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

Abstract

The present application provides a positive electrode material and a preparation method thereof, a positive electrode plate and a sodium ion battery. It belongs to the field of sodium ion battery technology, wherein the positive electrode material has a core-shell structure, the core of the core-shell structure is a Prussian blue particle, and the shell 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. The positive electrode material of the present application adopts a core-shell structure design of Prussian blue particles, sodium ferrous vanadium phosphate layer and carbon layer from the inside to the outside, and adopts the synergistic effect of low-cost Prussian blue particles and carbon layer with high-cost sodium ferrous vanadium phosphate, while having low cost, and having excellent rate performance, cycle performance and safety.

Description

Technical Field

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

[0002] Sodium resources are abundant, and sodium-ion batteries have similar working principles to lithium-ion batteries. Based on the above characteristics, 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 technology.

[0003] Prussian blue and its analogs have high theoretical capacities, but due to the presence of crystalline water, the release of water during the charge-discharge process affects the structural stability, resulting in a decay of the cycle performance and poor rate performance.

[0004] Therefore, it is urgent to develop a cathode material that has both excellent rate performance and cycle performance and has low cost. Summary of the Invention

[0005] In view of the technical problems existing in the background technology, the present application provides a positive electrode material and its preparation method, a positive electrode plate and a sodium ion battery, aiming to solve the technical problem that the positive electrode material for sodium ion batteries is difficult to have both excellent rate performance and cycle performance while having low cost.

[0006] In the first aspect, an embodiment of the present application provides a positive electrode material having a core-shell structure, wherein 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.

[0007] The positive electrode material of the present application adopts a core-shell structure design of Prussian blue particles, sodium vanadium iron phosphate layer and carbon layer from the inside to the outside, and the sodium vanadium iron phosphate layer and carbon layer are wrapped in sequence on the surface of Prussian blue, so that the positive electrode material effectively transmits sodium ions while greatly improving the rate performance. Specifically, Prussian blue particles serve as cores, and sodium vanadium iron phosphate layer and carbon layer serve as shells, which avoid the oxidation of ferrous ions in Prussian blue particles, improve the stability of the structure of Prussian blue, and reduce the risk of structural destruction of the positive electrode material during the charge-discharge cycle. At the same time, the carbon layer improves the overall conductivity of the positive electrode material, effectively improves the electron transfer efficiency, significantly improves the rate performance of the positive electrode material, and reduces the risk of thermal runaway of the positive electrode material during the charge-discharge process, thereby improving the safety of the positive electrode material. In addition, the present application adopts low-cost Prussian blue particles, carbon layer and high-cost sodium vanadium iron phosphate to synergize with each other, while having low cost, 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 vanadium iron phosphate layer is Na i Fe j V k (PO4) l , wherein, 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 crystalline water in the Prussian blue particles is 4.00%~10.00%.

[0009] In the present application, by using the Prussian blue particles of 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; wherein, a high content of sodium ions helps to improve the sodium ion storage capacity of the positive electrode material, and improves 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 vanadium iron phosphate of the above molecular formula as the sodium vanadium iron phosphate layer of the positive electrode material is beneficial to maintain the structural stability, ion conductivity and chemical stability of the sodium vanadium iron phosphate layer Stability, optimize the electrochemical properties of sodium vanadium ferrophosphate, especially improve the capacity retention ability of the positive electrode material at high rate, reduce the volume change of the positive electrode material during the charge-discharge process, and enhance the overall structural stability and cycle performance of the positive electrode material; control the crystallinity of the Prussian blue particles within the above range, reduce the defects of the Prussian blue particles, and help improve the electrochemical properties of the positive electrode material, such as cycle stability and rate performance; control the crystalline water content of the Prussian blue particles is conducive to maintaining the structural stability of the positive electrode material during the charge-discharge process, and help improve the cycle performance and capacity performance of the positive electrode material.

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

[0011] Prussian blue particles and sodium ferrous vanadium phosphate layer are the main active substances of the positive electrode material; among them, the higher theoretical capacity of Prussian blue particles is the basis for the high energy density of the positive electrode material; the sodium ferrous vanadium phosphate layer coats the Prussian blue particles, which not only contributes additional capacity to the positive electrode material, but also improves the sodium ion transport performance of the positive electrode material due to its sodium superion conductor properties, so that the sodium ferrous vanadium phosphate layer effectively improves the rate performance of the positive electrode material while maintaining the stability of the Prussian blue particles; the carbon layer not only enhances the electronic conductivity of the positive electrode material, but also further reduces the direct contact between the core and the sodium ferrous vanadium phosphate layer and the electrolyte, reduces the occurrence of side reactions, and enhances the chemical stability of the positive electrode material; by selecting the above-mentioned optimized mass ratio of Prussian blue particles, sodium ferrous vanadium phosphate layer and carbon layer, it is beneficial to promote the mutual coordination of Prussian blue particles, sodium ferrous vanadium phosphate layer and carbon layer, while improving the charging capacity and discharge capacity of the positive electrode material, effectively improving the rate performance and cycle stability of the positive electrode material.

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

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

[0014] In a second aspect, an embodiment of the present application provides a method for preparing a positive electrode material, which 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; and subjecting the first slurry to solid-liquid separation, washing, drying, and iron removal treatment in sequence to obtain a positive electrode material; wherein the pH value of the first solution is less than the pH value of the first slurry.

