Positive electrode material and preparation method thereof, positive electrode plate and sodium ion battery

Abstract

The invention provides a positive electrode material and a preparation method thereof, a positive electrode plate and a sodium ion battery. Belongs to the technical field of sodium ion batteries, the positive electrode material has a core-shell structure, the core of the core-shell structure is Prussian blue particles, the shell layer of the core-shell structure comprises a sodium ferrovanadium phosphate layer and a carbon layer, the sodium ferrovanadium phosphate layer coats the Prussian blue particles, and the carbon layer coats the sodium ferrovanadium phosphate layer. According to the positive electrode material disclosed by the invention, by adopting the design of the core-shell structure comprising the Prussian blue particles, the sodium ferrovanadium phosphate layer and the carbon layer from inside to outside in sequence, the low-cost Prussian blue particles, the carbon layer and the high-cost sodium ferrovanadium phosphate have a mutual synergistic effect, so that the positive electrode material has excellent rate capability, cycle performance and safety while the cost is relatively low.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and particularly relates to a cathode material, a preparation method thereof, a cathode electrode sheet and a sodium ion battery. Background Art

[0002] Sodium resources are abundant in reserves, and sodium ion batteries have a working principle similar to that of lithium ion batteries. Based on the above characteristics, the research on sodium ion batteries has received continuous attention in recent years. Sodium ion batteries are expected to become one of the new energy storage technologies that promote the low-cost application and sustainable development of energy storage technologies.

[0003] Prussian blue and its analogs have a relatively high theoretical capacity. However, due to the presence of crystal water, the structural stability is affected by the release of water during the charge-discharge process, resulting in the attenuation of cycle performance and poor rate performance.

[0004] Therefore, there is an urgent need to develop a cathode material that has excellent rate performance and cycle performance and at the same time has a relatively low cost. Summary of the Invention

[0005] In view of the technical problems existing in the background art, the present application provides a cathode material, a preparation method thereof, a cathode electrode sheet and a sodium ion battery, aiming to solve the technical problem that it is difficult for the cathode material for sodium ion batteries to simultaneously have excellent rate performance and cycle performance and at the same time have a relatively low cost.

[0006] In a first aspect, an embodiment of the present application provides a cathode material, which has a core-shell structure. The core of the core-shell structure is Prussian blue particles, and the shell layer of the core-shell structure includes a sodium iron vanadium phosphate layer and a carbon layer. Among them, the sodium iron vanadium phosphate layer is coated on the Prussian blue particles, and the carbon layer is coated on the sodium iron vanadium phosphate layer.

[0007] For the cathode material of the present application, through the design of the core-shell structure with Prussian blue particles, a sodium iron vanadium phosphate layer and a carbon layer from the inside to the outside in sequence, the Prussian blue surface is sequentially wrapped with a sodium iron vanadium phosphate layer and a carbon layer, thereby greatly improving the rate performance while effectively transporting sodium ions in the cathode material. Specifically, the Prussian blue particles are used as the core, and the sodium iron vanadium phosphate layer and the carbon layer are used as the shell, which avoids the oxidation of ferrous ions in the Prussian blue particles, improves the structural stability of Prussian blue, and reduces the risk of the structure of the cathode material being damaged during the charge-discharge cycle; at the same time, the carbon layer improves the overall conductivity of the cathode material, effectively improves the electron transport efficiency, significantly improves the rate performance of the cathode material, and reduces the risk of thermal runaway of the cathode material during the charge-discharge process, improving the safety of the cathode material; in addition, the present application uses low-cost Prussian blue particles, a carbon layer and high-cost sodium iron vanadium phosphate to interact synergistically, and while having a relatively low cost, has excellent rate performance, cycle performance and safety.

[0008] In some embodiments, the molecular formula of the Prussian blue particles is Na x Fe y [Fe(CN)6] z ·nH2O, where 1.80 ≤ x ≤ 3.20, 0.60 ≤ y ≤ 1.70, 0.80 ≤ z ≤ 1, 1.0 ≤ n ≤ 2.8; the molecular formula of the sodium iron vanadium phosphate layer is Na i Fe j V k (PO4) l , where 2.00 ≤ i ≤ 3.80, 0.80 ≤ j ≤ 2.00, 0.30 ≤ k ≤ 1.30, 2.70 ≤ l ≤ 3.80; and / or, the crystallinity of the Prussian blue particles is 0.80 - 0.99, and / or the mass fraction of crystal water in the Prussian blue particles is 4.00% - 10.00%.

[0009] In the present application, by using the Prussian blue particles with the above molecular formula as the core structure of the positive electrode material, it is beneficial to optimize the electronic conductivity and ion conductivity of the Prussian blue particles and maintain the structural stability of the positive electrode material during the charge-discharge process; among them, the high content of sodium ions helps to improve the sodium ion storage capacity of the positive electrode material and enhance the charge capacity and discharge capacity of the positive electrode material; the proportion of iron is beneficial to the redox activity of the Prussian blue particles; using the sodium iron vanadium phosphate with the above molecular formula as the sodium iron vanadium phosphate layer of the positive electrode material is beneficial to maintaining the structural stability, ion conductivity and chemical stability of the sodium iron vanadium phosphate layer, optimizing the electrochemical performance of the sodium iron vanadium phosphate, especially improving the capacity retention ability of the positive electrode material at high rates, and reducing the volume change of the positive electrode material during the charge-discharge process, enhancing the overall structural stability and cycle performance of the positive electrode material; controlling the crystallinity of the Prussian blue particles within the above range reduces the defects of the Prussian blue particles and helps to improve the electrochemical performance of the positive electrode material, such as cycle stability and rate performance, etc.; controlling the crystal water content of the Prussian blue particles is beneficial to maintaining the structural stability of the positive electrode material during the charge-discharge process and helps to improve the cycle performance and capacity performance of the positive electrode material, etc.

[0010] In some embodiments, the mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer and the carbon layer is (15 - 33):(65 - 82):(0.5 - 3).

[0011] Prussian blue particles and sodium iron vanadium phosphate layers are the main active substances of the cathode material; among them, the relatively high theoretical capacity of Prussian blue particles is the basis for the high energy density of the cathode material; the sodium iron vanadium phosphate layer coats the Prussian blue particles, which not only contributes additional capacity to the cathode material, but also improves the sodium ion transport performance of the cathode material due to its characteristics as a sodium superionic conductor, enabling the sodium iron vanadium phosphate layer to effectively improve the rate performance of the cathode material while maintaining the stability of Prussian blue particles; the carbon layer not only enhances the electronic conductivity of the cathode material, but also further reduces the direct contact between the core and the sodium iron vanadium phosphate layer and the electrolyte, reduces the occurrence of side reactions, and enhances the chemical stability of the cathode material; by selecting the above optimized mass ratios of Prussian blue particles, sodium iron vanadium phosphate layers and carbon layers, it is beneficial to promote the mutual coordination of Prussian blue particles, sodium iron vanadium phosphate layers and carbon layers, and while improving the charge capacity and discharge capacity of the cathode material, effectively improve the rate performance and cycle stability of the cathode material.

[0012] In some embodiments, the D10 particle size of the Prussian blue particles is 1.00 μm to 5.00 μm, the D50 particle size is 8.00 μm to 20.00 μm, and the D90 particle size is 18.00 μm to 50.00 μm.

[0013] Optimizing the distribution of the above D10 particle size, D50 particle size and D90 particle size helps to reduce the volume change of the cathode material during charge-discharge processes, reduce the structural stress between the cathode material particles, thereby enhancing the structural stability of the cathode material, improving the cycle performance of the cathode material and extending the cycle life.

[0014] In a second aspect, an embodiment of the present application provides a method for preparing a cathode material, the method for preparing the cathode material comprising: dissolving a ferrous source, sodium ferrocyanide, a vanadium source, a phosphorus source, a sodium source, and a reducing agent in a solvent to obtain a first solution; subjecting the first solution to hydrothermal treatment to obtain a first slurry; subjecting the first slurry to solid-liquid separation, washing, drying and iron removal treatment in sequence to obtain the cathode material; wherein, the pH value of the first solution is less than the pH value of the first slurry.

[0015] This application utilizes a hydrothermal treatment process. First, at a relatively low pH value, a sodium prussian blue battery material is obtained, and then at a relatively high pH value, sodium iron vanadium phosphate is synthesized to form a core-shell structured cathode material with the sodium prussian blue battery material as the core and sodium iron vanadium phosphate as the shell. By controlling the change of the pH value, on the one hand, prussian blue particles are formed in an environment with a relatively low pH value, which is conducive to their formation at a low saturation degree, resulting in larger-sized prussian blue particles with a more concentrated particle size distribution, higher crystallinity, and fewer defects, thereby improving the electrochemical performance of the cathode material; on the other hand, sodium iron vanadium phosphate that coats the prussian blue particles is formed under relatively high pH conditions, ensuring the uniformity, conductivity, and stability of the formed shell structure. Coating the prussian blue particles with sodium iron vanadium phosphate, a sodium superionic conductor, is beneficial to improving the sodium ion transport efficiency of the formed cathode material and further enhancing the electrochemical performance of the cathode material.

[0016] In some embodiments, the pH value of the first solution is 1.5 - 2; and / or, the ferrous source is selected from one or more of ferrous gluconate and ferrous citrate; and / or, the vanadium source is a metavanadate source; and / or, the phosphorus source is selected from one or more of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, and ammonium hydrogen phosphate; and / or, the sodium source is selected from one or more of sodium phosphate, sodium dihydrogen phosphate, and sodium hydrogen phosphate; and / or, the reducing agent is selected from one or more of ascorbic acid, sodium borohydride, and hydrazine hydrate; and / or, when the ferrous source is ferrous gluconate, the vanadium source is sodium metavanadate, the sodium source is the said sodium phosphate, and the reducing agent is ascorbic acid, the molar ratio of the ferrous source, sodium ferrocyanide, vanadium source, sodium source, and reducing agent is (1.2 - 1.5):1:(0.2 - 0.4):(1 - 1.5):(0.1 - 0.3); and / or, the molar ratio of the phosphorus source to the vanadium source is (0.1 - 0.5):1; and / or, the pH value of the first solution is adjusted with an acid, and the acid is selected from one or more of sulfuric acid and hydrochloric acid; and / or, the total mass fraction of each solute in the first solution is 25% - 35%.

