Phosphate 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 positive electrode material and a preparation method therefor, a positive electrode sheet, and a secondary battery. The phosphate positive electrode material comprises an inner core and a carbon layer coating the inner core. The inner core comprises a sodium vanadium fluorophosphate material. The compaction density of the phosphate positive electrode material is greater than or equal to 1.78 g / cm3, and the BET specific surface area thereof is greater than or equal to 9 m3 / g. The Raman spectrum of the phosphate positive electrode material has a D peak and a G peak, and the intensity ratio of the D peak to the G peak is (1.03-1.05):1. In the present application, the provided phosphate positive electrode material uses a sodium vanadium fluorophosphate material having a high working voltage as an inner core, and the inner core is coated with a carbon layer having a high crystallinity, which not only provides the phosphate positive electrode material with better electron conductivity and structural stability, but also improves the compaction density and BET specific surface area thereof, thereby facilitating the preparation of a secondary battery having high energy density.

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. Among the positive electrode materials for sodium-ion batteries, polyanionic compounds (PACs) exhibit excellent cycle life and have broad application prospects 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, the poor electronic conductivity of traditional phosphate cathode materials makes it difficult to improve the discharge capacity and rate performance of secondary batteries prepared using traditional phosphate cathode materials. 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 problem of poor electronic conductivity of traditional phosphate cathode materials.

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

[0006] The core material includes sodium vanadium fluorophosphate;

[0007] The compaction density of the phosphate cathode material is ≥1.78 g / cm³. 3 BET specific surface area is ≥9m² 2 / g;

[0008] The Raman spectrum of the phosphate cathode material has a D peak and a G peak, and the intensity ratio of the D peak to the G peak is (1.03~1.05):1.

[0009] In the technical solution of this application embodiment, the phosphate cathode material uses sodium vanadium fluorophosphate with high working voltage as the core, and a carbon layer with high crystallinity is coated on the core. This not only gives it better electronic conductivity and structural stability, but also improves its compaction density and BET specific surface area, thereby improving the specific capacity and rate performance of the secondary battery prepared using the phosphate cathode material, and thus obtaining a secondary battery with high energy density.

[0010] In some embodiments, the mass ratio of the core to the carbon layer is 100:(1.75 to 3.3).

[0011] In this embodiment, by controlling the mass ratio of the core to the carbon layer, higher electronic conductivity can be achieved, thereby improving the material's specific capacity, rate performance, and cycle performance. If the mass ratio of the carbon layer is too low, an effective coating structure and conductive network cannot be formed, resulting in an unsatisfactory improvement in electronic conductivity; if the mass ratio of the carbon layer is too high, it will reduce the content of electrochemically active substances, increase the material's resistance, and hinder the absorption of Na+. + The diffusion of phosphate reduces the overall electrochemical performance of the prepared phosphate cathode material.

[0012] In some embodiments, the thickness of the carbon layer is 1 nm to 3 nm.

[0013] In this embodiment, controlling the thickness of the carbon layer within a suitable range can improve electronic conductivity, enhance structural stability, and suppress side reactions, thereby improving the specific capacity, rate performance, and cycle performance of the prepared phosphate cathode material.

[0014] In some embodiments, the D50 particle size of the phosphate cathode material is 4 μm to 10 μm.

[0015] In this embodiment, the phosphate cathode material has a suitable particle size to ensure Na + It can be inserted and extracted relatively quickly, improving the specific capacity and rate performance of secondary batteries, and is easily dispersed in electrode slurry, which is conducive to forming a positive electrode active coating with high flatness and good uniformity, thereby improving the consistency and stability of the positive electrode sheet prepared using phosphate positive electrode material.

[0016] Secondly, embodiments of this application provide a method for preparing a phosphate cathode material, comprising the following steps:

[0017] Vanadium source, phosphorus source, sodium source, fluorine source, carbon source and aqueous phase are mixed and dispersed to prepare precursor slurry;

[0018] Phosphate cathode materials were prepared by drying and calcining the precursor slurry.

[0019] The carbon source includes a first carbon source and a second carbon source. The first carbon source and the second carbon source each independently include one or more of glucose, polyethylene glycol, sucrose and ascorbic acid. The first carbon source and the second carbon source are different from each other.

[0020] In the technical solution of this application embodiment, at least two carbon sources are selected for carbon coating treatment, thereby forming a highly crystalline carbon layer on the core surface of the phosphate cathode material, giving it better electronic conductivity and structural stability, and significantly improving the compaction density and BET specific surface area of ​​the phosphate cathode material. This is beneficial to improving the specific capacity and rate performance of the secondary battery prepared using the phosphate cathode material, and thus preparing a high-energy-density secondary battery.

[0021] In some embodiments, the first carbon source is glucose, and the second carbon source includes one or more of polyethylene glycol, sucrose, and ascorbic acid.

[0022] In this embodiment, glucose is used as the first carbon source and is compounded with at least one of polyethylene glycol, sucrose and ascorbic acid, which is beneficial to the formation of a highly crystalline carbon layer and ensures that the material has high compaction density, high BET specific surface area and excellent electrochemical performance.

[0023] In some embodiments, the mass ratio of the first carbon source to the second carbon source is (1-6):1.

[0024] In this embodiment, by controlling the ratio of the two carbon sources, a good balance can be achieved in terms of compaction density, BET specific surface area and electrochemical performance, thereby producing a phosphate cathode material that has high compaction density, high BET specific surface area and excellent electrochemical performance.

[0025] In some embodiments, the molar ratio of vanadium in the vanadium source, phosphorus in the phosphorus source, sodium in the sodium source, fluorine in the fluorine source, and carbon in the carbon source is 1:(0.8-1.2):(1.4-1.6):(1.4-1.6):(0.45-0.55).

[0026] In this embodiment, by controlling the molar ratio of each element in the raw materials, the raw materials can react fully to form a high-purity sodium vanadium fluorophosphate material, and a carbon layer with suitable thickness and carbon content can be formed on its surface.

[0027] In some embodiments, the vanadium source includes one or more of vanadium pentoxide, vanadium trioxide, vanadium dioxide, ammonium metavanadate, sodium metavanadate, and sodium orthovanadate; and / or, the phosphorus source includes one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; and / or, the sodium source includes one or more of sodium fluoride, sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate; and / or, the fluorine source includes one or more of sodium fluoride and ammonium fluoride.

[0028] In this embodiment, different vanadium, phosphorus, sodium and fluorine sources have different reactivity. By rationally selecting and combining different raw materials, the reaction rate of high-temperature solid-phase synthesis reaction can be controlled, the stoichiometry of phosphate cathode material can be precisely controlled, the crystal structure and surface morphology of the material can be optimized, thereby improving its electrochemical performance.

[0029] In some embodiments, the dispersion process includes one or more of ball milling and sand milling.

[0030] In this embodiment, ball milling and / or sand milling can make different materials mix evenly and make them fully contacted, and reduce the D50 particle size of the precursor slurry, which is beneficial to improving the subsequent reaction rate and product uniformity.

