Phosphate-based positive electrode material and preparation method therefor, positive electrode sheet, and secondary battery

Abstract

The present application relates to the field of battery materials, and provides a phosphate-based positive electrode material and a preparation method therefor, a positive electrode sheet, and a secondary battery. The phosphate-based positive electrode material comprises a core and a carbon layer coating the core. The core is expressed as Na3V2-x-yAxBy(PO4)2F3, wherein 0≤x≤0.5, 0≤y≤0.5, x and y are not simultaneously zero, the element A and the element B each independently comprise one or more of Fe, Mn, Ti, Al, and Mg, and the element A and the element B are different from each other. The phosphate-based positive electrode material has a compaction density of greater than or equal to 2.0 g / cm3 and a BET specific surface area of greater than or equal to 8 m2 / g. The core of the phosphate-based positive electrode material is doped with metal elements for a high-voltage plateau, and the core is coated with a carbon layer, thereby effectively improving the electron conductivity, compaction density and BET specific surface area of the phosphate-based positive electrode material, and enabling the phosphate-based positive electrode material to have excellent capacity, rate performance and cycle performance.

Description

Phosphate cathode materials and their preparation methods, cathode plates and secondary batteries Technical Field

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

[0002] Sodium-ion batteries (SIBs) are a type of battery that relies on sodium ions (Na+) for energy. + Sodium-ion batteries (SIBs) are secondary batteries that move between positive and negative electrodes to achieve charging and discharging. Their working principle is similar to that of mainstream lithium-ion batteries (LIBs), and they have attracted much attention due to the abundance and low cost of sodium ions in the Earth's crust, making them the most likely alternative to LIBs in grid-scale energy storage and power batteries for new energy vehicles. Among the positive electrode materials for sodium-ion batteries, polyanionic compounds (PACs) have excellent cycle life and show great promise for application in the SIB field.

[0003] Sodium superionic conductor (NASICON) is one of the representative PAC materials, possessing a stable three-dimensional open framework that enables the realization of Na+. + Rapid migration. Phosphate materials, such as sodium vanadium phosphate (Na3V2(PO4)3, NVP) and sodium vanadium fluorophosphate (Na3V2(PO4)2F3, NVPF), are typical NASICON materials and can be used as cathode materials for SIBs. However, traditional phosphate cathode materials suffer from drawbacks such as poor electronic conductivity, low compaction density, and small BET specific surface area, which limit their electrochemical performance. Summary of the Invention

[0004] In view of the technical problems existing in the background art, this application provides a phosphate cathode material and its preparation method, cathode sheet and secondary battery, aiming to solve the technical problems that traditional phosphate cathode materials have defects such as poor electronic conductivity, low compaction density and small BET specific surface area, which limit their electrochemical performance.

[0005] In a first aspect, embodiments of this application provide a phosphate cathode material, including a core and a carbon layer covering the core;

[0006] The kernel expression is Na 3+x+y V 2-x-y A x B y (PO4)2F3;

[0007] Where 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.5, and x and y are not both zero;

[0008] Element A and element B each independently include one or more of Fe, Mn, Ti, Al, and Mg, and elements A and B are different from each other;

[0009] The compaction density of phosphate cathode material is ≥2.0 g / cm³. 3 BET specific surface area is ≥8m² 2 / g.

[0010] In the technical solution of this application embodiment, the core of the phosphate cathode material is a sodium vanadium fluorophosphate material doped with at least one of element A and element B. The elements A and B are selected from metal elements such as Fe, Mn, Ti, Al and Mg, which are beneficial to improving the voltage platform. This helps to optimize the electronic conductivity of the phosphate cathode material and effectively improve the average voltage platform. At the same time, by controlling the compaction density and BET specific surface area of ​​the phosphate cathode material and coating the core with a carbon layer, the electronic conductivity, structural stability and energy density of the phosphate cathode material are further improved. This is beneficial to the excellent performance of the phosphate cathode material in terms of capacity, rate performance and cycle performance, thereby obtaining a high-energy-density sodium-ion battery.

[0011] In some embodiments, the carbon layer has a mass fraction of 1.8% to 3% in the phosphate cathode material; and / or, the thickness of the carbon layer is 0.8 nm to 1.8 nm.

[0012] In this embodiment, controlling the mass fraction and / or thickness of the carbon layer within the above-mentioned range is beneficial to ensuring that the carbon layer has a good coating effect on the core, which can improve electronic conductivity, enhance structural stability and suppress side reactions, thereby improving the electrochemical performance of the phosphate cathode material, such as specific capacity, rate performance and cycle performance.

[0013] In some embodiments, the phosphate cathode material has an initial charge specific capacity of ≥120mAh / g at 0.1C.

[0014] In some embodiments, the phosphate cathode material has an initial discharge specific capacity of ≥110 mAh / g at 0.1C.

[0015] In some embodiments, the phosphate cathode material has an initial coulombic efficiency of ≥90% at 0.1C.

[0016] In this embodiment, the phosphate cathode material has high charge specific capacity, discharge specific capacity and first coulombic efficiency, thereby ensuring that the cathode sheet and the secondary battery have the advantages of high specific capacity and good rate performance.

[0017] Secondly, embodiments of this application provide a method for preparing a phosphate cathode material, comprising the following steps:

[0018] A mixture of vanadium source, oxalic acid, phosphorus source, dopant, and solvent is obtained through reaction treatment.

[0019] The mixture is subjected to a first drying treatment and a first sintering treatment to obtain a precursor;

[0020] The precursor, sodium source, fluorine source and carbon source are mixed and ground to obtain a slurry;

[0021] The slurry undergoes a second drying treatment and a second sintering treatment to obtain the phosphate cathode material;

[0022] The dopant includes a first source and / or a second source, wherein the first metal element in the first source and the second metal element in the second source each independently include one or more of Fe, Mn, Ti, Al and Mg, and the first metal element and the second metal element are different from each other.

[0023] In the technical solution of this application embodiment, a two-step sintering process is performed, which makes it easier for the first metal element and / or the second metal element in the dopant to embed into the crystal structure of the sodium vanadium fluorophosphate material, thereby improving the structural stability of the phosphate cathode material. Furthermore, the first metal element and / or the second metal element facilitates more complete melting of the phosphate cathode material during high-temperature sintering, resulting in a denser and smoother surface morphology. This improves the compaction density, BET specific surface area, rate performance, and cycle performance of the phosphate cathode material. Simultaneously, the introduction of reducing oxalic acid during the precursor preparation process facilitates the reduction of vanadium in the vanadium source. Moreover, the oxalic acid leaves almost no residual carbon after sintering, allowing for better control of the carbon content and surface morphology of the phosphate cathode material, thus improving its compaction density and BET specific surface area. Therefore, the phosphate cathode material prepared in this application not only has significantly improved compaction density and BET specific surface area, but also has carbon layer coated on the core of the phosphate cathode material, which further improves the electronic conductivity, structural stability and energy density of the phosphate cathode material, thereby ensuring its excellent performance in specific capacity, rate performance and cycle performance, which is beneficial for the preparation of high energy density sodium-ion batteries.

[0024] In some embodiments, the step of mixing a vanadium source, oxalic acid, a phosphorus source, a dopant, and a solvent, and then reacting the mixture to obtain a solution includes:

[0025] The vanadium source and the solvent are mixed and subjected to a first temperature and a first time to obtain a first solution;

[0026] The first solution, the dopant, and the oxalic acid are mixed and subjected to a second temperature for a second time to obtain a second solution;

[0027] The second solution and the phosphorus source are mixed and subjected to a third temperature for a third time to obtain the mixture;

[0028] Wherein, the first temperature is 80℃~90℃, and the first time is 30min~90min;

[0029] The second temperature is 90℃~95℃, and the second time is 30min~60min;

[0030] The third temperature is 90℃~100℃, and the third time is 12min~36min.

[0031] In this embodiment, a first solution containing a vanadium source is used as the reaction substrate, and dopants, oxalic acid and phosphorus source are added step by step to carry out the reaction. This helps to ensure that the raw materials are mixed evenly and react fully, and makes it easier for the metal elements in the dopant to be embedded in the crystal structure of the precursor, which is beneficial to improving the purity and structural stability of the phosphate cathode material.

[0032] In some embodiments, when the dopant includes the first source and the second source, the molar ratio of vanadium in the vanadium source, oxalic acid, phosphorus in the phosphorus source, the first metal element in the first source, and the second metal element in the second source is (0.9-1.1):(2.5-3.5):1:(0-0.25):(0-0.25).

[0033] In this embodiment, by controlling the molar ratio of each element in the raw materials, it is beneficial to achieve full reaction between the raw materials and obtain a precursor with high purity.

[0034] In some embodiments, the vanadium source includes one or more of vanadium pentoxide, vanadium trioxide, vanadium dioxide, ammonium metavanadate, sodium metavanadate, sodium orthovanadate, and vanadium oxalate.

[0035] In some embodiments, the phosphorus source includes one or more of phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate.

[0036] In some embodiments, the first source includes one or more of oxides, hydroxides, and salt compounds containing the first metal element.

[0037] In some embodiments, the second source includes one or more of oxides, hydroxides, and salt compounds containing the second metal element.

[0038] In some embodiments, the molar ratio of phosphorus in the precursor, sodium in the sodium source, fluorine in the fluorine source, and carbon in the carbon source is 1:(1.3-1.5):(1.3-1.5):(0.48-0.9).

[0039] In this embodiment, by controlling the molar ratio of each element in the raw materials, the raw materials react fully to form a high-purity sodium vanadium fluorophosphate material, and a carbon layer with a suitable thickness and carbon content is formed on its surface.

[0040] In some embodiments, the sodium source includes one or more of sodium fluoride, sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium acetate, sodium oxalate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate.

[0041] In some embodiments, the fluorine source includes one or more of sodium fluoride, ammonium fluoride, and hydrogen fluoride.

[0042] In some embodiments, the carbon source includes one or more of glucose, polyethylene glycol, sucrose, and ascorbic acid.

[0043] In this embodiment, the selection of different carbon sources is beneficial to adjusting the compaction density, BET specific surface area and electrochemical performance of phosphate cathode materials, thereby improving the overall performance of phosphate cathode materials.

[0044] In some embodiments, the sintering temperature of the first sintering treatment is 700℃~750℃, and the holding time is 1h~3h.

[0045] In this embodiment, the first sintering treatment, which involves holding the material at 700℃ to 750℃ for 1 to 3 hours, helps to improve the crystallinity of the precursor, thereby increasing the purity of the phosphate cathode material and thus improving its electrochemical performance.