[0015] The present application utilizes a hydrothermal treatment process to first obtain a Prussian blue sodium battery material at a relatively low pH value, and then synthesize sodium vanadium ferrophosphate at a relatively high pH value, thereby forming a core-shell structure positive electrode material with the Prussian blue sodium battery material as the core and the sodium vanadium ferrophosphate as the shell. By controlling the change in pH value, on the one hand, the Prussian blue particles are formed in a relatively low pH environment, which is conducive to their formation at low saturation. The resulting Prussian blue particles are larger in size and have a more concentrated particle size distribution, higher crystallinity and fewer defects, thereby improving the electrochemical performance of the positive electrode material. On the other hand, under relatively high pH conditions, sodium vanadium ferrophosphate coated on the Prussian blue particles is formed to ensure the uniformity, conductivity and stability of the formed shell structure. The sodium vanadium ferrophosphate coating of the Prussian blue particles, which is a sodium superion conductor, is conducive to improving the sodium ion transmission efficiency of the formed positive electrode material, thereby further improving the electrochemical performance of the positive electrode 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 metavanadic acid source; and / or, the phosphorus source is selected from one or more of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, and ammonium monohydrogen 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, the ferrous source is ferrous gluconate, and the metavanadic acid source is metavanadic acid. When 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); 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 by 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] The present application is conducive to promoting the reaction of sodium ferrocyanide with the ferrous source and the sodium source by controlling the pH value of the first solution within the above range, and promoting the formation of Prussian blue particles with larger size, more concentrated particle size distribution, higher crystallinity and fewer defects; the selected ferrous source helps to improve its synergistic coordination with other components, and the above ferrous source serves not only as an iron source but also as a carbon source, which is carbonized to form a carbon layer in the subsequent high-temperature treatment and coated on the surface of the sodium vanadium iron phosphate layer, thereby improving the conductivity of the positive electrode material. The addition of sodium metavanadate introduces vanadium elements into the sodium vanadium phosphate, promotes the formation of the sodium vanadium iron phosphate layer, and helps to improve the electrical properties of the positive electrode material. By controlling the types of phosphorus source and sodium source within the above range, it is conducive to forming a stable and efficient sodium vanadium iron phosphate layer, improving the sodium ion storage capacity, ionic conductivity and rate performance of the sodium vanadium iron phosphate layer. Controlling the type of reducing agent within the above range helps to reduce metal ions and promote the formation of the sodium vanadium iron phosphate layer, wherein ascorbic acid also helps to convert into a carbon layer at a subsequent high temperature. When the reducing agent is ascorbic acid, controlling the molar ratio of the ferrous source, sodium ferrocyanide, vanadium source, sodium source, and reducing agent to (1.2-1.5):1:(0.2-0.4):(1-1.5):(0.1-0.3) facilitates the full formation of Prussian blue particles and the subsequent uniform formation of the sodium ferrovanadium phosphate layer and carbon layer, thereby improving the rate performance, cycle performance, and performance stability of the positive electrode material while achieving a lower manufacturing cost. Controlling the molar ratio of the phosphorus source to the vanadium source within the above range facilitates the formation of a stable sodium ferrovanadium phosphate layer, thereby improving the ionic conductivity and rate performance of the positive electrode material. The selection of the above acid facilitates adjusting the pH of the first solution under low-volume conditions, promoting optimal synthesis conditions for the Prussian blue particles. Controlling the total mass fraction of the solutes in the first solution within the above range not only facilitates the reaction during the hydrothermal treatment but also helps regulate the particle size distribution of the formed positive electrode material, resulting in a positive electrode material with good compaction density and electrochemical activity.

[0018] In some embodiments, 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 in sequence, and the first solution is subjected to a first hydrothermal reaction temperature, a first hydrothermal reaction pressure, and a first hydrothermal reaction time to obtain a first mixture; the first mixture is mixed with a first pH regulator, and the mixture is subjected to a second hydrothermal reaction temperature, a second hydrothermal reaction pressure, and 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 regulator is selected from one or more of ammonia and sodium hydroxide solution, wherein when the first pH regulator is selected from sodium hydroxide solution, the concentration of the first pH regulator is 0.5mol / L~1mol / L; and / or, when the first pH regulator is selected from ammonia, the first pH regulator is The agent is passed into the first mixture and mixed, and the mixing time of the first mixture and the first pH adjuster is 20 min~40 min; and / or, the temperature of the first hydrothermal reaction is 140°C~180°C, the pressure of the first hydrothermal reaction is 0.6 MPa~1.0 MPa, and the time of the first hydrothermal reaction is 3 h~6 h; and / or, a first stirring is performed during the first hydrothermal reaction, the speed of the first stirring is 50 r / min~150 r / min, and the time of the first stirring is 3 h~6 h; and / or, the temperature of the second hydrothermal reaction is 220°C~250°C, the pressure of the second hydrothermal reaction is 2.0 MPa~2.5 MPa, and the time of the second hydrothermal reaction is 14 h~15 h; and / or, a second stirring is performed during the second hydrothermal reaction, the speed of the second stirring is 100 r / min~200 r / min, and the time of the second stirring is 3 h~6 h.

