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

Abstract

The present application provides a positive electrode material, a preparation method therefor, a positive electrode sheet, and a sodium ion battery. The positive electrode material comprises composite particles, the molecular formula of the composite particles being: NaxFey(PO4)n(P2O7)m, wherein x is 4.2-4.5, y is 3.2-3.5, n is 2.2-2.5, and m is 0.9-1.1. The composite particles comprise an olivine-type phosphate and a polyanionic phosphate coated on the surface of the olivine-type phosphate, and the molar ratio of the olivine-type phosphate to the polyanionic phosphate is (0.2-0.5):1. By optimizing the molar ratio of the olivine-type phosphate to the polyanionic phosphate in an iron-based composite phosphate material and designing the microstructure of the composite particles formed by the olivine-type phosphate and the polyanionic phosphate, the present application achieves nanocrystallization at the crystal structure level and interfacial permeation between the two phases, thereby significantly increasing the capacity of the positive electrode material.

Description

Positive electrode materials, their preparation methods, positive electrode sheets and sodium-ion batteries Technical Field

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

[0002] In recent years, significant efforts have been devoted to developing cost-effective and high-performance cathode materials for commercially viable sodium-ion batteries, such as those used in new energy vehicles. In a related field, lithium-ion batteries, the LiFePO4 cathode has achieved tremendous commercial success. This type of iron-based phosphate material offers superior performance and is inexpensive, making it one of the best choices for cathode active materials in rechargeable batteries.

[0003] However, olivine-type NaFePO4 (NFP) exhibits poor thermodynamic properties and is considered electrochemically inert. In contrast, pyrophosphate Na2FeP2O7 materials, due to their open three-dimensional Na+ microstructure... + Channels, and P2O7 in its crystals 4- It exhibits strong inductive properties, thus allowing for higher operating voltages (~3.0V), but Na2FeP2O7 has a low reversible capacity (<100mAh·g). -1 This seriously hinders its practical application.

[0004] Both olivine-type phosphates and polyanionic phosphates have certain shortcomings when used as positive electrode active materials in sodium-ion batteries, resulting in inferior battery performance. Current technology provides a polyanionic positive electrode material comprising phosphates and a carbon layer coating the phosphate surface. The phosphates include NFPP, NFP, and NNFPO, and the ratio of NFP to NNFPO content is controlled through multiple short-time sintering processes. However, this technology only reduces impurity phases in the positive electrode material and does not achieve synergistic effects between different crystalline phases; therefore, the performance of the resulting positive electrode material still needs improvement.

[0005] Therefore, how to design the positive electrode active material of sodium-ion batteries at the crystalline phase level to give it superior electrochemical performance is one of the important technical problems that need to be solved in this field.

[0006] Summary of the Invention

[0007] The main objective of this invention is to provide a cathode material, its preparation method, a cathode sheet, and a sodium-ion battery, so as to solve the problem of poor electrical performance of cathode materials in existing sodium-ion batteries.

[0008] To achieve the above objectives, the first aspect of the present invention provides a cathode material comprising composite particles, wherein the molecular formula of the composite particles is: Na x Fe y (PO4) n (P2O7) m The values ​​of x are 4.2–4.5, y is 3.2–3.5, n is 2.2–2.5, and m is 0.9–1.1. The composite particles include olivine phosphate (NaFePO4, NFP) and polyanionic phosphate (Na4Fe3(PO4)2P2O7, i.e., NFPP) coated on the surface of olivine phosphate, and the molar ratio of olivine phosphate to polyanionic phosphate is (0.2–0.5):1.

[0009] This invention optimizes the cathode material, specifically the molar ratio of olivine phosphate to polyanionic phosphate in an iron-based composite phosphate material, to be (0.2–0.5):1. Simultaneously, it designs the microstructure of the composite particles formed by these two components, allowing the polyanionic phosphate phase to encapsulate the olivine phosphate crystals. This achieves nanocrystallization at the crystal structure level and interfacial penetration between the two phases, which can induce Na+... + The electrochemical activation of the olivine-type phosphate phase is achieved through the olivine-type phosphate phase, which significantly improves the capacity of iron-based composite phosphate (NxFPP).

[0010] Furthermore, the molar ratio of olivine phosphate to polyanionic phosphate is (0.2–0.4):1. When the proportion of olivine phosphate in the material structure is too high, the polyanionic phosphate crystal phase cannot completely encapsulate the olivine phosphate crystal, resulting in the presence of olivine phosphate crystals forming independently on the outside of the polyanionic phosphate crystal, becoming an impurity phase within the polyanionic phosphate crystal, leading to a lower capacity of the composite material. When the proportion of olivine phosphate is too low, there are correspondingly fewer olivine phosphate crystals inside the polyanionic phosphate crystal, making it difficult to fully penetrate the two-phase interface, resulting in fewer electrochemically activated olivine phosphate phases, thus making it difficult to significantly improve the capacity of the composite material.

[0011] Furthermore, in order to improve the conductivity of the obtained cathode material and its structural stability during the charge-discharge process, the cathode material also includes a carbon layer coated on the surface of the composite particles, and the mass fraction of the carbon layer is 1.5% to 3.0% based on the total weight of the cathode material of 100%.

[0012] Regarding the carbon coating layer, the preferred mass fraction is as described above, in order to form a more complete carbon coating layer, improve the electronic conductivity of the cathode material, thereby reducing its powder internal resistance and enhancing its electrochemical performance. Preferably, the carbon layer is an amorphous carbon portion formed from an organic carbon source and / or an inorganic carbon portion formed from an inorganic carbon source.

[0013] More preferably, the carbon layer is a combination of amorphous carbon portion and inorganic carbon portion, and the weight percentage of inorganic carbon portion is 2.5% to 5.0% based on the total weight of the carbon layer as 100%.

[0014] The cathode material provided by this invention preferably contains both amorphous carbon portions formed from organic carbon sources and inorganic carbon portions formed from inorganic carbon sources in its outermost carbon layer. The amorphous carbon portions are formed by uniformly coating composite particles with soluble organic carbon sources after calcination, while the inorganic carbon portions are interspersed within the amorphous carbon portions, forming a well-uniform carbon coating. The amorphous carbon portions have a higher specific surface area and better mechanical properties, while the more graphitized inorganic carbon portions provide better electronic conductivity. Together, they further improve the electrochemical performance of the cathode material.

[0015] Furthermore, the resistivity of the cathode material powder is 12000 Ω·cm to 93000 Ω·cm. The cathode material obtained by this invention has a lower resistivity, and when used as a cathode active material in sodium-ion batteries, it enables the corresponding batteries to exhibit higher charge specific capacity and discharge specific capacity.