[0046] In some embodiments, the sintering temperature of the second sintering treatment is 550℃~750℃, and the holding time is 8h~12h.

[0047] In this embodiment, a second sintering treatment is performed at 550℃ to 750℃ for 8 to 12 hours. This effectively controls energy consumption while avoiding the formation of too many impurity phases during the sintering process. This helps to improve the crystallinity and purity of the phosphate cathode material, thereby enhancing electrochemical performance such as charging specific capacity and discharging specific capacity.

[0048] Thirdly, embodiments of this application provide a positive electrode sheet, including the phosphate positive electrode material provided in the first aspect of this application, or the phosphate positive electrode material prepared by the method for preparing the phosphate positive electrode material provided in the second aspect of this application.

[0049] In this embodiment, the positive electrode sheet includes the above-mentioned phosphate positive electrode material, thus having the advantages of high charge specific capacity and discharge specific capacity, good rate performance and excellent cycle performance.

[0050] Fourthly, embodiments of this application provide a secondary battery, including the positive electrode sheet provided in the third aspect of this application.

[0051] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, thus possessing advantages such as high charging and discharging specific capacity, good rate performance, and excellent cycle performance.

[0052] 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

[0053] 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.

[0054] Figure 1 is a schematic flowchart of a method for preparing a phosphate cathode material in one embodiment;

[0055] Figure 2 shows the morphology of the phosphate cathode material in Example 1;

[0056] Figure 3 shows the morphology of the phosphate cathode material in Comparative Example 1.

[0057] Figure 4 shows the charge-discharge curves of the phosphate cathode material in Example 1;

[0058] Figure 5 shows the charge-discharge curves of the phosphate cathode material in Comparative Example 1. Detailed Implementation

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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).

[0065] In the description of the embodiments of this application, unless otherwise specified, the solvent or water in the "solution" or "base liquid" is at least one of distilled water, deionized water, pure water, and ultrapure water.

[0066] the term

[0067] Unless otherwise stated or in case of conflict, the terms or phrases used in this application shall have the following meanings:

[0068] Particle size: For spherical particles, particle size refers to the diameter of the spherical particle. For non-spherical particles, such as particles with an olivine morphology, particle size usually refers to the equivalent particle size (generally referred to as particle size), which can be obtained by scanning electron microscopy (SEM) or laser particle size analyzer. The equivalent particle size means that when a particle has a physical property that is the same as or similar to that of a homogeneous spherical particle, the diameter of the spherical particle is used to represent the diameter of the actual particle. Unless otherwise stated or contradictory, the particle size in this application refers to the equivalent particle size.

[0069] Particle size distribution parameter: In the particle size distribution curve, the particle size corresponding to the cumulative particle size distribution percentage reaching N% is called the DN particle size, indicating that particles smaller than this size account for N% of all particles, where N = 0 to 100. When N = 100, the D100 particle size represents the particle size corresponding to the cumulative particle size distribution percentage reaching 100%. When N = 50, the D50 particle size is the particle size corresponding to the cumulative particle size distribution percentage reaching 50%, representing the median particle size, indicating that particles smaller than and larger than this size each account for 50%. For example, a D50 particle size of 1 mm means that particles smaller than 1 mm and particles larger than 1 mm each account for 50% of all particles.

[0070] Sodium-ion batteries (SIBs) are facing increasing demands for capacity and compaction density during commercialization. High capacity and high compaction density translate to higher energy density after assembly. Currently, SIB electrode materials face significant challenges, such as insurmountable structural instability, sluggish ion diffusion, low operating voltage, and low energy / power density. To address these issues, researchers are primarily focused on designing and fabricating novel electrode materials with high adaptability and reversibility during large sodium ion intercalation / extraction processes. Layered transition metal oxides (TMOs) and polyanionic compounds (PACs) are two of the most promising candidates for SIB cathode materials. The theoretical capacity of PAC materials is slightly lower than that of TMO materials (such as NaMnO2 and P2-Na). 2 / 3 Fe 1 / 2 Mn 1 / 2 While PAC materials can utilize O2 and vanadium oxides (among others), they typically exhibit superior cycle life compared to TMO materials, meeting the requirements for ultra-long cycle life in grid-scale energy storage. Therefore, if the overall electrochemical performance of PAC materials (including high-rate capability, specific capacity, and operating voltage) is improved, their prospects in SIB applications will be very broad.

[0071] Sodium superionic conductor (NASICON) is a representative PAC material. It possesses a stable three-dimensional (3D) open framework composed of PO tetrahedra and MO octahedra (M representing a transition metal), enabling the realization of Na… + Rapid migration. Sodium vanadium phosphate (Na3V2(PO4)3, NVP) is a typical NASICON material with a theoretical capacity of 117.6 mAh / g and an operating voltage of 3.3V–3.4V, providing a material-based energy density of approximately 394 Wh / kg, significantly lower than the theoretical value of phosphate cathode materials in SIBs. Therefore, a major challenge for phosphate cathode materials applied in SIBs is to improve energy density to narrow the gap between theoretical and actual energy densities. Compared to fluorine-free NVP, fluorine-doped NASICON materials typically have higher operating voltages. For example, sodium vanadium fluorophosphate (Na3V2(PO4)2F3, NVPF) can provide an average operating voltage up to 3.95V, a theoretical capacity of 128 mAh / g, and a corresponding theoretical energy density as high as ~507 Wh / kg. Therefore, NVPF is a more attractive and promising high-energy-density NASICON material for use in SIBs. However, as a member of the NASICON family, NVPF also suffers from low electronic conductivity, as well as problems such as low compaction density and small BET specific surface area, which limit its capacity and rate performance and hinder it from achieving comprehensive and excellent electrochemical performance, including high energy density.

[0072] To address the limitations of traditional phosphate cathode materials, such as poor electronic conductivity, low compaction density, and small BET specific surface area, which restrict their electrochemical performance, this application provides a phosphate cathode material, its preparation method, a cathode electrode, and a secondary battery. The core of the phosphate cathode material is a sodium vanadium fluorophosphate material doped with metal elements, and a carbon layer is coated on the core. This effectively improves the electronic conductivity, compaction density, and BET specific surface area of ​​the phosphate cathode material, resulting in excellent capacity, rate performance, and cycle performance. Consequently, the capacity, rate performance, and cycle performance of the cathode electrode and the secondary battery are also improved.

[0073] In a first aspect, embodiments of this application provide a phosphate cathode material, including a core and a carbon layer covering the core;

[0074] The kernel expression is Na 3+x+y V 2-x-y A x B y (PO4)2F3;

[0075] Where 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.5, and x and y are not both zero;

[0076] Element A and element B each independently include one or more of Fe, Mn, Ti, Al, and Mg, and elements A and B are not the same.

[0077] The compaction density of phosphate cathode material is ≥2.0 g / cm³. 3 BET specific surface area is ≥8m² 2 / g.

[0078] In the technical solution of this application embodiment, the core of the phosphate cathode material is a sodium vanadium fluorophosphate material doped with at least one of element A and element B. The elements A and B are selected from metal elements such as Fe, Mn, Ti, Al and Mg, which are beneficial to improving the voltage platform. This helps to optimize the electronic conductivity of the phosphate cathode material and effectively improve the average voltage platform. At the same time, by controlling the compaction density and BET specific surface area of ​​the phosphate cathode material and coating the core with a carbon layer, the electronic conductivity, structural stability and energy density of the phosphate cathode material are further improved. This is beneficial to the excellent performance of the phosphate cathode material in terms of capacity, rate performance and cycle performance, thereby obtaining a high-energy-density sodium-ion battery.

[0079] As an example, the compaction density of phosphate cathode materials is ≥2.0 g / cm³. 3 Including but not limited to 2.0g / cm 3 2.02 g / cm 3 2.04 g / cm 3 2.06 g / cm 3 2.08 g / cm 3 2.1g / cm 3 2.12 g / cm 3 2.14 g / cm 3 2.16 g / cm 3 2.18 g / cm 3 Or 2.2g / cm 3 Furthermore, the preferred compaction density of the phosphate cathode material is 2.03 g / cm³. 3 ~2.13g / cm 3 Phosphate cathode materials have higher compaction density, which is beneficial for improving specific capacity and rate performance, and also helps to reduce the amount of phosphate cathode materials used, thus reducing the manufacturing cost of sodium-ion batteries.

[0080] As an example, the BET specific surface area of ​​phosphate cathode materials is ≥8m². 2 / g, including but not limited to 8m 2 / g, 8.5m 2 / g、9m 2 / g, 9.5 m 2 / g, 9.8 m 2 / g, 10 m 2 / g, 12 m 2 / g, 14 m 2 / g, 16 m 2 / g, 18 m 2 / g or 20 m 2 / g. Further, the BET specific surface area of the phosphate cathode material is preferably 10 m 2 / g ~ 15.5 m 2 / g. The phosphate cathode material has a moderate specific surface area, which is conducive to providing more active sites for the occurrence of electrochemical reactions and improving the transmission rate of sodium ions, helping the sodium-ion battery to have a higher specific capacity and better rate performance.

[0081] It can be understood that the values of x and y each independently include but are not limited to 0, 0.01, 0.05, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5, and x and y are not both zero at the same time. If x = 0 or y = 0, the core of the phosphate cathode material is doped with at least one metal cation. If 0 < x ≤ 0.5 and 0 < y ≤ 0.5, the core of the phosphate cathode material is doped with at least two different metal cations, thus forming a high-entropy doped sodium vanadium fluorophosphate material.

[0082] It can be understood that according to the value ranges of x and y, (2 - x - y):x:y = (1 - 2):(0 - 0.5):(0 - 0.5), and non-limiting examples include but are not limited to: 1:0.5:0.5, 1.1:0.5:0.4, 1.1:0.4:0.5, 1.2:0.4:0.4, 1.3:0.4:0.3, 1.3:0.3:0.4, 1.4:0.3:0.3, 1.5:0.3:0.2, 1.5:0.2:0.3, 1.6:0.2:0.2, 1.7:0.2:0.1, 1.7:0.1:0.2, 1.8:0.1:0.1, 1.9:0.05:0.05, 1.9:0.1:0 or 1.9:0:1.

