and gelatin. The wide availability of raw materials helps to reduce the manufacturing cost of sodium vanadium fluorophosphate cathode material and enables better control over the structure and morphology of the prepared sodium vanadium fluorophosphate cathode material, further improving the uniformity and stability of the material, thereby making the improvement of the conductivity and electron transport rate of the fluorophosphate cathode material more significant.

[0057] In some preferred embodiments, the mass ratio of organic carbon source to organic nitrogen source is (1:1) to (3:1); this allows for better synergy between the two sources, resulting in superior performance of the prepared sodium vanadium fluorophosphate cathode material. Specifically, the mass ratio of organic carbon source to organic nitrogen source can be 1.2:1, 1.5:1, 1.8:1, 2:1, 2.3:1, 2.5:1, 2.8:1, etc., but is not limited to these.

[0058] In some embodiments, the sum of the mass of the organic carbon source and the organic nitrogen source is 6%-18% of the mass of sodium vanadium fluorophosphate in step S1, which can make the sodium vanadium fluorophosphate cathode material prepared by the preparation method of this application have suitable carbon and nitrogen content. As an example, the sum of the mass of the organic carbon source and the organic nitrogen source is 7%, 9%, 10%, 12%, 14%, 16%, 17% of the mass of sodium vanadium fluorophosphate, etc., but is not limited thereto.

[0059] The aforementioned crosslinking agent plays a role in crosslinking and complexing ions during the gelation process, and can be selected from existing technologies. In some embodiments, the crosslinking agent is one or more of citric acid, tannic acid, tannic acid, and phytic acid, which play a role in crosslinking and complexing ions, thereby giving the prepared sodium vanadium fluorophosphate cathode material better electrochemical performance.

[0060] Preferably, the crosslinking agent comprises 15%-25% of the mass of sodium vanadium fluorophosphate. Controlling the amount of crosslinking agent added within this range not only ensures good crosslinking and ion complexation but also controls the carbon content in the sodium vanadium fluorophosphate cathode material, preventing the carbon content of the final product from being too high or too low. For example, the crosslinking agent may comprise 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, or 24% of the mass of sodium vanadium fluorophosphate, but is not limited to these values.

[0061] Sodium vanadium fluorophosphate used for gel treatment can be selected from existing technologies or prepared by existing methods, and this application does not have any special requirements. As an example, sodium vanadium fluorophosphate is prepared by the following method: V2O5 and H2C2O4·2H2O are mixed and dissolved in an aqueous solution according to the elemental molar ratio of sodium vanadium fluorophosphate (e.g., 1:3), and stirred at a certain temperature (e.g., 40℃-80℃) to form a blue solution. NaH2PO4, NaF and NH4F are dissolved in water (e.g., deionized water, distilled water or pure water) in a molar ratio of 2:1:2 to form a mixed solution. This mixed solution is poured into the above blue solution and stirred continuously at 80℃-100℃ to react until it is completely dissolved to form a blue-green solution, which is the sodium vanadium fluorophosphate solution.

[0062] In some embodiments, sodium vanadium fluorophosphate is mixed with an organic carbon source, an organic nitrogen source, and a crosslinking agent in the form of a sodium vanadium fluorophosphate solution for gel treatment. Alternatively, sodium vanadium fluorophosphate, the organic carbon source, the organic nitrogen source, and the crosslinking agent can be mixed with a solvent (such as deionized water, distilled water, or pure water) for gel treatment. Those skilled in the art can choose the appropriate implementation method according to specific conditions, and this application does not limit the choice.

[0063] In some embodiments, the gelation treatment temperature is 30℃-50℃, the gelation treatment time is 2h-6h, and the gelation treatment also includes stirring at a speed of 200rpm-500rpm. In this embodiment, by controlling the process conditions such as temperature and time of the gelation treatment, the organic carbon source, organic nitrogen source, and crosslinking agent can undergo a more efficient gelation reaction with sodium vanadium fluorophosphate, thereby improving the performance of the final product. Specifically, the gelation treatment temperature is 35℃, 40℃, 45℃, 48℃, etc., but not limited to these; the gelation treatment time is 2h, 3h, 4h, 5h, etc., but not limited to these; the stirring speed is 250rpm, 300rpm, 350rpm, 400rpm, 450rpm, etc., but not limited to these.

