Positive electrode particles and their preparation methods, positive electrode sheets and sodium batteries

Abstract

This application provides a positive electrode particle, a method for preparing the same, a positive electrode sheet, and a sodium battery. The positive electrode particle provided in this application has a first porosity P1 within half the radius of its maximum cross-section, a second porosity P2 between half and three-quarters of the radius of its maximum cross-section, and a third porosity P3 between three-quarters of the radius of its maximum cross-section and the surface of the positive electrode particle, wherein P1 < P2 < P3, and the maximum cross-section of the positive electrode particle refers to the cross-section along its maximum length direction.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a positive electrode particle and its preparation method, a positive electrode sheet, and a sodium battery. Background Technology

[0002] Sodium batteries, with their advantages of abundant and widely distributed sodium resources and low cost, are considered a highly promising large-scale energy storage solution and have received widespread attention in recent years. Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7), possessing a three-dimensional sodium ion diffusion channel and a sodium superionic conductor structure, exhibits high voltage plateau, high capacity, excellent rate capability, and excellent cycle stability, making it a very promising cathode material for large-scale production in sodium batteries. However, in related technologies, sodium iron pyrophosphate has struggled to simultaneously achieve both high compaction density and high rate performance. Summary of the Invention

[0003] This application provides a positive electrode particle with high compaction density and high rate performance.

[0004] In a first aspect, embodiments of this application provide a positive electrode particle, wherein the positive electrode particle has a first porosity P1 within a 1 / 2 radius range of its maximum cross-section, a second porosity P2 between a 1 / 2 radius and a 3 / 4 radius of its maximum cross-section, and a third porosity P3 between a 3 / 4 radius of its maximum cross-section and the surface of the positive electrode particle, wherein P1 < P2 < P3, and the maximum cross-section of the positive electrode particle refers to the cross-section along the maximum length direction of the positive electrode particle.

[0005] In some embodiments, the porosity of the positive electrode particle gradually increases from the inside to the outside in the radial direction at its maximum cross-section.

[0006] In some embodiments, the average porosity P of the maximum cross-section of the positive electrode particle ranges from 5% to 30%.

[0007] In some embodiments, the first porosity P1 ranges from 3% to 15%.

[0008] In some embodiments, the second porosity P2 ranges from 7% to 18%.

[0009] In some embodiments, the third porosity P3 ranges from 10% to 28%.

[0010] In some embodiments, the maximum cross-section of the positive electrode particle satisfies at least one of the following relationships:

[0011] (5P2+7P3) / P1>12; and

[0012] (4P1+5P2) / P3<9.

[0013] In some embodiments, the positive electrode particles are secondary particles of sodium iron pyrophosphate, which are composed of multiple primary sodium iron pyrophosphate particles; the chemical formula of the sodium iron pyrophosphate is: Na. x Fe y Me z (PO4) α (P2O7) β , wherein Me is at least one of Mg, Ca, Al, Ti, V, Mn, Cr, Cu, Zn, Zr, Nb, W, Mo, Sc, Y, 3.7≤x≤4.3, 2.8≤y+z≤3.0, 0≤z≤1.0, and 3.9≤α+2β≤4.1.

[0014] In some embodiments, the positive electrode particle is a secondary sodium iron pyrophosphate phosphate particle, which is composed of a plurality of primary sodium iron pyrophosphate phosphate particles; the primary sodium iron pyrophosphate phosphate particle includes an active core and a carbon layer, the active core is sodium iron pyrophosphate phosphate, and the carbon layer is wrapped around the surface of the active core, wherein the mass fraction of the carbon layer in the positive electrode particle ranges from 1.0 wt% to 3.0 wt%.

[0015] Secondly, embodiments of this application provide a method for preparing positive electrode particles, the method comprising:

[0016] It provides sodium, phosphorus, iron, and carbon sources;

[0017] Sodium, phosphorus, iron, and carbon sources are placed in a solvent and milled to obtain a slurry;

[0018] The slurry was spray-dried to obtain precursor powder; and

[0019] The precursor powder is pre-sintered and sintered sequentially to obtain positive electrode particles. The positive electrode particles have a first porosity P1 within 1 / 2 radius of the maximum cross-section, a second porosity P2 between 1 / 2 and 3 / 4 radius of the maximum cross-section, and a third porosity P3 between 3 / 4 radius of the maximum cross-section and the surface of the positive electrode particles. P1 < P2 < P3, and the maximum cross-section of the positive electrode particles refers to the cross-section along the maximum length direction of the positive electrode particles.