[0019] The first solution is subjected to a first hydrothermal reaction at a lower pH value, which is conducive to the formation of Prussian blue particles. Subsequently, the pH value is increased to 5.0-6.0 by a first pH adjuster (such as ammonia or sodium hydroxide solution), which helps to uniformly deposit a sodium ferrous vanadium phosphate layer on the Prussian blue particles, and is conducive to significantly improving the cycle stability and rate performance of the positive electrode material; the temperature and pressure of the second hydrothermal reaction are controlled to be higher than those of the first hydrothermal reaction. Higher temperature and pressure are conducive to accelerating the reaction and shortening the reaction time, which helps to quickly deposit and crystallize sodium ferrous vanadium phosphate on the surface of Prussian blue particles while ensuring the uniformity and integrity of the deposition of the sodium ferrous vanadium phosphate layer.

[0020] Using ammonia or sodium hydroxide solution as the first pH regulator is beneficial for precisely controlling the reaction conditions and ensuring that the pH value is maintained between 5.0 and 6.0 during the second hydrothermal reaction. Controlling the concentration of the sodium hydroxide solution between 0.5 mol / L and 1 mol / L contributes to more stable and controllable pH changes. The introduction time of ammonia is controlled between 20 minutes and 40 minutes, ensuring a smooth change in pH value and facilitating the uniform formation of the sodium vanadium ferrophosphate layer.

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

[0022] Controlling the temperature, pressure and time of the first hydrothermal reaction within the above ranges not only helps to form Prussian blue and improve the stability of Prussian blue, but also the higher temperature is beneficial to reduce the crystallization water in Prussian blue, thereby improving the capacity and cycle performance of the positive electrode material; controlling the temperature, pressure and time of the second hydrothermal reaction within the above ranges promotes uniform mixing of the reactants, is beneficial to the rapid and uniform deposition of the sodium vanadium ferrous phosphate layer, and controls the thickness of the sodium vanadium ferrous phosphate layer. Therefore, by comprehensively controlling the above conditions, it is beneficial to prepare a high-performance positive electrode material with a core-shell structure.

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

[0024] On the one hand, by controlling the drying conditions and ensuring that the free water content is less than or equal to 800 ppm, it is beneficial to reduce the side effects caused by free water, thereby maintaining the initial electrochemical properties of the positive electrode material and improving the electrochemical properties such as capacity and cycle stability of the positive electrode material when used in sodium ion batteries; on the other hand, the low free water content is beneficial to avoid the accelerated aging of the positive electrode material caused by free water and reduce the side reactions between the positive electrode material and the electrolyte, thereby extending the life of the sodium ion battery using the positive electrode material; at the same time, the lower free water content helps to improve the processing performance of the positive electrode material, such as increasing the compaction density, etc., thereby improving the energy density of the sodium ion battery using the positive electrode material.

[0025] In a third aspect, an embodiment of the present application provides a positive electrode plate, which includes the above-mentioned positive electrode material.

[0026] Since the positive electrode sheet in the present application includes the above-mentioned positive electrode material, the positive electrode sheet also has higher capacity, better cycle stability, higher rate performance, better safety and stability.

[0027] In a fourth aspect, an embodiment of the present application provides a sodium ion battery, comprising a positive electrode, wherein the positive electrode plate of the sodium ion battery is the above-mentioned positive electrode plate.

[0028] Applying the above-mentioned positive electrode plate to the sodium-ion battery in this 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 cost-effective and high-performance sodium-ion batteries, and is of great significance for promoting the commercialization of sodium battery technology.

[0029] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] To more clearly illustrate the technical solution of this application, the following is a brief introduction to the drawings used in this application. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be derived from these drawings without inventive effort.

[0031] Figure 1 This is a flow chart of the preparation process of the positive electrode material in Example 1 of the present application. DETAILED DESCRIPTION

[0032] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

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

[0034] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0035] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0036] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: 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 are in an "or" relationship.

[0037] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0038] In the description of the embodiments of the present application, technical terms such as "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", and "circumferential" indicate orientations or positional relationships, which are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the embodiments of the present application.

[0039] In the description of the embodiments of the present 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 the present application, unless otherwise specified, ppm means the mass of a test result accounts for parts per million of the mass of the sample.

[0041] Some specific examples are listed below. It should be noted that the examples described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0042] In a first aspect, an embodiment of the present application provides a positive electrode material having a core-shell structure, wherein 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.

[0043] The positive electrode material of the present application adopts a core-shell structure design of Prussian blue particles, sodium vanadium iron phosphate layer and carbon layer from the inside to the outside, and wraps the sodium vanadium iron phosphate layer and carbon layer in sequence on the surface of Prussian blue, so that the positive electrode material effectively transmits sodium ions while greatly improving the rate performance. Specifically, Prussian blue particles serve as cores, and sodium vanadium iron phosphate layer and carbon layer serve as shells, which avoids the oxidation of ferrous ions in Prussian blue particles, improves the structural stability of the Prussian blue core, and reduces the risk of structural destruction of the positive electrode material during the charge-discharge cycle. At the same time, the carbon layer improves the overall conductivity of the positive electrode material, effectively improves the electron transfer efficiency, significantly improves the rate performance of the positive electrode material, and reduces the risk of thermal runaway of the positive electrode material during the charge-discharge process, thereby improving the safety of the positive electrode material. In addition, the present application adopts low-cost Prussian blue particles, carbon layer and high-cost sodium vanadium iron phosphate to synergize with each other, while having low cost, excellent rate performance, cycle performance and safety.

[0044] Furthermore, 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 vanadium iron phosphate layer is Na i Fe j V k (PO4) l , among which, 2.00≤i≤3.80, 0.80≤j≤2.00, 0.30≤k≤1.30, 2.70≤l≤3.80.