[0064] In some embodiments, the atomization pressure of the spray freeze-drying is 0.1 MPa-0.4 MPa. By controlling the atomization pressure of the spray freeze-drying within the above range, the drying efficiency is improved, a nitrogen-doped carbon coating layer with a more uniform porous network structure is formed, and the loss of F during the drying process is further reduced, as well as the resulting impurity phases such as sodium vanadium phosphate, thereby improving the performance of the sodium vanadium fluorophosphate cathode material. Specifically, the atomization pressure of the spray freeze-drying can be 0.1 MPa, 0.15 MPa, 0.2 MPa, 0.25 MPa, 0.3 MPa, 0.35 MPa, etc., but is not limited to these.

[0065] In some embodiments, the above-mentioned spray freeze-drying uses a micro-high pressure atomizing pump to inject the gel slurry into the atomizer at a flow rate of 1 mL / min-5 mL / min, with an inlet air flow rate of 10 L / min-16 L / min, atomizing the gel slurry into fine droplets. The droplets enter a low-temperature medium container and freeze to obtain ice ball particles. In some embodiments, the above-mentioned low-temperature medium is one of -196℃ liquid nitrogen, -80℃ dry ice / ethanol, -40℃ ethanol, and -20℃ ethanol.

[0066] The dried sample is then subjected to carbonization treatment. In some embodiments, the carbonization treatment includes segmented carbonization, which sequentially comprises a first carbonization stage, a second carbonization stage, and a third carbonization stage. The temperature of the first carbonization stage is 100℃~120℃ (e.g., 105℃, 110℃, 115℃, etc.), and the holding time for the first carbonization stage is 2h-4h (e.g., 2.5h, 3h, 3.5h, etc.). The temperature of the second carbonization stage is 300℃~35℃. The first stage of carbonization is performed at 0℃ (e.g., 310℃, 320℃, 330℃, 340℃, etc.). The second stage of carbonization is held at 1-3 hours (e.g., 1.5 hours, 2 hours, 2.5 hours, etc.). The third stage of carbonization is performed at 600℃-750℃ (e.g., 620℃, 640℃, 660℃, 680℃, 700℃, 720℃, 740℃, etc.), and the holding time for the third stage is 5-8 hours (e.g., 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, etc.). This segmented carbonization process allows for the formation of a more uniform nitrogen-doped carbon coating layer. Preferably, the carbonization process is performed in an inert gas, which can be one or more of argon and nitrogen.

[0067] In some embodiments, the carbonized material is pulverized, sieved, and iron-removed to obtain sodium vanadium fluorophosphate cathode material. The pulverization method is not limited; an air jet mill or a mechanical pulverizer can be used. Sieving after pulverization improves the uniformity of the resulting sodium vanadium fluorophosphate cathode material particles, preventing large particles from adversely affecting electrical performance. Preferably, the particle size range of the sieved sodium vanadium fluorophosphate cathode material is: 0.8 μm ≤ D50 ≤ 14 μm, D100 ≤ 65 μm; more preferably, the particle size range is: 1 μm ≤ D50 ≤ 10 μm, D100 ≤ 35 μm.

[0068] Preferably, the ambient humidity during crushing, sieving, and iron removal is controlled to be below 15% to prevent the sodium vanadium fluorophosphate cathode material from absorbing excessive moisture during processing, which would reduce its electrical performance. Preferably, the content of magnetic foreign matter in the sodium vanadium fluorophosphate cathode material obtained after crushing, sieving, and iron removal is below 150 ppm, free sodium is below 100 ppm, and moisture is below 1000 ppm.

[0069] In some embodiments, the sodium vanadium fluorophosphate cathode material prepared by the method of this application is the sodium vanadium fluorophosphate cathode material as described above, and its structure, properties, etc. will not be repeated here.

[0070] Thirdly, this application provides a positive electrode sheet, including a current collector and an active material layer disposed on the surface of the current collector, wherein the active material layer includes any of the above-mentioned sodium vanadium fluorophosphate positive electrode materials or sodium vanadium fluorophosphate positive electrode materials prepared by any of the above-mentioned methods.

[0071] The positive electrode contains the aforementioned sodium vanadium fluorophosphate positive electrode material, thus exhibiting good stability and excellent electrochemical properties such as cycle performance and rate performance.

[0072] Fourthly, this application provides a sodium-ion battery containing the aforementioned positive electrode.