[0083] Understandably, element A and element B each independently include one or more of Fe, Mn, Ti, Al, and Mg, and element A and element B are distinct from each other. Combinations of element A and element B include, but are not limited to: Fe and Mn; Fe and Ti; Fe and Al; Fe and Mg; Mn and Ti; Mn and Al; Mn and Mg; Ti and Al; Ti and Mg; Al and Mg; Fe, Mn and Ti; Fe, Mn and Al; Fe, Mn and Mg; Fe, Ti and Al; Fe, Ti and Mg; Fe, Al and Mg; Fe, Mn, Ti and Al; Fe, Mn, Ti and Mg; Fe, Mn, Ti, Al and Mg, etc. When element A is selected from at least two metallic elements, the expression for the kernel can be Na3V. 2-x-y A1 x1 A2 x2 A3 x3...... An xn B y (PO4)2F3, where A1, A2, A3...An are distinct metallic elements, and 0 ≤ x1 + x2 + x3 + ... + xn ≤ 0.5. When element B is selected from at least two metallic elements, the kernel expression can be Na3V. 2-x-y A x B1 y1 B2 y2 B3 y3...... Bm yn (PO4)2F3, where B1, B2, B3...Bm are distinct metallic elements, and 0 ≤ y1 + y2 + y3 + ... + ym ≤ 0.5. When elements A and B are independently selected from two metallic elements, the kernel expression can be Na3V. 2-x-y A1 x1 A2 x2 B1 y1 B2 y2 (PO4)2F3, where A1, A2, B1, and B2 are distinct metallic elements, 0 ≤ x1 + x2 ≤ 0.5, and 0 ≤ y1 + y2 ≤ 0.5.

[0084] In the technical solutions of this application embodiment, the ionic radii of metals such as Fe, Mn, Ti, Al, and Mg are all similar to V. 3+ The ionic radii (0.062 nm) are similar, which is beneficial for expanding the Na+ ion radius. +The diffusion channels enhance the ionic conductivity of phosphate cathode materials, suppressing structural phase transitions during charge-discharge processes and thus reducing voltage decay caused by phase transitions. Compared to NVPFs without metal cation doping, doping sodium vanadium fluorophosphate with high-voltage plateau metal cations such as Fe, Mn, Ti, Al, and Mg effectively improves the average voltage plateau and specific capacity of the phosphate cathode material. Co-doping with specific types and proportions of metal cations also promotes more complete melting of the phosphate cathode material during high-temperature sintering, resulting in a denser and smoother surface morphology, thereby increasing the compaction density, BET specific surface area, and specific capacity of the sodium vanadium fluorophosphate material.

[0085] In some embodiments, element A is selected from Fe, and element B is selected from Ti.

[0086] In this embodiment, the compaction density and BET specific surface area of ​​the phosphate cathode material are further improved, thereby further improving the specific capacity, rate performance and cycle performance of the phosphate cathode material.

[0087] In some embodiments, 0.05 ≤ x ≤ 0.3, 0.05 ≤ y ≤ 0.3. Understandably, based on the values ​​of x and y, (2-xy):x:y = (1.4~1.9):(0.05~0.3):(0.05~0.3).

[0088] In this embodiment, controlling the doping ratio of element A and element B within the above-mentioned range is beneficial to obtaining a phosphate cathode material with high compaction density, low powder resistivity, and high average voltage plateau voltage.

[0089] In some embodiments, 0.05 ≤ x ≤ 0.25, 0.05 ≤ y ≤ 0.25. Understandably, based on the values ​​of x and y, (2-xy):x:y = (1.5~1.9):(0.05~0.25):(0.05~0.25).

[0090] In this embodiment, controlling the doping ratio of elements A and B within the above-mentioned range is beneficial to obtaining phosphate cathode materials with higher compaction density, lower powder resistivity, and higher average voltage plateau voltage.

[0091] In some embodiments, 0.1≤x≤0.2, 0.1≤y≤0.2. Understandably, based on the range of values ​​for x and y, (2-xy):x:y = (1.6~1.8):(0.1~0.2):(0.1~0.2).

[0092] In this embodiment, controlling the doping ratio of elements A and B within the above-mentioned range is more conducive to obtaining phosphate cathode materials with higher compaction density, lower powder resistivity, and higher average voltage plateau voltage, thereby further improving the electrochemical performance of phosphate cathode materials, such as charge specific capacity, discharge specific capacity, first coulombic efficiency, and rate performance.

[0093] In some embodiments, the core of the phosphate cathode material has one or more of the following expressions:

[0094] Na3V 1.8 Fe 0.1 Ti 0.1 (PO4)2F3;Na3V 1.7 Fe 0.2 Ti 0.1 (PO4)2F3;Na3V 1.6 Fe 0.2 Ti 0.2 (PO4)2F3;Na3V 1.9 Fe 0.05 Ti 0.05 (PO4)2F3;Na3V 1.4 Fe 0.3 Ti 0.3 (PO4)2F3;Na3V 1.8 Fe 0.1 Mn 0.1 (PO4)2F3;Na3V 1.8 Mn 0.1 Al 0.1 (PO4)2F3;Na3V 1.8 Ti 0.1 Mg 0.1 (PO4)2F3;Na3V 1.9 Fe 0.1 (PO4)2F3;Na3V 1.9 Ti 0.1 (PO4)2F3;Na3V 1.6 (FeAlMnTi) 0.1 (PO4)2F3;Na3V 1.65 Fe 0.1 Ti 0.25 (PO4)2F3;Na3V 1.65 Fe 0.25 Ti 0.1 (PO4)2F3; Na 2.95 V 1.8 Fe 0.1 Ti 0.1 (PO4)2F 2.95 Na 3.05 V 1.8Fe 0.1 Ti 0.1 (PO4)2F 3.05 Na 2.9 V 1.8 Fe 0.1 Ti 0.1 (PO4)2F 2.9 Na 3.1 V 1.8 Fe 0.1 Ti 0.1 (PO4)2F 3.1 .

[0095] In some embodiments, the carbon layer has a mass fraction of 1.8% to 3% in the phosphate cathode material.

[0096] In some embodiments, the thickness of the carbon layer is 0.8 nm to 1.8 nm.

[0097] In this embodiment, controlling the mass fraction and / or thickness of the carbon layer within the above-mentioned range is beneficial to ensuring that the carbon layer has a good coating effect on the core, which can improve electronic conductivity, enhance structural stability and suppress side reactions, thereby improving the electrochemical performance of the phosphate cathode material, such as specific capacity, rate performance and cycle performance.

[0098] As an example, the mass fraction of the carbon layer in the phosphate cathode material includes, but is not limited to, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, or 3%. Further, the mass fraction of the carbon layer in the phosphate cathode material is preferably 2.2% to 2.74%. Within this mass ratio range, the carbon layer has a better coating effect on the core, thereby further improving the electrochemical performance of the phosphate cathode material, such as specific capacity, rate performance, and cycle performance.

[0099] As an example, the thickness of the carbon layer includes, but is not limited to, 0.8 nm, 0.9 nm, 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, or 1.8 nm. Furthermore, the mass ratio of the core to the carbon layer is preferably 0.8 nm to 1.5 nm. Within this thickness range, the carbon layer is beneficial for further improving electronic conductivity, resulting in superior electrochemical performance of the phosphate cathode material.

[0100] In some embodiments, the D50 particle size of the phosphate cathode material is 6 μm to 10 μm, including but not limited to 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm or 10 μm.

[0101] In this embodiment, the phosphate cathode material has a suitable D50 particle size to ensure Na + It can be inserted and extracted relatively quickly, improving the specific capacity and rate performance of phosphate cathode materials, and is easily dispersed in electrode slurry, which is conducive to forming a cathode active coating with high flatness and good uniformity, thereby improving the flatness and stability of cathode sheets with this phosphate cathode material.

[0102] In some embodiments, the resistivity of the phosphate cathode material powder is 100 Ω·cm to 300 Ω·cm, including but not limited to 100 Ω·cm, 120 Ω·cm, 140 Ω·cm, 160 Ω·cm, 180 Ω·cm, 200 Ω·cm, and 300 Ω·cm, and more preferably 100 Ω·cm to 200 Ω·cm.

[0103] In some embodiments, the average voltage plateau of the phosphate cathode material is 3.6V to 3.7V, including but not limited to 3.6V, 3.61V, 3.62V, 3.63V, 3.64V, 3.65V, 3.67V, 3.68V, 3.69V or 3.7V, and more preferably 3.65V to 3.7V.

[0104] In some embodiments, the phosphate cathode material has an initial charge specific capacity of ≥120mAh / g at 0.1C, including but not limited to 120mAh / g, 121mAh / g, 122mAh / g, 123mAh / g, 124mAh / g, 125mAh / g, 126mAh / g, 127mAh / g, 128mAh / g, 129mAh / g or 130mAh / g, and more preferably 122mAh / g to 127mAh / g.

[0105] In some embodiments, the phosphate cathode material has an initial discharge specific capacity of ≥110mAh / g at 0.1C, including but not limited to 110mAh / g, 111mAh / g, 112mAh / g, 113mAh / g, 114mAh / g, 115mAh / g, 116mAh / g, 117mAh / g, 118mAh / g, 119mAh / g or 120mAh / g, and more preferably 111mAh / g to 116mAh / g.

[0106] In some embodiments, the phosphate cathode material has an initial coulombic efficiency of ≥90% at 0.1C, including but not limited to 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%, and more preferably 90.1% to 93%.

[0107] In some embodiments, the phosphate cathode material has an initial discharge specific capacity of ≥107 mAh / g at 1C, including but not limited to 107 mAh / g, 108 mAh / g, 109 mAh / g, 110 mAh / g, 111 mAh / g, 112 mAh / g, 113 mAh / g, 114 mAh / g, 115 mAh / g, 116 mAh / g, 117 mAh / g, 118 mAh / g, 119 mAh / g, or 120 mAh / g, and more preferably 107 mAh / g to 114 mAh / g.

[0108] In some embodiments, the phosphate cathode material has an initial discharge specific capacity of ≥102 mAh / g at 5C, including but not limited to 102 mAh / g, 103 mAh / g, 104 mAh / g, 105 mAh / g, 106 mAh / g, 107 mAh / g, 108 mAh / g, 109 mAh / g, 110 mAh / g, 111 mAh / g, 112 mAh / g, 113 mAh / g, 114 mAh / g or 115 mAh / g, and more preferably 102 mAh / g to 110 mAh / g.

[0109] In some embodiments, the 1C / 0.1C rate performance of the phosphate cathode material is ≥94%, including but not limited to 94%, 95%, 96%, 97%, 98%, 99% or 100%, and more preferably 94.46% to 98.5%.

[0110] In some embodiments, the phosphate cathode material retains ≥70% of its capacity during room temperature cycling at 1C rate for 1000 cycles, including but not limited to 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, or 90%, and more preferably 70% to 80%.