[0045] In the present application, by using the Prussian blue particles of 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; wherein, a high content of sodium ions helps to improve the sodium ion storage capacity of the positive electrode material, and improves 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 vanadium iron phosphate of the above molecular formula as the sodium vanadium iron phosphate layer of the positive electrode material is beneficial to maintain the structural stability, ion conductivity and chemical stability of the sodium vanadium iron phosphate layer Stability, optimize the electrochemical properties of sodium vanadium ferrophosphate, especially improve the capacity retention ability of the positive electrode material at high rate, reduce the volume change of the positive electrode material during the charge-discharge process, and enhance the overall structural stability and cycle performance of the positive electrode material; control the crystallinity of the Prussian blue particles within the above range, reduce the defects of the Prussian blue particles, and help improve the electrochemical properties of the positive electrode material, such as cycle stability and rate performance; control the crystalline water content of the Prussian blue particles is conducive to maintaining the structural stability of the positive electrode material during the charge-discharge process, and help improve the cycle performance and capacity performance of the positive electrode material.

[0046] In addition, the molecular formula of 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 Any one or more of [Fe(CN)6]1·1.1H2O.

[0047] Furthermore, 1.80≤x≤3.20, preferably 1.87≤x≤3.17, and more preferably 2.12≤x≤2.37. Furthermore, 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. Furthermore, y can be 0.67, 0.86, 1.00, 1.01, 1.02, 1.05, 1.18 or 1.69.

[0049] Preferably, z is 0.80≤z≤1, preferably 0.89≤z≤1, and more preferably z = 1. Furthermore, z may be 0.89 or 1.

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

[0051] Furthermore, the molecular formula of the sodium vanadium iron 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 .

[0052] Furthermore, 2.00≤i≤3.80, preferably 2.17≤i≤3.58, and more preferably 3.11≤i≤3.21. Furthermore, 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. Furthermore, 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. Furthermore, 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. Furthermore, 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, and 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 and helps to improve the electrochemical properties of the positive electrode material, such as cycle stability and rate performance.

[0058] Furthermore, the crystallinity of the Prussian blue particles may 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 crystalline water in the Prussian blue particles is 4.00% to 10.00%, preferably 4.32% to 9.98%, and further preferably 6.29% to 6.78%.

[0060] Controlling the crystalline water content of 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.

[0061] In addition, the mass fraction of crystalline water 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 optimally controlling the range of the above conditions, when the positive electrode material is applied to sodium ion batteries, it is beneficial to significantly improve the energy density, cycle life and safety of sodium ion batteries, while reducing production costs, providing strong support for the commercial application of sodium ion batteries.

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

[0064] Prussian blue particles and sodium ferrous vanadium phosphate layer are the main active substances of the positive electrode material; among them, the higher theoretical capacity of Prussian blue particles is the basis for the high energy density of the positive electrode material; the sodium ferrous vanadium phosphate layer coats the Prussian blue particles, which not only contributes additional capacity to the positive electrode material, but also improves the sodium ion transport performance of the positive electrode material due to its sodium superion conductor properties, so that the sodium ferrous vanadium phosphate layer effectively improves the rate performance of the positive electrode material while maintaining the stability of the Prussian blue particles; the carbon layer not only enhances the electronic conductivity of the positive electrode material, but also further reduces the direct contact between the core and the sodium ferrous vanadium phosphate layer and the electrolyte, reduces the occurrence of side reactions, and enhances the chemical stability of the positive electrode material; by selecting the above-mentioned optimized mass ratio of Prussian blue particles, sodium ferrous vanadium phosphate layer and carbon layer, it is beneficial to promote the mutual coordination of Prussian blue particles, sodium ferrous vanadium phosphate layer and carbon layer, while improving the charging capacity and discharge capacity of the positive electrode material, effectively improving the rate performance and cycle stability of the positive electrode material.

[0065] In addition, the mass ratio of the Prussian blue particles, the sodium vanadium iron phosphate layer, and the carbon layer is preferably (15-30): (65-80): (1-3). Furthermore, the mass ratio of the Prussian blue particles, the sodium vanadium iron phosphate layer, and the carbon layer is preferably (21.68-25.46): (73.17-76.45): (1.37-1.87). Specifically, the mass ratio of the Prussian blue particles, the sodium vanadium ferrophosphate 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 vanadium iron phosphate layer, and the carbon layer can be any point value in the range of (15-33): (65-82): (0.5-3), which will not be elaborated here.

[0066] In some embodiments, the D10 particle size of the Prussian blue particles is 1.00 μm to 5.00 μm, preferably 1.57 μm to 4.79 μm, further preferably 3.00 μm to 4.00 μm, further preferably 3.17 μm to 3.99 μm; the 50 particle size of the Prussian blue particles is 8.00 μm to 20.00 μm, preferably 8.95 μm to 18.69 μm, further preferably 3.00 μm to 4.00 μm, further preferably 3.17 μm to 3.99 μm. In one step, it is preferably 10.00μm~15.00μm, and further, it is preferably 12.57μm~13.72μm; the D90 particle size of the Prussian blue particles is 18.00μm~50.00μm, preferably 18.95μm~49.57μm, further, it is preferably 20.00μm~30.00μm, and further, it is preferably 23.59μm~26.93μm.

[0067] Optimizing the distribution of the above D10, D50, and D90 particle sizes helps reduce the volume change of the positive electrode material during the charge-discharge process and reduces the structural stress between the positive electrode material particles, thereby enhancing the structural stability of the positive electrode material, improving the cycle performance of the positive electrode material, and extending the cycle life.