[0016] A second aspect of the present invention provides a method for preparing a cathode material, comprising: preparing a first slurry by dissolving a sodium source, an iron source, and a phosphorus source; subjecting the first slurry to a first drying and a first calcination to obtain a precursor; dissolving the precursor and a carbon source to prepare a second slurry; and subjecting the second slurry to a second drying and a second calcination to obtain the cathode material.

[0017] The preparation process provided by this invention employs a two-step method. First, sodium, iron, and phosphorus sources are used to prepare a precursor. Then, a carbon source is formed with the precursor to create a homogeneous slurry. After a second drying and a second calcination, the carbon source uniformly coats the outer surface of the pure-phase grains of the precursor, resulting in a cathode material with low powder resistivity. This helps improve the electronic conductivity and capacity of the obtained cathode material. Simultaneously, during the preparation process, the pure-phase precursor obtained after the first drying and first calcination has already formed grains of a certain size. The secondary carbon coating does not affect its grain structure. This two-step preparation process facilitates obtaining the cathode material with the coating structure provided by this invention and improves the compaction density and tap density of the obtained cathode material.

[0018] In some embodiments, the molar ratio of sodium source, iron source and phosphorus source in the first slurry is (4.2~4.86):(3.2~3.5):(4.2~4.5). In this addition ratio, the sodium source is in excess compared to the final composite particles, which helps to avoid Na loss during high-temperature sintering and obtain composite particles with more complete crystal structure.

[0019] Furthermore, to achieve higher purity of the precursor crystal phase obtained from the first calcination, better structural stability and ion diffusion performance, and ultimately higher energy density in sodium-ion batteries, the first calcination preferably includes a first calcination and a second calcination performed sequentially. The first calcination is conducted at a temperature of 400℃–500℃ for 1.5h–2.5h; the second calcination is conducted at a temperature of 550℃–650℃ for 10h–14h. This calcination method and the set temperature and time parameters facilitate the sequential formation of olivine-type phosphate and polyanionic phosphate, promote the formation of composite particles with polyanionic phosphate coating the surface of olivine-type phosphate, and facilitate better composite formation between the two crystal phases, thereby significantly improving the electrochemical performance of the obtained cathode material. Simultaneously, to reduce the introduction of impurities, it is preferable that the first calcination is carried out under a protective atmosphere to obtain a cathode material with higher electrochemical performance.

[0020] Preferably, the sodium source is selected from one or more of sodium bicarbonate, sodium phosphate, sodium monohydrogen phosphate, and sodium dihydrogen phosphate; the iron source is selected from one or more of anhydrous ferric phosphate, polyhydrate ferric phosphate, monohydrogen phosphate, and ferrous oxalate; and the phosphorus source is selected from one or more of anhydrous ferric phosphate, ammonium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, polyhydrate ferric phosphate, and monohydrogen phosphate.

[0021] Furthermore, the carbon source includes an organic carbon source and / or an inorganic carbon source, preferably an organic carbon source selected from one or more of citric acid, glucose, sucrose, soluble starch, and polyethylene glycol; and / or, an inorganic carbon source selected from one or more of graphite, carbon nanotubes, and graphene. These carbon sources can undergo a more complete graphitization reaction during the second calcination process, resulting in a more uniform coating and thus significantly optimizing the electrochemical performance of the obtained cathode material.

[0022] Preferably, the carbon source includes soluble starch, polyethylene glycol, and graphene; more preferably, the polyethylene glycol is PEG-2000; even more preferably, the ratio of the amount of soluble starch, PEG-2000, and graphene added in the carbon source is (1.2-1.5):1:(0.2-0.4). The inventors optimized the types of carbon sources used through numerous experiments to obtain the above-mentioned carbon source combination. The amorphous carbon layer formed after sintering of soluble starch has high porosity but low powder resistivity; the carbon layer formed after sintering of PEG-2000 is more dense. The combined use of the two helps to improve the compaction density of the material, reduce the powder resistivity of the material, and ultimately improve the electrochemical performance of the obtained cathode material in sodium-ion batteries. Meanwhile, graphene, due to its unique sheet-like structure (two-dimensional structure), is beneficial for coating the surface of the precursor and forming point-to-surface contact with the precursor. Meanwhile, the combined use of soluble starch, PEG-2000, and graphene helps to reduce the BET specific surface area of ​​the cathode material, decrease its water absorption, and improve its processing performance. Considering the unavoidable losses during subsequent calcination and the synergistic effect of the three components, using them in the aforementioned proportions is beneficial for better synergistic effects of graphene, polyethylene glycol, and soluble starch after calcination, thereby obtaining a uniformly coated carbon layer with good conductivity and improving the electrochemical performance of the resulting cathode material.

[0023] Furthermore, the second calcination includes the following sequential processes: a first calcination at a temperature of 40℃ to 60℃ for 1 to 2 hours; a second calcination at a temperature of 100℃ to 130℃ for 0.5 to 1.5 hours; a third calcination at a temperature of 180℃ to 210℃ for 2 to 4 hours; and a fourth calcination at a temperature of 500℃ to 600℃ for 2 to 6 hours.

[0024] This invention optimizes the specific steps in the second calcination process into a four-stage calcination, which helps to achieve the segmented decomposition of the carbon source mixed with the precursor. This avoids excessive decomposition within a certain temperature range, which would generate excessive gas and cause significant expansion of the material volume, leading to a decrease in packing ratio and a decline in electrochemical performance. Furthermore, segmented calcination results in slower gas escape, better preventing the rapid shrinkage of the resulting cathode material after the formation of numerous void structures, which would otherwise lead to a decrease in compaction density, an increase in BET specific surface area, and a deterioration in electrochemical performance.

[0025] Furthermore, the heating rates for the first, second, third, and fourth calcination stages are each independently 3°C / min to 10°C / min. This heating rate can better adapt to the state of the composite material corresponding to the above four calcination stages, so as to obtain a cathode material with superior phase purity and crystallinity, improve its performance in sodium-ion batteries, and ultimately optimize the capacity of the resulting battery.

[0026] A third aspect of the present invention provides a positive electrode sheet comprising the aforementioned positive electrode material, or a positive electrode material prepared by a method comprising the aforementioned positive electrode material. The aforementioned positive electrode material, or the obtained positive electrode material, has a lower powder resistivity, thereby enabling the positive electrode sheet in which it is located to possess superior overall performance.

[0027] A fourth aspect of the present invention provides a sodium-ion battery comprising the aforementioned positive electrode sheet. Because the positive electrode material in the positive electrode sheet obtained by the present invention has superior performance, the sodium-ion battery in which it is located exhibits higher energy density, higher capacity, and higher efficiency, while also possessing excellent cycle stability.