[0020] In some embodiments, the slurry comprises solid particles, wherein the volume median particle size Dv50 of the solid particles is in the range of 100nm≤Dv50≤300nm.

[0021] In some embodiments, the step of sequentially pre-sintering and sintering the precursor powder to obtain cathode particles includes:

[0022] The precursor powder is heated to 250°C to 400°C at a first heating rate for pre-sintering to obtain an intermediate; and

[0023] The intermediate is heated to 500°C to 650°C at a second heating rate and sintered to obtain positive electrode particles.

[0024] In some embodiments, the first heating rate ranges from 2°C / min to 10°C / min; the second heating rate ranges from 1°C / min to 5°C / min.

[0025] Thirdly, embodiments of this application provide a positive electrode sheet, the positive electrode sheet comprising:

[0026] Positive current collector; and

[0027] A positive electrode active layer, wherein the positive electrode active layer includes the positive electrode particles described in the embodiments of this application.

[0028] Fourthly, this application provides a sodium battery, which includes: an electrolyte, a positive electrode, a separator, and a negative electrode as described in this application.

[0029] The cathode particle in this embodiment has a first porosity P1 within half the radius of its maximum cross-section, a second porosity P2 between half and three-quarters of its maximum cross-section, and a third porosity P3 between three-quarters of its maximum cross-section and the surface of the cathode particle, wherein P1 < P2 < P3, and the maximum cross-section of the cathode particle refers to the cross-section along its maximum length direction. This application achieves a gradient increase in pore size from the inside to the outside of the cathode particle, resulting in a lower overall porosity and higher compaction density, thus enabling the sodium battery to have a higher energy density. Furthermore, the pore size distribution exhibits a gradient increase from the inside out, resulting in a higher pore size, larger specific surface area, and greater roughness on the surface of the cathode particles. This facilitates electrolyte wetting of the cathode particles, improving their electrolyte wettability. Moreover, the pore structure with a decreasing porosity gradient from the outside in allows the electrolyte to gradually penetrate the interior of the cathode particles through capillary action after initial surface wetting. This ensures that even the lower porosity areas within the cathode particles are well wetted by the electrolyte, resulting in excellent electrolyte wettability and thus improved kinetic and rate performance of the sodium battery. In summary, this application, through its design of a gradient increase in porosity from the inside out, achieves a high powder compaction density in the cathode particles, enabling the sodium battery to possess both high energy density and excellent kinetic and rate performance. Attached Figure Description

[0030] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of the structure of an energy storage system according to an embodiment of this application.

[0032] Figure 2 This is a schematic diagram of the structure of an energy storage system according to another embodiment of this application.

[0033] Figure 3 This is a schematic diagram of the structure of an energy storage device according to an embodiment of this application.

[0034] Figure 4 This is a schematic diagram of the structure of a sodium battery according to an embodiment of this application.

[0035] Figure 5 For the application of an embodiment of a sodium battery Figure 4A schematic diagram of the cross-sectional structure along the AA direction.

[0036] Figure 6 This is a cross-sectional view of the negative electrode sheet according to one embodiment of the application.

[0037] Figure 7 This is a cross-sectional view of the positive electrode sheet according to an embodiment of this application.

[0038] Figure 8 This is a schematic diagram of the structure of a positive electrode particle according to an embodiment of this application.

[0039] Figure 9 This is a schematic diagram of the structure of the positive electrode particle according to another embodiment of this application.

[0040] Figure 10 This is a schematic flowchart illustrating a method for preparing positive electrode particles according to an embodiment of this application.

[0041] Figure 11 This is a schematic diagram of the pre-sintering and sintering process in the preparation method of positive electrode particles according to an embodiment of this application.

[0042] Explanation of reference numerals in the attached figures:

[0043] 100 - Energy storage system; 110 - High-voltage cable; 120 - First energy conversion device; 130 - Second energy conversion device; 140 - Third energy conversion device; 150 - First user load; 160 - Second user load; 200 - Energy storage device; 210 - Single cell; 300 - Sodium battery; 320 - Separator; 330 - Negative electrode sheet; 331 - Negative current collector; 332 - Negative active layer; 340 - Shell; 350 - End cap assembly; 400 - Pore sheet; 410 - Pore current collector; 420 - Pore active layer; 500 - Pore particles; 510 - Pores; 520 - Primary sodium iron pyrophosphate particles; 521 - Active core; 522 - Carbon layer. Detailed Implementation

[0044] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0045] The terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0046] The technical solutions in the embodiments of this application will now be described with reference to the accompanying drawings.