[0073] Because this sodium-ion battery includes the aforementioned positive electrode, its electrochemical performance, such as cycle performance and rate performance, is significantly improved. It performs particularly well under high-rate charge-discharge conditions, making it well-suited for various application scenarios.

[0074] Fifthly, this application provides an electrical device comprising the aforementioned sodium-ion battery. This electrical device, such as an electric vehicle, exhibits better power and stability due to the use of the sodium-ion battery described above.

[0075] 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.

[0076] I. Preparation Method

[0077] Example 1

[0078] First, 0.5511 g of V₂O₅ and 1.1403 g of H₂C₂O₄·2H₂O were weighed and dissolved in 50 mL of distilled water at a molar ratio of 3:1. The mixture was stirred at 60 °C for 2 h to obtain a blue VOC₂O₄ solution. Then, 2.5810 g of NaH₂PO₄, 0.8608 g of Na₂SO₄, and 0.5141 g of NaF were weighed and dissolved in distilled water at a molar ratio of 3:1:2 to obtain 50 mL of solution. This solution was mixed with the VOC₂O₄ solution and heated to 100 °C. The mixture was then refluxed for 8 h to transform the solution into a blue-green color, yielding the NVPF solution.

[0079] Then, organic carbon source carboxymethyl cellulose and organic nitrogen source chitosan were mixed at a mass ratio of 3:1, with the total addition amount being 7% of the mass of NVPF in the NVPF solution. The mixture was then dissolved in the NVPF solution, and 20% of the mass of NVPF phytic acid was added as a crosslinking agent to the NVPF solution for gelation treatment. The gelation treatment temperature was 50℃, the stirring speed was 500 rpm, and the gelation treatment time was 2 hours to obtain a gel slurry.

[0080] Next, the above-mentioned gel slurry was pressurized by a micro high-pressure atomizing pump at a pressure of 0.4 MPa and a flow rate of 5 mL / min, and injected into the atomizer. The inlet flow rate was 10 L / min, which atomized the gel slurry into fine droplets. The droplets were rapidly sprayed into a container containing liquid nitrogen at -196°C and quickly frozen. The sample was then placed in a vacuum freeze dryer at -80°C and a vacuum degree of 8 Pa for 48 hours to obtain a dried sample.

[0081] Finally, the obtained dried sample was placed in a tube furnace and subjected to segmented carbonization under high-purity argon gas. The procedure was to hold at 120℃ for 4 hours, hold at 350℃ for 2 hours, and sinter at 750℃ for 5 hours. The resulting sample was then crushed, sieved, and iron removed to obtain sodium vanadium fluorophosphate cathode material.

[0082] The microstructure of the sodium vanadium fluorophosphate cathode material is shown in Figure 2. In this SEM image, it can be clearly seen that the nitrogen-doped carbon coating layer forms a porous carbon aerogel network that uniformly coats the surface of the NVPF particles in the sodium vanadium fluorophosphate cathode material prepared in Example 1. This porous structure not only provides abundant channels for sodium ion transport but also increases the surface area, thereby improving electron conductivity.

[0083] Example 2

[0084] First, 0.5511 g of V₂O₅ and 1.1403 g of H₂C₂O₄·2H₂O were weighed and dissolved in 50 mL of distilled water at a molar ratio of 3:1. The mixture was stirred at 80 °C for 2 h to obtain a blue VOC₂O₄ solution. Then, 2.5810 g of NaH₂PO₄, 0.8608 g of Na₂SO₄, and 0.5141 g of NaF were weighed and dissolved in distilled water at a molar ratio of 3:1:2 to obtain 50 mL of solution. This solution was mixed with the VOC₂O₄ solution and heated to 80 °C. The mixture was then refluxed for 8 h to transform into a blue-green solution, yielding the NVPF solution.

[0085] Then, organic carbon source sucrose and organic nitrogen source chitosan were mixed at a mass ratio of 2:1, with the total addition amount being 8% of the mass of NVPF in the NVPF solution. The mixture was then dissolved in the NVPF solution, and 16% of the mass of tannic acid of NVPF was added as a crosslinking agent to the NVPF solution for gelation treatment. The gelation treatment temperature was 50℃, the stirring speed was 500 rpm, and the gelation treatment time was 3 hours to obtain a gel slurry.