[0111] In this embodiment, the phosphate cathode material has high charge specific capacity, discharge specific capacity, first coulombic efficiency, rate performance and cycle performance, thereby ensuring that the cathode sheet and the secondary battery have the advantages of high specific capacity, good rate performance and excellent cycle performance.

[0112] Referring to Figure 1, in a second aspect, this application provides a method for preparing a phosphate cathode material, including the following steps:

[0113] S1, vanadium source, oxalic acid, phosphorus source, dopant and solvent are mixed and reacted to obtain a mixed solution;

[0114] S2. The mixture undergoes a first drying treatment and a first sintering treatment to obtain the precursor;

[0115] S3, precursor, sodium source, fluorine source and carbon source are mixed and ground to obtain slurry;

[0116] S4. The slurry undergoes a second drying treatment and a second sintering treatment to obtain a phosphate cathode material;

[0117] The dopant includes a first source and / or a second source, wherein the first metal element in the first source and the second metal element in the second source each independently include one or more of Fe, Mn, Ti, Al and Mg, and the first metal element and the second metal element are different from each other.

[0118] In the technical solution of this application embodiment, a two-step sintering process is performed, which makes it easier for the first metal element and / or the second metal element in the dopant to embed into the crystal structure of the sodium vanadium fluorophosphate material, thereby improving the structural stability of the phosphate cathode material. Furthermore, the first metal element and / or the second metal element facilitates more complete melting of the phosphate cathode material during high-temperature sintering, resulting in a denser and smoother surface morphology. This improves the compaction density, BET specific surface area, rate performance, and cycle performance of the phosphate cathode material. Simultaneously, the introduction of reducing oxalic acid during the precursor preparation process facilitates the reduction of vanadium in the vanadium source. Moreover, the oxalic acid leaves almost no residual carbon after sintering, allowing for better control of the carbon content and surface morphology of the phosphate cathode material, thus improving its compaction density and BET specific surface area. Therefore, the phosphate cathode material prepared in this application not only has significantly improved compaction density and BET specific surface area, but also has carbon layer coated on the core of the phosphate cathode material, which further improves the electronic conductivity, structural stability and energy density of the phosphate cathode material, thereby ensuring its excellent performance in specific capacity, rate performance and cycle performance, which is beneficial for the preparation of high energy density sodium-ion batteries.

[0119] In some embodiments, the method for preparing phosphate cathode materials provided in this application is used to prepare the phosphate cathode materials as described above.

[0120] The preparation method of phosphate cathode material is described in detail below using a step-by-step approach.

[0121] S1, vanadium source, oxalic acid, phosphorus source, dopant and solvent are mixed and reacted to obtain a mixed solution.

[0122] In some embodiments, step S1, which involves mixing a vanadium source, oxalic acid, a phosphorus source, a dopant, and a solvent, and then reacting the mixture to obtain a final solution, includes:

[0123] The vanadium source and solvent are mixed and subjected to a first temperature and a first time to obtain a first solution;

[0124] The first solution, dopant, and oxalic acid are mixed and subjected to a second temperature and a second time to obtain a second solution;

[0125] The second solution and the phosphorus source are mixed and subjected to a third temperature and a third time to obtain a mixed solution;

[0126] The first temperature is 80℃~90℃, and the first time is 30min~90min;

[0127] The second temperature is 90℃~95℃, and the second time is 30min~60min;

[0128] The third temperature is 90℃~100℃, and the third time is 12min~36min.

[0129] In this embodiment, a first solution containing a vanadium source is used as the reaction substrate, and dopants, oxalic acid and phosphorus source are added step by step to carry out the reaction. This helps to ensure that the raw materials are mixed evenly and react fully, and makes it easier for the metal elements in the dopant to be embedded in the crystal structure of the precursor, which is beneficial to improving the purity and structural stability of the phosphate cathode material.

[0130] As an example, the first temperature includes, but is not limited to, 80℃, 81℃, 82℃, 83℃, 84℃, 85℃, 86℃, 87℃, 88℃, 89℃, or 90℃, and the first time includes, but is not limited to, 30min, 40min, 50min, 60min, 70min, 80min, or 90min; the second temperature is 90℃, 91℃, 92℃, 93℃, 94℃, or 95℃, and the second time is 30min, 35min, 40min, 45min, 50min, 55min, or 60min; the third temperature is 90℃, 91℃, 92℃, 93℃, 94℃, 95℃, 96℃, 97℃, 98℃, 99℃, or 100℃, and the third time is 12min, 16min, 20min, 24min, 28min, 32min, or 36min.

[0131] In some embodiments, when the dopant includes a first source and a second source, the molar ratio of vanadium in the vanadium source, oxalic acid, phosphorus in the phosphorus source, the first metal element in the first source, and the second metal element in the second source is (0.9–1.1):(2.5–3.5):1:(0–0.5):(0–0.5), and the molar ratio of the first metal element in the first source and the second metal element in the second source is not zero. It is understood that in other embodiments, when the dopant includes either a first source or a second source, the molar ratio of vanadium in the vanadium source, oxalic acid, phosphorus in the phosphorus source, and the first metal element in the first source (or the second metal element in the second source) is (0.9–1.1):(2.5–3.5):1:(0–0.5), and the molar ratio of the first metal element in the first source (and / or the second metal element in the second source) is not zero.

[0132] In this embodiment, by controlling the molar ratio of each element in the raw materials, it is beneficial to achieve full reaction between the raw materials and obtain a precursor with high purity.

[0133] As examples, the molar ratio of vanadium in the vanadium source to phosphorus in the phosphorus source includes, but is not limited to, 0.9:1, 0.92:1, 0.94:1, 0.96:1, 0.98:1, 1:1, 1.02:1, 1.04:1, 1.06:1, 1.08:1, or 1.1:1; the molar ratio of oxalic acid to phosphorus in the phosphorus source includes, but is not limited to, 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, 3:1, 3.1:1, 3.2:1, 3.3:1, 3.4:1, or 3.5:1; the phosphorus in the phosphorus source... The molar ratio of the element to the first metallic element in the first source includes, but is not limited to, 1:0, 1:0.05, 1:0.1, 1:0.15, 1:0.2, 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45, or 1:0.5; the molar ratio of the phosphorus element in the phosphorus source to the second metallic element in the second source includes, but is not limited to, 1:0, 1:0.05, 1:0.1, 1:0.15, 1:0.2, 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45, or 1:0.5.

[0134] In some embodiments, the molar ratio of phosphorus in the phosphorus source, the first metal element in the first source, and the second metal element in the second source is 1:(0.05-0.3):(0.05-0.3).

[0135] In this embodiment, controlling the amount of feed from the first source and the second source within the above-mentioned range is beneficial to obtaining phosphate cathode materials with high compaction density, low powder resistivity, and high average voltage plateau voltage.

[0136] In some embodiments, the molar ratio of phosphorus in the phosphorus source, the first metal element in the first source, and the second metal element in the second source is 1:(0.05-0.25):(0.05-0.25).

[0137] In this embodiment, controlling the amount of feed from the first source and the second source within the above-mentioned range is beneficial to obtaining phosphate cathode materials with higher compaction density, lower powder resistivity, and higher average voltage plateau voltage.

[0138] In some embodiments, the molar ratio of phosphorus in the phosphorus source, the first metal element in the first source, and the second metal element in the second source is 1:(0.1-0.2):(0.1-0.2).

[0139] In this embodiment, controlling the amount of the first source and the second source within the above-mentioned range is more conducive to obtaining phosphate cathode materials with higher compaction density, lower powder resistivity, and higher average voltage plateau voltage, thereby further improving the electrochemical performance of phosphate cathode materials.

[0140] In some embodiments, the vanadium source includes one or more of vanadium pentoxide, vanadium trioxide, vanadium dioxide, ammonium metavanadate, sodium metavanadate, sodium orthovanadate, and vanadium oxalate.

[0141] Understandably, the vanadium source can be selected from any one of the aforementioned vanadium sources, or from a combination of at least two of them, such as: a combination of vanadium pentoxide and vanadium trioxide, a combination of vanadium pentoxide and vanadium dioxide, a combination of vanadium pentoxide and ammonium metavanadate, a combination of vanadium pentoxide and sodium metavanadate, a combination of vanadium pentoxide and sodium orthovanadate, a combination of vanadium pentoxide and vanadium oxalate, or a combination of vanadium pentoxide, vanadium trioxide, vanadium dioxide, and ammonium metavanadate, etc. Further, the preferred vanadium source is vanadium pentoxide.

[0142] In some embodiments, the phosphorus source includes one or more of phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate.

[0143] Understandably, the phosphorus source can be selected from any one of the phosphorus sources mentioned above, or from a combination of at least two of them, such as: a combination of phosphoric acid and diammonium hydrogen phosphate, a combination of phosphoric acid and ammonium dihydrogen phosphate, a combination of phosphoric acid and ammonium phosphate, a combination of phosphoric acid and disodium hydrogen phosphate, a combination of phosphoric acid and sodium dihydrogen phosphate, a combination of phosphoric acid and sodium phosphate, a combination of diammonium hydrogen phosphate and ammonium dihydrogen phosphate, a combination of diammonium hydrogen phosphate and disodium hydrogen phosphate, a combination of diammonium hydrogen phosphate and sodium dihydrogen phosphate, a combination of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate, etc. Further, ammonium dihydrogen phosphate is preferred as the phosphorus source.

[0144] In some embodiments, the first source includes one or more of oxides, hydroxides, and salt compounds containing a first metal element, and the second source includes one or more of oxides, hydroxides, and salt compounds containing a second metal element. The salt compounds include one or more of carbonates, sulfates, nitrates, hydrochlorides, acetates, oxalates, and ethoxides. Further, the first source includes one or more of oxides and carbonates containing a first metal element, and the second source includes one or more of oxides and carbonates containing a second metal element. Using oxides and / or carbonates as dopants helps avoid introducing anionic impurities, thereby ensuring higher purity of the phosphate cathode material.

[0145] In some embodiments, the first source and the second source each independently include one or more of ferric oxide, iron tetroxide, manganese dioxide, manganese tetroxide, manganese carbonate, titanium dioxide, aluminum oxide, magnesium oxide, and magnesium carbonate, and the first metal element in the first source and the second metal element in the second source are different from each other.