[0017] By controlling the pH value of the first solution within the above range, it is beneficial to promote the reaction between sodium ferrocyanide, iron source and sodium source, and promote the formation of Prussian blue particles with larger size, more concentrated particle size distribution, higher crystallinity and fewer defects. The selected iron source above helps to improve the synergy with other components. Moreover, the above iron source not only serves as an iron source but also as a carbon source, and forms a carbon layer by carbonization during subsequent high-temperature treatment, which coats the surface of the sodium iron vanadium phosphate layer, thereby improving the conductivity of the cathode material. The addition of sodium metavanadate introduces vanadium elements into sodium vanadate phosphate, promotes the formation of the sodium iron vanadium phosphate layer, and is beneficial to improving the electrical properties of the cathode material. By controlling the types of phosphorus source and sodium source within the above range, it is beneficial to form a stable and efficient sodium iron vanadium phosphate layer, and improve the sodium ion storage capacity, ionic conductivity and rate performance of the sodium iron vanadium phosphate layer. Controlling the type of reducing agent within the above range helps to reduce metal ions and promote the formation of the sodium iron vanadium phosphate layer. Among them, ascorbic acid also helps to be converted into a carbon layer at high temperature subsequently. When the reducing agent is ascorbic acid, controlling the molar ratio of the iron source, sodium ferrocyanide, vanadium source, sodium source to the reducing agent to be (1.2-1.5):1:(0.2-0.4):(1-1.5):(0.1-0.3) helps to fully form Prussian blue particles and uniformly form the subsequent sodium iron vanadium phosphate layer and carbon layer, thereby improving the rate performance, cycle performance and performance stability of the cathode material while achieving a lower manufacturing cost. Controlling the molar ratio of the phosphorus source to the vanadium source within the above range is beneficial to forming a stable sodium iron vanadium phosphate layer, thereby improving the ionic conductivity and rate performance of the cathode material. The selection of the above acid is beneficial to adjusting the pH value of the first solution under the condition of small amount of use, and promoting the optimal synthesis conditions of Prussian blue particles. By controlling the total mass fraction of each solute in the first solution within the above range, it is not only beneficial to the progress of the reaction in hydrothermal treatment, but also beneficial to regulating the particle size distribution of the formed cathode material, and promoting the cathode material to have good tap density and electrochemical activity.

[0018] In some embodiments, the pH value of the first slurry is 5.0 to 6.0; and / or, the hydrothermal treatment includes a first hydrothermal reaction and a second hydrothermal reaction carried out in sequence. The first solution is subjected to the first hydrothermal reaction at the first hydrothermal reaction temperature and the first hydrothermal reaction pressure for the first hydrothermal reaction time to obtain a first mixture; the first mixture is mixed with a first pH regulator and then subjected to the second hydrothermal reaction at the second hydrothermal reaction temperature and the second hydrothermal reaction pressure for the second hydrothermal reaction time to obtain the first slurry; the second hydrothermal reaction temperature is higher than the first hydrothermal reaction temperature, and the second hydrothermal reaction pressure is higher than the first hydrothermal reaction pressure; and / or, the first pH regulator is selected from one or more of ammonia gas and sodium hydroxide solution. When the first pH regulator is selected from sodium hydroxide solution, the concentration of the first pH regulator is 0.5 mol / L to 1 mol / L; and / or, when the first pH regulator is selected from ammonia gas, the first pH regulator is introduced into the first mixture for mixing, and the mixing time of the first mixture and the first pH regulator is 20 min to 40 min; and / or, the temperature of the first hydrothermal reaction is 140°C to 180°C, the pressure of the first hydrothermal reaction is 0.6 MPa to 1.0 MPa, and the time of the first hydrothermal reaction is 3 h to 6 h; and / or, a first stirring is carried out during the first hydrothermal reaction, the rotation speed of the first stirring is 50 r / min to 150 r / min, and the time of the first stirring is 3 h to 6 h; and / or, the temperature of the second hydrothermal reaction is 220°C to 250°C, the pressure of the second hydrothermal reaction is 2.0 MPa to 2.5 MPa, and the time of the second hydrothermal reaction is 14 h to 15 h; and / or, a second stirring is carried out during the second hydrothermal reaction, the rotation speed of the second stirring is 100 r / min to 200 r / min, and the time of the second stirring is 3 h to 6 h.

[0019] Performing the first hydrothermal reaction on the first solution at a lower pH value is conducive to the formation of Prussian blue particles. Subsequently, the pH value is raised to 5.0 to 6.0 by the first pH regulator (such as ammonia gas or sodium hydroxide solution), which helps to uniformly deposit the sodium iron vanadium phosphate layer on the Prussian blue particles, and is conducive to significantly improving the cycle stability and rate performance of the cathode material; controlling the temperature and pressure of the second hydrothermal reaction to be higher than those of the first hydrothermal reaction, the higher temperature and pressure are conducive to accelerating the reaction and shortening the reaction time, which helps to rapidly deposit and crystallize the sodium iron vanadium phosphate on the surface of the Prussian blue particles while ensuring the uniformity and integrity of the deposition of the sodium iron vanadium phosphate layer.

[0020] Using ammonia gas or sodium hydroxide solution as the first pH regulator is beneficial to precisely control the reaction conditions and ensure that the pH value is maintained between 5.0 and 6.0 in the second hydrothermal reaction; the concentration of the sodium hydroxide solution is controlled between 0.5 mol / L and 1 mol / L, which helps for more stable and controllable pH value changes, and the introduction time of ammonia gas is controlled between 20 min and 40 min, ensuring a smooth change in the pH value and being conducive to the uniform formation of the sodium iron vanadium phosphate layer.

[0021] Through the comprehensive control of the above conditions, it is beneficial to improve the purity of the cathode material, effectively reduce the crystal water in Prussian blue, increase the sodium ion content, and improve the electrochemical performance of the cathode material, such as capacity, cycle stability, rate performance, etc.

[0022] Controlling the temperature, pressure, and time of the first hydrothermal reaction within the above ranges not only helps the formation of Prussian blue and improves the stability of Prussian blue, but also the relatively high temperature is beneficial to reducing the crystal water in Prussian blue and improving the capacity and cycle performance of the cathode material; controlling the temperature, pressure, and time of the second hydrothermal reaction within the above ranges promotes the uniform mixing of reactants, is conducive to the rapid and uniform deposition of the sodium iron vanadium phosphate layer, and controls the thickness of the sodium iron vanadium phosphate layer. Therefore, through the comprehensive control of the above conditions, it is beneficial to prepare a high-performance cathode material with a core-shell structure.

[0023] In some embodiments, the free water content of the dried product is less than or equal to 800 ppm.

[0024] On the one hand, by controlling the drying conditions to ensure that the free water content is less than or equal to 800 ppm, it is beneficial to reduce the side effects brought by free water, thereby maintaining the initial electrochemical performance of the cathode material and improving the electrochemical performance such as capacity and cycle stability when the cathode material is applied to sodium-ion batteries; on the other hand, the low free water content is beneficial to avoiding the accelerated aging of the cathode material caused by free water and reducing the side reactions between the cathode material and the electrolyte, prolonging the life of the sodium-ion battery using this cathode material; at the same time, the relatively low free water content helps to improve the processing performance of the cathode material, such as being conducive to increasing the compaction density, etc., and improving the energy density of the sodium-ion battery using this cathode material.

[0025] In the third aspect, the embodiments of the present application provide a positive electrode tab, which includes the above-mentioned cathode material.

[0026] Since the positive electrode tab in the present application includes the above-mentioned cathode material, therefore, this positive electrode tab also has a relatively high capacity, good cycle stability, high rate performance, good safety and stability.

[0027] In the fourth aspect, the embodiments of the present application provide a sodium-ion battery, including a positive electrode, and the positive electrode tab of this sodium-ion battery is the above-mentioned positive electrode tab.

[0028] Applying the above-mentioned positive electrode sheet to the sodium-ion battery in the present application can comprehensively improve the performance of the sodium-ion battery, including but not limited to energy density, cycle stability, rate performance, safety and reliability, as well as the efficiency and consistency of the manufacturing process, providing a solid technical foundation for the development of high-performance and cost-effective sodium-ion batteries, and being of great significance for promoting the commercialization process of sodium battery technology.

[0029] The above description is only an overview of the technical solution of the present application. In order to be able to more clearly understand the technical means of the present application, it can be implemented according to the content of the specification. And in order to make the above and other purposes, features and advantages of the present application more obvious and understandable, the specific embodiments of the present application are specifically exemplified below. Brief Description of the Drawings

[0030] In order to more clearly illustrate the technical solution of the present application, the drawings used in the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0031] Figure 1 It is a process flow diagram for the preparation of the positive electrode material in Example 1 of the present application. Detailed Embodiments

[0032] The embodiments of the technical solution of the present application will be described in detail below with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, so they are only examples and cannot be used to limit the protection scope of the present application.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present application; the terms "including" and "having" and any variations thereof in the specification and claims of the present application are intended to cover non-exclusive inclusion.

[0034] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary-secondary relationship of the indicated technical features. In the description of the embodiments of the present application, "a plurality of" means more than two unless otherwise specifically defined.

[0035] References to "embodiments" in this document mean that the specific features, structures, or characteristics described in connection with the embodiments can be included in at least one embodiment of this application. The phrase appears in various places in the specification and does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0036] In the description of the embodiments of this application, the term "and / or" is merely a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects before and after are in an "or" relationship.

[0037] In the description of the embodiments of this application, the term "plural" means two or more (including two). Similarly, "multiple groups" means two or more groups (including two groups), and "multiple pieces" means two or more pieces (including two pieces).

[0038] In the description of the embodiments of this application, the orientation or positional relationships indicated by technical terms such as "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. are only for the convenience of describing the embodiments of this application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be construed as a limitation on the embodiments of this application.