[0031] In some embodiments, the solid content of the precursor slurry is 20% to 30%; and / or, the D50 particle size of the precursor slurry is 0.4 μm to 0.6 μm.

[0032] In this embodiment, adjusting the solid content of the precursor slurry can regulate the rate of water evaporation during drying, improve production efficiency and reduce energy consumption, and promote the transformation of the precursor slurry into a dried material with a relatively uniform particle size distribution. Adjusting the D50 particle size of the precursor slurry can improve the dispersion uniformity and stability of the raw materials, optimize the particle size distribution of the dried material, and help improve the reaction uniformity during calcination, thereby improving the electrochemical performance of the material.

[0033] In some embodiments, the drying process includes the following steps: spray drying the precursor slurry, wherein the inlet air temperature of the spray drying is 240°C to 260°C and the outlet air temperature is 90°C to 100°C, to obtain a dried material.

[0034] In this embodiment, controlling the inlet and outlet air temperatures of the spray dryer can effectively remove moisture from the dried material and prevent excessively high outlet air temperatures from causing the decomposition of some carbon sources and reducing the carbon content of the product.

[0035] In some embodiments, the calcination treatment includes the following steps: heating to 550°C to 750°C and holding at that temperature for 6 to 12 hours in a protective atmosphere.

[0036] In this embodiment, high-temperature calcination in a protective atmosphere avoids reactions between the material and moisture and oxygen during the heating, holding, and cooling stages, which helps improve the purity of the finished product and reduces adverse effects on the subsequent preparation process and electrical performance of the positive electrode and secondary battery. Calcination at 550℃ to 750℃ helps prevent the formation of excessive impurities during sintering due to excessively high or low temperatures, thus improving the crystallinity of the finished product and increasing the charge-discharge specific capacity. Holding time of 6h to 12h helps ensure the purity of the finished product and the uniformity of carbon coating, thereby improving the electrical performance of the material.

[0037] In some embodiments, after calcination, the method further includes the following steps: pulverizing, sieving, and removing iron from the calcined product to prepare a phosphate cathode material with a D50 particle size of 4 μm to 10 μm.

[0038] 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.

[0039] 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.

[0040] In this embodiment, the positive electrode sheet includes the aforementioned phosphate positive electrode material, which is beneficial for improving the specific capacity and rate performance of the secondary battery prepared using the positive electrode sheet.

[0041] Fourthly, embodiments of this application provide a secondary battery, including the positive electrode sheet provided in the third aspect of this application.

[0042] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, thus possessing the advantages of high specific capacity and good rate performance.

[0043] 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

[0044] 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.

[0045] Figure 1 is a schematic flowchart of a method for preparing a phosphate cathode material according to an embodiment of this application;

[0046] Figure 2 is a phase diagram of the phosphate cathode material of Example 1 of this application;

[0047] Figure 3 shows the phase composition of the phosphate cathode material of Comparative Example 1 of this application.

[0048] Figure 4 shows the phase composition of the phosphate cathode material of Comparative Example 2 of this application.

[0049] Figure 5 shows the morphology of the phosphate cathode material in Example 1 of this application;

[0050] Figure 6 shows the morphology of the phosphate cathode material of Comparative Example 1 of this application.

[0051] Figure 7 shows the morphology of the phosphate cathode material of Comparative Example 2 of this application.

[0052] Figure 8 is the Raman spectrum of the phosphate cathode material of Example 1 of this application. Detailed Implementation

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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).

[0059] the term

[0060] Unless otherwise stated or in case of conflict, the terms or phrases used in this application shall have the following meanings:

[0061] 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). The particle size measured by a laser particle size analyzer is the equivalent diameter of the particle. 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.

[0062] 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.

[0063] Crystallinity of carbon materials: Carbon materials exhibit two main characteristic peaks in Raman spectroscopy: the D peak and the G peak; the D peak is typically located at 1350 cm⁻¹.-1 The G peak is around 1580 cm⁻¹, which is related to the disordered structure and defects of carbon materials; the G peak is usually around 1580 cm⁻¹. -1 The ratio of the intensity of the D peak to the intensity of the G peak is related to the graphitized structure of carbon materials. D / I G It can characterize the crystallinity of carbon materials; I D / I G A lower value indicates a higher degree of crystallinity in the carbon material, with a greater proportion of graphitized structures; I D / I G A higher value indicates that the carbon material has a lower degree of crystallinity and contains more disordered structures and defects.

[0064] 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.

[0065] 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 LIBs. Therefore, a major challenge for phosphate cathode materials used in SIBs is to improve energy density to narrow the gap between theoretical and actual energy densities. To achieve higher energy densities in phosphate cathode materials, increasing the operating voltage is generally more efficient and easier than adjusting the capacity, because reducing the molecular weight per electron transfer is very difficult. 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 of up to 3.95V, a theoretical capacity of 128 mAh / g, and a corresponding theoretical energy density of up to ~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 inherits the drawback of low electronic conductivity, which limits its capacity and rate performance and hinders its ability to achieve comprehensive and superior electrochemical performance, including high energy density.

[0066] To address the technical problem of poor electronic conductivity in traditional phosphate cathode materials, which limits their discharge capacity and rate performance, this application provides a phosphate cathode material, its preparation method, a cathode electrode, and a secondary battery. The phosphate cathode material has a highly crystalline carbon layer coated on the surface of sodium vanadium fluorophosphate, which not only imparts better electronic conductivity and structural stability but also increases its compaction density and BET specific surface area, thereby improving the specific capacity and rate performance of the cathode electrode and the secondary battery.

[0067] In a first aspect, embodiments of this application provide a phosphate cathode material, which includes a core and a carbon layer covering the core; the core includes sodium vanadium fluorophosphate; wherein the compaction density of the phosphate cathode material is ≥1.78 g / cm³. 3 BET specific surface area is ≥9m² 2 / g; The Raman spectrum of the phosphate cathode material has D peak and G peak, and the intensity ratio of D peak to G peak is (1.03~1.05):1.

[0068] In the technical solution of this application embodiment, the phosphate cathode material uses sodium vanadium fluorophosphate with high working voltage as the core, and a carbon layer with high crystallinity is coated on the core. This not only gives it better electronic conductivity and structural stability, but also improves its compaction density and BET specific surface area, thereby improving the specific capacity and rate performance of the secondary battery prepared using the phosphate cathode material, and thus obtaining a secondary battery with high energy density.

[0069] In this embodiment, the compaction density of the phosphate cathode material is ≥1.78 g / cm³. 3 including but not limited to 1.78g / cm³ 3 1.8g / cm 3 1.9g / cm 3 2.0g / cm 3 2.1g / cm 3 2.2g / cm 3 2.3g / cm 3 2.4g / cm 3 Or 2.5g / cm 3 Further preferred value is 1.94 g / cm³. 3 ~2.06g / cm 3 Phosphate cathode materials have a high compaction density, which can improve the specific capacity and rate performance of secondary batteries made using phosphate cathode materials.