[0068] Furthermore, the D10 particle size of the Prussian blue particles may 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 may be any one or more of 8.95 μm, 11.37 μm, 11.48 μm, 12.57 μm, 13.11 μm, 13.26 μm, 13.58 μm, 13.72 μm, 15.74 μm and 18.69 μm.

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

[0071] Furthermore, the carbon content of the positive electrode material may be 0.50% to 2.50%, preferably 0.68% to 2.36%, and more preferably 1.37% to 1.87%. Specifically, it may 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 positive electrode material can be 7.10 m 2 / g~19.00m 2 / g, preferably 7.21m 2 / g~18.79m 2 / g, more preferably 13.1m 2 / g~15.12m 2 / g. Specifically, it can be 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 positive electrode material may be 160 ppm to 700 ppm, preferably 178 ppm to 678 ppm, and more preferably 426 ppm to 634 ppm. Specifically, it may be any one of 178 ppm, 216 ppm, 411 ppm, 426 ppm, 428 ppm, 429 ppm, 469 ppm, 634 ppm, and 678 ppm.

[0074] Furthermore, the compacted density of the positive electrode material may 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 may 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 may 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 may 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 may 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 may 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 may be 1000 ppm to 2800 ppm, preferably 1104 ppm to 2725 ppm, and further preferably 1104 ppm to 1325 ppm. Specifically, it may 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 may be 9.00-11.00, preferably 9.21-10.89, and more preferably 9.32-9.56. Specifically, it may 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 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; and subjecting the first solution to hydrothermal treatment to obtain a first slurry.

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

[0081] The present application utilizes a hydrothermal treatment process to first obtain a Prussian blue sodium battery material at a relatively low pH value, and then synthesize sodium vanadium ferrophosphate at a relatively high pH value, thereby forming a core-shell structure positive electrode material with the Prussian blue sodium battery material as the core and the sodium vanadium ferrophosphate as the shell. By controlling the change in pH value, on the one hand, the Prussian blue particles are formed in a relatively low pH environment, which is conducive to their formation at low saturation. The resulting Prussian blue particles are larger in size and have a more concentrated particle size distribution, higher crystallinity and fewer defects, thereby improving the electrochemical performance of the positive electrode material. On the other hand, under relatively high pH conditions, sodium vanadium ferrophosphate coated on the Prussian blue particles is formed to ensure the uniformity, conductivity and stability of the formed shell structure. The sodium vanadium ferrophosphate coating of the Prussian blue particles, which is a sodium superion conductor, is conducive to improving the sodium ion transmission efficiency of the formed positive electrode material, thereby further improving the electrochemical performance of the positive electrode material.

[0082] In some embodiments, the method for preparing the positive electrode material provided in the present application, the positive electrode material prepared is the positive electrode material as described above.

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

[0084] The present application controls the pH value of the first solution within the above range, which is conducive to promoting the reaction of sodium ferrocyanide with the ferrous source and the sodium source, and promoting the formation of Prussian blue particles with larger size, more concentrated particle size distribution, higher crystallinity and fewer defects.

[0085] The pH value of the first solution may be 1.5, 1.6, 1.7, 1.8, 1.9 or 2.0, or any value within the range of 1.5 to 2, which will not be described in detail here.

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

[0087] The above-selected ferrous source helps to improve its synergistic coordination with other components, and the above-mentioned ferrous source serves not only as an iron source but also as a carbon source. In the subsequent high-temperature treatment, it is carbonized to form a carbon layer, which is coated on the surface of the sodium vanadium ferrous phosphate layer, thereby improving the conductivity of the positive electrode material.

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

[0089] The addition of sodium metavanadate introduces vanadium element into sodium vanadium phosphate, promotes the formation of sodium vanadium iron phosphate layer, and is beneficial to improving the electrical properties of the positive electrode material.

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

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

[0092] By controlling the types of phosphorus source and sodium source within the above range, it is beneficial to form a stable and efficient sodium ferrous vanadium phosphate layer, and improve the sodium ion storage capacity, ionic conductivity and rate performance of the sodium ferrous 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 reducing agent within the above range is helpful to reduce metal ions and promote the formation of sodium ferrovanadium phosphate layer, wherein ascorbic acid also helps to convert into a carbon layer at a subsequent high temperature.

[0095] In some embodiments, 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, 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 (1.2-1.5):1:(0.2-0.4):(1-1.5):(0.1-0.3) facilitates the full formation of Prussian blue particles and the subsequent uniform formation of sodium ferrovanadium phosphate and carbon layers, thereby improving the rate performance, cycle performance, and performance stability of the cathode material while achieving lower manufacturing costs. In particular, controlling the molar ratio of ferrous gluconate to sodium ferrocyanide (Na4Fe(CN)6) to be greater than 1 facilitates the full reaction of Na4Fe(CN)6, reducing the difficulty of wastewater treatment, and the remaining unreacted iron and vanadium react with phosphate and sodium ions to obtain sodium ferrovanadium phosphate 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 resulting Prussian blue particles is higher than that of traditional processes, 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 conducive to forming a stable sodium ferrovanadium phosphate layer, thereby improving the ionic conductivity and rate performance of the positive electrode material.

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

[0100] The selection of the above acid is conducive to adjusting the pH value of the first solution under the condition of using a small amount, thereby optimizing the synthesis conditions of Prussian blue particles.