[0028] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0030] Figure 1 shows the SEM results of the cathode material obtained in Example 1;

[0031] Figure 2 shows the SEM results of the cathode material obtained in Example 14;

[0032] Figure 3 shows the SEM results of the cathode material obtained in Example 15;

[0033] Figure 4 is a process flow diagram of the preparation method of the cathode material provided in the embodiment. Detailed Implementation

[0034] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0036] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0037] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0038] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0039] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0040] As described in the background section, existing sodium-ion battery cathode materials suffer from poor electrical performance. To address this problem, the first aspect of this invention provides a cathode material comprising composite particles with the molecular formula: Na... x Fe y (PO4) n (P2O7) mThe values ​​of x are 4.2–4.5, y is 3.2–3.5, n is 2.2–2.5, and m is 0.9–1.1. The composite particles include olivine phosphate (NaFePO4, NFP) and polyanionic phosphate (Na4Fe3(PO4)2P2O7, i.e., NFPP) coated on the surface of olivine phosphate, and the molar ratio of olivine phosphate to polyanionic phosphate is (0.2–0.5):1.

[0041] In some embodiments, the molecular formula of the composite particles is Na. x Fe y (PO4) n (P2O7) m x is 4.2, 4.3, 4.4, or 4.5; y is 3.2, 3.3, 3.4, or 3.5; n is 2.2, 2.3, 2.4, or 2.5; m is 0.9 or 1.0.

[0042] This invention optimizes the molar ratio of olivine phosphate to polyanionic phosphate in the cathode material, i.e., the iron-based composite phosphate material, to (0.2–0.5):1, and designs the microstructure of the composite particles formed by the two, so that the polyanionic phosphate phase encapsulates the nanocrystals of olivine phosphate, achieving nanocrystalization at the crystal structure level and interfacial penetration between the two phases. This can induce Na+... + Electrochemical activation of the olivine-type phosphate phase is achieved through the use of an olivine-type phosphate phase, significantly improving the capacity of iron-based composite phosphate (precursor, NxFPP). Specifically, due to the superior electrochemical performance of polyanionic phosphates, especially in terms of theoretical specific capacity, voltage plateau, and cycle stability, coating them with olivine-type phosphate significantly enhances the overall electrochemical performance of the resulting material. This results in better charge-discharge performance of sodium-ion batteries when used as cathode materials.

[0043] Furthermore, regarding structural stability, the coating of the polyanionic phosphate phase provides additional structural support to the olivine-type phosphate phase, reducing structural damage during charge-discharge processes and thus enhancing the material's cycle stability. Simultaneously, the polyanionic phosphate coating improves the Na+ content of the olivine-type phosphate. + The encapsulated structure facilitates ion diffusion, improving both charging and discharging rates. Furthermore, this coating structure reduces side reactions between olivine phosphate and the electrolyte, thus extending battery life.

[0044] Furthermore, taking into account the different electrochemical properties, cycle stability, ion diffusion efficiency, and synthesis process costs of the polyanionic phosphate phase and the olivine phosphate phase, this invention strictly controls the molar ratio of olivine phosphate to polyanionic phosphate to be (0.2-0.5):1, so as to achieve complete coating of the polyanionic phosphate phase on the surface of the olivine phosphate phase and avoid unsatisfactory capacity improvement due to the formation of impurities or the inability to achieve penetration at the interface between the two phases.

[0045] In other words, the cathode material with the above-mentioned structure provided by the present invention exhibits significant advantages in terms of energy density, electrochemical performance, structural stability and safety.

[0046] Furthermore, the molar ratio of olivine phosphate to polyanionic phosphate is (0.2–0.4):1. Through extensive experimentation and creative work, the inventors further adjusted this molar ratio of olivine phosphate to polyanionic phosphate, optimizing it to (0.2–0.4):1. This is because when the proportion of olivine phosphate in the material structure is too high, the polyanionic phosphate crystalline phase cannot completely encapsulate the olivine phosphate nanocrystalline phase. The olivine phosphate forms crystals independently and is distributed outside the polyanionic phosphate crystals, becoming an impurity phase within the polyanionic phosphate crystals, resulting in a lower capacity of the composite material. When the proportion of olivine phosphate is too low, the olivine phosphate nanocrystalline phase inside the polyanionic phosphate crystalline phase is correspondingly less, making it difficult to form sufficient two-phase interface penetration. This results in less electrochemically activated olivine phosphate phase, thus failing to significantly improve the capacity of the composite material.

[0047] It should be further explained that, in practical applications, the cathode material provided by this invention suffers from unavoidable P loss and PO4 loss during the sintering process during its preparation. 3- Transformation into P2O7 4- Due to the uncertainty, the relationship between the molecular formula of the composite particles and the molar ratio of olivine phosphate and polyanionic phosphate may not be completely consistent, but the above relationship corresponds within the detection limit.

[0048] Specifically, the molecular formula of the composite particles is Na. 4.2 Fe 3.2 (PO4) 2.2 P2O7, where the molar ratio of olivine phosphate to polyanionic phosphate is 0.2:1; or the composite particles have the molecular formula Na. 4.3 Fe 3.3 (PO4) 2.3 P2O7, where the molar ratio of olivine phosphate to polyanionic phosphate is 0.3:1; or the composite particles have the molecular formula Na.4.4 Fe 3.4 (PO4) 2.4 P2O7, where the molar ratio of olivine phosphate to polyanionic phosphate is 0.4:1; or the composite particles have the molecular formula Na. 4.5 Fe 3.5 (PO4) 2.5 P2O7, at which point the molar ratio of olivine phosphate to polyanionic phosphate is 0.5:1.

[0049] Furthermore, to improve the conductivity and structural stability of the obtained cathode material during charge and discharge, the cathode material also includes a carbon layer coated on the surface of the composite particles. The carbon layer has a mass fraction of 1.5% to 3.0% (e.g., 1.5%, 2.0%, 2.5%, 3.0%) based on the total weight of the cathode material. Regarding the coated carbon layer, the preferred mass fraction is as described above, so as to form a more complete carbon coating layer without excessively increasing the resistance to sodium ion transport, thereby improving its electrochemical performance. Preferably, the carbon layer is an amorphous carbon portion formed from an organic carbon source and / or an inorganic carbon portion formed from an inorganic carbon source; more preferably, the carbon layer is a combination of amorphous carbon portions and inorganic carbon portions, and the inorganic carbon portion accounts for 2.5% to 5.0% (e.g., 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%) based on the total weight of the carbon layer. The cathode material provided by this invention preferably contains both amorphous carbon portions formed from organic carbon sources and inorganic carbon portions formed from inorganic carbon sources in its outermost carbon layer. The amorphous carbon portions are formed by uniformly coating composite particles with soluble organic carbon sources after calcination, while the inorganic carbon portions are interspersed within the amorphous carbon portions, forming a well-uniform carbon coating. The amorphous carbon portions have a higher specific surface area and better mechanical properties, while the more graphitized inorganic carbon portions provide better electronic conductivity. Together, they further improve the electrochemical performance of the cathode material.