[0070] In this embodiment, the BET specific surface area of ​​the phosphate cathode material is ≥9m². 2 / g, including but not limited to 9m 2 / g, 9.5m 2 / g, 9.8m 2 / g, 10m 2 / g、12m 2 / g、14m 2 / g, 16m 2 / g、18m 2 / g、20m 2 / g、22m 2 / g、24m 2 / g、26m 2 / g、28m 2 / g or 30m 2 / g, further preferably 9.8m 2 / g~16m 2 / g. Among them, phosphate cathode materials have a relatively moderate specific surface area, which means that they have more active sites to enable the chemical reaction to occur, and can improve the lithium ion transport rate. This allows secondary batteries made with phosphate cathode materials to exhibit higher specific capacity and better rate performance.

[0071] In this embodiment, the intensity ratio (I D / I G ) of the D peak and the G peak of the phosphate cathode material in the Raman spectrum is (1.03 to 1.05):1, including but not limited to 1.03:1, 1.032:1, 1.034:1, 1.036:1, 1.038:1, 1.04:1, 1.042:1, 1.044:1, 1.046:1, 1.048:1 or 1.05:1. Among them, the value of I D / I G of the phosphate cathode material in the Raman spectrum is relatively low, indicating that the crystallinity of its carbon layer is relatively high, which can improve its electron conductivity and structural stability, and thus can improve the specific capacity, rate performance and cycle performance of the secondary battery prepared by using this phosphate cathode material.

[0072] In some embodiments, the expression of the sodium vanadium fluorophosphate material is: Na 3+x+y V 2-x-y A x B y (PO4)2F3; where 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.5; element A and element B each independently include one of Fe, Mn, Ti, Al and Mg, and element A and element B are different from each other.

[0073] In this embodiment, both x and y represent stoichiometric ratios, and the values of x and y can each independently be 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. Preferably, 0 < x ≤ 0.5, 0 < y ≤ 0.5.

[0074] It can be understood that if x = 0 and y = 0, the expression of the sodium vanadium fluorophosphate material is Na3V2(PO4)2F3 (NVPF), which means that this sodium vanadium fluorophosphate material does not contain any doped metal cations. If x = 0 or y = 0, a single metal cation is doped in this sodium vanadium fluorophosphate material. If 0 < x ≤ 0.5 and 0 < y ≤ 0.5, two different metal cations are doped in this sodium vanadium fluorophosphate material, thus forming high-entropy doping.

[0075] In this embodiment, by doping with specific types and proportions of metal cations, the sodium vanadium fluorophosphate material can be more completely melted during high-temperature sintering, resulting in a denser surface morphology and thus improving the compaction density, BET specific surface area, and specific capacity of the sodium vanadium fluorophosphate material. Specifically, compared with NVPF without metal cation doping, doping with high-voltage plateau metal cations such as Fe, Mn, Ti, Al, and Mg in sodium vanadium fluorophosphate material effectively improves the average voltage plateau and specific capacity of the material. Among them, the ionic radii of 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 are enhanced, suppressing structural phase transitions during charging and discharging, thereby reducing voltage decay caused by phase transitions. Simultaneously, the doping of multi-metal cations can also increase the diffusion capacity of Na. + The diffusion rate is increased, thereby improving the ionic conductivity of sodium vanadium fluorophosphate, which in turn enhances the specific capacity and rate performance of sodium vanadium fluorophosphate.

[0076] In some embodiments, the mass ratio of the core to the carbon layer is 100:(1.75 to 3.3), including but not limited to 100:1.75, 100:1.8, 100:1.9, 100:2, 100:2.1, 100:2.2, 100:2.3, 100:2.4, 100:2.5, 100:2.6, 100:2.7, 100:2.8, 100:2.9, 100:3, 100:3.1, 100:3.2, or 100:3.3. Preferably, the mass ratio of the core to the carbon layer is 100:(2 to 3).

[0077] In this embodiment, by controlling the mass ratio of the core to the carbon layer, higher electronic conductivity can be achieved, thereby improving the specific capacity, rate performance, and cycle performance of the prepared phosphate cathode material. If the mass ratio of the carbon layer is too low, an effective coating structure and conductive network cannot be formed, resulting in an unsatisfactory improvement in electronic conductivity; if the mass ratio of the carbon layer is too high, it will reduce the content of electrochemically active materials, increase the material resistance, and hinder the absorption of Na+. + The diffusion of phosphate reduces the overall electrochemical performance of the prepared phosphate cathode material.

[0078] In some embodiments, the thickness of the carbon layer is 1 nm to 3 nm, including but not limited to 1 nm, 1.2 nm, 1.5 nm, 1.8 nm, 2 nm, 2.2 nm, 2.5 nm, 2.8 nm or 3 nm, and preferably, the thickness of the carbon layer is 1.95 nm to 2.8 nm.

[0079] In this embodiment, controlling the thickness of the carbon layer within a suitable range can improve electronic conductivity, enhance structural stability, and suppress side reactions, thereby improving the specific capacity, rate performance, and cycle performance of the prepared phosphate cathode material.

[0080] In some embodiments, the D50 particle size of the phosphate cathode material is 4 μm to 10 μm, including but not limited to 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm or 10 μm. Preferably, the D50 particle size of the phosphate cathode material is 5 μm to 6 μm.

[0081] In this embodiment, the phosphate cathode material has a suitable particle size to ensure Na + It can be inserted and extracted relatively quickly, which can effectively improve the specific capacity and rate performance of secondary batteries made with phosphate cathode materials. At the same time, because phosphate cathode materials are easy to disperse in electrode slurry, it is beneficial to form a cathode active coating with high flatness and good uniformity, which is beneficial to improve the consistency and stability of cathode sheets made with phosphate cathode materials.

[0082] Referring to Figure 1, in a second aspect, this application provides a method for preparing a phosphate cathode material, including the following steps:

[0083] S1: Vanadium source, phosphorus source, sodium source, fluorine source, carbon source and aqueous phase are mixed and dispersed to prepare precursor slurry;

[0084] S2: Drying and calcining the precursor slurry to prepare phosphate cathode material;

[0085] The carbon source includes a first carbon source and a second carbon source. The first carbon source and the second carbon source each independently include one or more of glucose, polyethylene glycol, sucrose and ascorbic acid. The first carbon source and the second carbon source are different from each other.

[0086] In the technical solution of this application embodiment, at least two carbon sources are selected for carbon coating treatment, thereby forming a highly crystalline carbon layer on the core surface of the phosphate cathode material, giving it better electronic conductivity and structural stability, and significantly improving the compaction density and BET specific surface area of ​​the phosphate cathode material. This is beneficial to improving the specific capacity and rate performance of the secondary battery prepared using the phosphate cathode material, and thus preparing a high-energy-density secondary battery.

[0087] The preparation method of phosphate cathode material is described in detail below using a step-by-step approach.

[0088] S1: Mix vanadium source, phosphorus source, sodium source, fluorine source, carbon source and water, and grind them to prepare a precursor slurry.