[0146] As an example, the combination of the first source and the second source includes, but is not limited to: a combination of ferric oxide and manganese dioxide, a combination of ferric oxide and manganese tetroxide, a combination of ferric oxide and manganese carbonate, a combination of ferric oxide and titanium dioxide, a combination of ferric oxide and aluminum oxide, a combination of ferric oxide and magnesium oxide, a combination of ferric oxide and magnesium carbonate, a combination of ferric oxide and manganese dioxide, a combination of ferric oxide and manganese tetroxide, a combination of ferric oxide and manganese carbonate, a combination of ferric oxide and titanium dioxide, a combination of ferric oxide and aluminum oxide, a combination of ferric oxide and magnesium oxide, a combination of ferric oxide and magnesium carbonate, a combination of manganese dioxide and titanium dioxide, a combination of manganese dioxide and aluminum oxide, a combination of manganese dioxide and magnesium oxide, a combination of manganese dioxide and magnesium carbonate, a combination of titanium dioxide and aluminum oxide, a combination of titanium dioxide and magnesium oxide, a combination of aluminum oxide and magnesium oxide, etc.

[0147] In some embodiments, the solvent may be selected from one or more of distilled water, deionized water, pure water and ultrapure water, and may further be selected as deionized water.

[0148] S2. The mixture undergoes a first drying treatment and a first sintering treatment to obtain the precursor.

[0149] In some embodiments, the step of subjecting the mixture to a first drying treatment includes: subjecting the mixture to a first spray drying treatment to obtain a first dried material.

[0150] In some embodiments, the sintering temperature of the first sintering treatment is 700℃~750℃, and the holding time is 1h~3h.

[0151] In this embodiment, the first sintering treatment, which involves holding the material at 700℃ to 750℃ for 1 to 3 hours, helps to improve the crystallinity of the precursor, thereby increasing the purity of the phosphate cathode material and thus improving its electrochemical performance.

[0152] As an example, the sintering temperature of the first sintering treatment includes, but is not limited to, 700℃, 705℃, 710℃, 715℃, 720℃, 725℃, 730℃, 735℃, 740℃, 745℃ or 750℃, more preferably 750℃; the holding time includes, but is not limited to, 1h, 1.5h, 2h, 2.5h or 3h, more preferably 1.5h.

[0153] In some embodiments, the first sintering process is carried out under a protective atmosphere, wherein the protective gas in the protective atmosphere includes at least one of nitrogen, helium, neon, argon, and xenon, and more preferably nitrogen or argon. High-temperature sintering in a protective atmosphere helps to prevent the precursor from reacting with moisture and oxygen during the heating, holding, and cooling stages, thereby improving the purity of the precursor.

[0154] In some embodiments, the precursor comprises a vanadium phosphate material, which is expressed as V 1-x-y A x B y PO4. The meanings and ranges of values ​​for A, B, x, and y are as described above.

[0155] S3, precursor, sodium source, fluorine source and carbon source are mixed and ground to obtain slurry.

[0156] In some embodiments, the molar ratio of phosphorus in the precursor, sodium in the sodium source, fluorine in the fluorine source, and carbon source is 1:(1.3-1.5):(1.3-1.5):(0.08-0.15).

[0157] In some embodiments, the molar ratio of phosphorus in the precursor, sodium in the sodium source, fluorine in the fluorine source, and carbon in the carbon source is 1:(1.3-1.5):(1.3-1.5):(0.48-0.9).

[0158] In this embodiment, by controlling the molar ratio of each element in the raw materials, it is beneficial to enable the raw materials to react fully and form a high-purity sodium vanadium fluorophosphate material, and to coat its surface with a carbon layer of moderate thickness and carbon content.

[0159] As examples, the molar ratio of phosphorus in the precursor to sodium in the sodium source includes, but is not limited to, 1:1.3, 1:1.32, 1:1.34, 1:1.36, 1:1.38, 1:1.4, 1:1.42, 1:1.44, 1:1.46, 1:1.48, and 1:1.5; the molar ratio of phosphorus in the precursor to fluorine in the fluorine source includes, but is not limited to, 1:1.3 and 1:1. 0.32, 1:1.34, 1:1.36, 1:1.38, 1:1.4, 1:1.42, 1:1.44, 1:1.46, 1:1.48, 1:1.5; the molar ratio of phosphorus in the precursor to carbon in the carbon source includes, but is not limited to, 1:0.48, 1:0.54, 1:0.6, 1:0.66, 1:0.72, 1:0.78, 1:0.84 or 1:0.9.

[0160] In some embodiments, the sodium source includes one or more of sodium fluoride, sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium acetate, sodium oxalate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate.

[0161] Understandably, the sodium source can be selected from any one of the sodium sources mentioned above, or from a combination of at least two of them, such as: a combination of sodium fluoride and sodium hydroxide, a combination of sodium fluoride and sodium carbonate, a combination of sodium fluoride and sodium bicarbonate, a combination of sodium fluoride and sodium nitrate, a combination of sodium fluoride and sodium acetate, a combination of sodium fluoride and sodium oxalate, a combination of sodium fluoride and sodium acetate, a combination of sodium fluoride and sodium dihydrogen phosphate, a combination of sodium fluoride and disodium hydrogen phosphate, a combination of sodium fluoride and sodium phosphate, a combination of sodium fluoride, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate, etc. Further, sodium fluoride is preferred as the sodium source.

[0162] In some embodiments, the fluorine source includes one or more of sodium fluoride, ammonium fluoride, and hydrogen fluoride.

[0163] Understandably, the fluorine source can be selected from any one of the above-mentioned fluorine sources, or from a combination of at least two of them, such as a combination of sodium fluoride and ammonium fluoride, a combination of sodium fluoride and hydrogen fluoride, and a combination of sodium fluoride, ammonium fluoride, and hydrogen fluoride. Further, sodium fluoride is preferred as the fluorine source.

[0164] In some embodiments, the carbon source includes one or more of glucose, polyethylene glycol, sucrose, and ascorbic acid.

[0165] In this embodiment, the selection of different carbon sources is beneficial to adjusting the compaction density, BET specific surface area and electrochemical performance of phosphate cathode materials, thereby improving the overall performance of phosphate cathode materials.

[0166] Understandably, the carbon source can be selected from any one of the aforementioned carbon sources, or from a combination of at least two of them, such as: a combination of glucose and polyethylene glycol, a combination of glucose and sucrose, a combination of glucose and ascorbic acid, a combination of polyethylene glycol and sucrose, a combination of polyethylene glycol and ascorbic acid, a combination of sucrose and ascorbic acid, and a combination of glucose, polyethylene glycol, sucrose, and ascorbic acid, etc. Furthermore, glucose is preferred as the carbon source, as it can form a fine-particle, dense carbon layer, which is beneficial for increasing the compaction density of the phosphate cathode material.

[0167] In some embodiments, the grinding process includes one or more of ball milling and sand milling.

[0168] In this embodiment, ball milling and / or sand milling are used to mix different materials evenly and ensure full contact, thereby reducing the D50 particle size of the solid particles in the slurry, which is beneficial to improving the subsequent reaction rate and product uniformity.

[0169] In some embodiments, the step of mixing the precursor, sodium source, fluorine source and carbon source and grinding them to obtain a slurry includes: mixing the precursor, sodium source, fluorine source and carbon source, adding water and ball milling to obtain a slurry with a solid content of 15% to 20% and a D50 particle size of 3 μm to 5 μm.

[0170] Understandably, the solid content of a slurry is the mass fraction of solids in the slurry. As an example, the solid content of a slurry includes, but is not limited to, 15%, 16%, 17%, 18%, 19%, and 20%; the D50 particle size of the solid particles in the slurry includes, but is not limited to, 3 μm, 3.5 μm, 4 μm, 4.5 μm, or 5 μm.

[0171] S4. The slurry undergoes a second drying treatment and a second sintering treatment to obtain a phosphate cathode material;

[0172] In some embodiments, the step of subjecting the slurry to a second drying treatment includes: subjecting the slurry to a second spray drying treatment, wherein the inlet air temperature is 240°C to 260°C and the outlet air temperature is 90°C to 100°C, to obtain a second dried material.

[0173] In this embodiment, controlling the inlet and outlet air temperatures of the second spray dryer is beneficial for fully removing moisture from the second drying material and avoids excessively high outlet air temperatures from causing partial decomposition of the carbon source, thereby reducing the carbon content of the final phosphate cathode material.

[0174] As an example, the inlet air temperature of the second spray drying process includes, but is not limited to, 240°C, 242°C, 244°C, 246°C, 248°C, 250°C, 252°C, 254°C, 256°C, 258°C, or 260°C; the outlet air temperature of the spray drying process includes, but is not limited to, 90°C, 91°C, 92°C, 93°C, 94°C, 95°C, 96°C, 97°C, 98°C, 99°C, or 100°C.

[0175] In some embodiments, the sintering temperature of the second sintering treatment is 550℃~750℃, and the holding time is 8h~12h.

[0176] In this embodiment, a second sintering treatment is performed at 550℃ to 750℃ for 8 to 12 hours. This effectively controls energy consumption while avoiding the formation of too many impurity phases during the sintering process. This helps to improve the crystallinity and purity of the phosphate cathode material, thereby enhancing electrochemical performance such as charging specific capacity and discharging specific capacity.

[0177] As an example, the sintering temperature of the second sintering treatment includes, but is not limited to, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, or 750℃; the holding time includes, but is not limited to, 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, or 12h.

[0178] In some embodiments, the second sintering process is carried out under a protective atmosphere, wherein the protective gas in the protective atmosphere includes at least one of nitrogen, helium, neon, argon, and xenon, and more preferably nitrogen or argon. High-temperature sintering in a protective atmosphere helps to avoid the reaction of the phosphate cathode material with moisture and oxygen during the heating, holding, and cooling stages, thereby improving the purity of the phosphate cathode material and reducing adverse effects on the subsequent cathode electrode and secondary battery fabrication processes and electrical performance.

[0179] In some embodiments, after the second sintering treatment, the method further includes the following steps: pulverizing and sieving the sintered material obtained after the second sintering treatment to obtain a phosphate cathode material with a D50 particle size of 6μm to 8μm and a D100 particle size of ≤30μm.

[0180] In this embodiment, controlling the D50 particle size of the phosphate cathode material is beneficial to Na + The rapid transport of the material can improve its dispersion uniformity in the electrode slurry, which is beneficial for forming a positive electrode active coating with high flatness and good uniformity, thereby improving the consistency and stability of the positive electrode sheet.

[0181] As an example, the D50 particle size of phosphate cathode materials includes, but is not limited to, 6μm, 6.5μm, 7μm, 7.5μm or 8μm; the D100 particle size of phosphate cathode materials includes, but is not limited to, 30μm, 28μm, 25μm, 22μm, 20μm, 18μm, 15μm, 12μm or 10μm.