[0050] Furthermore, the resistivity of the cathode material powder is preferably 17000 Ω·cm to 33000 Ω·cm, such as 17050 Ω·cm, 17600 Ω·cm, 18400 Ω·cm, 18900 Ω·cm, 19900 Ω·cm, and 32070 Ω·cm; the tap density is preferably 1.14 g / mL to 1.19 g / mL, such as 1.14 g / mL, 1.17 g / mL, 1.18 g / mL, and 1.19 g / mL; and the compaction density is 1.7 g / cm³. 3 ~2.2g / cm 3 The preferred value is 1.95 g / cm³. 3 ~2.18g / cm 3For example, 1.98g / cm 3 2.08 g / cm 3 2.12 g / cm 3 2.13 g / cm 3 2.15g / cm 3 2.16 g / cm 3 The potting ratio is 60.0%–76.5%, preferably 72.3%–76.1%, such as 72.30%, 75.01%, 75.38%, 75.47%, 76.00%, and 76.03%; the specific surface area is 9 m². 2 / g~50m 2 / g, preferably 11.5m 2 / g~35.7m 2 / , for example, 11.71m 2 / g, 12.00m 2 / g, 12.30m 2 / g, 12.85m 2 / g, 14.63m 2 / g, 35.67m 2 / g. The cathode material obtained by this invention has a higher compaction density, a lower specific surface area, and especially a lower resistivity. When used as a cathode active material for sodium-ion batteries, it enables the corresponding batteries to exhibit higher charge specific capacity and discharge specific capacity.

[0051] A second aspect of the present invention provides a method for preparing a cathode material, comprising: preparing a first slurry by dissolving a sodium source, an iron source, and a phosphorus source; subjecting the first slurry to a first drying and a first calcination to obtain a precursor; dissolving the precursor and a carbon source to prepare a second slurry; and subjecting the second slurry to a second drying and a second calcination to obtain the cathode material.

[0052] The preparation process provided by this invention employs a two-step process. First, a precursor is prepared from sodium, iron, and phosphorus sources, which is a composite of olivine-type phosphate and polyanionic phosphate. Then, a carbon source is formed with the precursor to create a homogeneous slurry. After a second drying and a second calcination, the carbon source uniformly coats the outer surface of the pure-phase grains of the precursor, resulting in a cathode material with low powder resistivity, which helps to improve the electronic conductivity of the obtained cathode material. Therefore, the cathode material prepared by this method, namely the NxFPP / C composite material, has a higher capacity. Simultaneously, during the preparation process, the pure-phase precursor obtained after the first drying and the first calcination has already formed grains of a certain size. The secondary carbon coating does not affect the grain structure of the precursor. The above two-step preparation process is beneficial for obtaining the cathode material with the coating structure provided by this invention and improves the compaction density and tap density of the obtained cathode material.

[0053] In the commonly used one-step preparation process, apart from insoluble iron and some carbon sources, the other raw materials in the slurry are soluble in water to form a homogeneous slurry. During drying, the water evaporates rapidly, and the soluble sodium salts, phosphate salts, and carbon sources mix and coat the outside of the insoluble iron and some carbon sources. During sintering, the carbon layer formed by the carbon source is trapped in the mixture, which to some extent hinders the migration and fusion of other ions. At the same time, due to the growth and fusion of precursor grains, the carbon layer cannot uniformly coat the outside of the precursor grains, resulting in a high powder resistivity. In addition, the carbon source trapped in the inorganic phase hinders the growth of precursor grains to some extent, and the volatilization of the carbon source during sintering leaves behind a porous structure. Smaller precursor grains embedded in these structures cannot form effective filling and support, resulting in low compaction and tap density of the composite material formed by the one-step method.

[0054] It should be further explained that, in practical applications, the cathode material provided by this invention suffers from unavoidable P loss and PO4 loss during the sintering process during its preparation. 3- Transformation into P2O7 4- Due to the uncertainty, the relationship between the molecular formula of the composite particles and the molar ratio of olivine phosphate and polyanionic phosphate may not be completely consistent, but the above relationship corresponds within the detection limit.

[0055] In some embodiments, the molar ratio of sodium source, iron source, and phosphorus source in the first slurry is (4.2–4.86):(3.2–3.5):(4.2–4.5), such as 4.2:3.2:4.2, 4.5:3.3:4.2, and 4.8:3.2:4.5. In this addition ratio, when sodium source is partially in excess compared to the final composite particles, the preferred excess sodium source mass fraction is 5.0%–5.5%, such as 5.0%, 5.1%, 5.2%, 5.3%, 5.4%, and 5.5%. This is because, through extensive experimentation, the inventors have found that this helps to avoid Na loss during high-temperature sintering and obtain composite particles with a more complete crystalline core.

[0056] Furthermore, to achieve higher crystal phase purity, better structural stability and ion diffusion performance in the precursor obtained from the first calcination, and ultimately higher energy density in sodium-ion batteries, the first calcination preferably includes a first calcination and a second calcination performed sequentially. The first calcination is performed at a temperature of 400℃–500℃, such as 440℃, 450℃, or 460℃, for 1.5h–2.5h; the second calcination is performed at a temperature of 550℃–650℃, such as 580℃, 600℃, or 620℃, for 10h–14h, such as 11h, 12h, or 13h. This calcination method and the setting of temperature and time parameters facilitate the sequential formation of olivine-type phosphate and polyanionic phosphate, promote the formation of composite particles with polyanionic phosphate coating the surface of olivine-type phosphate, and facilitate better composite formation between the two crystal phases, thereby significantly improving the electrochemical performance of the obtained cathode material. Meanwhile, in order to reduce the introduction of impurities, it is preferable to carry out the first calcination under a protective atmosphere to obtain a cathode material with higher electrochemical performance.

[0057] Furthermore, the carbon source includes organic carbon sources and / or inorganic carbon sources. Even further, the inventors have conducted extensive experiments to optimize the types of carbon sources and found that when the organic carbon source is selected from one or more of citric acid, glucose, sucrose, soluble starch and polyethylene glycol; and / or the inorganic carbon source is selected from one or more of graphite, carbon nanotubes and graphene, the above carbon sources can undergo a more complete graphitization reaction during the second calcination process, resulting in a more uniform coating degree, thereby significantly optimizing the electrochemical performance of the obtained cathode material.