[0089] In some embodiments, the molar ratio of vanadium in the vanadium source, phosphorus in the phosphorus source, sodium in the sodium source, fluorine in the fluorine source, and carbon in the carbon source is 1:(0.8-1.2):(1.4-1.6):(1.4-1.6):(0.45-0.55).

[0090] In this embodiment, by controlling the molar ratio of each element in the raw materials, the raw materials can react fully to form a high-purity sodium vanadium fluorophosphate material, and a carbon layer with suitable thickness and carbon content is formed on its surface. Specifically, the molar ratio of vanadium in the vanadium source to phosphorus in the phosphorus source can be any ratio within the range of 1:0.8, 1:0.85, 1:0.9, 1:0.95, 1:1, 1:1.05, 1:1.1, 1:1.15, 1:1.2, or 1:(0.8~1.2); the molar ratio of vanadium in the vanadium source to sodium in the sodium source, and the molar ratio of vanadium in the vanadium source to fluorine in the fluorine source, can each independently be 1:1.4, 1:1.42, 1:1.44, 1:1.46, or 1:1.4. 8. The ratio of vanadium in the vanadium source to carbon in the carbon source can be any ratio within the range of 1:1.5, 1:1.52, 1:1.54, 1:1.56, 1:1.58, 1:1.6 or 1:(1.4 to 1.6); the molar ratio of vanadium in the vanadium source to carbon in the carbon source can be any ratio within the range of 1:0.45, 1:0.46, 1:0.47, 1:0.48, 1:0.49, 1:0.50, 1:0.51, 1:0.52, 1:0.53, 1:0.54, 1:0.55 or 1:(0.45 to 0.55).

[0091] In some embodiments, the vanadium source includes one or more of vanadium pentoxide, vanadium trioxide, vanadium dioxide, ammonium metavanadate, sodium metavanadate, and sodium orthovanadate. It is understood that the vanadium source can be selected from any one of the above-mentioned 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, vanadium trioxide, vanadium dioxide, and ammonium metavanadate, etc. Preferably, the vanadium source is vanadium pentoxide.

[0092] In some embodiments, the phosphorus source includes one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate. It is understood that the phosphorus source can be selected from any one of the above-mentioned phosphorus sources, or from a combination of at least two of them, such as: a combination of diammonium hydrogen phosphate and ammonium dihydrogen phosphate, a combination of diammonium hydrogen phosphate and ammonium 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. Preferably, the phosphorus source is diammonium hydrogen phosphate.

[0093] In some embodiments, the sodium source includes one or more of sodium fluoride, sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate. It is understood that the sodium source can be selected from any one of the above-mentioned sodium sources, or from combinations 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 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. Preferably, the sodium source is sodium fluoride.

[0094] In some embodiments, the fluorine source includes one or more of sodium fluoride and ammonium fluoride. It is understood that the fluorine source may be selected from sodium fluoride, ammonium fluoride, and combinations of sodium fluoride and ammonium fluoride. Preferably, the fluorine source is sodium fluoride.

[0095] In this embodiment, different vanadium, phosphorus, sodium and fluorine sources have different reactivity. By rationally selecting and combining different raw materials, the reaction rate of high-temperature solid-phase synthesis reaction can be controlled, the stoichiometry of phosphate cathode material can be precisely controlled, the crystal structure and surface morphology of the material can be optimized, thereby improving its electrochemical performance.

[0096] In some embodiments, the first carbon source is glucose, and the second carbon source includes one or more of polyethylene glycol, sucrose, and ascorbic acid. Understandably, the second carbon source can be selected from any one of polyethylene glycol, sucrose, and ascorbic acid, or from at least two of polyethylene glycol, sucrose, and ascorbic acid, for example: a combination of polyethylene glycol and sucrose, a combination of polyethylene glycol and ascorbic acid, a combination of sucrose and ascorbic acid, or a combination of polyethylene glycol, sucrose, and ascorbic acid. The molecular weight of the polyethylene glycol can be from 100 to 10000, for example, 100, 200, 400, 600, 800, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, or 10000; preferably, the molecular weight of the polyethylene glycol is from 2000 to 6000.

[0097] This application reveals that different carbon sources, when calcined at high temperatures, result in significantly different carbon layer structures, which greatly influence the compaction density, BET specific surface area, and electrochemical performance of phosphate cathode materials. Using glucose as the primary carbon source, and compounding it with at least one of polyethylene glycol, sucrose, and ascorbic acid, facilitates the formation of highly crystalline carbon layers and ensures that the prepared phosphate cathode material exhibits high compaction density, high BET specific surface area, and excellent electrochemical performance.

[0098] Please refer to Table 1. When glucose is used as the carbon source, the resulting carbon layer has finer particles and a denser surface, resulting in higher compaction density and a smaller BET specific surface area. However, excessively high surface density is detrimental to Na+. + The diffusion of phosphate leads to poor electrochemical performance of the prepared phosphate cathode material.

[0099] Furthermore, when polyethylene glycol (PEG) is used as a carbon source, PEG, as a polymeric carbon source, is prone to incomplete breakage of long carbon chains due to insufficient sintering temperature, thereby forming a carbon layer with a network structure. This can promote electrolyte penetration and improve electronic conductivity, resulting in lower compaction density, larger BET specific surface area, and better electrochemical performance.

[0100] Furthermore, when sucrose is used as a carbon source, it will first be converted into caramel with higher viscosity during the low-temperature heating stage, which inhibits the growth of crystals in the material and exhibits a needle-like or flocculent structure. This results in low crystallinity of the carbon layer, poor electronic conductivity and structural stability, thus leading to high compaction density, BET specific surface area between glucose and PEG, and poor electrochemical performance.

[0101] Furthermore, when ascorbic acid is used as a carbon source, the carbon layer formed by ascorbic acid, also known as vitamin C (VC), has large particles. Excessive particles lead to instability in the carbon layer structure. Therefore, the compaction density and BET specific surface area are between those of sucrose and PEG, resulting in poor electrochemical performance.

[0102] Table 1. Effects of different carbon sources on the performance of phosphate cathode materials

[0103] In some embodiments, the mass ratio of the first carbon source and the second carbon source is (1 to 6):1, including but not limited to 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1 or 6:1.

[0104] In this embodiment, by controlling the ratio of the two carbon sources, a good balance can be achieved in terms of compaction density, BET specific surface area and electrochemical performance, thereby producing a phosphate cathode material that has high compaction density, high BET specific surface area and excellent electrochemical performance.

[0105] In some embodiments, the water may be selected from one or more of distilled water, reverse osmosis water, deionized water, pure water and ultrapure water, preferably, the water is deionized water.

[0106] In some embodiments, the dispersion process includes one or more of ball milling and sand milling.

[0107] In this embodiment, ball milling and / or sand milling can make different materials mix evenly and make them fully contacted, and reduce the D50 particle size of the precursor slurry, which is beneficial to improving the subsequent reaction rate and product uniformity.