[0182] Thirdly, embodiments of this application provide a positive electrode sheet, including the phosphate positive electrode material provided in the first aspect of this application, or the phosphate positive electrode material prepared by the method for preparing the phosphate positive electrode material provided in the second aspect of this application.

[0183] In this embodiment, the positive electrode sheet includes the above-mentioned phosphate positive electrode material, thus having the advantages of high charge specific capacity and discharge specific capacity, good rate performance and excellent cycle performance.

[0184] Fourthly, embodiments of this application provide a secondary battery, including the positive electrode sheet provided in the third aspect of this application.

[0185] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, thus possessing advantages such as high charge specific capacity and discharge specific capacity, good rate performance, and excellent cycle performance.

[0186] 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.

[0187] I. Preparation Method

[0188] Example 1

[0189] The preparation method of the phosphate cathode material provided in this embodiment is as follows:

[0190] (1) Vanadium pentoxide (V2O5) and deionized water were mixed and heated and stirred at 90°C for 0.5 h to completely dissolve vanadium pentoxide (V2O5) and obtain an orange-red first solution; a dopant was added to the first solution and oxalic acid (H2C2O4) was slowly added. After the solution changed color, it was heated and stirred at 90°C for 1 h to obtain a dark green second solution; ammonium dihydrogen phosphate (NH4H2PO4) was slowly added to the second solution. After the solution turned into a clear blue, it was heated and stirred at 90°C for 0.5 h to obtain a clear blue mixed solution.

[0191] The dopant includes a first source and a second source. The first source is ferric oxide (Fe2O3), and the second source is titanium dioxide (TiO2). The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:3:1:0.1:0.1.

[0192] (2) The mixture is spray-dried to obtain a light green first dried material; the first dried material is placed in a nitrogen atmosphere sintering furnace and sintered at 750°C for 1.5 hours to obtain a dark green precursor.

[0193] (3) The precursor, sodium fluoride, carbon source and deionized water were ball-milled to make the materials uniformly mixed and to obtain a slurry with a solid content of 18% and a D50 particle size of 4μm. The slurry was spray-dried with the inlet air temperature controlled at 255℃ and the outlet air temperature controlled at 90℃ to obtain a gray-green second dried material. The second dried material was placed in a nitrogen atmosphere sintering furnace and sintered at 700℃ for 10h to obtain a sintered material. The sintered material was crushed and sieved to obtain a phosphate cathode material with a D50 particle size of 6μm and a D100 particle size ≤30μm.

[0194] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.4:1.4:0.6, and the carbon source is glucose.

[0195] Example 2

[0196] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0197] (1) Mix vanadium pentoxide (V2O5) and deionized water, heat and stir at 80°C for 0.5 h to completely dissolve vanadium pentoxide (V2O5) and obtain an orange-red first solution; add a dopant to the first solution, slowly add oxalic acid (H2C2O4), wait for the solution to change color, heat and stir at 90°C for 0.5 h to obtain a dark green second solution; slowly add ammonium dihydrogen phosphate (NH4H2PO4) to the second solution, wait for the solution to turn into a clear blue, continue to heat and stir at 90°C for 12 min to obtain a clear blue mixed solution.

[0198] The dopant includes a first source and a second source. The first source is ferric oxide (Fe2O3), and the second source is titanium dioxide (TiO2). The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:3:1:0.2:0.1.

[0199] (2) The mixture is spray-dried to obtain a light green first dried material; the first dried material is placed in a nitrogen atmosphere sintering furnace and sintered at 700°C for 1 hour to obtain a dark green precursor.

[0200] (3) The precursor, sodium fluoride, carbon source and deionized water were ball-milled to make the materials uniformly mixed and to obtain a slurry with a solid content of 15% and a D50 particle size of 3μm. The slurry was spray-dried with the inlet air temperature controlled at 250℃ and the outlet air temperature controlled at 90℃ to obtain a gray-green second dried material. The second dried material was placed in a nitrogen atmosphere sintering furnace and sintered at 550℃ for 8 hours to obtain a sintered material. The sintered material was crushed and sieved to obtain a phosphate cathode material with a D50 particle size of 6μm and a D100 particle size ≤30μm.

[0201] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.4:1.4:0.6, and the carbon source is glucose.

[0202] Example 3

[0203] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0204] (1) Vanadium pentoxide (V2O5) and deionized water were mixed and heated and stirred at 90°C for 1.5 h to completely dissolve vanadium pentoxide (V2O5) and obtain an orange-red first solution; a dopant was added to the first solution and oxalic acid (H2C2O4) was slowly added. After the solution changed color, it was heated and stirred at 95°C for 1 h to obtain a dark green second solution; ammonium dihydrogen phosphate (NH4H2PO4) was slowly added to the second solution. After the solution turned into a clear blue, it was heated and stirred at 100°C for 36 min to obtain a clear blue mixed solution.

[0205] The dopant includes a first source and a second source. The first source is ferric oxide (Fe2O3), and the second source is titanium dioxide (TiO2). The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:3:1:0.2:0.2.

[0206] (2) The mixture is spray-dried to obtain a light green first dried material; the first dried material is placed in a nitrogen atmosphere sintering furnace and sintered at 750°C for 3 hours to obtain a dark green precursor.

[0207] (3) The precursor, sodium fluoride, carbon source and deionized water were ball-milled to make the materials uniformly mixed and to obtain a slurry with a solid content of 20% and a D50 particle size of 5μm. The slurry was spray-dried with the inlet air temperature controlled at 260℃ and the outlet air temperature controlled at 90℃ to obtain a gray-green second dried material. The second dried material was placed in a nitrogen atmosphere sintering furnace and sintered at 750℃ for 12h to obtain a sintered material. The sintered material was crushed and sieved to obtain a phosphate cathode material with a D50 particle size of 6μm and a D100 particle size ≤30μm.

[0208] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.4:1.4:0.6, and the carbon source is glucose.

[0209] Example 4

[0210] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0211] The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:3:1:0.05:0.05.

[0212] Example 5

[0213] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0214] The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:3:1:0.3:0.3.

[0215] Example 6

[0216] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0217] The first source is iron(III) oxide (Fe3O4), the second source is manganese(III) oxide (Mn3O4), and the molar ratio of vanadium in vanadium(II) oxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and manganese in the second source is 1:3:1:0.1:0.1.

[0218] Example 7

[0219] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0220] The first source is manganese carbonate (MnCO3), the second source is aluminum oxide (Al2O3), and the molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), manganese in the first source, and aluminum in the second source is 1:3:1:0.1:0.1.

[0221] Example 8

[0222] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0223] The first source is titanium dioxide (TiO2), the second source is magnesium oxide (MgO), and the molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), titanium in the first source, and magnesium in the second source is 1:3:1:0.1:0.1.

[0224] Example 9

[0225] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0226] The first source is ferric oxide (Fe2O3), and no second source is added. The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), and iron in the first source is 1:3:1:0.1.

[0227] Example 10

[0228] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0229] No first source was added. The second source was titanium dioxide (TiO2). The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), and titanium in the second source was 1:3:1:0.1.

[0230] Example 11

[0231] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0232] The first source is ferric oxide (Fe2O3) and aluminum oxide (Al2O3), the second source is titanium dioxide (TiO2) and manganese dioxide (MnO2), and the molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, aluminum in the first source, titanium in the second source, and manganese in the second source is 1:3:1:0.1:0.1:0.1:0.1.

[0233] Example 12

[0234] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0235] The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:2.5:1:0.1:0.1.

[0236] Example 13

[0237] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0238] The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:3.5:1:0.1:0.1.

[0239] Example 14

[0240] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0241] In step (2), the sintering temperature of the first sintering treatment is 700℃ and the holding time is 3h.

[0242] Example 15

[0243] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0244] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.4:1.4:0.48, and the carbon source is glucose.

[0245] Example 16

[0246] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0247] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.4:1.4:0.9, and the carbon source is glucose.

[0248] Example 17

[0249] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0250] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon source is 1:1.4:1.4:0.1, and the carbon source is PEG-2000.

[0251] Example 18

[0252] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0253] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.4:1.4:0.6, and the carbon source is ascorbic acid.

[0254] Example 19

[0255] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that the types of vanadium source, phosphorus source, sodium source, and fluorine source are different, as detailed below:

[0256] (1) Mix ammonium metavanadate (NH4VO3) and deionized water, heat and stir at 80°C for 0.5 h to completely dissolve ammonium metavanadate (NH4VO3) to obtain an orange-red first solution; add dopant to the first solution, slowly add oxalic acid (H2C2O4), wait for the solution to change color, heat and stir at 90°C for 1 h to obtain a dark green second solution; slowly add diammonium hydrogen phosphate ((NH4)2HPO4) to the second solution, wait for the solution to turn into a clear blue, continue to heat and stir at 90°C for 0.5 h to obtain a clear blue mixed solution.

[0257] The dopant includes a first source and a second source. The first source is ferric oxide (Fe2O3), and the second source is titanium dioxide (TiO2). The molar ratio of vanadium in ammonium metavanadate (NH4VO3), oxalic acid, phosphorus in diammonium hydrogen phosphate ((NH4)2HPO4), iron in the first source, and titanium in the second source is 1:3:1:0.1:0.1.

[0258] (2) The mixture is spray-dried to obtain a light green first dried material; the first dried material is placed in a nitrogen atmosphere sintering furnace and sintered at 750°C for 1.5 hours to obtain a dark green precursor.

[0259] (3) The precursor, sodium hydroxide, ammonium fluoride, carbon source and deionized water were ball-milled to make the materials uniformly mixed and to obtain a slurry with a solid content of 15% and a D50 particle size of 4μm. The slurry was spray-dried and the inlet air temperature was controlled at 250℃ and the outlet air temperature was controlled at 90℃ to obtain a gray-green second dried material. The second dried material was placed in a nitrogen atmosphere sintering furnace and sintered at 700℃ for 10h to obtain a sintered material. The sintered material was crushed and sieved to obtain a phosphate cathode material with a D50 particle size of 6μm and a D100 particle size ≤30μm.

[0260] The molar ratio of phosphorus in the precursor, sodium in sodium hydroxide, fluorine in ammonium fluoride, and carbon in the carbon source is 1:1.4:1.4:0.1, and the carbon source is glucose.

[0261] Example 20

[0262] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0263] The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 0.9:3:1:0.1:0.1.

[0264] Example 21

[0265] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0266] The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:3:1:0.1:0.25.

[0267] Example 22

[0268] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0269] The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:3:1:0.25:0.1.