[0039] In the description of the embodiments of this application, unless otherwise specified, the "solvent" is selected from at least one of distilled water, deionized water, pure water, and ultrapure water.

[0040] In the description of the embodiments of this application, unless otherwise specified, ppm represents the parts per million of the mass of a certain test result in the mass of the sample.

[0041] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are only used to explain this application and cannot be construed as a limitation on this application. For those embodiments where specific techniques or conditions are not indicated, the techniques or conditions described in the literature in this field or according to the product specification are followed. For reagents or instruments whose manufacturers are not indicated, they are all conventional products that can be obtained through commercial purchase.

[0042] In a first aspect, an embodiment of the present application provides a cathode material. The cathode material has a core-shell structure. The core of the core-shell structure is Prussian blue particles, and the shell layer of the core-shell structure includes a sodium iron vanadium phosphate layer and a carbon layer. Among them, the sodium iron vanadium phosphate layer coats the Prussian blue particles, and the carbon layer coats the sodium iron vanadium phosphate layer.

[0043] For the cathode material of the present application, through the design of a core-shell structure with Prussian blue particles, a sodium iron vanadium phosphate layer, and a carbon layer from the inside to the outside in sequence, the Prussian blue surface is wrapped with a sodium iron vanadium phosphate layer and a carbon layer in sequence. Thus, while effectively transporting sodium ions in the cathode material, the rate performance is greatly improved. Specifically, the Prussian blue particles serve as the core, and the sodium iron vanadium phosphate layer and the carbon layer serve as the shell, avoiding the oxidation of ferrous ions in the Prussian blue particles, enhancing the stability of the structure of the Prussian blue core, and reducing the risk of the structure of the cathode material being damaged during the charge-discharge cycle; at the same time, the carbon layer improves the overall conductivity of the cathode material, effectively enhances the electron transport efficiency, significantly improves the rate performance of the cathode material, reduces the risk of thermal runaway of the cathode material during the charge-discharge process, and improves the safety of the cathode material; in addition, the present application uses low-cost Prussian blue particles, carbon layers, and high-cost sodium iron vanadium phosphate to interact synergistically, having excellent rate performance, cycle performance, and safety while having a relatively low cost.

[0044] Further, in some embodiments, the molecular formula of the above Prussian blue particles is Na x Fe y [Fe(CN)6] z ·nH2O, where 1.80 ≤ x ≤ 3.20, 0.60 ≤ y ≤ 1.70, 0.80 ≤ z ≤ 1, 1.0 ≤ n ≤ 2.8; the molecular formula of the sodium iron vanadium phosphate layer is Na i Fe j V k (PO4) l , where 2.00 ≤ i ≤ 3.80, 0.80 ≤ j ≤ 2.00, 0.30 ≤ k ≤ 1.30, 2.70 ≤ l ≤ 3.80.

[0045] In this application, by using Prussian blue particles with the above molecular formula as the core structure of the cathode material, it is beneficial to optimize the electronic conductivity and ionic conductivity of the Prussian blue particles and maintain the structural stability of the cathode material during the charge-discharge process; among them, a high content of sodium ions helps to improve the sodium ion storage capacity of the cathode material and enhance the charge capacity and discharge capacity of the cathode material; the proportion of iron is conducive to the redox activity of the Prussian blue particles; using sodium iron vanadium phosphate with the above molecular formula as the sodium iron vanadium phosphate layer of the cathode material is beneficial to maintaining the structural stability, ionic conductivity and chemical stability of the sodium iron vanadium phosphate layer, optimizing the electrochemical performance of the sodium iron vanadium phosphate, especially improving the capacity retention ability of the cathode material at high rates, and reducing the volume change of the cathode material during the charge-discharge process, enhancing the overall structural stability and cycle performance of the cathode material; controlling the crystallinity of the Prussian blue particles within the above range reduces the defects of the Prussian blue particles, which helps to improve the electrochemical performance of the cathode material, such as cycle stability and rate performance, etc.; controlling the crystal water content of the Prussian blue particles is beneficial to maintaining the structural stability of the cathode material during the charge-discharge process and helps to improve the cycle performance and capacity performance of the cathode material, etc.

[0046] In addition, the molecular formula of the Prussian blue particles can be Na 2.25 Fe 1.01 [Fe(CN)6]1·1.4H2O, Na 2.20 Fe 1.02 [Fe(CN)6]1·1.5H2O, Na 2.37 Fe 1.00 [Fe(CN)6]1·1.3H2O, Na 2.20 Fe 1.05 [Fe(CN)6]1·1.6H2O, Na 2.98 Fe 0.67 [Fe(CN)6]1·1.0H2O, Na 3.17 Fe 1.69 [Fe(CN)6]1·2.5H2O, Na 2.12 Fe 1.00 [Fe(CN)6]1·1.5H2O, Na 1.95 Fe 0.86 [Fe(CN)6]1·1.3H2O, Na 1.87 Fe 1.18 [Fe(CN)6] 0.89 ·1.9H2O and Na 2.68 Fe 1.18 [Fe(CN)6]1·1.1H2O, or any one or more of them.

[0047] In addition, 1.80 ≤ x ≤ 3.20, preferably 1.87 ≤ x ≤ 3.17, more preferably 2.12 ≤ x ≤ 2.37. Further, x can be 1.87, 1.95, 2.12, 2.20, 2.25, 2.37, 2.68, 2.98 or 3.17.

[0048] 0.60 ≤ y ≤ 1.70, preferably 0.67 ≤ y ≤ 1.69, more preferably 1.00 ≤ y ≤ 1.05. Further, y can be 0.67, 0.86, 1.00, 1.01, 1.02, 1.05, 1.18 or 1.69.

[0049] Preferably 0.80 ≤ z ≤ 1, preferably 0.89 ≤ z ≤ 1, more preferably z = 1. Further, z can be 0.89 or 1.

[0050] 1.0 ≤ n ≤ 2.8, preferably 1.0 ≤ n ≤ 2.5, more preferably 1.3 ≤ n ≤ 1.6. Further, n can be 1.0, 1.1, 1.3, 1.4, 1.5, 1.6, 1.9 or 2.5.

[0051] Further, the molecular formula of the sodium iron vanadium phosphate layer can be Na 3.20 Fe 1.27 V 0.81 (PO4) 3.18 、Na 3.15 Fe 1.2 V 0.8 (PO4) 3.2 、Na 3.11 Fe 1.39 V 0.64 (PO4) 3.11 、Na 3.21 Fe 1.26 V 0.81 (PO4) 3.19 、Na 3.00 Fe 1.89 V 0.37 (PO4) 3.68 、Na 3.58 Fe 1.17 V 1.26 (PO4) 3.58 、Na 3.14 Fe 1.21 V 0.80 (PO4) 3.11 、Na 3.01 Fe 0.95 V 1.08 (PO4) 3.01 、Na 2.78 Fe 1.76 V 0.43 (PO4)2.91 and Na 2.17 Fe 0.82 V 0.43 (PO4) 2.82 Any one or more of them.

[0052] In addition, 2.00 ≤ i ≤ 3.80, preferably 2.17 ≤ i ≤ 3.58, more preferably 3.11 ≤ i ≤ 3.21. Further, i can be 2.17, 2.78, 3.00, 3.01, 3.11, 3.14, 3.15, 3.20, 3.21 or 3.58.

[0053] 0.80 ≤ j ≤ 2.00, preferably 0.82 ≤ j ≤ 1.89, more preferably 1.20 ≤ j ≤ 1.39. Further, j can be 0.82, 0.95, 1.17, 1.20, 1.21, 1.26, 1.27, 1.39, 1.76 or 1.89.

[0054] 0.30 ≤ k ≤ 1.30, preferably 0.37 ≤ k ≤ 1.26, more preferably 0.64 ≤ k ≤ 0.81. Further, k can be 0.37, 0.43, 0.64, 0.80, 0.81, 1.08 or 1.26.

[0055] 2.70 ≤ l ≤ 3.80, preferably 2.82 ≤ l ≤ 3.68, more preferably 3.11 ≤ l ≤ 3.20. Further, l can be 2.82, 2.91, 3.01, 3.11, 3.18, 3.19, 3.20, 3.58 or 3.68.

[0056] In some embodiments, the crystallinity of the Prussian blue particles is 0.80 - 0.99, preferably 0.84 - 0.98, more preferably 0.94 - 0.97.

[0057] Controlling the crystallinity of the Prussian blue particles within the above range reduces the defects of the Prussian blue particles, which helps to improve the electrochemical performance of the cathode material, such as cycle stability and rate performance.

[0058] In addition, the crystallinity of the Prussian blue particles can be any one or more of 0.84, 0.89, 0.91, 0.94, 0.96, 0.97 and 0.98.

[0059] In some embodiments, the mass fraction of the crystal water in the Prussian blue particles is 4.00% - 10.00%, preferably 4.32% - 9.98%, more preferably 6.29% - 6.78%.

[0060] Controlling the water of crystallization content of Prussian blue particles is beneficial to maintaining the structural stability of the cathode material during the charge-discharge process, and helps to improve the cycle performance and capacity performance of the cathode material, etc.

[0061] In addition, the mass fraction of water of crystallization in the Prussian blue particles can be any one or more of 4.32%, 4.52%, 5.42%, 6.29%, 6.67%, 6.68%, 6.74%, 6.78%, 8.68% and 9.98%.

[0062] Therefore, by preferably controlling the ranges of the above conditions, when the cathode material is applied to a sodium-ion battery, it is beneficial to significantly improve the energy density, cycle life and safety of the sodium-ion battery, while reducing the production cost, providing strong support for the commercial application of the sodium-ion battery.

[0063] Furthermore, in some embodiments, the mass ratio of the above Prussian blue particles, sodium iron vanadium phosphate layer and carbon layer is (15~33):(65~82):(0.5~3).