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

[0102] By controlling the total mass fraction of each solute in the first solution within the above range, it is not only beneficial to the reaction in the hydrothermal treatment, but also beneficial to regulating the particle size distribution of the formed positive electrode material, thereby enabling the positive electrode material to have good compaction density and electrochemical activity.

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

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

[0105] The first solution is subjected to a first hydrothermal reaction at a lower pH value, which is conducive to the formation of Prussian blue particles. Subsequently, the pH value is raised to 5.0~6.0 by a first pH adjuster (such as ammonia or sodium hydroxide solution), which helps to uniformly deposit a sodium vanadium iron phosphate layer on the Prussian blue particles, which is beneficial to significantly improve the cycle stability and rate performance of the positive electrode material.

[0106] In addition, the pH value of the first slurry may 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 any value within the range of 5.0 to 6.0, which will not be described in detail here.

[0107] In some embodiments, the hydrothermal treatment includes a first hydrothermal reaction and a second hydrothermal reaction performed sequentially.

[0108] The first hydrothermal reaction step includes: subjecting the first solution to a first hydrothermal reaction temperature, a first hydrothermal reaction pressure, and a first hydrothermal reaction time to obtain a first mixture.

[0109] The second hydrothermal reaction step includes: mixing the first mixture with a first pH regulator, and then reacting the mixture at a second hydrothermal reaction temperature and a second hydrothermal reaction pressure for a second hydrothermal reaction time to obtain a first slurry.

[0110] 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] The temperature and pressure of the second hydrothermal reaction are controlled to be higher than those of the first hydrothermal reaction. Higher temperature and pressure are conducive to accelerating the reaction and shortening the reaction time, which helps to quickly deposit and crystallize sodium vanadium ferrophosphate on the surface of Prussian blue particles while ensuring the uniformity and integrity of the deposition of the sodium vanadium ferrophosphate layer.

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

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

[0114] In addition, when the first pH adjuster is selected from sodium hydroxide solution, the molar concentration of the first pH adjuster 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 any point value within the range of 0.5 mol / L to 1 mol / L, which will not be repeated here.

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

[0116] The introduction time of ammonia is controlled within 20 min to 40 min, which ensures a smooth change of pH value and is conducive to the uniform formation of sodium vanadium iron phosphate layer.

[0117] In addition, the introduction time of the above-mentioned first pH adjuster 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 any point value within the range of 20 min to 40 min, which is not repeated here.

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

[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 to form Prussian blue and improve the stability of Prussian blue, but also higher temperatures are beneficial to reduce the crystallization water in Prussian blue and improve the capacity and cycle performance of the positive electrode material.

[0121] In addition, the temperature of the first hydrothermal reaction may 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 within the range of 140°C to 180°C, which will not be further described here.

[0122] The pressure of the first hydrothermal reaction may be 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa or 1.0 MPa, or any value within the range of 0.6 MPa to 1.0 MPa, which will not be described in detail here.

[0123] The time of the 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 repeated here.

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

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

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

[0127] The first stirring time may 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 value within the range of 3 h to 6 h, which will not be described in detail 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 uniform mixing of the reactants, facilitates rapid and uniform deposition of the sodium ferrous vanadium phosphate layer, and controls the thickness of the sodium ferrous vanadium phosphate layer. Therefore, by comprehensively controlling the above conditions, it is beneficial to prepare a high-performance positive electrode material with a core-shell structure.

[0130] In addition, the temperature of the 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 any point value within the range of 220°C to 250°C, which will not be repeated 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 any point value within the range of 2.0 MPa to 2.5 MPa, which will not be repeated here.

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

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

[0134] Controlling the stirring speed and time rate of the second hydrothermal reaction within the above ranges helps to promote uniform mixing of the reactants and uniform formation of the sodium vanadium ferrophosphate layer, thereby improving the electrochemical performance of the positive electrode material.

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

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

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

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

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

[0140] In some embodiments, the solid material is dried in a vacuum or nitrogen flow at a temperature of 105° C. to 130° C.

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

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

[0143] In some embodiments, the process of drying, removing iron from the solid material, and packaging the solid material comprises:

[0144] The solid material is dried in a vacuum or nitrogen flow until the free water content of the material is less than 800 ppm, and then the drying is stopped. The solid material is sieved using a 60-mesh sieve to a 200-mesh sieve, and iron is removed using an electromagnetic iron remover. The solid material is packaged in a constant temperature and humidity room with a humidity of ≤10% and a temperature of 25±5°C. The solid material is vacuum packaged to obtain a Prussian blue and sodium vanadium iron phosphate composite positive electrode material.

[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 sheet in the present application includes the above-mentioned positive electrode material, the positive electrode sheet also has higher capacity, better cycle stability, higher rate performance, better safety and stability.

[0147] In a fourth aspect, an embodiment of the present application provides a sodium ion battery, comprising a positive electrode, wherein 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 this 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 cost-effective and high-performance sodium-ion batteries, and is of great significance for promoting the commercialization of sodium battery technology.