[0086] Next, the above-mentioned gel slurry was pressurized by a micro high-pressure atomizing pump at a pressure of 0.1 MPa and a flow rate of 1 mL / min, and injected into the atomizer. The inlet flow rate was 16 L / min, which atomized the gel slurry into fine droplets. The droplets were rapidly sprayed into a container containing a -80°C dry ice / ethanol solution and quickly frozen. The container was then placed in a vacuum freeze dryer at -80°C and a vacuum degree of 8 Pa for 48 hours to obtain a dried sample.

[0087] Finally, the obtained dry sample was placed in a tube furnace and subjected to segmented carbonization under high-purity argon gas. The procedure was to hold at 120℃ for 2 hours, hold at 350℃ for 3 hours, and sinter at 750℃ for 5 hours. The resulting sample was then crushed, sieved, and iron removed to obtain sodium vanadium fluorophosphate cathode material.

[0088] Example 3

[0089] First, 0.5511 g of V₂O₅ and 1.1403 g of H₂C₂O₄·2H₂O were weighed and dissolved in 50 mL of distilled water at a molar ratio of 3:1. The mixture was stirred at 40 °C for 2 h to obtain a blue VOC₂O₄ solution. Then, 2.5810 g of NaH₂PO₄, 0.8608 g of Na₂SO₄, and 0.5141 g of NaF were weighed and dissolved in distilled water at a molar ratio of 3:1:2 to obtain 50 mL of solution. This solution was mixed with the VOC₂O₄ solution and heated to 90 °C. The mixture was then refluxed for 6 h to transform into a blue-green solution, yielding the NVPF solution.

[0090] Then, organic carbon source cyclodextrin and organic nitrogen source chitosan were mixed at a mass ratio of 1:1, with the total addition amount being 8% of the mass of NVPF in the NVPF solution. The mixture was then dissolved in the NVPF solution, and 15% of the mass of NVPF was added as a crosslinking agent to the NVPF solution for gelation treatment. The gelation treatment temperature was 50℃, the stirring speed was 400 rpm, and the gelation treatment time was 2 hours to obtain a gel slurry.

[0091] Next, the above-mentioned gel slurry was pressurized by a micro high-pressure atomizing pump at a pressure of 0.1 MPa and a flow rate of 1 mL / min, and injected into the atomizer. The inlet flow rate was 16 L / min, which atomized the gel slurry into fine droplets. The droplets were rapidly sprayed into a container containing a -40°C ethanol solution and quickly frozen. The container was then placed in a vacuum freeze dryer at -80°C and a vacuum degree of 8 Pa for 48 hours to obtain a dried sample.

[0092] Finally, the obtained dry sample was placed in a tube furnace and subjected to segmented carbonization under high-purity nitrogen. The procedure was to hold at 120℃ for 3 hours, hold at 350℃ for 2 hours, and sinter at 750℃ for 8 hours. The resulting sample was then crushed, sieved, and iron removed to obtain sodium vanadium fluorophosphate cathode material.

[0093] Example 4

[0094] First, 0.5511 g of V₂O₅ and 1.1403 g of H₂C₂O₄·2H₂O were weighed and dissolved in 50 mL of distilled water at a molar ratio of 3:1. The mixture was stirred at 40 °C for 2 h to obtain a blue VOC₂O₄ solution. Then, 2.5810 g of NaH₂PO₄, 0.8608 g of Na₂SO₄, and 0.5141 g of NaF were weighed and dissolved in distilled water at a molar ratio of 3:1:2 to obtain 50 mL of solution. This solution was mixed with the VOC₂O₄ solution and heated to 80 °C. The mixture was then refluxed for 6 h to transform the solution into a blue-green color, yielding the NVPF solution.

[0095] Then, organic carbon source carboxymethyl cellulose and organic nitrogen source gelatin were mixed at a mass ratio of 3:1, with the total addition amount being 8% of the mass of NVPF in the NVPF solution. The mixture was then dissolved in the NVPF solution, and 16% of the mass of tannic acid of NVPF was added as a crosslinking agent to the NVPF solution for gelation treatment. The gelation treatment temperature was 30°C, the stirring speed was 300 rpm, and the gelation treatment time was 4 hours to obtain a gel slurry.

[0096] Next, the above-mentioned gel slurry was pressurized by a micro high-pressure atomizing pump at a pressure of 0.2 MPa and a flow rate of 2 mL / min, and injected into the atomizer. The inlet flow rate was 12 L / min, which atomized the gel slurry into fine droplets. The droplets were rapidly sprayed into a container containing a -20°C ethanol solution and quickly frozen. The sample was then placed in a vacuum freeze dryer at -80°C and a vacuum degree of 8 Pa for 48 hours to obtain a dried sample.