[0058] In several typical embodiments, the preferred carbon source includes soluble starch, polyethylene glycol, and graphene; more preferably, the polyethylene glycol is PEG-2000; even more preferably, the ratio of the amount of soluble starch, PEG-2000, and graphene added in the carbon source is (1.2-1.5):1:(0.2-0.4), such as 1.24:1:0.4, 1.4:1:0.2, 1.48:1:0.2, and preferably (1.25-1.5):1:(0.2-0.3). The inventors optimized the carbon source types through extensive experiments to obtain the aforementioned carbon source combination. Soluble starch, after sintering, forms an amorphous carbon layer with high porosity but low powder resistivity; PEG-2000, after sintering, forms a denser carbon layer. The combined use of these two materials helps increase the compaction density of the material, reduce its powder resistivity, and ultimately improve the electrochemical performance of the resulting cathode material in sodium-ion batteries. Simultaneously, graphene, due to its unique sheet-like structure (two-dimensional structure), is beneficial for coating the NxFPP surface and forming point-to-surface contact with NxFPP. Furthermore, the combined use of soluble starch, PEG-2000, and graphene helps reduce the BET specific surface area of ​​the cathode material, decreases its water absorption, and improves its processing performance. Considering the unavoidable losses during subsequent calcination and the synergistic effect of the three components, using them in the aforementioned proportions is beneficial for better synergistic effects of graphene, polyethylene glycol, and soluble starch after calcination. This results in a uniformly coated carbon layer with good conductivity, improving the electrochemical performance of the obtained cathode material. However, adding too much graphene not only fails to significantly improve the material's capacity and powder resistivity, but also, due to its high cost, excessive graphene addition leads to higher production costs for the composite material, hindering the low-cost advantage of sodium-ion batteries.

[0059] Specifically, the sodium source used in this invention is selected from one or more of sodium bicarbonate, sodium phosphate, sodium monohydrogen phosphate, and sodium dihydrogen phosphate; the iron source is selected from one or more of anhydrous ferric phosphate, polyhydrate ferric phosphate, monohydrogen phosphate, and ferrous oxalate; and the phosphorus source is one or more of anhydrous ferric phosphate, ammonium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, polyhydrate ferric phosphate, and monohydrogen phosphate.

[0060] Furthermore, the second calcination includes the following sequential processes: a first-stage calcination at a temperature of 40℃ to 60℃, such as 45℃, 50℃, and 55℃, with a holding time of 1h to 2h; a second-stage calcination at a temperature of 100℃ to 130℃, such as 110℃, 120℃, and 130℃, with a holding time of 0.5h to 1.5h; a third-stage calcination at a temperature of 180℃ to 210℃, such as 190℃, 200℃, and 210℃, with a holding time of 2h to 4h; and a fourth-stage calcination at a temperature of 500℃ to 600℃, such as 540℃, 550℃, and 560℃, with a holding time of 2h to 6h. This invention optimizes the specific steps in the second calcination process into a four-stage calcination, which helps to achieve the segmented decomposition of the carbon source mixed with the precursor. This avoids excessive decomposition within a certain temperature range, which would generate excessive gas and cause significant expansion of the material volume, leading to a decrease in packing ratio and a decline in electrochemical performance. Furthermore, segmented calcination results in slower gas escape, better preventing the rapid shrinkage of the resulting cathode material after the initial formation of numerous void structures, which would otherwise lead to a decrease in compaction density, an increase in BET specific surface area, and a deterioration in electrochemical performance.

[0061] Furthermore, in order to adapt to the state of the composite material corresponding to the above four calcination processes, so as to obtain a cathode material with superior phase purity and crystallinity, improve its performance in sodium-ion batteries, and ultimately optimize the capacity of the resulting battery, it is preferred that the heating rates of the first, second, third, and fourth calcination stages are each independently 3℃ / min to 10℃ / min, such as 6℃ / min, 6.5℃ / min, and 7.0℃ / min.

[0062] A third aspect of the present invention provides a positive electrode sheet comprising the aforementioned positive electrode material, or a positive electrode material prepared by a method comprising the aforementioned positive electrode material. The aforementioned positive electrode material, or the obtained positive electrode material, has a lower powder resistivity, thereby enabling the positive electrode sheet in which it is located to possess superior overall performance.

[0063] A fourth aspect of the present invention provides a sodium-ion battery comprising the aforementioned positive electrode sheet. Because the positive electrode material in the positive electrode sheet obtained by the present invention has superior performance, the sodium-ion battery in which it is located exhibits higher energy density, higher capacity, and higher efficiency, while also possessing excellent cycle stability.

[0064] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0065] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0066] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0067] I. Preparation Method

[0068] Example 1

[0069] A method for preparing a cathode material:

[0070] (1) Using sodium bicarbonate as the sodium source, anhydrous ferric phosphate as the iron and phosphorus source, and ammonium dihydrogen phosphate as the phosphorus source, according to Na… 4.2 Fe 3.2 (PO4) 2.2 The feedstock is prepared using the molecular formula P2O7 (i.e., the molar ratio of olivine phosphate to polyanionic phosphate is 0.2:1), with a sodium source in excess of 5.5%, i.e., a molar ratio of sodium source: iron source: phosphorus source = 4.2:3.2:4.2. Water is used as the dispersion liquid to obtain the first slurry.

[0071] (2) The first slurry is spray-dried and then calcined for the first time. The first calcination is carried out by heating from room temperature to 450℃ at 5℃ / min for 2 hours, and the second calcination is carried out by heating from room temperature to 600℃ at 5℃ / min for 12 hours to obtain the precursor.

[0072] (3) Soluble starch, polyethylene glycol PEG-2000, and graphene were used as carbon sources. The carbon layer content in the final cathode material was 3%, and the weight ratio of soluble starch, polyethylene glycol PEG-2000, and graphene was 1.4:1:0.2. The carbon sources were mixed with the precursors and water was used as the dispersion to prepare the second slurry.

[0073] (4) The second slurry was spray-dried and then subjected to a second calcination at a heating rate of 6.5℃ / min, for a total of four calcination stages, to obtain the cathode material. The conditions were as follows:

[0074] The first stage of calcination is carried out at a temperature of 50℃ for 1.5 hours.

[0075] The second stage of calcination is carried out at a temperature of 120℃ for 1 hour.

[0076] The third stage of calcination is carried out at a temperature of 200℃ for 3 hours.

[0077] The fourth stage of calcination is carried out at a temperature of 550℃ for 4 hours.