[0108] In some embodiments, the dispersion treatment includes the following steps: feeding a mixture containing vanadium source, phosphorus source, sodium source, fluorine source, carbon source and water into a ball mill, and ball milling for 1 hour to 3 hours at a working frequency of 10 Hz to 50 Hz to obtain a ball mill slurry; feeding the ball mill slurry into a sand mill, and ball milling at a main motor speed of 400 rpm to 1300 rpm and a circulation flow rate of 2000 m³ / h. 3 / h~5000m 3 Sand milling for 1 to 3 hours under the condition of / h yields precursor slurry.

[0109] In some embodiments, the solid content of the precursor slurry is 20% to 30%, including but not limited to 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or any value within the range of 20% to 30%.

[0110] In this embodiment, adjusting the solid content of the precursor slurry can regulate the rate of water evaporation during drying, improve production efficiency and reduce energy consumption, and promote the transformation of the precursor slurry into a dry material with a relatively uniform particle size distribution, which is beneficial to improving the reaction uniformity during calcination and thus improving the electrochemical performance of the material.

[0111] In some embodiments, the D50 particle size of the precursor slurry is 0.4 μm to 0.6 μm, including but not limited to 0.4 μm, 0.42 μm, 0.45 μm, 0.48 μm, 0.5 μm, 0.52 μm, 0.55 μm, 0.58 μm, 0.6 μm or any value within the range of 0.4 μm to 0.6 μm. Preferably, the D50 particle size of the precursor slurry is 0.4 μm.

[0112] In this embodiment, adjusting the D50 particle size of the precursor slurry can improve the dispersion uniformity and stability of the raw materials, optimize the particle size distribution of the dried material, and improve the reaction uniformity during the calcination process, thereby improving the electrochemical performance of the material.

[0113] S2: Drying and calcining the precursor slurry to prepare phosphate cathode material;

[0114] In some embodiments, the drying process includes the following steps: spray drying the precursor slurry, wherein the inlet air temperature of the spray drying is 240°C to 260°C and the outlet air temperature is 90°C to 100°C, to obtain a dried material.

[0115] In this embodiment, the inlet air temperature for spray drying can be any value within the range of 240℃, 242℃, 244℃, 246℃, 248℃, 250℃, 252℃, 254℃, 256℃, 258℃, 260℃, or 240℃~260℃; the outlet air temperature for spray drying can be any value within the range of 90℃, 91℃, 92℃, 93℃, 94℃, 95℃, 96℃, 97℃, 98℃, 99℃, 100℃, or 90℃~100℃. By controlling the inlet and outlet air temperatures of the spray dryer, moisture in the dried material can be effectively removed, and excessively high outlet air temperatures can be avoided to prevent the decomposition of some carbon sources and reduce the carbon content of the product.

[0116] In some embodiments, the calcination treatment includes the following steps: heating the dried material to 550°C to 750°C and holding it at that temperature for 6 to 12 hours in a protective atmosphere.

[0117] In this embodiment, the protective gas in the protective atmosphere can be at least one of nitrogen, helium, neon, argon, and xenon, preferably nitrogen or argon. High-temperature calcination in a protective atmosphere avoids reactions between the material and moisture and oxygen during the heating, holding, and cooling stages, which helps improve the purity of the finished product and reduces adverse effects on the subsequent preparation process and electrical performance of the positive electrode and secondary battery.

[0118] In this embodiment, the calcination temperature can be any value within the range of 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, or 550℃ to 750℃. Calcination at 550℃ to 750℃ helps avoid excessive impurity phase formation during sintering due to excessively high or low temperatures, thus improving the crystallinity of the finished product and increasing the charge / discharge specific capacity. Preferably, the calcination temperature is 550℃ to 650℃. Calcination at this temperature reduces the energy consumption of the calcination process while ensuring the crystallinity and charge / discharge specific capacity of the finished product, thereby reducing the cost of the phosphate cathode material.

[0119] In this embodiment, the heat preservation time can be 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, 12h, or any value within the range of 6h to 12h. A heat preservation time of 6h to 12h helps to ensure the purity of the finished product and the uniformity of carbon coating, thereby improving the electrochemical performance of the material. Preferably, the heat preservation time is 8h to 12h.

[0120] In some embodiments, after calcination, the method further includes the following steps: pulverizing, sieving, and removing iron from the calcined product to prepare a phosphate cathode material with a D50 particle size of 4 μm to 10 μm.

[0121] In this embodiment, the D50 particle size of the phosphate cathode material can be 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, or 10μm. Preferably, the D50 particle size of the phosphate cathode material is 5μm to 6μm. By controlling the D50 particle size of the phosphate cathode material, it is beneficial to Na + The rapid transport of the material can improve its dispersion uniformity in the electrode slurry, which is beneficial to the formation of a positive electrode active coating with high flatness and good uniformity, thereby improving the consistency and stability of the positive electrode sheet prepared using phosphate positive electrode material.

[0122] In some embodiments, the D100 particle size of the phosphate cathode material is ≤30μm.

[0123] In some embodiments, the ambient humidity is ≤10% during the crushing, screening and iron removal processes.

[0124] 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.

[0125] In this embodiment, the positive electrode sheet includes the above-mentioned phosphate positive electrode material, thus having the advantages of high specific capacity and good rate performance.

[0126] Fourthly, embodiments of this application provide a secondary battery, including the positive electrode sheet provided in the third aspect of this application.

[0127] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, thus possessing the advantages of high specific capacity and good rate performance.

[0128] In some embodiments, the initial charge specific capacity of the secondary battery at 0.1C is ≥109mAh / g, including but not limited to 109mAh / g, 110mAh / g, 112mAh / g, 114mAh / g, 116mAh / g, 118mAh / g, 120mAh / g, 122mAh / g, 124mAh / g, 126mAh / g, 128mAh / g or 130mAh / g, and more preferably 122mAh / g to 128mAh / g.

[0129] In some embodiments, the initial discharge specific capacity of the secondary battery at 0.1C is ≥95mAh / g, including but not limited to 95mAh / g, 96mAh / g, 98mAh / g, 100mAh / g, 102mAh / g, 104mAh / g, 106mAh / g, 108mAh / g, 110mAh / g, 112mAh / g, 114mAh / g, 116mAh / g, 118mAh / g or 120mAh / g, and more preferably 113mAh / g to 117mAh / g.

[0130] In some embodiments, the secondary battery has an initial coulombic efficiency of ≥87% at 0.1C, including but not limited to 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100%, and more preferably 91.31% to 92.74%.

[0131] In some embodiments, the initial discharge specific capacity of the secondary battery at 1C is ≥84mAh / g, including but not limited to 84mAh / g, 86mAh / g, 88mAh / g, 90mAh / g, 92mAh / g, 95mAh / g, 98mAh / g, 100mAh / g, 102mAh / g, 105mAh / g, 108mAh / g, 110mAh / g, 112mAh / g or 115mAh / g, and more preferably 107mAh / g to 112mAh / g.

[0132] In some embodiments, the 1C / 0.1C rate performance of the secondary battery is ≥88%, including but not limited to 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100%, and more preferably 93.91% to 95.72%.