[0097] Finally, the obtained dry sample was placed in a tube furnace and subjected to segmented carbonization under high-purity argon gas. The procedure was to hold at 120℃ for 2 hours, hold at 350℃ for 2 hours, and sinter at 750℃ for 8 hours. The resulting sample was then crushed, sieved, and iron removed to obtain sodium vanadium fluorophosphate cathode material.

[0098] Example 5

[0099] First, 0.5511 g of V₂O₅ and 1.1403 g of H₂C₂O₄·2H₂O were weighed and dissolved in 50 mL of distilled water at a molar ratio of 3:1. The mixture was stirred at 40 °C for 2 h until a blue VOC₂O₄ solution was obtained. Then, 2.5810 g of NaH₂PO₄, 0.8608 g of Na₂SO₄, and 0.5141 g of NaF were weighed and dissolved in distilled water at a molar ratio of 3:1:2 to obtain 50 mL of solution. This solution was mixed with the VOC₂O₄ solution and heated to 80 °C. The mixture was then refluxed for 10 h to transform into a blue-green solution, yielding the NVPF solution.

[0100] Then, organic carbon source sucrose and organic nitrogen source gelatin were mixed at a mass ratio of 1:1, with the total addition amount being 8% of the mass of NVPF in the NVPF solution. The mixture was then dissolved in the NVPF solution, and 15% of the mass of phytic acid was added to the NVPF solution for gelation treatment. The gelation treatment temperature was 40℃, the stirring speed was 500 rpm, and the gelation treatment time was 2.5 h to obtain a gel slurry.

[0101] Next, the above-mentioned gel slurry was pressurized by a micro high-pressure atomizing pump at a pressure of 0.3 MPa and a flow rate of 2 mL / min, and injected into the atomizer. The inlet flow rate was 14 L / min, which atomized the gel slurry into fine droplets. The droplets were rapidly sprayed into a container containing liquid nitrogen at -196°C and quickly frozen. The sample was then placed in a vacuum freeze dryer at -80°C and a vacuum degree of 8 Pa for 48 hours to obtain a dried sample.

[0102] Finally, the obtained dried sample was placed in a tube furnace and subjected to segmented carbonization treatment under a high-purity nitrogen atmosphere. The procedure was to hold at 120℃ for 4 hours, hold at 350℃ for 2 hours, and sinter at 750℃ for 6 hours. The resulting sample was then crushed, sieved, and iron removed to obtain sodium vanadium fluorophosphate cathode material.

[0103] Example 6

[0104] The difference from Example 1 is that the total amount of organic carbon source carboxymethyl cellulose and organic nitrogen source chitosan added is 20% of the mass of NVPF in the NVPF solution.

[0105] Example 7

[0106] The difference from Example 1 is that the total amount of organic carbon source carboxymethyl cellulose and organic nitrogen source chitosan added is 4% of the mass of NVPF in the NVPF solution.

[0107] Example 8

[0108] The difference from Example 1 is that the organic carbon source carboxymethyl cellulose and the organic nitrogen source chitosan are mixed in a mass ratio of 5:1.

[0109] Example 9

[0110] The difference from Example 1 is that the organic carbon source carboxymethyl cellulose and the organic nitrogen source chitosan are mixed in a mass ratio of 1:2.

[0111] Example 10

[0112] The difference from Example 1 is that the gel slurry is pressurized by a micro high-pressure atomizing pump at a pressure of 0.2 MPa and a flow rate of 14 mL / min, and the inlet air flow rate is 8 L / min.

[0113] Example 11

[0114] One difference from Example 1 is that the gel slurry is pressurized by a micro high-pressure atomizing pump at a pressure of 0.6 MPa and a flow rate of 6 mL / min, and the inlet flow rate is 15 L / min.

[0115] Secondly, the obtained dried sample is placed in a tube furnace and subjected to segmented carbonization under high-purity argon gas. The procedure is to hold at 120℃ for 4 hours, hold at 350℃ for 2 hours, and sinter at 650℃ for 5 hours.

[0116] Example 12

[0117] The difference from Example 1 is that the obtained dried sample was placed in a tube furnace and subjected to segmented carbonization under high-purity argon gas. The procedure was to hold at 120°C for 2 hours, then rapidly increase the temperature to 750°C at 10°C / min, and sinter at 750°C for 2 hours.