[0149] Some specific examples are listed below. It should be noted that the examples described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0150] Example 1

[0151] Reference Figure 1 The preparation process flow chart of the positive electrode material is shown in the figure. The specific preparation process is as follows:

[0152] Step (1): Ferrous gluconate (C 12 H 22 O 14Fe·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, wherein 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 of the first solution 1.8;

[0153] Step (2): adding the above materials into a hydrothermal reactor, performing a first stirring, maintaining the first stirring speed at 100 r / min, heating to a temperature of 160°C and a pressure of 0.7 MPa, and performing a first hydrothermal reaction under these conditions for 5 hours to obtain a first mixture; then continuing to introduce ammonia gas for 30 minutes, so that the pH value of the first mixture is 5.5; continuing to heat to a temperature of 235°C, performing a second stirring, the second stirring speed is 100 r / min, the second stirring time is 4 hours, increasing the pressure to 2.2 MPa, and continuing to perform a second hydrothermal reaction under these conditions for 10 hours to obtain a first slurry;

[0154] Step (3): After the first slurry is cooled, the pressure is released, and then it is poured out and filtered. The filtration is performed using a filter press, and the washing is stopped after washing until the conductivity of the washing water is ≤200 μS / cm to obtain a solid material;

[0155] Step (4): the solid material is dried in a vacuum or nitrogen stream at a temperature of 115°C until the free water content of the material is less than 800 ppm, and then the drying is stopped. The solid material is sieved using a 100-mesh sieve and iron removal is performed using an electromagnetic iron remover. The solid material is packaged in a constant temperature and humidity room at a humidity of ≤10% and a temperature of 25±5°C, and is vacuum packaged to obtain a Prussian blue and sodium vanadium iron phosphate composite positive electrode material.

[0156] The final test data is 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 vanadium iron 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 crystalline water in Prussian blue particles is 6.29%.

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

[0161] The D10 particle size of the 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 adjust the pH of the first solution to 1.5; wherein 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 pure water is added to adjust the mass fraction of the solute to 25%;

[0167] Step (2): adding the above materials into a hydrothermal reactor, heating to a temperature of 140°C and a pressure of 0.6 MPa, and reacting under these conditions for 3 hours. During the hydrothermal reaction, a first stirring is performed at the same time, and the stirring speed is maintained at 50 r / min to obtain a first mixture; then ammonia gas is continuously introduced for 20 minutes, so that the pH value of the first mixture is 6; continuing to heat to a temperature of 220°C, performing a second stirring, the second stirring speed is 50 r / min, the second stirring time is 6 hours, and the pressure is increased to 2.0 MPa. Under these conditions, a second hydrothermal reaction is continued for 8 hours to obtain a first slurry;

[0168] Step (3): cooling the first slurry, releasing the pressure, pouring it out, filtering it, and washing it to obtain a solid material; wherein the filtration is performed using a filter press or a centrifuge, and washing is stopped after the conductivity of the washing water is ≤200 μS / cm;

[0169] Step (4): The solid material is dried in a vacuum or in a nitrogen flow, and after screening, iron removal and packaging, a Prussian blue and sodium vanadium iron phosphate composite positive electrode material is obtained; wherein the drying temperature is 105°C, and the drying is stopped after the free water content of the material is less than 800ppm. The screening is carried out using a 60-mesh sieve, and the 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 of ≤10% and a temperature of 25±5°C, and the packaging is vacuum packed.

[0170] The final test data is 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 vanadium iron 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 crystalline water in Prussian blue particles is 6.74%.

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

[0175] The D10 particle size of the 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) 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 are added to pure water to obtain a first solution, and sulfuric acid is added to adjust the pH of the first solution to 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; and pure water is added to adjust the mass fraction of the solute to 35%;

[0181] Step (2) adding the above materials into a hydrothermal reactor, performing a first stirring, heating to a temperature of 180° C. and a pressure of 1.0 MPa, and performing a first hydrothermal reaction under these conditions for 6 hours. During the hydrothermal reaction, maintaining a first stirring speed of 150 r / min, to obtain a first mixture; then continuing to introduce ammonia gas, and the time of introducing ammonia gas is 40 minutes so that the pH value of the first mixture is 5; continuing to heat to a temperature of 250° C., performing a second stirring, the second stirring speed is 150 r / min, the second stirring time is 3 hours, increasing the pressure to 2.5 MPa, and continuing to perform a second hydrothermal reaction under these conditions for 12 hours to obtain a first slurry;

[0182] Step (3) cooling the first slurry, releasing the pressure, pouring it out, filtering it, and washing it to obtain a solid material; wherein the filtration is performed using a filter press or a centrifuge, and the washing is stopped after the conductivity of the washing water is ≤200 μS / cm;

[0183] In step (4), the solid material is dried in a vacuum or in a nitrogen gas flow, and after screening, iron removal and packaging, a Prussian blue and sodium vanadium iron phosphate composite positive electrode material is obtained; wherein the drying temperature is 130°C, and the drying is stopped after the free water content of the material is less than 800 ppm. Screening is performed using a 200-mesh sieve, iron removal is performed using an electromagnetic iron remover, and packaging is performed in a constant temperature and humidity room with a humidity of ≤10% and a temperature of 25±5°C, and vacuum packaging is used.

[0184] Final test data such as:

[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 vanadium iron 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 crystalline water in Prussian blue particles is 6.67%.

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

[0189] The D10 particle size of the 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] The difference from Example 1 is that the pH value of the first solution is 2, and the positive electrode material is finally obtained.

[0194] The final test data is as follows:

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

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

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

[0198] The mass ratio of Prussian blue particles, sodium vanadium iron phosphate layer and 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] The difference from Example 1 is that the pH value of the first solution is 1, and the positive electrode material is finally obtained.

[0204] The final test data is as follows:

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

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

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

[0208] The mass ratio of Prussian blue particles, sodium vanadium iron phosphate layer and 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 positive electrode material is finally obtained.