[0133] In this embodiment, the secondary battery has high charge specific capacity, discharge specific capacity, initial coulombic efficiency, and rate performance, which is beneficial to improving the electrochemical performance of the electrical device using the secondary battery.

[0134] 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.

[0135] I. Preparation Method

[0136] Example 1

[0137] Please refer to Table 2. The preparation method of the phosphate cathode material in this embodiment is as follows:

[0138] (1) Mix vanadium source, phosphorus source, sodium source, fluorine source and carbon source, add deionized water to obtain a mixture with a solid content of 25%; put the mixture into a ball mill for coarse grinding, and ball mill for 2 hours at a working frequency of 30Hz to make the material uniformly mixed to obtain ball mill slurry; transfer the ball mill slurry to a sand mill for fine grinding, and sand mill for 2 hours at a speed of 1000rpm and a circulation flow rate of 3000 to obtain a precursor slurry with a D50 particle size of 0.4μm.

[0139] Among them, the vanadium source is vanadium pentoxide, the phosphorus source is diammonium hydrogen phosphate, the sodium source and the fluorine source are both sodium fluoride, and the carbon source is glucose and PEG-2000 with a mass ratio of 4:1. The molar ratio of vanadium, phosphorus, sodium, fluorine and carbon is 1:1:1.5:1.5:0.5.

[0140] (2) Spray drying of the precursor slurry, controlling the inlet air temperature to 250℃ and the outlet air temperature to 90℃, to obtain dried material; transfer the dried material into a sintering furnace, introduce dry nitrogen as a protective gas, heat to 600℃ and hold for 10h to obtain sintered material; crush, screen and remove iron from the sintered material to obtain phosphate cathode material with D50 particle size of 5.1μm and D100 particle size <30μm.

[0141] Examples 2-14

[0142] Examples 2-14 are basically the same as Example 1, with the following differences:

[0143] Example 2: The carbon source used was glucose and PEG-2000 in a mass ratio of 6:1;

[0144] Example 3: The carbon source used was glucose and PEG-2000 in a mass ratio of 5:1;

[0145] Example 4: The carbon source was glucose and PEG-2000 in a mass ratio of 3:1;

[0146] Example 5: The carbon source used was glucose and PEG-2000 in a mass ratio of 2:1;

[0147] Example 6: The carbon source was glucose and PEG-2000 in a mass ratio of 1:1;

[0148] Example 7: The carbon source used was glucose and PEG-2000 in a mass ratio of 1:2;

[0149] Example 8: The carbon source was glucose and sucrose in a mass ratio of 1:1;

[0150] Example 9: The carbon source was selected from glucose and sucrose in a mass ratio of 4:1;

[0151] Example 10: The carbon source was glucose and sucrose in a mass ratio of 6:1;

[0152] Example 11: The carbon source was selected from glucose and vitamin C in a mass ratio of 1:1;

[0153] Example 12: The carbon source was glucose and vitamin C in a mass ratio of 4:1;

[0154] Example 13: The carbon source was selected from glucose and vitamin C in a mass ratio of 6:1;

[0155] Example 14: The carbon source used was VC and PEG-2000 with a mass ratio of 4:1;

[0156] Example 15: During the process of mixing vanadium, phosphorus, sodium, fluorine and carbon sources, the molar ratio of vanadium, phosphorus, sodium, fluorine and carbon is 1:1:1.5:1.5:0.4;

[0157] Example 16: During the process of mixing vanadium, phosphorus, sodium, fluorine and carbon sources, the molar ratio of vanadium, phosphorus, sodium, fluorine and carbon is 1:1:1.5:1.5:0.6.

[0158] Comparative Examples 1-4

[0159] Comparative Examples 1-4 are basically the same as Example 1, with the following differences:

[0160] Comparative Example 1: Glucose was used as the carbon source;

[0161] Comparative Example 2: PEG-2000 was used as the carbon source;

[0162] Comparative Example 3: Sucrose was used as the carbon source;

[0163] Comparative Example 4: The carbon source used was VC;

[0164] II. Testing Methods

[0165] 1. Property testing of phosphate cathode materials

[0166] (1) X-ray diffraction (XRD) test: Phase analysis was performed using a Bruker D8 Advance X-ray diffractometer manufactured in Germany. The results are shown in Figures 2 to 4. Among them, Figure 2 is the phase result diagram of the phosphate cathode material of Example 1; Figure 3 is the phase result diagram of the phosphate cathode material of Comparative Example 1; and Figure 4 is the phase result diagram of the phosphate cathode material of Comparative Example 2.

[0167] (2) Scanning Electron Microscopy (SEM) Test: Morphological analysis was performed using a MERLIN Compact or Quanta 200FEG field emission scanning electron microscope manufactured by Zeiss. The results are shown in Figures 5-7. Figure 5 shows the morphological results of the phosphate cathode material of Example 1; Figure 6 shows the morphological results of the phosphate cathode material of Comparative Example 1; Figure 7 shows the morphological results of the phosphate cathode material of Comparative Example 2.

[0168] (3) 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. The results are shown in Table 3.

[0169] (4) Carbon content: The carbon content was tested using an infrared carbon-sulfur meter of model HCS-140 manufactured by Shanghai Dekai Company. The results are shown in Table 3.

[0170] (5) Raman testing: Raman spectroscopy analysis was performed using an in Via model Raman spectrometer manufactured by Renishaw, UK. The results are shown in Figures 8-10 and Table 3. Among them, Figure 8 is the Raman spectrum of the phosphate cathode material of Example 1.

[0171] (6) Particle size test: Particle size analysis was performed using a Mastersizer 2000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK. The results are shown in Table 4.

[0172] (7) Compacted density: The compacted density was tested using a UTM7305 battery powder compaction density tester provided by Shenzhen Sansi Zongheng Technology Co., Ltd. The results are shown in Table 4.

[0173] (8) Specific surface area: The specific surface area was tested using a BELSORP MaxII specific surface area analyzer manufactured by Japan-McQKBAY. The results are shown in Table 4.

[0174] 2. Properties of secondary batteries

[0175] 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 12μm to form a cathode slurry layer with a thickness of 80μm. Then it was placed in an oven at 110℃ and dried for 10h. After drying, it was punched into a circular electrode sheet with a diameter of 15mm and rolled to a compaction density of 1.8g / cm3 to obtain the cathode electrode sheet.

[0176] 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.

[0177] The rate performance of button half-cells was tested using the CT3002A battery performance testing system manufactured by Wuhan Landian Electronics Technology Co., Ltd. The test temperature was 25℃, the voltage range was 2~4.35V, and the rate range was 0.1C~3C. The results are shown in Table 4.