[0064] The Prussian blue particles and the sodium iron vanadium phosphate layer are the main active substances of the cathode material; among them, the relatively high theoretical capacity of the Prussian blue particles is the basis for the high energy density of the cathode material; the sodium iron vanadium phosphate layer coats the Prussian blue particles, not only contributing additional capacity to the cathode material, but also improving the sodium ion transport performance of the cathode material due to its characteristics as a sodium superionic conductor, enabling the sodium iron vanadium phosphate layer to effectively enhance the rate performance of the cathode material while maintaining the stability of the Prussian blue particles; the carbon layer not only enhances the electronic conductivity of the cathode material, but also further reduces the direct contact between the core and the sodium iron vanadium phosphate layer and the electrolyte, reducing the occurrence of side reactions and enhancing the chemical stability of the cathode material; by selecting the above optimized mass ratio of the Prussian blue particles, sodium iron vanadium phosphate layer and carbon layer, it is beneficial to promote the mutual coordination of the Prussian blue particles, sodium iron vanadium phosphate layer and carbon layer, effectively improving the rate performance and cycle stability of the cathode material while increasing the charge capacity and discharge capacity of the cathode material.

[0065] In addition, preferably, the mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer, and the carbon layer can be (15 - 30):(65 - 80):(1 - 3). Further preferably, the mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer, and the carbon layer can be (21.68 - 25.46):(73.17 - 76.45):(1.37 - 1.87). Specifically, the mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer, and the carbon layer can be any one or more of 23.11:75.26:1.63, 25.46:73.17:1.37, 21.68:76.45:1.87, 22.05:76.24:1.71, 19.47:78.17:2.36, 29.67:68.86:1.47, 23.89:74.70:1.41, 17.57:81.45:0.98, 32.51:66.81:0.68, and 24.98:73.13:1.89. Of course, the mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer, and the carbon layer can be any point value within (15 - 33):(65 - 82):(0.5 - 3), which will not be elaborated here.

[0066] In some embodiments, the D10 particle size of the above Prussian blue particles is 1.00 μm - 5.00 μm, preferably 1.57 μm - 4.79 μm, further preferably 3.00 μm - 4.00 μm, and even further preferably 3.17 μm - 3.99 μm; the D50 particle size of the Prussian blue particles is 8.00 μm - 20.00 μm, preferably 8.95 μm - 18.69 μm, further preferably 10.00 μm - 15.00 μm, and even further preferably 12.57 μm - 13.72 μm; the D90 particle size of the Prussian blue particles is 18.00 μm - 50.00 μm, preferably 18.95 μm - 49.57 μm, further preferably 20.00 μm - 30.00 μm, and even further preferably 23.59 μm - 26.93 μm.

[0067] Optimizing the distribution of the above D10 particle size, D50 particle size, and D90 particle size helps reduce the volume change of the cathode material during charge - discharge, reduce the structural stress between the cathode material particles, thereby enhancing the structural stability of the cathode material, improving the cycling performance of the cathode material, and prolonging the cycle life.

[0068] Further, the D10 particle size of the Prussian blue particles can be any one or more of 1.57 μm, 2.89 μm, 3.12 μm, 3.17 μm, 3.61 μm, 3.67 μm, 3.89 μm, 3.99 μm, and 4.79 μm.

[0069] Furthermore, the D50 particle size of the Prussian blue particles can be any one or more of 8.95μm, 11.37μm, 11.48μm, 12.57μm, 13.11μm, 13.26μm, 13.58μm, 13.72μm, 15.74μm, and 18.69μm.

[0070] Furthermore, the D90 particle size of the Prussian blue particles can be any one or more of 18.95μm, 22.89μm, 23.59μm, 25.16μm, 25.89μm, 26.78μm, 26.93μm, 27.83μm, 28.47μm, and 49.57μm.

[0071] Furthermore, the mass fraction of carbon element in the cathode material can be 0.50% - 2.50%, preferably 0.68% - 2.36%, and more preferably 1.37% - 1.87%. Specifically, it can be any one of 0.68%, 0.98%, 1.37%, 1.41%, 1.47%, 1.63%, 1.71%, 1.87%, 1.89%, and 2.36%.

[0072] Furthermore, the BET specific surface area of the cathode material can be 7.10m 2 / g - 19.00m 2 / g, preferably 7.21m 2 / g - 18.79m 2 / g, and more preferably 13.1m 2 / g - 15.12m 2 / g. Specifically, it can be any one of 7.21m 2 / g, 9.57m 2 / g, 11.5m 2 / g, 12.3m 2 / g, 13.1m 2 / g, 13.5m 2 / g, 14.3m 2 / g, 15.12m 2 / g, and 18.79m 2 / g.

[0073] Furthermore, the free moisture content of the cathode material can be 160ppm - 700ppm, preferably 178ppm - 678ppm, and more preferably 426ppm - 634ppm. Specifically, it can be any one of 178ppm, 216ppm, 411ppm, 426ppm, 428ppm, 429ppm, 469ppm, 634ppm, and 678ppm.

[0074] Furthermore, the tap density of the positive electrode material can be 1.60 g / mL to 2.50 g / mL, preferably 1.76 g / mL to 2.35 g / mL, and more preferably 2.01 g / mL to 2.09 g / mL. Specifically, it can be any one of 1.76 g / mL, 1.78 g / mL, 2.01 g / mL, 2.02 g / mL, 2.04 g / mL, 2.05 g / mL, 2.09 g / mL, and 2.35 g / mL.

[0075] Furthermore, the iron dissolution amount of the positive electrode material can be 10.0 ppm to 90.0 ppm, preferably 10.2 ppm to 89.6 ppm, and more preferably 14.3 ppm to 17.5 ppm. Specifically, it can be any one of 10.2 ppm, 10.4 ppm, 13.8 ppm, 14.3 ppm, 16.3 ppm, 16.4 ppm, 16.45 ppm, 17.1 ppm, 17.5 ppm, and 89.6 ppm.

[0076] Furthermore, the vanadium dissolution amount of the positive electrode material can be 3.0 ppm to 55 ppm, preferably 3.7 ppm to 52 ppm, and more preferably 5.1 ppm to 6.5 ppm. Specifically, it can be any one of 3.7 ppm, 4.2 ppm, 5.1 ppm, 5.2 ppm, 5.3 ppm, 5.8 ppm, 6.5 ppm, 11.5 ppm, 12.6 ppm, and 52 ppm.

[0077] Furthermore, the free sodium content of the positive electrode material can be 1000 ppm to 2800 ppm, preferably 1104 ppm to 2725 ppm, and more preferably 1104 ppm to 1325 ppm. Specifically, it can be any one of 1104 ppm, 1148 ppm, 1167 ppm, 1211 ppm, 1219 ppm, 1299 ppm, 1325 ppm, 1978 ppm, 2148 ppm, and 2725 ppm.

[0078] Furthermore, the pH value of the positive electrode material can be 9.00 to 11.00, preferably 9.21 to 10.89, and more preferably 9.32 to 9.56. Specifically, it can be any one of 9.21, 9.32, 9.34, 9.42, 9.49, 9.56, 10.59, 10.67, and 10.89.

[0079] In a second aspect, an embodiment of the present application provides a method for preparing a positive electrode material. The method for preparing the positive electrode material includes: dissolving a ferrous source, sodium ferrocyanide, a vanadium source, a phosphorus source, a sodium source, and a reducing agent in a solvent to obtain a first solution; subjecting the first solution to hydrothermal treatment to obtain a first slurry;

[0080] The first slurry is successively subjected to solid-liquid separation, washing, drying and iron removal treatment to obtain a cathode material; wherein, the pH value of the first solution is less than the pH value of the first slurry.

[0081] In this application, a hydrothermal treatment process is utilized. First, at a lower pH value, a sodium prussian blue battery material is obtained, and then at a higher pH value, sodium iron vanadium phosphate is synthesized to form a core-shell structured cathode material with the sodium prussian blue battery material as the core and sodium iron vanadium phosphate as the shell. By controlling the change of the pH value, on the one hand, prussian blue particles are formed in an environment with a lower pH value, which is beneficial to their formation at a low saturation degree, and the obtained prussian blue particles have larger sizes, more concentrated particle size distributions, higher crystallinity and fewer defects, improving the electrochemical performance of the cathode material; on the other hand, sodium iron vanadium phosphate coating the prussian blue particles is formed under higher pH value conditions, ensuring the uniformity, conductivity and stability of the formed shell structure. Coating the prussian blue particles with sodium iron vanadium phosphate as a sodium superionic conductor is beneficial to improving the sodium ion transport efficiency of the formed cathode material and further improving the electrochemical performance of the cathode material.

[0082] In some embodiments, for the preparation method of the cathode material provided in this application, the prepared cathode material is the cathode material as described above.

[0083] In some embodiments, the pH value of the above-mentioned first solution is 1.5 - 2.

[0084] By controlling the pH value of the first solution within the above range in this application, it is beneficial to promote the reaction of sodium ferrocyanide with the iron source and sodium source, and promote the formation of prussian blue particles with larger sizes, more concentrated particle size distributions, higher crystallinity and fewer defects.

[0085] The pH value of the above-mentioned first solution can be 1.5, 1.6, 1.7, 1.8, 1.9 or 2.0, or can also be any point value within the range of 1.5 - 2, which will not be elaborated here.

[0086] In some embodiments, the iron source is selected from one or more of ferrous gluconate and ferrous citrate.

[0087] Selecting the above iron source helps to improve the synergistic cooperation with other components, and the above iron source not only serves as an iron source but also as a carbon source, forming a carbon layer by carbonization during subsequent high-temperature treatment and coating on the surface of the sodium iron vanadium phosphate layer, thereby improving the conductivity of the cathode material.

[0088] In some embodiments, the vanadium source is a metavanadate source.

[0089] The addition of sodium metavanadate introduces vanadium elements into sodium vanadium phosphate, promotes the formation of the sodium iron vanadium phosphate layer, and is beneficial to improving the electrical performance of the cathode material.

[0090] In some embodiments, the phosphorus source is selected from one or more of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0091] In some embodiments, the sodium source is selected from one or more of sodium phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0092] By controlling the types of the phosphorus source and the sodium source within the above ranges, it is beneficial to form a stable and efficient sodium iron vanadium phosphate layer, and improve the sodium ion storage capacity, ionic conductivity, and rate performance of the sodium iron vanadium phosphate layer.