[0213] The final test data is as follows:

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

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

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

[0217] The mass ratio of Prussian blue particles, sodium vanadium iron phosphate layer and 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 positive electrode material is finally obtained.

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

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

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

[0225] The mass ratio of Prussian blue particles, sodium vanadium iron 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 positive electrode material is finally obtained.

[0230] The final test data is as follows:

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

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

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

[0234] The mass ratio of Prussian blue particles, sodium vanadium iron 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 finally the positive electrode material is obtained.

[0239] Final test data such as:

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

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

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

[0243] The mass ratio of Prussian blue particles, sodium vanadium iron phosphate layer and 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 finally the positive electrode material is obtained.

[0248] Final test data such as:

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

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

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

[0252] The mass ratio of Prussian blue particles, sodium vanadium iron phosphate layer and 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 the positive electrode material finally obtained does not contain a Prussian blue phase.

[0257] Comparative Example 2

[0258] The difference from Example 1 is that sodium metavanadate is not added in step (1), and the positive electrode material finally obtained does not contain sodium vanadium iron phosphate phase.

[0259] 1. Test Method

[0260] 1. Physical and chemical performance test of cathode materials

[0261] Iron and vanadium dissolution test: 1 g of the test sample was added to 100 mL of a 0.1 mol / L hydrogen fluoride-ethanol solution, stirred and dissolved at 45°C for 30 minutes, and then filtered. The iron and vanadium contents of the obtained filtrate were measured, which were the iron and vanadium dissolution amounts, respectively.

[0262] Free sodium content: tested by potentiometric titration.

[0263] Free water content: tested using the Karl Fischer (KF) method.

[0264] Elemental carbon content: tested using a carbon-sulfur analyzer.

[0265] Compaction density: tested with a compaction density meter, the test pressure is 3T, and the pressing time is 30S.

[0266] pH value: Test according to GB / T 9724, General rules for determination of pH value of chemical reagents.

[0267] Contents of Na, Fe, V, and P: tested using inductively coupled plasma optical emission spectrometry;

[0268] N content: tested using a nitrogen oxygen analyzer.

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

[0270] Specific surface area (BET): tested using a BET tester and nitrogen adsorption method.

[0271] 2. Property test of secondary batteries

[0272] The positive electrode materials obtained in the examples and comparative examples were mixed with polyvinylidene fluoride and conductive carbon black in a mass ratio of 85:7:8, respectively, and coated on aluminum foil. Then, pole pieces were prepared with a compaction density of 2.2 g / mL. A sodium sheet was used as the negative electrode, and the electrolyte was a 1 mol / L sodium perchlorate solution. The cells were assembled into button batteries and measured at 25±0.1°C.

[0273] Using the constant current charge and discharge mode, charge and discharge tests were conducted at current densities of 0.1C, 1C, and 2C, with a charge cut-off voltage of 3.8 V and a discharge cut-off voltage of 2.0 V. The initial charge capacity at 0.1C, the initial discharge capacity at 0.1C, the initial charge capacity at 2C, the initial discharge capacity at 2C, and the capacity retention rate after 1000 cycles at 1C are shown in Table 11.

[0274] II. Analysis of test results of various embodiments and comparative examples

[0275] Table 11

[0276]

[0277] As can be seen from the above table, the positive electrode material obtained in the embodiment of the present application has a more significantly improved charge specific capacity, discharge specific capacity, and better rate performance and cycle performance when applied to sodium ion batteries compared to the positive electrode material obtained in the comparative example.

[0278] When the positive electrode materials of Examples 1 and 4 are positive electrode materials obtained when the pH value of the first solution is within the range of 1.5 to 2, and Examples 5 and 6 are positive electrode materials obtained when the pH value of the first solution is outside the range of 1.5 to 2, the positive electrode materials of Examples 5 and 6, when applied to sodium ion batteries, are inferior to those of the sodium ion batteries using the positive electrode materials of Examples 1 and 4 in terms of charge specific capacity, discharge specific capacity, rate performance and cycle performance.

[0279] From the data when the positive electrode materials of Examples 1 and 7 are applied to sodium ion batteries, and compared with the data when Example 8 is applied to sodium ion batteries, it can be seen that when the molar ratio of the ferrous source, sodium ferrocyanide (Na4Fe(CN)6), vanadium source, sodium source, and reducing agent is 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 cycle performance are significantly improved.

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

[0281] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode material, characterized in that The positive electrode material has a core-shell structure, wherein 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; 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, and 1.0≤n≤2.8; The mass ratio of the Prussian blue particles, the sodium vanadium ferrophosphate layer, and the carbon layer is (19.47-32.51): (66.81-78.17): (0.68-2.36).

2. The positive electrode material according to claim 1, characterized in that 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 crystalline 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 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.

4. 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; 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.

5. The preparation method according to claim 4, 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, ammonium dihydrogen phosphate, and ammonium monohydrogen 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 the solutes in the first solution is 25% to 35%.

6. The preparation method according to claim 4, 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, wherein the first solution is subjected to a first hydrothermal reaction temperature, a first hydrothermal reaction pressure, and 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 temperature, a second hydrothermal reaction pressure, and 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 and mixed, 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.

7. The preparation method according to claim 4, characterized in that The free water content of the dried product is less than or equal to 800 ppm.

8. A positive electrode plate, characterized in that: The positive electrode sheet comprises the positive electrode material according to any one of claims 1 to 3.

9. A sodium ion battery, characterized in that: The positive electrode sheet of the sodium ion battery is the positive electrode sheet according to claim 8.

Images