[0178] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0179] Table 2. Carbon sources and dopants for phosphate cathode materials

[0180] Table 3. Carbon layer parameters of phosphate cathode materials

[0181] Table 4. Performance Comparison of Phosphate Cathode Materials and Corresponding Secondary Batteries

[0182] As shown in Figures 2-4, compared with Comparative Examples 1-2, Example 1, using glucose and PEG-2000 as a composite carbon source, showed virtually no impurity peaks at 2θ = 35° and 37°, demonstrating that the composite carbon source could yield a phosphate cathode material with higher purity. As shown in Figures 5-7, in Comparative Example 1, using glucose as a single carbon source, the carbon layer had multiple fine single-crystal particles with an uneven surface; in Comparative Example 2, using PEG-2000 as a single carbon source, the carbon layer exhibited a chain-like network structure; in Example 1, using glucose and PEG-2000 as a composite carbon source, the carbon layer consisted of very fine particles with a less dense network structure, exhibiting advantages such as a smoother surface, higher density, and better crystallinity.

[0183] As shown in Tables 2-3 and Figure 8, the morphology of the carbon layer of the phosphate cathode materials in Examples 1-16 changes with the composition and mass ratio of the two carbon sources. The carbon layer thickness is between 1.4 nm and 3.0 nm, the carbon content is between 1.75% and 3.30%, the intensity ratio of the D peak to the G peak is 1.03 to 1.05, and it has a high degree of crystallinity.

[0184] As shown in Table 4, in Examples 1-6, the first carbon source was glucose, and the second carbon source was PEG-2000. The mass ratio of the two was controlled within the range of (1-6):1, resulting in a compaction density of 1.94 g / cm³ for the phosphate cathode material. 3 ~2.06g / cm 3 The specific surface area of ​​BET is 9.8 m². 2 / g~16m 2The phosphate cathode material has the advantages of high compaction density and large BET specific surface area. Secondary batteries prepared using the phosphate cathode materials of Examples 1-6 exhibit the following characteristics: 0.1C initial charge specific capacity of 122 mAh / g to 128 mAh / g, 0.1C initial discharge specific capacity of 113 mAh / g to 117 mAh / g, 0.1C initial coulombic efficiency of 91.31% to 92.74%, 1C initial discharge specific capacity of 107 mAh / g to 112 mAh / g, and 1C / 0.1C rate performance of 93.91% to 95.72%. These properties demonstrate high specific capacity, high initial coulombic efficiency, and excellent rate performance. Therefore, using glucose and PEG-2000 as a composite carbon source can significantly improve the compaction density and BET specific surface area of ​​the prepared phosphate cathode material, thereby improving the specific capacity, initial coulombic efficiency, and rate performance of secondary batteries prepared using this phosphate cathode material. Among them, when the mass ratio of glucose to PEG-2000 in Example 1 was controlled at 4:1, the prepared phosphate cathode material had the highest compaction density. The secondary battery prepared using this phosphate cathode material had the highest 0.1C charge-discharge specific capacity, the highest 1C discharge specific capacity, and the highest 1C / 0.1C rate performance, resulting in the best overall performance.

[0185] Compared with Examples 1-6, the mass ratio of glucose to PEG-2000 in Example 7 was controlled at 1:2, which resulted in a significant decrease in the compaction density and BET specific surface area of ​​the prepared phosphate cathode material. Consequently, the 0.1C charge-discharge specific capacity, 0.1C initial coulombic efficiency, 1C discharge specific capacity, and 1C / 0.1C rate performance of the secondary battery prepared using this phosphate cathode material all decreased significantly. This demonstrates that the amount of glucose in the composite carbon source was too small, which is not conducive to improving the compaction density and BET specific surface area of ​​the prepared phosphate cathode material, and thus leads to a deterioration in the electrochemical performance of the secondary battery prepared using this phosphate cathode material.

[0186] Compared to Examples 1-7, Comparative Example 1 used glucose as a single carbon source, and Comparative Example 2 used PEG-2000 as a single carbon source. This resulted in a significant decrease in the compaction density and BET specific surface area of ​​the prepared phosphate cathode material, leading to a significant decrease in the 0.1C charge / discharge specific capacity, 1C discharge specific capacity, and 1C / 0.1C rate performance of the secondary battery prepared using this phosphate cathode material. This demonstrates that using both glucose and PEG-2000 as carbon sources for carbon coating, and appropriately controlling the mass ratio of the two carbon sources, can significantly improve the compaction density and BET specific surface area of ​​the phosphate cathode material, ensuring that the secondary battery prepared using this phosphate cathode material exhibits excellent performance in charge / discharge specific capacity, initial coulombic efficiency, and rate performance.

[0187] In Examples 8-10, the first carbon source was glucose, and the second carbon source was sucrose, with their mass ratio controlled within the range of (1-6):1, resulting in a compaction density of 1.944 g / cm³ for the phosphate cathode material. 3 ~2.04g / cm 3 BET has a specific surface area of ​​10m². 2 / g~14.8m 2 The phosphate cathode material, with its high compaction density and large BET specific surface area, exhibits advantages such as high compaction density and large BET specific surface area. Secondary batteries prepared using the phosphate cathode materials of Examples 8-10 show the following characteristics: 0.1C initial charge specific capacity of 121 mAh / g to 125 mAh / g, 0.1C initial discharge specific capacity of 112 mAh / g to 115 mAh / g, 0.1C initial coulombic efficiency of 90.32% to 92.56%, 1C initial discharge specific capacity of 105 mAh / g to 110 mAh / g, and 1C / 0.1C rate performance of 93.75% to 95.40%. These properties demonstrate high specific capacity, high initial coulombic efficiency, and excellent rate performance. Therefore, using glucose and sucrose as a composite carbon source can improve the compaction density and BET specific surface area of ​​the prepared phosphate cathode material, thereby contributing to improved specific capacity, initial coulombic efficiency, and rate performance of secondary batteries prepared using this phosphate cathode material. Compared with Comparative Examples 1 and 3, the compaction density and BET specific surface area of ​​the phosphate cathode materials prepared in Examples 8-10 were significantly improved, resulting in a significant improvement in the 0.1C charge-discharge specific capacity, 1C discharge specific capacity, and 1C / 0.1C rate performance of the secondary batteries prepared using this phosphate cathode material. This demonstrates that using glucose and sucrose as carbon sources for carbon coating treatment is also beneficial to improving the compaction density, BET specific surface area, and electrochemical performance of the secondary batteries using this phosphate cathode material.

[0188] In Examples 11-13, the first carbon source was glucose, and the second carbon source was vitamin C. The mass ratio of the two was controlled within the range of (1-6):1, resulting in a compaction density of 1.95 g / cm³ for the phosphate cathode material. 3 ~2.043g / cm 3 The specific surface area of ​​BET is 9.9 m². 2 / g~14.1m 2The phosphate cathode material has the advantages of high compaction density and large BET specific surface area. Secondary batteries prepared using the phosphate cathode materials of Examples 11-13 exhibit the following characteristics: 0.1C initial charge specific capacity of 120 mAh / g to 122 mAh / g, 0.1C initial discharge specific capacity of 109 mAh / g to 111 mAh / g, 0.1C initial coulombic efficiency of 90.83% to 90.98%, 1C initial discharge specific capacity of 102 mAh / g to 106 mAh / g, and 1C / 0.1C rate performance of 93.57% to 95.49%. These properties demonstrate high specific capacity, high initial coulombic efficiency, and excellent rate performance. Therefore, using glucose and VC as a composite carbon source can significantly improve the compaction density and BET specific surface area of ​​the prepared phosphate cathode material, thereby improving the specific capacity, initial coulombic efficiency, and rate performance of secondary batteries prepared using this phosphate cathode material. Compared with Comparative Examples 1 and 4, the compaction density and BET specific surface area of ​​the phosphate cathode materials prepared in Examples 11-13 were significantly improved, resulting in a significant improvement in the 0.1C charge-discharge specific capacity, 1C discharge specific capacity, and 1C / 0.1C rate performance of the secondary batteries prepared using this phosphate cathode material. This confirms that carbon coating treatment using glucose and VC as two carbon sources is also beneficial to improving the compaction density, BET specific surface area, and electrochemical performance of the secondary batteries using this phosphate cathode material.