[0078] After the second calcination described above, the two organic carbon sources, soluble starch and polyethylene glycol PEG-2000, in the carbon layer are transformed into amorphous carbon parts, while the inorganic carbon source, graphene, is transformed into inorganic carbon parts. The mass ratio of the three contributing to the carbon layer is 4.5:5:0.5. That is, based on the total weight of the carbon layer being 100%, the weight percentage of the inorganic carbon parts is 5.0%.

[0079] The SEM characterization results of the obtained cathode material are shown in Figure 1.

[0080] The process flow for preparing this cathode material is shown in Figure 4.

[0081] Example 2

[0082] A method for preparing a cathode material:

[0083] The only difference between this embodiment and Embodiment 1 is the amount of each raw material added, so that the molar ratio of olivine phosphate to polyanionic phosphate in the composite particles of the resulting cathode material is 0.3:1. The molecular formula of the resulting composite particles is Na. 4.3 Fe 3.3 (PO4) 2.3 P2O7.

[0084] Example 3

[0085] A method for preparing a cathode material:

[0086] The only difference between this embodiment and Embodiment 1 is the amount of each raw material added, so that the molar ratio of olivine phosphate to polyanionic phosphate in the composite particles of the resulting cathode material is 0.4:1. The molecular formula of the resulting composite particles is Na. 4.4 Fe 3.4 (PO4) 2.4 P2O7.

[0087] Example 4

[0088] A method for preparing a cathode material:

[0089] The only difference between this embodiment and Embodiment 1 is the amount of each raw material added, so that the molar ratio of olivine phosphate to polyanionic phosphate in the composite particles of the resulting cathode material is 0.5:1. The molecular formula of the resulting composite particles is Na. 4.5 Fe 3.5 (PO4) 2.5P2O7.

[0090] Example 5

[0091] A method for preparing a cathode material:

[0092] The only difference between this embodiment and Example 1 is that the carbon content in the carbon source added in step (3) is still 3% in the final cathode material, and the weight ratio of soluble starch, polyethylene glycol PEG-2000 and graphene added is changed to 1.48:1:0.2.

[0093] After the second calcination, the two organic carbon sources, soluble starch and polyethylene glycol (PEG-2000), in the carbon layer are transformed into amorphous carbon parts, while the inorganic carbon source, graphene, is transformed into inorganic carbon parts. The mass ratio of the three contributing to the carbon layer is 4.75:5:0.25. That is, based on the total weight of the carbon layer being 100%, the weight percentage of the inorganic carbon parts is 2.5%.

[0094] Example 6

[0095] A method for preparing a cathode material:

[0096] The only difference between this embodiment and Example 1 is that the carbon content in the carbon source added in step (3) is still 3% in the final cathode material, and the weight ratio of soluble starch, polyethylene glycol PEG-2000 and graphene added is changed to 1.24:1:0.4.

[0097] After the second calcination, the two organic carbon sources, soluble starch and polyethylene glycol PEG-2000, in the carbon layer are transformed into amorphous carbon parts, while the inorganic carbon source, graphene, is transformed into inorganic carbon parts. The mass ratio of the three contributing to the carbon layer is 4:5:1. That is, based on the total weight of the carbon layer being 100%, the weight percentage of the inorganic carbon parts is 10%.

[0098] Example 7

[0099] A method for preparing a cathode material:

[0100] The only difference between this embodiment and Embodiment 1 is that the carbon source added in step (3) is changed to have a carbon layer content of 1.5% in the final positive electrode material, and the carbon source only includes soluble starch and polyethylene glycol PEG-2000, and the ratio of the amount of soluble starch to polyethylene glycol PEG-2000 added is 1.56:1. The mass ratio of the carbon layer provided by soluble starch and polyethylene glycol PEG-2000 in the obtained carbon layer is 1:1, and the inorganic carbon part is 0%.

[0101] Example 8

[0102] A method for preparing a cathode material:

[0103] The only difference between this embodiment and Embodiment 1 is that the carbon source added in step (3) includes only soluble starch, and the carbon layer provided by the soluble starch accounts for 3% of the obtained positive electrode material.

[0104] Example 9

[0105] A method for preparing a cathode material:

[0106] The only difference between this embodiment and Example 1 is that the carbon source added in step (3) includes only polyethylene glycol PEG-2000, and the carbon layer provided by polyethylene glycol PEG-2000 accounts for 3% of the obtained cathode material.

[0107] Example 10

[0108] A method for preparing a cathode material:

[0109] The only difference between this embodiment and Embodiment 1 is that the total amount of carbon source in step (3) is changed so that the mass fraction of carbon layer in the final cathode material is changed to 1%.

[0110] Example 11

[0111] A method for preparing a cathode material:

[0112] The only difference between this embodiment and Embodiment 1 is that the total amount of carbon source in step (3) is changed so that the mass fraction of carbon layer in the final cathode material is changed to 4%.

[0113] Example 12

[0114] A method for preparing a cathode material:

[0115] The only difference between this embodiment and Embodiment 1 is that the calcination conditions in step (4) are different, specifically:

[0116] The first stage of calcination is carried out at a temperature of 30℃ for 2.5 hours.

[0117] The second stage of calcination is carried out at a temperature of 80℃ for 2 hours.

[0118] The third stage of calcination is carried out at a temperature of 240℃ for 1 hour.

[0119] The fourth stage of calcination is carried out at a temperature of 650℃ for 1 hour.

[0120] Example 13

[0121] A method for preparing a cathode material:

[0122] The only difference between this embodiment and Embodiment 1 is that the calcination conditions in step (4) are different, specifically:

[0123] The first stage of calcination is carried out at a temperature of 70℃ for 0.5 hours.

[0124] The second stage of calcination was carried out at a temperature of 140℃ for 0.3 hours.

[0125] The third stage of calcination is carried out at a temperature of 150℃ for 5 hours.

[0126] The fourth stage of calcination is carried out at a temperature of 450℃ for 7 hours.

[0127] Example 14

[0128] A method for preparing a cathode material:

[0129] The only difference between this embodiment and Embodiment 1 is that the calcination conditions in step (4) are different, specifically:

[0130] The first stage of calcination is carried out at a temperature of 200℃ for 3 hours.

[0131] The second stage of calcination was carried out at a temperature of 550℃ for 4 hours.

[0132] The SEM characterization results of the obtained cathode material are shown in Figure 2.

[0133] Example 15

[0134] A method for preparing a cathode material:

[0135] The only difference between this embodiment and Embodiment 1 is that the calcination conditions in step (4) are different, specifically:

[0136] Only one calcination stage was performed at a temperature of 550℃ for 4 hours.

[0137] The SEM characterization results of the obtained cathode material are shown in Figure 3.