[0118] Comparative Example 1

[0119] The difference from Example 1 is that, after atomization, the gel material is atomized using high-temperature gas gel atomization particles at a temperature exceeding 120°C instead of rapid freezing and freeze-drying, and the dried sample is obtained by spray drying. The remaining steps are the same as in Example 1, and sodium vanadium fluorophosphate cathode material is prepared.

[0120] The microstructure of the sodium vanadium fluorophosphate cathode material is shown in Figure 3. The non-porous carbon layer formed on its surface simply covers the surface of the NVPF particles, forming a relatively smooth and aggregated carbon layer. Although this structure also provides some protection, its ion and electron transport performance is poor compared to porous carbon aerogel coatings.

[0121] Comparative Example 2

[0122] The difference from Example 1 is that the gel slurry was directly dried in a vacuum drying oven at 80°C for 10 hours to obtain a dried sample. The remaining steps were the same as in Example 1 to prepare a nitrogen-doped carbon aerogel-coated sodium vanadium fluorophosphate cathode material.

[0123] The microstructure of the sodium vanadium fluorophosphate cathode material is shown in Figure 4. In this comparative example, the porous carbon obtained by vacuum drying technology is simply mixed with NVPF particles without forming a distinct coating layer. Although this structure can introduce the advantages of porous carbon, due to the lack of a clear coating layer, its structural stability and ion / electron transport performance are inferior to those of the nitrogen-doped carbon coating layer in the sodium vanadium fluorophosphate cathode material obtained in the examples.

[0124] II. Testing Methods

[0125] 1. Property testing of sodium vanadium fluorophosphate cathode material

[0126] Carbon content test: A carbon-sulfur analyzer was used for testing.

[0127] Nitrogen content test: The N content was determined by high-temperature combustion method using an elemental analyzer, in accordance with standard GB / T37588-2019.

[0128] Particle size testing: A laser particle size analyzer is used to measure the intensity of scattered light produced when a laser beam passes through a dispersed particle sample. The resulting scattering spectrum is analyzed and calculated to determine the particle size distribution.

[0129] Specific surface area test: The BET gas adsorption method was used, with nitrogen as the adsorbate and helium as the carrier gas. When the mixed gas flows through the solid material, the solid material physically adsorbs nitrogen at low temperatures, while the carrier gas is not adsorbed. The amount of adsorption at different relative pressures was measured, and the specific surface area was calculated.

[0130] Average aperture test: Observed and calculated using SEM.

[0131] I D / I G : Test the Raman spectrum of the sample to be tested, and determine the intensity of peak D based on I. D With the intensity of peak G I G Calculate I D / I G The value is as follows: Take 10mg of sample with a spatula and place it in the middle of a slide. Place another slide on top of the sample and press for a while until the sample is compacted and does not disperse. The measurement range is 50-4000nm, and a 405nm laser is used.

[0132] Porosity testing method: Refer to TCSTM 00553—2022, Method for determining porosity of lightweight porous materials.

[0133] Powder resistivity: The four-probe method was used to test the resistivity at a pressure of 10 MPa.

[0134] The sodium ion diffusion coefficient was obtained by electrochemical impedance spectroscopy (EIS).

[0135] The test results of the sodium vanadium fluorophosphate cathode materials prepared in each embodiment and comparative example are listed in Table 1 below.

[0136] 2. Electrochemical performance testing

[0137] The sodium vanadium fluorophosphate cathode material prepared in the above examples and the cathode material prepared in the comparative example were mixed with conductive carbon black (Super P) and polyvinylidene fluoride (PVDF) at a mass ratio of 7:2:1 to form a uniform slurry. This slurry was then dispersed in N-methyl-2-pyrrolidone (NMP) solvent and coated onto aluminum foil using a doctor blade or a four-sided coating applicator to form a cathode sheet. The slurry was then coated onto the aluminum foil current collector using a sheet-making machine to form a 150 nm thick film layer, which was vacuum dried at 120 °C for 8 hours. The coated aluminum foil was then cut into 12 mm diameter circular electrode sheets and assembled into CR2032 button batteries. A sodium metal sheet was used as the counter electrode, and 1 M NaClO4 was used as the electrolyte. This electrolyte consisted of a 1:1 volume ratio mixture of propylene carbonate and ethylene carbonate, and contained 5% by mass of fluoroethylene carbonate as an additive. Whatman glass fiber was used as the separator.