[0093] In some embodiments, the reducing agent is selected from one or more of ascorbic acid, sodium borohydride, and hydrazine hydrate.

[0094] Controlling the type of the reducing agent within the above range helps to reduce metal ions and promote the formation of the sodium iron vanadium phosphate layer. Among them, ascorbic acid also helps to be converted into a carbon layer at a subsequent high temperature.

[0095] In some embodiments, when the ferrous source is ferrous gluconate, the metavanadate source is sodium metavanadate, the sodium source is sodium phosphate, and the reducing agent is ascorbic acid, the molar ratio of the ferrous source, sodium ferrocyanide, vanadium source, sodium source, and reducing agent is (1.2~1.5):1:(0.2~0.4):(1~1.5):(0.1~0.3).

[0096] Controlling the molar ratio of the ferrous source, sodium ferrocyanide, vanadium source, sodium source, and reducing agent to be (1.2~1.5):1:(0.2~0.4):(1~1.5):(0.1~0.3) helps to fully form Prussian blue particles and uniformly form the subsequent sodium iron vanadium phosphate layer and carbon layer, thereby improving the rate performance, cycle performance, and performance stability of the cathode material while achieving a lower manufacturing cost; in particular, controlling the molar ratio of ferrous gluconate to sodium ferrocyanide (Na4Fe(CN)6) to be greater than 1 helps to fully react Na4Fe(CN)6, reduce the difficulty of wastewater treatment, and obtain sodium iron vanadium phosphate with the remaining unreacted iron, vanadium, phosphate ions, and sodium ions under higher pH conditions; in addition, by using sodium salts such as sodium phosphate and sodium metavanadate, Prussian blue particles are synthesized in a sodium-rich system, and the sodium content of the obtained Prussian blue particles is higher than that of the traditional process, thereby improving the capacity of the cathode material.

[0097] In some embodiments, the molar ratio of the phosphorus source to the vanadium source is (0.1~0.5):1;

[0098] Controlling the molar ratio of the phosphorus source to the vanadium source within the above range is beneficial to form a stable sodium iron vanadium phosphate layer, thereby improving the ionic conductivity and rate performance of the cathode material.

[0099] In some embodiments, the pH value of the first solution is adjusted with an acid, and the acid is selected from one or more of sulfuric acid and hydrochloric acid;

[0100] The selection of the above acids is conducive to adjusting the pH value of the first solution under the condition of a small amount of use, and promoting the optimal synthesis conditions of Prussian blue particles.

[0101] In some embodiments, the total mass fraction of each solute in the first solution is 25% - 35%.

[0102] By controlling the total mass fraction of each solute in the first solution within the above range, it is not only conducive to the progress of the reaction in the hydrothermal treatment, but also conducive to regulating the particle size distribution of the formed cathode material, and promoting the cathode material to have good tap density and electrochemical activity.

[0103] In addition, the total mass fraction of each solute in the above first solution can be 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34% or 35%, or can also be any point value within the range of 25% - 35%, which will not be elaborated here.

[0104] Furthermore, in some embodiments, the pH value of the above first slurry is 5.0 - 6.0;

[0105] Performing the first hydrothermal reaction on the first solution at a lower pH value is conducive to the formation of Prussian blue particles. Subsequently, the pH value is raised to 5.0 - 6.0 through the first pH value regulator (such as ammonia gas or sodium hydroxide solution), which helps to uniformly deposit the sodium iron vanadium phosphate layer on the Prussian blue particles, and is conducive to significantly improving the cycle stability and rate performance of the cathode material.

[0106] In addition, the pH value of the above first slurry can be 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9 or 6.0, or can also be any point value within the range of 5.0 - 6.0, which will not be elaborated here.

[0107] In some embodiments, the hydrothermal treatment includes a first hydrothermal reaction and a second hydrothermal reaction carried out in sequence.

[0108] The steps of the first hydrothermal reaction include: the first solution obtains a first mixture after the first hydrothermal reaction time at the first hydrothermal reaction temperature and the first hydrothermal reaction pressure.

[0109] The steps of the second hydrothermal reaction include: the first mixture is mixed with the first pH value regulator, and a first slurry is obtained after the second hydrothermal reaction time at the second hydrothermal reaction temperature and the second hydrothermal reaction pressure.

[0110] Among them, the second hydrothermal reaction temperature is higher than the first hydrothermal reaction temperature, and the second hydrothermal reaction pressure is higher than the first hydrothermal reaction pressure.

[0111] Controlling the temperature and pressure of the second hydrothermal reaction to be higher than those of the first hydrothermal reaction, the higher temperature and pressure are beneficial to accelerating the reaction and shortening the reaction time, which helps the rapid deposition and crystallization of sodium iron vanadium phosphate on the surface of Prussian blue particles while ensuring the uniformity and integrity of the deposition of the sodium iron vanadium phosphate layer.

[0112] In some embodiments, the first pH regulator is selected from one or more of ammonia gas and sodium hydroxide solution. Among them, when the first pH regulator is selected from sodium hydroxide solution, the concentration of the first pH regulator is 0.5 mol / L to 1 mol / L.

[0113] Using ammonia gas or sodium hydroxide solution as the first pH regulator is beneficial to precisely controlling the reaction conditions and ensuring that the pH value is maintained between 5.0 and 6.0 in the second hydrothermal reaction; controlling the concentration of the sodium hydroxide solution at 0.5 mol / L to 1 mol / L helps for a more stable and controllable change in the pH value.

[0114] In addition, when the first pH regulator is selected from sodium hydroxide solution, the molar concentration of the first pH regulator can be 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L or 1 mol / L, or can also be any point value within the range of 0.5 mol / L to 1 mol / L, which will not be elaborated here.

[0115] In some embodiments, when the first pH regulator is selected from ammonia gas, the first pH regulator is introduced into the first mixture for mixing, and the mixing time of the first mixture and the first pH regulator is 20 min to 40 min;

[0116] Controlling the introduction time of ammonia gas at 20 min to 40 min ensures a stable change in the pH value and is beneficial to the uniform formation of the sodium iron vanadium phosphate layer.

[0117] In addition, the introduction time of the above-mentioned first pH regulator is 20 min, 21 min, 22 min, 23 min, 24 min, 25 min, 26 min, 27 min, 28 min, 29 min, 30 min, 31 min, 32 min, 33 min, 34 min, 35 min, 36 min, 37 min, 38 min, 39 min or 40 min, or can also be any point value within the range of 20 min to 40 min, which will not be elaborated here.

[0118] Through the comprehensive control of the above conditions, it is beneficial to improve the purity of the cathode material, effectively reduce the crystal water in Prussian blue, increase the sodium ion content, and improve the electrochemical performance of the cathode material, such as capacity, cycle stability, rate performance, etc.

[0119] In some embodiments, the temperature of the first hydrothermal reaction is 140°C to 180°C, the pressure of the first hydrothermal reaction is 0.6 MPa to 1.0 MPa, and the time of the first hydrothermal reaction is 3 h to 6 h.

[0120] Controlling the temperature, pressure, and time of the first hydrothermal reaction within the above ranges not only helps the formation of Prussian blue and improves the stability of Prussian blue, but also the higher temperature is beneficial to reducing the crystal water in Prussian blue and improving the capacity and cycle performance of the cathode material.

[0121] In addition, the temperature of the above first hydrothermal reaction can be 140°C, 142°C, 145°C, 148°C, 150°C, 155°C, 158°C, 160°C, 165°C, 168°C, 170°C, 172°C, 175°C or 180°C, or any point value within the range of 140°C to 180°C, which will not be elaborated here.

[0122] The pressure of the above first hydrothermal reaction can be 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa or 1.0 MPa, or any point value within the range of 0.6 MPa to 1.0 MPa, which will not be elaborated here.

[0123] The time of the above first hydrothermal reaction can be 3 h, 3.2 h, 3.5 h, 4 h, 4.2 h, 4.5 h, 5 h, 5.2 h, 5.5 h or 6 h, or any point value within the range of 3 h to 6 h, which will not be elaborated here.

[0124] In some embodiments, a first stirring is performed during the first hydrothermal reaction, the rotation speed of the first stirring is 50 r / min to 150 r / min, and the time of the first stirring is 3 h to 6 h.

[0125] Controlling the stirring speed and time of the first hydrothermal reaction within the above ranges not only helps to promote the uniform mixing of reactants and the uniform formation of Prussian blue particles, but also helps to control the particle size and morphology of Prussian blue particles, thereby being beneficial to improving the electrochemical performance of the cathode material.

[0126] In addition, the rotation speed of the above first stirring can be 50 r / min, 60 r / min, 70 r / min, 80 r / min, 90 r / min, 100 r / min, 110 r / min, 120 r / min, 130 r / min, 140 r / min or 150 r / min, or can be any point value within the range of 50 r / min to 150 r / min, which will not be elaborated here.

[0127] The time of the above first stirring can be 3 h, 3.2 h, 3.5 h, 4 h, 4.2 h, 4.5 h, 5 h, 5.2 h, 5.5 h or 6 h, or can be any point value within the range of 3 h to 6 h, which will not be elaborated here.

[0128] In some embodiments, the temperature of the second hydrothermal reaction is 220°C to 250°C, the pressure of the second hydrothermal reaction is 2.0 MPa to 2.5 MPa, and the time of the second hydrothermal reaction is 14 h to 15 h.

[0129] Controlling the temperature, pressure and time of the second hydrothermal reaction within the above ranges promotes the uniform mixing of reactants, is conducive to the rapid and uniform deposition of the sodium iron vanadium phosphate layer, and controls the thickness of the sodium iron vanadium phosphate layer. Therefore, by comprehensively controlling the above conditions, it is conducive to preparing a high-performance cathode material with a core-shell structure.

[0130] In addition, the temperature of the above second hydrothermal reaction can be 220°C, 222°C, 225°C, 230°C, 235°C, 240°C, 242°C, 245°C or 250°C, or can be any point value within the range of 220°C to 250°C, which will not be elaborated here.