[0270] Example 23

[0271] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0272] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.3:1.3:0.6, and the carbon source is glucose.

[0273] Example 24

[0274] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0275] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.5:1.5:0.6, and the carbon source is glucose.

[0276] Example 25

[0277] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0278] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.2:1.2:0.6, and the carbon source is glucose.

[0279] Example 26

[0280] The preparation method of the phosphate cathode material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0281] The molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.6:1.6:0.6, and the carbon source is glucose.

[0282] Comparative Example 1

[0283] The preparation method of the phosphate cathode material provided in this comparative example is the same as or similar to that in Example 1, except that:

[0284] No dopant was added in step (1). The specific preparation method is as follows:

[0285] (1) Mix vanadium pentoxide (V2O5) and deionized water, heat and stir at 90°C for 0.5 h to completely dissolve vanadium pentoxide (V2O5) to obtain an orange-red first solution; slowly add oxalic acid (H2C2O4) to the first solution, wait for the solution to change color, and then heat and stir at 90°C for 1 h to obtain a dark green second solution; slowly add ammonium dihydrogen phosphate (NH4H2PO4) to the second solution, wait for the solution to turn into a clear blue, and then continue to heat and stir at 90°C for 0.5 h to obtain a clear blue mixed solution.

[0286] The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, and phosphorus in ammonium dihydrogen phosphate (NH4H2PO4) is 1:3:1.

[0287] (2) is the same as step (2) in Example 1.

[0288] (3) is the same as step (3) in Example 1.

[0289] Comparative Example 2

[0290] The preparation method of the phosphate cathode material provided in this comparative example is the same as or similar to that in Example 1, except that:

[0291] No oxalic acid was added in step (1). The specific preparation method is as follows:

[0292] (1) Mix vanadium pentoxide (V2O5) and deionized water, heat and stir at 90°C for 0.5 h to completely dissolve vanadium pentoxide (V2O5) to obtain an orange-red first solution; add a dopant to the first solution and heat and stir at 90°C for 1 h to obtain a red second solution; slowly add ammonium dihydrogen phosphate (NH4H2PO4) to the second solution, wait for the solution to turn light blue, and continue to heat and stir at 90°C for 0.5 h to obtain a clear blue mixture.

[0293] The dopant includes a first source and a second source. The first source is ferric oxide (Fe2O3), and the second source is titanium dioxide (TiO2). The molar ratio of vanadium in vanadium pentoxide (V2O5), phosphorus in ammonium dihydrogen phosphate (NH4H2PO4), iron in the first source, and titanium in the second source is 1:1:0.1:0.1.

[0294] (2) is the same as step (2) in Example 1.

[0295] (3) is the same as step (3) in Example 1.

[0296] Comparative Example 3

[0297] The preparation method of the phosphate cathode material provided in this comparative example is the same as or similar to that in Example 1, except that:

[0298] In step (1), oxalic acid is replaced with an equal amount of citric acid.

[0299] Comparative Example 4

[0300] The preparation method of the phosphate cathode material provided in this comparative example is the same as or similar to that in Example 1, except that:

[0301] In step (2), the first dried material was not subjected to a first sintering treatment. The specific preparation method is as follows:

[0302] (1) The mixture was prepared according to step (1) of Example 1.

[0303] (2) The mixture is spray-dried to obtain a light green first dried material.

[0304] (3) The first dry material, sodium fluoride, carbon source and deionized water are ball-milled to make the materials uniformly mixed to obtain a slurry with a solid content of 15% and a D50 particle size of 4μm; the slurry is spray-dried with the inlet air temperature controlled at 250℃ and the outlet air temperature controlled at 90℃ to obtain a gray-green second dry material; the second dry material is placed in a nitrogen atmosphere sintering furnace and sintered at 700℃ for 10h to obtain a sintered material; the sintered material is crushed and sieved to obtain a phosphate cathode material with a D50 particle size of 6μm and a D100 particle size ≤30μm.

[0305] The molar ratio of phosphorus in the first drying material, sodium and fluorine in sodium fluoride, and carbon in the carbon source is 1:1.4:1.4:0.1, and the carbon source is glucose.

[0306] Comparative Example 5

[0307] The preparation method of the phosphate cathode material provided in this comparative example is the same as or similar to that in Example 1, except that:

[0308] No dopant was added in step (1), but a dopant was added in step (3). The specific preparation method is as follows:

[0309] (1) Mix vanadium pentoxide (V2O5) and deionized water, heat and stir at 90°C for 0.5 h to completely dissolve vanadium pentoxide (V2O5) to obtain an orange-red first solution; slowly add oxalic acid (H2C2O4) to the first solution, wait for the solution to change color, and then heat and stir at 90°C for 0.5 h to obtain a dark green second solution; slowly add ammonium dihydrogen phosphate (NH4H2PO4) to the second solution, wait for the solution to turn into a clear blue, and then continue to heat and stir at 90°C for 0.5 h to obtain a clear blue mixed solution.

[0310] The molar ratio of vanadium in vanadium pentoxide (V2O5), oxalic acid, and phosphorus in ammonium dihydrogen phosphate (NH4H2PO4) is 1:3:1.

[0311] (2) The mixture is spray-dried to obtain a light green first dried material; the first dried material is placed in a nitrogen atmosphere sintering furnace and sintered at 750°C for 1.5 hours to obtain a dark green precursor.

[0312] (3) The precursor, sodium fluoride, carbon source, dopant and deionized water were ball-milled to make the materials uniformly mixed and to obtain a slurry with a solid content of 15% and a D50 particle size of 4μm. The slurry was spray-dried with the inlet air temperature controlled at 250℃ and the outlet air temperature controlled at 90℃ to obtain a gray-green second dried material. The second dried material was placed in a nitrogen atmosphere sintering furnace and sintered at 700℃ for 10h to obtain a sintered material. The sintered material was crushed and sieved to obtain a phosphate cathode material with a D50 particle size of 6μm and a D100 particle size of <30μm.

[0313] The carbon source is glucose; the dopants include a first source and a second source, the first source being ferric oxide (Fe2O3) and the second source being titanium dioxide (TiO2); the molar ratio of phosphorus in the precursor, sodium and fluorine in sodium fluoride, carbon in the carbon source, iron in the first source and titanium in the second source is 1:1.4:1.4:0.6:0.1:0.1.

[0314] II. Testing Methods

[0315] 1. Property testing of phosphate cathode materials

[0316] (1) Scanning electron microscopy (SEM) test: Morphological characterization was performed using a field emission scanning electron microscope (MERLIN Compact, Quanta 200FEG) manufactured by Zeiss.

[0317] (2) Transmission electron microscopy (TEM) test: Morphological analysis was performed using a FEI Tecnai G2 F20 X-Twin transmission electron microscope manufactured by Thermo Fisher Scientific, USA, and the thickness of the carbon layer was measured.

[0318] (3) Element content: The content of each element was tested using an inductively coupled plasma optical emission spectrometer (ICP-OES), and the carbon content was tested using an infrared carbon-sulfur analyzer.

[0319] (4) Compacted density: The compacted density was tested using a UTM7305 battery powder compaction density tester provided by Shenzhen Sansi Zongheng Technology Co., Ltd. The test pressure was 3T and the pressing time was 30S.

[0320] (5) Specific surface area: The specific surface area was tested using a BELSORP MaxII specific surface area analyzer manufactured by Japan-McQKBAY.

[0321] (6) Powder resistivity: The four-probe method was used for testing, and the test pressure was 10 MPa.

[0322] 2. Properties of secondary batteries

[0323] Phosphate cathode material was mixed with conductive carbon black and PVDF binder at a mass ratio of 90:5:5 to obtain cathode slurry. The cathode slurry was coated onto an aluminum foil with a thickness of 12 μm to form a cathode slurry layer with a thickness of 80 μm. The slurry was then dried in an oven at 110℃ for 10 hours. After drying, it was punched into circular electrode sheets with a diameter of 15 mm and compacted to a density of 1.8 g / cm³. 3 The positive electrode sheet is obtained by rolling.

[0324] Sodium hexafluorophosphate (NaPF6) was used as the sodium salt, and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1 were used as organic solvents to dissolve the sodium salt and prepare an electrolyte with a concentration of 1M. A 99% pure sodium sheet was used as the counter electrode, and the battery was assembled with the positive electrode and the electrolyte in an LG2400 / 1000TS glove box manufactured by Wig Gas Purification Technology (Suzhou) Co., Ltd. to obtain a coin cell half-cell.

[0325] The CT3002A battery performance testing system manufactured by Wuhan Landian Electronics Technology Co., Ltd. was used to test the rate performance of button half-cells. The test temperature was 25℃, the voltage range was 2V~4.35V, and the rate range was 0.1C~3C.

[0326] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0327] The test results are shown in Figures 2 to 5 and Tables 1 to 2.

[0328] Table 1

[0329] Table 2

[0330] Table 1 shows that the mass fraction of each element in the phosphate cathode materials of Examples 1-26 is controlled within the following ranges: Na 15.47%-17.54%, V 17.95%-24.11%, A ≤2.99%, B ≤3%, P 14.14%-14.82%, F 13.34%-17.26%, and C 2.2%-2.74%. TEM testing shows that the carbon layer thickness is 0.8 nm-1.5 nm. This demonstrates that the phosphate cathode material includes a core and a carbon layer covering the core, and the core satisfies the following expression: Na3V 2-x-y A x B y (PO4)2F3, where 0≤x≤0.5, 0≤y≤0.5, and x and y are not both zero.

[0331] As shown in Table 2, the compaction density of the phosphate cathode materials in Examples 1-26 is 2 g / cm³. 3 ~2.1g / cm 3 BET has a specific surface area of ​​10m². 2 / g~15.3m 2 / g, powder resistivity 100Ω·cm~300Ω·cm, average voltage plateau 3.61V~3.7V, 0.1C initial charge specific capacity 115mAh / g~127mAh / g, 0.1C initial discharge specific capacity 105mAh / g~116mAh / g, 0.1C initial coulombic efficiency 90%~93.04%, 1C initial discharge specific capacity 100mAh / g~114mAh / g, 5C initial discharge specific capacity 92mAh / g~110mAh / g, 1C / 0.1C rate performance 91.4%~98.5%, and room temperature cycling capacity retention 66%~80% at 1C rate after 1000 cycles.