[0138] Example 16

[0139] A method for preparing a cathode material:

[0140] The only difference between this embodiment and Example 1 is that the sodium source, iron source, phosphorus source and carbon source used are mixed at the same time, water is used as a dispersant to obtain a slurry, and the cathode material is obtained under the drying and calcination conditions in step (2) of Example 1.

[0141] Example 17

[0142] A method for preparing a cathode material:

[0143] The only difference between this embodiment and embodiment 1 is that the heating rate of the four calcination stages in step (4) is changed to 3℃ / min.

[0144] Example 18

[0145] A method for preparing a cathode material:

[0146] The only difference between this embodiment and embodiment 1 is that the heating rate of the four calcination stages in step (4) is changed to 10℃ / min.

[0147] Example 19

[0148] A method for preparing a cathode material:

[0149] The only difference between this embodiment and embodiment 1 is that the heating rate of the four calcination stages in step (4) is changed to 2℃ / min.

[0150] Example 20

[0151] A method for preparing a cathode material:

[0152] The only difference between this embodiment and embodiment 1 is that the heating rate of the four calcination stages in step (4) is changed to 12℃ / min.

[0153] Example 21

[0154] A method for preparing a cathode material:

[0155] The only difference between this embodiment and Embodiment 1 is that the calcination conditions in step (2) are different, specifically:

[0156] The first calcination involves heating from room temperature to 350℃ at a rate of 5℃ / min for 3 hours, followed by a second calcination at a rate of 5℃ / min from 350℃ to 500℃ for 11 hours to obtain the precursor.

[0157] Example 22

[0158] A method for preparing a cathode material:

[0159] The only difference between this embodiment and Embodiment 1 is that the calcination conditions in step (2) are different, specifically:

[0160] The first calcination was carried out at room temperature, with the temperature increased to 550℃ at 5℃ / min for 1 hour, and the second calcination was carried out at 700℃ at 5℃ / min for 13 hours to obtain the precursor.

[0161] Comparative Example 1

[0162] A method for preparing a cathode material:

[0163] The only difference between this comparative example and Example 1 is that the amount of each raw material added is changed so that the molar ratio of olivine phosphate to polyanionic phosphate in the composite particles of the obtained cathode material is 0:1.

[0164] Comparative Example 2

[0165] A method for preparing a cathode material:

[0166] The only difference between this comparative example and Example 1 is that the amount of each raw material added is changed so that the molar ratio of olivine phosphate to polyanionic phosphate in the composite particles of the obtained cathode material is 0.1:1.

[0167] Comparative Example 3

[0168] A method for preparing a cathode material:

[0169] The only difference between this comparative example and Example 1 is that the amount of each raw material added is changed so that the molar ratio of olivine phosphate to polyanionic phosphate in the composite particles of the obtained cathode material is 0.8:1.

[0170] Comparative Example 4

[0171] A method for preparing a cathode material:

[0172] The only difference between this comparative example and Example 1 is that the amount of each raw material added is changed so that the molar ratio of olivine phosphate to polyanionic phosphate in the composite particles of the obtained cathode material is 1:1.

[0173] II. Testing Methods

[0174] D50: GB / T 19077, Laser diffraction method for particle size analysis.

[0175] BET: GB / T 19587, Determination of specific surface area of ​​solid substances by gas adsorption BET method.

[0176] Packing ratio: GB / T 5000-2017, Determination of powder packing density of powder metallurgy parts.

[0177] Compacted density: GB / T 24533, Determination of compacted density of powder.

[0178] Tap density: GB / T 5162-2021, Determination of tap density of powder.

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

[0180] Battery performance test:

[0181] (1) Preparation of the positive electrode sheet: The positive electrode materials obtained in the examples and comparative examples were mixed with acetylene black (as a conductive agent) and polyvinylidene fluoride (PVDF, as a binder) at a mass ratio of 88:6:6, with the mass accurate to 0.001g, respectively, to form a positive electrode slurry. Aluminum foil was used as the current collector for the positive electrode sheet. The positive electrode material, acetylene black, PVDF and N-methylpyrrolidone were stirred to form a slurry to obtain the positive electrode slurry. The positive electrode slurry was uniformly coated on the aluminum foil, dried in an oven at 100℃, and then cut into electrode sheets with a diameter of 18mm and a thickness of 0.10mm. The electrode sheets were weighed with the mass accurate to 0.0001g.

[0182] (2) Battery assembly: In an inert gas glove box with water and oxygen contents of ≤0.0005%, a sodium metal sheet was used as the negative electrode material, a polypropylene microporous membrane was used as the separator, and a sodium-ion battery electrolyte consisting of 1 mol / L sodium perchlorate (NaClO4) and a mixed carbonate-based organic solvent [ethylene carbonate (EC) / diethyl carbonate (DEC) / fluoroethylene carbonate (FEC) (the volume ratio of EC, DEC, and FEC is 1:1:0.05)] was assembled and sealed to form a test battery.

[0183] (3) Battery testing: The test battery was subjected to charge-discharge cycles at 25°C using a sodium-ion battery electrochemical performance tester (Wuhan Landian CT2001A battery testing system).

[0184] (a) 0.2C rate current charging, charging limit voltage 4.0V;

[0185] (b) Discharge at a rate of 0.2C, with a discharge termination voltage of 2.0V.

[0186] The results of the above tests are shown in Tables 1 and 2.

[0187] Table 1

[0188] Table 2

[0189] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0190] As can be seen from the above results, the embodiments of the present invention have achieved the preparation of a sodium-ion battery cathode material with excellent electrochemical performance. Specifically:

[0191] Comparing the various embodiments with the comparative examples, it can be seen that the positive electrode material prepared in this application, wherein the molar ratio of olivine phosphate to polyanionic phosphate is (0.2~0.5):1, and the polyanionic phosphate phase encapsulates the nanocrystals of olivine phosphate, has superior performance compared with conventional iron-based phosphate sodium battery materials. After being used as a positive electrode material to prepare sodium-ion batteries, it exhibits higher first charge specific capacity and first discharge specific capacity.

[0192] Comparing Examples 1 to 3 with Example 4 and Comparative Examples 1 to 4, it can be seen that when the molar ratio of olivine phosphate to polyanionic phosphate is (0.2 to 0.5):1, especially (0.2 to 0.4):1, the coordination between the two phases of olivine phosphate and polyanionic phosphate is better, and the electrochemical performance of the resulting cathode material is superior.