[0131] The pressure of the second hydrothermal reaction can be 2.0 MPa, 2.1 MPa, 2.2 MPa, 2.3 MPa, 2.4 MPa or 2.5 MPa, or can be any point value within the range of 2.0 MPa to 2.5 MPa, which will not be elaborated here.

[0132] The time of the second hydrothermal reaction can be 14 h, 14.5 h or 15 h, or can be any point value within the range of 14 h to 15 h, which will not be elaborated here.

[0133] In some embodiments, a second stirring is performed during the second hydrothermal reaction, the rotation speed of the second stirring is 100 r / min to 200 r / min, and the time of the second stirring is 3 h to 6 h.

[0134] Controlling the stirring speed and time speed of the second hydrothermal reaction within the above ranges helps to promote the uniform mixing of reactants, enables the uniform formation of the sodium iron vanadium phosphate layer, and thus improves the electrochemical performance of the cathode material.

[0135] In addition, the rotation speed of the above-mentioned second stirring can be 100 r / min, 110 r / min, 120 r / min, 130 r / min, 140 r / min, 150 r / min, 160 r / min, 170 r / min, 180 r / min, 190 r / min or 200 r / min, or can be any point value within the range of 100 r / min to 200 r / min, which will not be elaborated here.

[0136] The time of the second stirring can be 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h or 6 h, or can be any point value within the range of 3 h to 6 h, which will not be elaborated here.

[0137] Therefore, by comprehensively controlling the above conditions, a high-performance cathode material with a core-shell structure can be prepared.

[0138] In some embodiments, the process of subjecting the first slurry to solid-liquid separation and washing in sequence includes: after the first slurry is cooled, the pressure is relieved, and then it is poured out and filtered and washed. Filtration can be carried out using a filter press, and washing is stopped after the conductivity of the washing water ≤ 200 μS / cm to obtain a solid material.

[0139] In some embodiments, the free water content of the product obtained by the above drying is less than or equal to 800 ppm.

[0140] In some embodiments, the solid material is dried in a vacuum or in a nitrogen gas stream, and the temperature during drying is 105 °C to 130 °C.

[0141] On the one hand, by controlling the drying conditions to ensure that the free water content is less than or equal to 800 ppm, it is beneficial to reduce the side effects brought by free water, thereby maintaining the initial electrochemical performance of the cathode material and improving the electrochemical performance such as capacity and cycle stability when the cathode material is applied to sodium-ion batteries; on the other hand, a low free water content is beneficial to avoid the accelerated aging of the cathode material caused by free water and reduce the side reaction between the cathode material and the electrolyte, prolonging the life of the sodium-ion battery using this cathode material; at the same time, a lower free water content helps to improve the processing performance of the cathode material, such as being beneficial to increasing the compaction density, etc., and improving the energy density of the sodium-ion battery using this cathode material.

[0142] The free water content of the product obtained by the above drying can be 5 ppm, 10 ppm, 20 ppm, 50 ppm, 80 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm or 700 ppm, 750 ppm, or can be any point value less than or equal to 800 ppm, which will not be elaborated here.

[0143] In some embodiments, the process of drying, iron removal, and packaging of solid materials includes:

[0144] The solid material is dried in a vacuum or in a nitrogen gas stream until the free water content of the material is less than 800 ppm, and then the drying is stopped. After screening, a 60-mesh to 200-mesh sieve is used. Iron removal is carried out using an electromagnetic iron remover. Packaging is carried out in a constant temperature and humidity room with a humidity ≤ 10% and a temperature of 25 ± 5°C. Vacuum packaging is used to obtain the composite cathode material of Prussian blue and sodium vanadium phosphate.

[0145] In a third aspect, an embodiment of the present application provides a positive electrode plate, which includes the above-mentioned positive electrode material.

[0146] Since the positive electrode plate in the present application includes the above-mentioned positive electrode material, therefore, the positive electrode plate also has a high capacity, good cycle stability, high rate performance, good safety and stability.

[0147] In a fourth aspect, an embodiment of the present application provides a sodium-ion battery, which includes a positive electrode, and the positive electrode plate of the sodium-ion battery is the above-mentioned positive electrode plate.

[0148] Applying the above-mentioned positive electrode plate to the sodium-ion battery in the present application can comprehensively improve the performance of the sodium-ion battery, including but not limited to energy density, cycle stability, rate performance, safety and reliability, as well as the efficiency and consistency of the manufacturing process, providing a solid technical foundation for the development of high-performance and cost-effective sodium-ion batteries, and having important significance for promoting the commercialization process of sodium battery technology.

[0149] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. For those without specific techniques or conditions noted in the embodiments, the techniques or conditions described in the literature in the field or according to the product specifications are followed. For reagents or instruments without the manufacturer noted, they are all conventional products that can be obtained through commercial purchase.

[0150] Example 1

[0151] Refer to Figure 1 The preparation process flow chart of the positive electrode material shown to prepare the positive electrode material. The specific preparation process is as follows:

[0152] Step (1): Ferrous gluconate (C 12 H 22 O 14Ferrous gluconate (Fe·2H2O), sodium ferrocyanide (Na4Fe(CN)6), sodium metavanadate, sodium phosphate, phosphoric acid (the molar ratio of phosphoric acid to sodium metavanadate is 0.3:1), and ascorbic acid are added to pure water to obtain a first solution. Among them, the molar ratio of ferrous gluconate, sodium ferrocyanide (Na4Fe(CN)6), sodium metavanadate, sodium phosphate, and ascorbic acid is 1.3:1:0.3:1.3:0.2; pure water is added to make the mass concentration of the solute 30.5%, and sulfuric acid is added to make the pH value of the first solution 1.8;

[0153] Step (2): Add the above materials into a hydrothermal reactor, perform a first stirring, maintain the first stirring speed at 100 r / min, heat up to a temperature of 160 °C and a pressure of 0.7 MPa, and carry out a first hydrothermal reaction for 5 h under these conditions to obtain a first mixture; then continue to introduce ammonia for 30 min to make the pH value of the first mixture 5.5; continue to heat up to a temperature of 235 °C, perform a second stirring, the second stirring speed is 100 r / min, the time of the second stirring is 4 h, increase the pressure to 2.2 MPa, and continue to carry out a second hydrothermal reaction for 10 h under these conditions to obtain a first slurry;

[0154] Step (3): After the first slurry is cooled, relieve the pressure, then pour it out, filter it. The filtration is carried out using a filter press, wash it, and stop washing until the conductivity of the washing water ≤ 200 μS / cm to obtain a solid material;

[0155] Step (4): Dry the solid material under vacuum or in a nitrogen gas stream. The drying temperature is 115 °C. Stop drying until the free water content of the material is lower than 800 ppm. After screening, the screening is carried out using a 100-mesh sieve, and iron removal is carried out using an electromagnetic iron remover. The packaging is carried out in a constant temperature and humidity room with a humidity ≤ 10% and a temperature of 25 ± 5 °C. Vacuum packaging is used to obtain a composite cathode material of Prussian blue and sodium iron vanadium phosphate.

[0156] The final test data are as follows:

[0157] The molecular formula of Prussian blue particles is Na 2.25 Fe 1.01 [Fe(CN)6]1·1.4H2O;

[0158] The molecular formula of the sodium iron vanadium phosphate layer is Na 3.20 Fe 1.27 V 0.81 (PO4) 3.18 ;

[0159] The crystallinity of Prussian blue particles is 0.97, and the mass content of crystal water in Prussian blue particles is 6.29%.

[0160] The mass ratio of Prussian blue particles, sodium iron vanadium phosphate layer and carbon layer is 23.11:75.26:1.63.

[0161] The D10 particle size of Prussian blue particles is 3.67 μm, the D50 particle size is 12.57 μm, and the D90 particle size is 23.59 μm.

[0162] Table 1

[0163]

[0164] Example 2

[0165] The preparation process is as follows:

[0166] Step (1): Ferrous gluconate (C 12 H 22 O 14 Fe·2H2O), sodium ferrocyanide (Na4Fe(CN)6), sodium metavanadate, sodium phosphate, phosphoric acid (the molar ratio of phosphoric acid to sodium metavanadate is 0.1:1), and ascorbic acid are added to pure water to obtain a first solution, and sulfuric acid is added to make the pH value of the first solution 1.5; among them, the molar ratio of ferrous gluconate, sodium ferrocyanide (Na4Fe(CN)6), sodium metavanadate, sodium phosphate, and ascorbic acid is 1.2:1:0.2:1:0.1; pure water is added to make the mass fraction of the solute 25%;

[0167] Step (2): Add the above materials into a hydrothermal reaction kettle, heat up to a temperature of 140 °C and a pressure of 0.6 MPa, and react under these conditions for 3 h. During the hydrothermal reaction, first stirring is carried out simultaneously, and the stirring speed is maintained at 50 r / min to obtain a first mixture; then ammonia gas is continuously introduced for 20 min to make the pH value of the first mixture 6; continue to heat up to a temperature of 220 °C, carry out second stirring, the second stirring speed is 50 r / min, the second stirring time is 6 h, increase the pressure to 2.0 MPa, and continue to carry out the second hydrothermal reaction for 8 h under these conditions to obtain a first slurry;

[0168] Step (3): After the first slurry is cooled, depressurized, then poured out, filtered, and washed to obtain solid materials; among them, filtration is carried out using a filter press or a centrifuge, and washing is stopped until the conductivity of the washing water ≤ 200 μS / cm;

[0169] Step (4): Dry the solid material under vacuum or in a nitrogen gas stream, followed by screening, iron removal, and packaging to obtain the composite cathode material of Prussian blue and sodium iron vanadium phosphate; wherein, the drying temperature is 105 °C, and the drying is stopped after the free water content of the material is lower than 800 ppm. Screening is carried out using a 60-mesh sieve, iron removal is carried out using an electromagnetic iron remover, and packaging is carried out in a constant temperature and humidity room with a humidity ≤ 10% and a temperature of 25 ± 5 °C, using vacuum packaging.