[0138] Its electrochemical performance was evaluated by constant current charge-discharge cycling in the Land test system. The first low-current constant current charge-discharge test was conducted at a rate of 0.1C. The constant current charge-discharge rate in the cycle test was set to 1C. The rate test ranged from 0.1C to 20C, and the voltage range was 2.0 to 4.0V.

[0139] The efficiency value is obtained by calculating the ratio of the initial discharge specific capacity at 0.1C to the initial charge specific capacity at 0.1C.

[0140] The electrochemical performance test results of the sodium vanadium fluorophosphate cathode materials prepared in each embodiment and comparative example are listed in Table 2 below.

[0141] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0142] Table 1

[0143] Table 2

[0144] As can be seen from the above, by applying the technical solution of this application, the sodium vanadium fluorophosphate cathode material contains a nitrogen-doped carbon coating layer. The ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating layer is controlled within the range of 1 to 1.9, and the porosity of the sodium vanadium fluorophosphate cathode material is controlled within the range of 12% to 33%. This results in a relatively uniform porous network structure in the nitrogen-doped carbon coating layer, which is beneficial for effectively controlling the volume change and agglomeration of sodium vanadium fluorophosphate particles during charge and discharge, thereby enhancing the structural stability of the sodium vanadium fluorophosphate cathode material. Simultaneously, the porous amorphous carbon coating structure and the introduction of nitrogen atoms in the nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material of this application help control the powder resistivity of the sodium vanadium fluorophosphate cathode material within the range of 27000 Ω·cm to 73000 Ω·cm, which helps enhance the conductivity and ion diffusion of the sodium vanadium fluorophosphate cathode material, giving it excellent electrochemical performance in terms of cycle performance and rate capability.

[0145] Specifically, as can be seen from the electrochemical performance test data in Table 2, the main difference between Examples 1-5 and Comparative Examples 1-2 of this application is that Comparative Examples 1 and 2 did not use spray freeze-drying to dry the gel slurry. There was no uniform porous carbon aerogel network forming a nitrogen-doped carbon coating layer on the surface of the sodium vanadium fluorophosphate particles, resulting in poor ion and electron transport performance and high powder resistivity. Therefore, the rate performance and cycle performance of the sodium-ion batteries prepared with the cathode materials of these two comparative examples are far worse than those of the sodium vanadium fluorophosphate cathode materials prepared in Examples 1-5, especially under high-rate charge-discharge conditions.

[0146] As shown in Tables 1 and 2, due to the different amounts of organic carbon and nitrogen sources added in Examples 1, 6, and 7, the carbon and nitrogen contents in the prepared sodium vanadium fluorophosphate cathode materials vary significantly. When the carbon and nitrogen contents in the sodium vanadium fluorophosphate cathode material are too low, the coating effect of the nitrogen-doped carbon coating layer is poor, resulting in low electronic conductivity and low capacity. When the carbon content in the sodium vanadium fluorophosphate cathode material is too high, it is not conducive to ion transport, resulting in low capacity of the finished sodium vanadium fluorophosphate cathode material. Controlling the carbon and nitrogen contents in the sodium vanadium fluorophosphate cathode material within a certain range is beneficial to improving the capacity and cycle performance of the sodium vanadium fluorophosphate cathode material.

[0147] As can be seen from the results in Tables 1 and 2, the different ratios of organic nitrogen source and organic carbon source added in Examples 1, 8 and 9 affected the electrochemical performance of the obtained sodium vanadium fluorophosphate cathode material. Within a certain range, the carbon and nitrogen content in the nitrogen-doped carbon coating layer of the sodium vanadium fluorophosphate cathode material can form a porous network structure that is more conducive to electron and ion conduction, thereby further improving the electrochemical performance of the sodium vanadium fluorophosphate cathode material.

[0148] Based on the results of Examples 10-12 and Example 1 in Tables 1 and 2, it can be seen that the particle size and specific surface area of ​​the sodium vanadium fluorophosphate cathode material have a certain influence on its electrochemical performance. The diffusion path of sodium ions is related to the particle size of the cathode material. When the particle size is too large, it is easy to cause the sodium ion diffusion path to grow, which leads to a decrease in the electrochemical performance of the sodium vanadium fluorophosphate cathode material. When the particle size of the sodium vanadium fluorophosphate cathode material is too small, the specific surface area increases. The sodium vanadium fluorophosphate cathode material is easy to absorb water, which makes it difficult to dry the cathode sheet, and also causes a decrease in electrochemical performance.