[0189] In Example 14, the first carbon source was VC, and the second carbon source was PEG-2000. The mass ratio of the two was controlled at 4:1, resulting in a compaction density of 1.88 g / cm³ for the phosphate cathode material. 3 The specific surface area of ​​BET is 12.2 m². 2 The secondary battery prepared using the phosphate cathode material of Example 14 exhibits the following performance characteristics: 0.1C initial charge specific capacity of 115 mAh / g, 0.1C initial discharge specific capacity of 105 mAh / g, 0.1C initial coulombic efficiency of 91.3%, 1C initial discharge specific capacity of 99 mAh / g, and 1C / 0.1C rate performance of 94.28%. These performance characteristics are generally lower than those of Examples 1, 9, and 12. This demonstrates that with a carbon source mass ratio of 4:1, using glucose as the primary carbon source yields a phosphate cathode material with higher compaction density and a larger BET specific surface area, which is beneficial for obtaining a secondary battery with superior electrochemical performance.

[0190] Compared to Example 1, Example 15 reduced the amount of carbon source, decreasing the carbon layer thickness of the phosphate cathode material to 1.4 nm and the carbon content to 1.75%. This resulted in a significant decrease in the compaction density and BET specific surface area of ​​the phosphate cathode material, leading to a significant reduction in the 0.1C charge / discharge specific capacity, 0.1C initial coulombic efficiency, 1C discharge specific capacity, and 1C / 0.1C rate performance of the secondary battery prepared using this phosphate cathode material. Example 16 increased the amount of carbon source, increasing the carbon layer thickness of the phosphate cathode material to 2.66 nm and the carbon content to 3.3%. While the BET specific surface area of ​​the phosphate cathode material improved, the compaction density decreased significantly. Therefore, although the secondary battery prepared using this phosphate cathode material showed improvements in initial coulombic efficiency and 1C / 0.1C rate performance, both the 0.1C charge / discharge specific capacity and the 1C discharge specific capacity were significantly reduced. Therefore, by controlling the total amount of the two carbon sources, a balance can be achieved between the compaction density and BET specific surface area of ​​the phosphate cathode material and the charge / discharge capacity, first coulombic efficiency and rate performance of the secondary battery prepared using the phosphate cathode material, resulting in a phosphate cathode material and secondary battery with superior overall performance.

[0191] In summary, this application uses two carbon sources for carbon coating treatment. By adjusting the composition and mass ratio of the carbon sources, a dense and highly crystalline carbon layer can be formed on the core surface, giving the phosphate cathode material better electronic conductivity and structural stability. This significantly improves the compaction density and BET specific surface area of ​​the phosphate cathode material, resulting in excellent charge-discharge specific capacity, first coulombic efficiency, and rate performance of the secondary battery prepared using the phosphate cathode material. Consequently, a secondary battery with high energy density is obtained.

[0192] 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 cathode material, characterized in that, Includes the kernel and the carbon layer covering the kernel; The core material includes sodium vanadium fluorophosphate. The compaction density of the phosphate cathode material is ≥1.78 g / cm³. 3 BET specific surface area is ≥9m² 2 / g; The Raman spectrum of the phosphate cathode material has a D peak and a G peak, and the intensity ratio of the D peak to the G peak is (1.03~1.05):

1.

2. The phosphate cathode material according to claim 1, characterized in that, One or more of the following conditions must be met: (1) The mass ratio of the core to the carbon layer is 100:(1.75~3.3); (2) The thickness of the carbon layer is 1 nm to 3 nm; (3) The D50 particle size of the phosphate cathode material is 4μm to 10μm.

3. A method for preparing a phosphate cathode material, characterized in that, Includes the following steps: Vanadium source, phosphorus source, sodium source, fluorine source, carbon source and aqueous phase are mixed and dispersed to prepare precursor slurry; The precursor slurry is dried and calcined to prepare the phosphate cathode material; The carbon source includes a first carbon source and a second carbon source, each of which independently includes one or more of glucose, polyethylene glycol, sucrose, and ascorbic acid, and the first carbon source and the second carbon source are different from each other.

4. The method for preparing the phosphate cathode material according to claim 3, characterized in that, The first carbon source is glucose, and the second carbon source includes one or more of polyethylene glycol, sucrose, and ascorbic acid.

5. The method for preparing the phosphate cathode material according to claim 4, characterized in that, The mass ratio of the first carbon source to the second carbon source is (1-6):

1.

6. The method for preparing the phosphate cathode material according to any one of claims 3 to 5, characterized in that, The molar ratio of vanadium in the vanadium source, phosphorus in the phosphorus source, sodium in the sodium source, fluorine in the fluorine source, and carbon in the carbon source is 1:(0.8-1.2):(1.4-1.6):(1.4-1.6):(0.45-0.55).

7. The method for preparing the phosphate cathode material according to claim 6, characterized in that, One or more of the following conditions must be met: (1) The vanadium source includes one or more of vanadium pentoxide, vanadium trioxide, vanadium dioxide, ammonium metavanadate, sodium metavanadate and sodium orthovanadate; (2) The phosphorus source includes one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; (3) The sodium source includes one or more of sodium fluoride, sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate; (4) The fluorine source includes one or more of sodium fluoride and ammonium fluoride.

8. The method for preparing the phosphate cathode material according to claim 7, characterized in that, One or more of the following conditions must be met: (1) The dispersion treatment includes one or more of ball milling and sand milling; (2) The solid content of the precursor slurry is 20% to 30%; (3) The D50 particle size of the precursor slurry is 0.4μm to 0.6μm; (4) The drying process includes the following steps: spray drying the precursor slurry, wherein the inlet air temperature of the spray drying is 240℃~260℃ and the outlet air temperature is 90℃~100℃, to obtain dried material; (5) The calcination treatment includes the following steps: heating to 550℃~750℃ in a protective atmosphere and holding for 6h~12h; (6) After the calcination treatment, the following steps are also included: pulverizing, sieving and removing iron from the calcined product to prepare a phosphate cathode material with a D50 particle size of 4μm to 10μm.

9. A positive electrode sheet, characterized in that, It includes the phosphate cathode material according to claim 1 or 2, or the phosphate cathode material prepared by the method according to any one of claims 3 to 8.

10. A secondary battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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