[0170] The final test data are as follows:

[0171] The molecular formula of Prussian blue particles is Na 2.20 Fe 1.02 [Fe(CN)6]1·1.5H2O;

[0172] The molecular formula of the sodium iron vanadium phosphate layer is Na 3.15 Fe 1.2 V 0.8 (PO4) 3.2 ;

[0173] The crystallinity of Prussian blue particles is 0.96, and the mass content of crystal water in Prussian blue particles is 6.74%.

[0174] The mass ratio of Prussian blue particles, sodium iron vanadium phosphate layer, and carbon layer is 25.46:73.17:1.37.

[0175] The D10 particle size of Prussian blue particles is 3.99 μm, the D50 particle size is 13.26 μm, and the D90 particle size is 26.93 μm.

[0176] Table 2

[0177]

[0178] Example 3

[0179] The preparation process is as follows:

[0180] Step (1) Add ferrous gluconate (C 12 H 22 O 14 Fe·2H2O), sodium ferrocyanide (Na4Fe(CN)6), sodium metavanadate, sodium phosphate, phosphoric acid (the molar ratio of phosphoric acid to sodium metavanadate is 0.5:1), and ascorbic acid to pure water to obtain the first solution, and add sulfuric acid to make the pH value of the first solution 2; wherein, the molar ratio of ferrous gluconate, sodium ferrocyanide (Na4Fe(CN)6), sodium metavanadate, sodium phosphate, and ascorbic acid is 1.5:1:0.4:1.5:0.3; add pure water to make the mass fraction of the solute 35%;

[0181] Step (2): Add the above materials into a hydrothermal reactor, conduct the first stirring, heat up to a temperature of 180°C and a pressure of 1.0 MPa, and carry out the first hydrothermal reaction for 6 h under these conditions. During the hydrothermal reaction, maintain the first stirring speed at 150 r / min to obtain the first mixture; then continue to introduce ammonia gas for 40 min to make the pH value of the first mixture 5; continue to heat up to a temperature of 250°C, conduct the second stirring, with the second stirring speed being 150 r / min and the second stirring time being 3 h, increase the pressure to 2.5 MPa, and continue to carry out the second hydrothermal reaction for 12 h under these conditions to obtain the first slurry.

[0182] Step (3): After cooling the first slurry, relieve the pressure, then pour it out, filter, and wash to obtain the solid material; among them, filtration is carried out using a filter press or a centrifuge, and washing is stopped until the conductivity of the washing water ≤ 200 μS / cm.

[0183] Step (4): Dry the solid material under vacuum or in a nitrogen gas stream, and after screening, iron removal, and packaging, obtain the composite cathode material of Prussian blue and sodium iron vanadium phosphate; among them, the drying temperature is 130°C, and drying is stopped until the free water content of the material is lower than 800 ppm. Screening is carried out using a 200-mesh sieve, iron removal is carried out using an electromagnetic iron remover, and packaging is carried out in a constant temperature and humidity room with a humidity ≤ 10% and a temperature of 25 ± 5°C, using vacuum packaging.

[0184] The final test data are as follows:

[0185] The molecular formula of Prussian blue particles is Na 2.37 Fe 1.00 [Fe(CN)6]1·1.3H2O;

[0186] The molecular formula of the sodium iron vanadium phosphate layer is Na 3.11 Fe 1.39 V 0.64 (PO4) 3.11 ;

[0187] The crystallinity of Prussian blue particles is 0.94, and the mass content of crystal water in Prussian blue particles is 6.67%.

[0188] The mass ratio of Prussian blue particles, sodium iron vanadium phosphate layer, and carbon layer is 21.68:76.45:1.87.

[0189] The D10 particle size of Prussian blue particles is 3.89 μm, the D50 particle size is 13.58 μm, and the D90 particle size is 25.89 μm.

[0190] Table 3

[0191]

[0192] Example 4

[0193] It is different from Example 1 in that the pH value of the first solution is 2, and a cathode material is finally obtained.

[0194] The final test data are as follows:

[0195] The molecular formula of the Prussian blue particles is Na 2.20 Fe 1.05 [Fe(CN)6]1·1.6H2O;

[0196] The molecular formula of the sodium iron vanadium phosphate layer is Na 3.21 Fe 1.26 V 0.81 (PO4) 3.19 ;

[0197] The crystallinity of the Prussian blue particles is 0.96, and the mass content of crystal water in the Prussian blue particles is 6.68%.

[0198] The mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer and the carbon layer is 22.05:76.24:1.71.

[0199] The D10 particle size of the Prussian blue particles is 3.17 μm, the D50 particle size is 13.72 μm, and the D90 particle size is 26.78 μm.

[0200] Table 4

[0201]

[0202] Example 5

[0203] It is different from Example 1 in that the pH value of the first solution is 1, and a cathode material is finally obtained.

[0204] The final test data are as follows:

[0205] The molecular formula of the Prussian blue particles is Na 2.98 Fe 0.67 [Fe(CN)6]1·1.0H2O;

[0206] The molecular formula of the sodium iron vanadium phosphate layer is Na 3.00 Fe 1.89 V 0.37 (PO4) 3.68 ;

[0207] The crystallinity of the Prussian blue particles is 0.84, and the mass content of crystal water in the Prussian blue particles is 4.32%.

[0208] The mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer and the carbon layer is 19.47:78.17:2.36.

[0209] The D10 particle size of the Prussian blue particles is 3.61 μm, the D50 particle size is 11.37 μm, and the D90 particle size is 22.89 μm. Table 5

[0210]

[0211] Example 6

[0212] The difference from Example 1 is that the pH value of the first solution is 3, and the cathode material is finally obtained.

[0213] The final test data are as follows:

[0214] The molecular formula of the Prussian blue particles is Na 3.17 Fe 1.69 [Fe(CN)6]1·2.5H2O;

[0215] The molecular formula of the sodium iron vanadium phosphate layer is Na 3.58 Fe 1.17 V 1.26 (PO4) 3.58 ;

[0216] The crystallinity of the Prussian blue particles is 0.91, and the mass content of the crystal water in the Prussian blue particles is 9.98%.

[0217] The mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer and the carbon layer is 29.67:68.86:1.47.

[0218] The D10 particle size of the Prussian blue particles is 4.79 μm, the D50 particle size is 18.69 μm, and the D90 particle size is 49.57 μm. Table 6

[0219]

[0220] Example 7

[0221] The difference from Example 1 is that in step (1), the molar ratio of ferrous gluconate, sodium ferrocyanide (Na4Fe(CN)6), sodium metavanadate, sodium phosphate, and ascorbic acid is 1.2:1:0.2:1:0.1, and the cathode material is finally obtained.

[0222] The molecular formula of the Prussian blue particles is Na 2.12 Fe 1.00 [Fe(CN)6]1·1.5H2O;

[0223] The molecular formula of the sodium iron vanadium phosphate layer is Na 3.14 Fe 1.21 V 0.80 (PO4) 3.11 ;

[0224] The crystallinity of the Prussian blue particles is 0.97, and the mass content of crystal water in the Prussian blue particles is 6.78%.

[0225] The mass ratio of the Prussian blue particles, sodium iron vanadium phosphate layer and carbon layer is 23.89:74.70:1.41.

[0226] The D10 particle size of the Prussian blue particles is 3.61 μm, the D50 particle size is 13.11 μm, and the D90 particle size is 25.16 μm. Table 7

[0227]

[0228] Example 8

[0229] The difference from Example 1 is that in step (1), the molar ratio of ferrous gluconate, sodium ferrocyanide (Na4Fe(CN)6), sodium metavanadate, sodium phosphate, and ascorbic acid is 1.1:1:0.5:1:0.4, and the cathode material is finally obtained.

[0230] The final test data are as follows:

[0231] The molecular formula of the Prussian blue particles is Na 1.95 Fe 0.86 [Fe(CN)6]1·1.3H2O;

[0232] The molecular formula of the sodium iron vanadium phosphate layer is Na 3.01 Fe 0.95 V 1.08 (PO4) 3.01 ;

[0233] The crystallinity of the Prussian blue particles is 0.91, and the mass content of crystal water in the Prussian blue particles is 5.42%.

[0234] The mass ratio of the Prussian blue particles, sodium iron vanadium phosphate layer and carbon layer is 17.57:81.45:0.98.

[0235] The D10 particle size of the Prussian blue particles is 3.12 μm, the D50 particle size is 11.48 μm, and the D90 particle size is 27.83 μm. Table 8

[0236]

[0237] Example 9

[0238] The difference from Example 1 is that the temperature of the first hydrothermal reaction is 130 °C, and the temperature of the second hydrothermal reaction is 200 °C, and the cathode material is finally obtained.

[0239] The final test data are as follows:

[0240] The molecular formula of the Prussian blue particles is Na 1.87 Fe 1.18 [Fe(CN)6] 0.89 ·1.9H2O;

[0241] The molecular formula of the sodium iron vanadium phosphate layer is Na 2.78 Fe 1.76 V 0.43 (PO4) 2.91 ;

[0242] The crystallinity of the Prussian blue particles is 0.89, and the mass content of crystal water in the Prussian blue particles is 8.68%.

[0243] The mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer and the carbon layer is 32.51:66.81:0.68.

[0244] The D10 particle size of the Prussian blue particles is 1.57 μm, the D50 particle size is 8.95 μm, and the D90 particle size is 18.95 μm. Table 9

[0245]

[0246] Example 10

[0247] The difference from Example 1 is that the temperature of the first hydrothermal reaction is 235 °C and the temperature of the second hydrothermal reaction is 160 °C, and the cathode material is finally obtained.

[0248] The final test data are as follows:

[0249] The molecular formula of the Prussian blue particles is Na 2.68 Fe 1.18 [Fe(CN)6]1·1.1H2O;

[0250] The molecular formula of the sodium iron vanadium phosphate layer is Na 2.17 Fe 0.82 V 0.43 (PO4) 2.82 ;

[0251] The crystallinity of the Prussian blue particles is 0.98, and the mass content of crystal water in the Prussian blue particles is 4.52%.

[0252] The mass ratio of the Prussian blue particles, the sodium iron vanadium phosphate layer and the carbon layer is 24.98:73.13:1.89.