[0193] Comparing Examples 1 to 3 with Examples 5 to 9, it is evident that the preferred carbon layer comprises an amorphous carbon portion formed from an organic carbon source and an inorganic carbon portion formed from an inorganic carbon source, with the inorganic carbon portion accounting for 2.5% to 5.0% by weight. This facilitates the synergistic effect of the two carbon layers with different properties and functions, further enhancing the electrochemical performance of the cathode material. In particular, when the preferred carbon source is specifically soluble starch, polyethylene glycol, and graphene, and the ratio of the added amount of soluble starch, PEG-2000, and graphene is (1.2 to 1.5): 1: (0.2 to 0.4), more preferably (1.25 to 1.5): 1: (0.2 to 0.3), it is beneficial to better synergize the superior properties of graphene, polyethylene glycol, and soluble starch after calcination, thereby obtaining a uniformly coated carbon layer with good conductivity and improving the electrochemical performance of the resulting cathode material.

[0194] Comparing Examples 1 to 3 with Examples 10 and 11, it can be seen that when the total weight of the cathode material is 100% and the mass fraction of the carbon layer is 1.5% to 3.0%, the electron transport capability and stability of the obtained cathode material can be better balanced, thereby improving its electrochemical performance.

[0195] Comparing Examples 1 to 3 with Example 16, it can be seen that the embodiments of this application use a two-step method, which, compared with the one-step method commonly used in the art, is more conducive to improving the electronic conductivity, compaction density, and tap density of the obtained cathode material, thereby obtaining a cathode material with higher electrochemical performance.

[0196] Comparing Examples 1 to 3 with Examples 21 and 22, it can be seen that during the formation of the precursor in the first calcination process, controlling the calcination process to be a first calcination and a second calcination performed sequentially, with the first calcination temperature being 400℃~500℃ and the time being 1.5h~2.5h; and the second calcination temperature being 550℃~650℃ and the time being 10h~14h, can promote the formation of a precursor with high phase purity and improve the electrochemical performance of the obtained cathode material.

[0197] Comparing Examples 1 to 3 with Examples 12 to 15, it can be seen that optimizing the second calcination into four stages during the formation of polyanionic phosphate in the second calcination process, while precisely controlling its temperature range and calcination time, can help achieve the segmented decomposition of the carbon source mixed with the precursor, while suppressing the degradation of the electrochemical performance of the obtained cathode material; at the same time, it can also enable better recombination between the two crystal phases, thereby significantly improving the electrochemical performance of the obtained cathode material.

[0198] A comparison of Examples 1 to 3 with Examples 17 to 20 shows that a heating rate of 3°C / min to 10°C / min can improve the phase purity and crystallinity of the obtained cathode material, thereby enabling it to exhibit higher electrochemical performance.

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

Claims

1. A positive electrode material, characterized in that, The positive electrode material comprises composite particles, the molecular formula of which is: Na x Fe y (PO4) n (P2O7) m Where x is 4.2–4.5, y is 3.2–3.5, n is 2.2–2.5, and m is 0.9–1.1; The composite particles include olivine phosphate and polyanionic phosphate coated on the surface of the olivine phosphate, and the molar ratio of the olivine phosphate to the polyanionic phosphate is (0.2-0.5):

1.

2. The cathode material according to claim 1, characterized in that, The molar ratio of the olivine-type phosphate to the polyanionic phosphate is (0.2-0.4):

1.

3. The cathode material according to claim 1 or 2, characterized in that, The cathode material further includes a carbon layer coated on the surface of the composite particles, and the mass fraction of the carbon layer is 1.5% to 3.0% based on the total weight of the cathode material as 100%. Preferably, the carbon layer is an amorphous carbon portion formed by an organic carbon source and / or an inorganic carbon portion formed by an inorganic carbon source; More preferably, the carbon layer is a combination of the amorphous carbon portion and the inorganic carbon portion, and the inorganic carbon portion accounts for 2.5% to 5.0% of the total weight of the carbon layer (100%).

4. The cathode material according to any one of claims 1 to 3, characterized in that, The resistivity of the positive electrode material powder is 12000Ω·cm~93000Ω·cm.

5. A method for preparing a positive electrode material, characterized in that, The method for preparing the cathode material includes: Sodium source, iron source and phosphorus source are formulated into the first slurry; The first slurry is subjected to a first drying and a first calcination to obtain a precursor; The precursor and carbon source are formulated into a second slurry; The second slurry is subjected to a second drying and a second calcination to obtain the positive electrode material.

6. The method for preparing the cathode material according to claim 5, characterized in that, In the first slurry, the molar ratio of sodium, iron, and phosphorus is (4.2–4.86):(3.2–3.5):(4.2–4.5); Preferably, the first calcination includes a first calcination and a second calcination performed sequentially, and, The first calcination is carried out at a temperature of 400℃ to 500℃ for 1.5h to 2.5h; the second calcination is carried out at a temperature of 550℃ to 650℃ for 10h to 14h. More preferably, the first calcination is carried out under a protective atmosphere.

7. The method for preparing the cathode material according to claim 6, characterized in that, The sodium source is selected from one or more of sodium bicarbonate, sodium phosphate, sodium monohydrogen phosphate, and sodium dihydrogen phosphate; the iron source is selected from one or more of anhydrous ferric phosphate, polyhydrate ferric phosphate, monohydrogen phosphate, and ferrous oxalate; the phosphorus source is selected from one or more of anhydrous ferric phosphate, ammonium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, polyhydrate ferric phosphate, and monohydrogen phosphate. Preferably, the carbon source includes an organic carbon source and / or an inorganic carbon source; Preferably, the organic carbon source is selected from one or more of citric acid, glucose, sucrose, soluble starch and polyethylene glycol; and / or, the inorganic carbon source is selected from one or more of graphite, carbon nanotubes and graphene. Preferably, the carbon source includes the soluble starch, the polyethylene glycol, and the graphene; More preferably, the polyethylene glycol is PEG-2000.

8. The method for preparing the cathode material according to any one of claims 5 to 7, characterized in that, The second calcination includes the following steps performed sequentially: The first stage of calcination is carried out at a temperature of 40℃~60℃ and a holding time of 1h~2h. The second stage of calcination is carried out at a temperature of 100℃~130℃, and the holding time is 0.5h~1.5h. The third stage of calcination is carried out at a temperature of 180℃~210℃, and the holding time is 2h~4h. The fourth stage of calcination is carried out at a temperature of 500℃~600℃ and a holding time of 2h~6h. Preferably, the heating rates of the first calcination stage, the second calcination stage, the third calcination stage, and the fourth calcination stage are each independently 3℃ / min to 10℃ / min.

9. A positive electrode sheet, characterized in that, The positive electrode sheet comprises the positive electrode material according to any one of claims 1 to 4, or the positive electrode material prepared by the method of preparing the positive electrode material according to any one of claims 5 to 8.

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

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