[0149] 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 sodium vanadium fluorophosphate positive electrode material, characterized in that, The material comprises sodium vanadium fluorophosphate particles and a nitrogen-doped carbon coating layer on the surface of the sodium vanadium fluorophosphate particles. The ratio of Ig to Id in the Raman spectrum of the nitrogen-doped carbon coating layer is 1 to 1.

9. The porosity of the sodium vanadium fluorophosphate cathode material is 12% to 33%, and the powder resistivity of the sodium vanadium fluorophosphate cathode material is 27000 Ω·cm to 73000 Ω·cm.

2. The sodium vanadium fluorophosphate cathode material according to claim 1, characterized in that, The mass of carbon in the nitrogen-doped carbon coating layer accounts for 1% to 4% of the mass of the sodium vanadium fluorophosphate cathode material. The nitrogen element in the nitrogen-doped carbon coating accounts for 0.2% to 1.5% of the mass of the sodium vanadium fluorophosphate cathode material. 3.The sodium vanadium fluorophosphate positive electrode material of claim 1 or 2, characterized in that, The D50 particle size of the sodium vanadium fluorophosphate cathode material is 0.8 μm to 14 μm, and the D100 particle size of the sodium vanadium fluorophosphate cathode material is less than 65 μm; The specific surface area of the sodium vanadium fluorophosphate positive electrode material is 12 m 2 / g~62 m 2 / g; The nitrogen-doped carbon coating of the sodium vanadium fluorophosphate cathode material has an average pore size of 1.9 nm to 6 nm. The resistivity of the sodium vanadium fluorophosphate cathode material is 27500 Ω·cm to 62000 Ω·cm.

4. A method for preparing a sodium vanadium fluorophosphate positive electrode material, characterized in that, include: Sodium vanadium fluorophosphate, organic carbon source, organic nitrogen source and crosslinking agent are mixed and gelled to obtain gel slurry; The gel slurry is dried and carbonized to obtain the sodium vanadium fluorophosphate cathode material. The drying process includes spray freeze drying.

5. The method for preparing sodium vanadium fluorophosphate cathode material according to claim 4, characterized in that, The organic carbon source is one or more of carboxymethyl cellulose, starch, sucrose, and cyclodextrin; The organic nitrogen source is one or more of chitosan and gelatin; The mass ratio of the organic carbon source to the organic nitrogen source is (1:1)-(3:1); The sum of the masses of the organic carbon source and the organic nitrogen source is 6%-18% of the mass of the sodium vanadium fluorophosphate.

6. The method for preparing sodium vanadium fluorophosphate cathode material according to claim 4, characterized in that, The crosslinking agent is one or more of citric acid, tannic acid, tannic acid and phytic acid; The mass of the crosslinking agent is 15%-25% of the mass of the sodium vanadium fluorophosphate.

7. The method for preparing sodium vanadium fluorophosphate cathode material according to any one of claims 4 to 6, characterized in that, The gel treatment temperature is 30℃-50℃, and the gel treatment time is 2h-6h.

8. The method for preparing sodium vanadium fluorophosphate cathode material according to claim 4, characterized in that, The atomization pressure of the spray freeze-drying is 0.1 MPa-0.4 MPa; The carbonization process includes a segmented carbonization process, which sequentially includes a first carbonization process, a second carbonization process, and a third carbonization process. The temperature of the first carbonization process is 100℃~120℃, and the holding time of the first carbonization process is 2h-4h. The temperature of the second carbonization process is 300℃~350℃, and the holding time of the second carbonization process is 1h-3h. The temperature of the third carbonization process is 600℃~750℃, and the holding time of the third carbonization process is 5-8h.

9. A positive electrode sheet, comprising a current collector and an active material layer disposed on the surface of the current collector, characterized in that, The active material layer comprises sodium vanadium fluorophosphate cathode material according to any one of claims 1 to 3 or sodium vanadium fluorophosphate cathode material prepared by the preparation method of sodium vanadium fluorophosphate cathode material according to any one of claims 4 to 8.

10. A sodium-ion battery, characterized in that, It contains the positive electrode sheet as described in claim 9.

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