[0253] The D10 particle size of the Prussian blue particles is 2.89 μm, the D50 particle size is 15.74 μm, and the D90 particle size is 28.47 μm. Table 10

[0254]

[0255] Comparative Example 1

[0256] The difference from Example 1 is that ferrous gluconate is not added in step (1), and finally a cathode material is obtained, in which there is no phase of Prussian blue.

[0257] Comparative Example 2

[0258] The difference from Example 1 is that sodium metavanadate is not added in step (1), and finally a cathode material is obtained, in which there is no phase of sodium iron vanadium phosphate.

[0259] I. Test Methods

[0260] 1. Physical and Chemical Property Tests of Cathode Materials

[0261] Test of iron dissolution and vanadium dissolution: Add 1 g of the sample to be tested into 100 mL of hydrofluoric acid-ethanol solution with a concentration of 0.1 mol / L, stir and dissolve at a temperature of 45 °C for 30 min, then filter, and measure the iron element content and vanadium element content in the obtained filtrate, which are the iron dissolution amount and vanadium dissolution amount respectively.

[0262] Content of free sodium: Test by potentiometric titration method.

[0263] Content of free water: Test by Karl Fischer (KF) method.

[0264] Content of elemental carbon: Test by carbon and sulfur analyzer.

[0265] Apparent density: Test by apparent density tester, the test pressure is 3 T, and the pressing time is 30 S.

[0266] pH value: Test with reference to General Rules for the Determination of pH Value of Chemical Reagents in GB / T 9724.

[0267] Contents of Na, Fe, V, and P: Test by inductively coupled plasma emission spectrometer;

[0268] N content: Test by nitrogen and oxygen analyzer.

[0269] Particle size (D10 particle size, D50 particle size, D90 particle size): Test by laser particle size analyzer using laser diffraction method for particle size analysis.

[0270] Specific surface area (BET): Test by BET tester using nitrogen adsorption method.

[0271] 2. Property Tests of Secondary Batteries

[0272] The cathode materials obtained in the examples and comparative examples were respectively mixed with polyvinylidene fluoride and conductive carbon black at a mass ratio of 85:7:8, coated on aluminum foil, and then electrode sheets were prepared with a compaction density of 2.2 g / mL. Sodium sheets were used as the anode, and 1 mol / L sodium perchlorate solution was used as the electrolyte. They were assembled into coin cells and measured in an environment of 25 ± 0.1 °C.

[0273] Using the constant current charge-discharge mode, charge-discharge tests were carried out at current densities of 0.1C, 1C, and 2C respectively. The charge cut-off voltage was 3.8V, and the discharge cut-off voltage was 2.0V. Among them, the results of the first charge capacity at 0.1C, the first discharge capacity at 0.1C, the first charge capacity at 2C, the first discharge capacity at 2C, and the capacity retention rate after 1000 cycles at 1C are shown in Table 11.

[0274] II. Analysis of the test results of each example and comparative example

[0275] Table 11

[0276]

[0277] As can be seen from the above table, the cathode materials obtained in the examples of the present application, compared with the cathode materials obtained in the comparative examples, have significantly improved charge specific capacity, discharge specific capacity, as well as more excellent rate performance and cycling performance when applied to sodium-ion batteries.

[0278] When the cathode materials of Example 1 and Example 4 were obtained when the pH value of the first solution was in the range of 1.5 - 2, and the cathode materials of Example 5 and Example 6 were obtained when the pH value of the first solution was outside the range of 1.5 - 2. When the cathode materials of Example 5 and Example 6 were applied to sodium-ion batteries, the sodium-ion batteries were inferior to those using the cathode materials of Example 1 and Example 4 in terms of charge specific capacity, discharge specific capacity, rate performance, and cycling performance.

[0279] From the comparison of the data when the cathode materials of Example 1 and Example 7 were applied to sodium-ion batteries with the data when Example 8 was applied to sodium-ion batteries, when the molar ratio of ferrous source, sodium ferrocyanide (Na4Fe(CN)6), vanadium source, sodium source, and reducing agent was in the range of (1.2 - 1.5):1:(0.2 - 0.4):(1 - 1.5):(0.1 - 0.3), the charge specific capacity, discharge specific capacity, rate performance, and cycling performance were significantly improved.

[0280] From the data of the cathode materials in Examples 1 to 3 when applied to sodium-ion batteries, compared with the data of Examples 9 and 10 when applied to sodium-ion batteries, it can be seen that when the reaction temperature of the first hydrothermal reaction is controlled at 140°C to 180°C and the temperature of the second hydrothermal reaction is controlled at 220°C to 250°C, it is beneficial to improve the electrochemical performance such as the charge specific capacity, discharge specific capacity, rate performance, and cycle performance.

[0281] It should be noted that this application is not limited to the above embodiments. The above embodiments are only examples, and embodiments with the same structure and the same effect as the technical idea within the technical scope of this application are included in the technical scope of this application. In addition, within the scope not departing from the gist of this application, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways constructed by combining some constituent elements of the embodiments are also included in the scope of this application.

Claims

1. A positive electrode material, characterized in that: The positive electrode material has a core-shell structure, the core of the core-shell structure is a Prussian blue particle, and the shell layer of the core-shell structure includes a sodium ferrous vanadium phosphate layer and a carbon layer, wherein the sodium ferrous vanadium phosphate layer is coated on the Prussian blue particle, and the carbon layer is coated on the sodium ferrous vanadium phosphate layer.

2. The positive electrode material according to claim 1, characterized in that The molecular formula of the Prussian blue particles is Na x Fe y [Fe(CN)6] z nH2O, where 1.80≤x≤3.20, 0.60≤y≤1.70, 0.80≤z≤1, 1.0≤n≤2.8; The molecular formula of the sodium ferrovanadium phosphate layer is Na i Fe j V k (PO4) l , where 2.00≤i≤3.80, 0.80≤j≤2.00, 0.30≤k≤1.30, 2.70≤l≤3.80; And / or, the crystallinity of the Prussian blue particles is 0.80-0.99, and the mass fraction of crystal water in the Prussian blue particles is 4.00%-10.00%.

3. The positive electrode material according to claim 1 or 2, characterized in that The mass ratio of the Prussian blue particles, the sodium vanadium iron phosphate layer and the carbon layer is (15-33): (65-82): (0.5-3).

4. The positive electrode material according to claim 1 or 2, characterized in that The D10 particle size of the Prussian blue particles is 1.00 μm to 5.00 μm, the D50 particle size is 8.00 μm to 20.00 μm, and the D90 particle size is 18.00 μm to 50.00 μm.

5. A method for preparing a positive electrode material, characterized in that: The preparation method of the positive electrode material comprises: A ferrous source, sodium ferrocyanide, a vanadium source, a phosphorus source, a sodium source, and a reducing agent are dissolved in a solvent to obtain a first solution; The first solution is subjected to hydrothermal treatment to obtain a first slurry; The first slurry is sequentially subjected to solid-liquid separation, washing, drying and iron removal treatment to obtain the positive electrode material; Wherein, the pH value of the first solution is lower than the pH value of the first slurry.

6. The preparation method according to claim 5, characterized in that: The pH value of the first solution is 1.5-2; And / or, the ferrous source is selected from one or more of ferrous gluconate and ferrous citrate; And / or, the vanadium source is a metavanadic acid source; And / or, the phosphorus source is selected from one or more of phosphoric acid, ammonium phosphate, diammonium phosphate, and monoammonium phosphate; And / or, the sodium source is selected from one or more of sodium phosphate, sodium dihydrogen phosphate, and sodium monohydrogen phosphate; And / or, the reducing agent is selected from one or more of ascorbic acid, sodium borohydride, and hydrazine hydrate; And / or, when the ferrous source is ferrous gluconate, the metavanadic acid source is sodium metavanadate, the sodium source is sodium phosphate, and the reducing agent is ascorbic acid, the molar ratio of the ferrous source, the sodium ferrocyanide, the vanadium source, the sodium source and the reducing agent is (1.2-1.5):1:(0.2-0.4):(1-1.5):(0.1-0.3); And / or, the molar ratio of the phosphorus source to the vanadium source is (0.1-0.5):1; and / or, adjusting the pH value of the first solution with an acid, wherein the acid is selected from one or more of sulfuric acid and hydrochloric acid; And / or, the total mass fraction of each solute in the first solution is 25%~35%.

7. The preparation method according to claim 5, characterized in that: The pH value of the first slurry is 5.0-6.0; And / or, the hydrothermal treatment includes a first hydrothermal reaction and a second hydrothermal reaction performed sequentially, and the first solution is subjected to a first hydrothermal reaction temperature and a first hydrothermal reaction pressure for a first hydrothermal reaction time to obtain a first mixture; The first mixture is mixed with a first pH adjuster, and subjected to a second hydrothermal reaction at a second hydrothermal reaction temperature and a second hydrothermal reaction pressure for a second hydrothermal reaction time to obtain a first slurry; The second hydrothermal reaction temperature is higher than the first hydrothermal reaction temperature, and the second hydrothermal reaction pressure is higher than the first hydrothermal reaction pressure; And / or, the first pH adjuster is selected from one or more of ammonia gas and sodium hydroxide solution, wherein when the first pH adjuster is selected from sodium hydroxide solution, the concentration of the first pH adjuster is 0.5 mol / L~1 mol / L; And / or, when the first pH adjuster is selected from ammonia, the first pH adjuster is introduced into the first mixture for mixing, and the mixing time of the first mixture and the first pH adjuster is 20 min to 40 min; and / or, the temperature of the first hydrothermal reaction is 140° C. to 180° C., the pressure of the first hydrothermal reaction is 0.6 MPa to 1.0 MPa, and the time of the first hydrothermal reaction is 3 h to 6 h; And / or, the temperature of the second hydrothermal reaction is 220° C. to 250° C., the pressure of the second hydrothermal reaction is 2.0 MPa to 2.5 MPa, and the time of the second hydrothermal reaction is 14 h to 15 h.

8. The preparation method according to claim 5, characterized in that: The free water content of the dried product is less than or equal to 800 ppm.

9. A positive electrode sheet, characterized in that: The positive electrode sheet comprises the positive electrode material according to any one of claims 1 to 4.

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

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