[0332] A comparison of Figures 2 and 3 shows that the phosphate cathode material of Example 1 has a dense and smooth surface without any crystal particles, thus exhibiting higher compaction density and BET specific surface area. In contrast, the carbon layer surface of the phosphate cathode material of Comparative Example 1 shows porous wrinkles and obvious crystal particles, which negatively impacts its compaction density and BET specific surface area. A comparison of Figures 4 and 5 shows that, compared to Comparative Example 1, the phosphate cathode material of Example 1 exhibits a higher and more stable discharge plateau in its charge-discharge curve. Table 2 shows that, compared to Comparative Example 1, the phosphate cathode material of Example 1 is doped with Fe and Ti elements, which significantly improves its compaction density, BET specific surface area, and electrochemical performance. This indicates that the co-doping treatment of Fe and Ti elements not only optimizes the electronic conductivity of the phosphate cathode material and effectively improves the average voltage plateau, but also gives the phosphate cathode material higher compaction density and a larger BET specific surface area, thereby ensuring excellent performance in capacity, rate capability, and cycle life.

[0333] Compared with Example 1, Comparative Example 5 did not add a dopant in step (1), but added a dopant in step (3). During the co-sintering process of the precursor and the dopant, the metal cations in the dopant were difficult to fully embed into the crystal structure. This not only significantly reduced the compaction density and BET specific surface area, but also led to structural instability of the phosphate cathode material, a significant decrease in electronic conductivity, a reduction in the average voltage plateau, and a substantial decrease in the discharge specific capacity, initial coulombic efficiency, rate performance, and cycle performance of the phosphate cathode material. This indicates that doping the precursor with a dopant is beneficial for the more complete embedding of metal cations into the crystal structure of the phosphate cathode material, thereby increasing the compaction density and BET specific surface area of ​​the phosphate cathode material, and improving its structural stability, electronic conductivity, and average voltage plateau, resulting in superior electrochemical performance.

[0334] The difference between the phosphate cathode materials in Examples 1-5 and Examples 21-22 lies in the doping amount of Fe and Ti elements. According to the corresponding results in Tables 1 and 2, when 0.05≤x≤0.2 and 0.05≤y≤0.2, it is more conducive to obtaining phosphate cathode materials with lower powder resistivity and higher average voltage plateau voltage.

[0335] The difference between the phosphate cathode materials in Examples 1 and 6-8 lies in the different types of elements A and B. Compared with Example 1, the phosphate cathode materials in Examples 6-8 show a decrease in compaction density, a certain increase in powder resistivity, and a decrease in average voltage plateau. Simultaneously, they exhibit varying degrees of decrease in capacity, initial coulombic efficiency, rate performance, and cycle performance. This indicates that compared to combinations such as Fe+Mn, Mn+Al, and Ti+Mg, co-doping with a combination of Fe and Ti elements results in phosphate cathode materials with superior compaction density, BET specific surface area, and electrochemical performance.

[0336] The difference between Examples 1, 12, and 13 lies in the amount of oxalic acid used in step (1). Compared to Example 1, the amount of oxalic acid used in Example 12 was reduced, resulting in a decrease in carbon content and carbon layer thickness. In Example 13, the amount of oxalic acid used was increased, resulting in an increase in carbon content and carbon layer thickness. Furthermore, the compaction density, BET specific surface area, and electrochemical performance of the phosphate cathode materials in Examples 12 and 13 decreased. This demonstrates that adding an appropriate amount of oxalic acid during the preparation of phosphate cathode materials helps to precisely control the carbon content and carbon layer thickness, thereby improving the compaction density, BET specific surface area, and electrochemical performance of the phosphate cathode materials. Compared to Example 1, Comparative Example 2 did not add oxalic acid, while Comparative Example 3 replaced oxalic acid with an equal amount of citric acid. The compaction density, BET specific surface area, and electrochemical performance of the phosphate cathode materials showed a significant deterioration, indicating that oxalic acid not only plays a reducing role but also controls the carbon content and surface morphology of the phosphate cathode materials, thereby improving the overall performance of the phosphate cathode materials.

[0337] Compared with Example 1, Comparative Example 4 did not undergo the first sintering treatment, resulting in a decrease in the crystallinity of the precursor and a decrease in the purity of the subsequently prepared phosphate cathode material. Consequently, the carbon content, carbon layer thickness, compaction density, BET specific surface area, and electrochemical performance of the phosphate cathode material obtained in Comparative Example 4 decreased significantly.

[0338] The difference between the phosphate cathode materials in Examples 1 and 15-16 lies in the amount of carbon source used in step (3). Based on the results in Tables 1 and 2, a molar ratio of phosphorus in the precursor to carbon in the carbon source (glucose) of 1:0.6 is more favorable for obtaining phosphate cathode materials with higher compaction density, lower powder resistivity, and higher average voltage plateau voltage. The difference between the phosphate cathode materials in Examples 1 and 17-18 lies in the type of carbon source used in step (3). Based on the results in Tables 1 and 2, compared to PEG-2000 and ascorbic acid, using glucose as the carbon source is more favorable for obtaining phosphate cathode materials with higher compaction density, lower powder resistivity, and higher average voltage plateau voltage.

[0339] Compared with Examples 25-26, Examples 1 and Examples 23-24 have higher compaction densities, indicating that controlling the phosphorus element in the precursor, the sodium element in the sodium source, and the fluorine element in the fluorine source within the range of 1:(1.3-1.5):(1.3-1.5) is more conducive to improving the compaction density of the phosphate cathode material and obtaining better electrochemical performance.

[0340] In summary, this application selects specific types and proportions of element A and / or element B to dope the phosphate cathode material, and adjusts the amount of oxalic acid and the types and amounts of vanadium, phosphorus, sodium, fluorine, and carbon sources. This is beneficial for the phosphate cathode material to have better electronic conductivity and structural stability, resulting in a significant improvement in the compaction density and BET specific surface area of ​​the phosphate cathode material. Consequently, it exhibits excellent performance in charge-discharge specific capacity, initial coulombic efficiency, rate performance, and cycle performance, which is conducive to the preparation of high-energy-density sodium-ion batteries.

[0341] 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 phosphate positive electrode material, characterized by, Includes the kernel and the carbon layer covering the kernel; The kernel expression is Na3V 2-x-y A x B y (PO4)2F3; Where 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.5, and x and y are not both zero; Element A and element B each independently include one or more of Fe, Mn, Ti, Al, and Mg, and element A and element B are different from each other; The compaction density of the phosphate cathode material is ≥2.0 g / cm³. 3 BET specific surface area is ≥8m² 2 / g.

2. The phosphate cathode material of claim 1, wherein, One or more of the following conditions must be met: (1) The carbon layer in the phosphate cathode material has a mass fraction of 1.8% to 3%; (2) The thickness of the carbon layer is 0.8 nm to 1.8 nm.

3. The phosphate cathode material of claim 1, wherein, One or more of the following conditions must be met: (1) The phosphate cathode material has an initial charge specific capacity of ≥120mAh / g at 0.1C; (2) The phosphate cathode material has an initial discharge specific capacity of ≥110mAh / g at 0.1C; (3) The initial coulombic efficiency of the phosphate cathode material at 0.1C is ≥90%.

4. A method for producing a phosphate positive electrode material, characterized by, Includes the following steps: A mixture of vanadium source, oxalic acid, phosphorus source, dopant, and solvent is obtained through reaction treatment. The mixture is subjected to a first drying treatment and a first sintering treatment to obtain a precursor; The precursor, sodium source, fluorine source and carbon source are mixed and ground to obtain a slurry; The slurry undergoes a second drying treatment and a second sintering treatment to obtain the phosphate cathode material; The dopant includes a first source and / or a second source, wherein the first metal element in the first source and the second metal element in the second source each independently include one or more of Fe, Mn, Ti, Al and Mg, and the first metal element and the second metal element are different from each other.

5. The method for preparing the phosphate cathode material according to claim 4, characterized in that, The steps of mixing vanadium source, oxalic acid, phosphorus source, dopant, and solvent, and then reacting them to obtain a mixed solution include: The vanadium source and the solvent are mixed and subjected to a first temperature and a first time to obtain a first solution; The first solution, the dopant, and the oxalic acid are mixed and subjected to a second temperature for a second time to obtain a second solution; The second solution and the phosphorus source are mixed and subjected to a third temperature for a third time to obtain the mixture; Wherein, the first temperature is 80℃~90℃, and the first time is 30min~90min; The second temperature is 90℃~95℃, and the second time is 30min~60min; The third temperature is 90℃~100℃, and the third time is 12min~36min.

6. The method of claim 5, wherein the phosphate-based cathode material is prepared by a process comprising: mixing a lithium source, a transition metal source, and a phosphate source to form a mixture; and heating the mixture to form the phosphate-based cathode material. One or more of the following conditions must be met: (1) When the dopant includes the first source and the second source, the molar ratio of vanadium in the vanadium source, oxalic acid, phosphorus in the phosphorus source, the first metal element in the first source and the second metal element in the second source is (0.9~1.1):(2.5~3.5):1:(0~0.25):(0~0.25); (2) The vanadium source includes one or more of vanadium pentoxide, vanadium trioxide, vanadium dioxide, ammonium metavanadate, sodium metavanadate, sodium orthovanadate, and vanadium oxalate. (3) The phosphorus source includes one or more of phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; (4) The first source includes one or more of oxides, hydroxides and salt compounds containing the first metal element; (5) The second source includes one or more of oxides, hydroxides and salts containing the second metal element.

7. The method of producing a phosphate-based positive electrode material according to any one of claims 4 to 6, characterized by, One or more of the following conditions must be met: (1) The molar ratio of phosphorus in the precursor, sodium in the sodium source, fluorine in the fluorine source and carbon in the carbon source is 1:(1.3~1.5):(1.3~1.5):(0.48~0.9); (2) The sodium source includes one or more of sodium fluoride, sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium acetate, sodium oxalate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate. (3) The fluorine source includes one or more of sodium fluoride, ammonium fluoride and hydrogen fluoride; (4) The carbon source includes one or more of glucose, polyethylene glycol, sucrose and ascorbic acid.

8. The method of producing a phosphate-based positive electrode material according to any one of claims 4 to 6, characterized by, One or more of the following conditions must be met: (1) The sintering temperature of the first sintering treatment is 700℃~750℃, and the holding time is 1h~3h; (2) The sintering temperature of the second sintering treatment is 550℃~750℃, and the holding time is 8h~12h.

9. A positive electrode sheet characterized by comprising: The material includes the phosphate cathode material according to any one of claims 1 to 3, or the phosphate cathode material prepared by the method according to any one of claims 4 to 8.

10. A secondary battery characterized by comprising: Includes the positive electrode sheet as described in claim 9.

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