[0047] It should be noted that, for ease of explanation, the same reference numerals denote the same components in the embodiments of this application, and for the sake of brevity, detailed descriptions of the same components are omitted in different embodiments.

[0048] Because the energy we need is highly time- and space-dependent, in order to utilize energy rationally and improve energy efficiency, it is necessary to store one form of energy in the same way or by converting it into another, and then release it in a specific energy form based on future application needs. Currently, the main way to generate green electricity is to develop green energy sources such as photovoltaics and wind power to replace fossil fuels.

[0049] Currently, the generation of green electricity generally relies on solar, wind, and hydropower. However, wind and solar power are generally characterized by strong intermittency and large fluctuations, which can cause grid instability, insufficient power during peak demand periods, and excessive power during off-peak periods. Unstable voltage can also damage the power grid. Therefore, insufficient electricity demand or insufficient grid capacity may lead to the problem of "wind and solar curtailment." Solving these problems requires energy storage. This involves converting electrical energy into other forms of energy through physical or chemical means and storing it. When needed, this energy can be converted back into electrical energy and released. Simply put, energy storage is like a large "power bank," storing electrical energy when solar and wind power are abundant and releasing the stored electricity when needed.

[0050] Taking electrochemical energy storage as an example, this solution provides an energy storage device for use in energy storage systems. The energy storage device is equipped with a set of chemical batteries, which mainly use the chemical elements in the batteries as energy storage media. The charging and discharging process is accompanied by the chemical reaction or change of the energy storage media. Simply put, the electrical energy generated by wind and solar energy is stored in the chemical batteries. When the use of external electrical energy reaches its peak, the stored electricity is released for use, or transferred to places with a shortage of electricity for use.

[0051] Current energy storage applications are quite widespread, including generation-side energy storage, grid-side energy storage, and consumption-side energy storage. The corresponding types of energy storage devices include:

[0052] (1) Large-scale energy storage power stations applied to wind power and photovoltaic power stations can assist renewable energy power generation in meeting grid connection requirements and improve the utilization rate of renewable energy. As a high-quality active / reactive power regulation power source on the power supply side, energy storage power stations can achieve load matching of power in time and space, enhance the absorption capacity of renewable energy, reduce instantaneous power changes, reduce the impact on the power grid, improve the absorption of new energy power generation, and are of great significance in power grid system backup, alleviating peak load power supply pressure and peak regulation and frequency regulation.

[0053] (2) Energy storage containers applied on the grid side mainly function as peak shaving, frequency regulation and grid congestion relief. In terms of peak shaving, they can achieve peak shaving and valley filling of electricity load, that is, charging the energy storage battery when the electricity load is low and releasing the stored electricity during the peak electricity load period, thereby achieving a balance between power production and consumption.

[0054] (3) Small energy storage cabinets applied to the electricity consumption side mainly function as self-consumption of electricity, peak-valley price arbitrage, capacity cost management, and improvement of power supply reliability. Depending on the application scenario, electricity consumption side energy storage can be divided into industrial and commercial energy storage cabinets, household energy storage devices, energy storage charging piles, etc., which are generally used in conjunction with distributed photovoltaics. Industrial and commercial users can use energy storage for peak-valley price arbitrage and capacity cost management. In the electricity market implementing peak-valley pricing, by charging the energy storage system when the electricity price is low and discharging the energy storage system when the electricity price is high, peak-valley price arbitrage can be achieved, reducing electricity costs. In addition, industrial enterprises subject to two-part tariffs can use energy storage systems to store energy during off-peak hours and discharge during peak loads, thereby reducing peak power and the maximum demand declared, achieving the goal of reducing capacity charges. Household photovoltaics with energy storage can improve the level of self-consumption of electricity. Due to high electricity prices and poor power supply stability, the demand for household photovoltaic installations is driven. Given that photovoltaic power generation occurs during the day, while user load is generally higher at night, configuring energy storage can better utilize photovoltaic power, improve self-consumption levels, and reduce electricity costs. Furthermore, energy storage is needed in areas such as communication base stations and data centers for backup power.

[0055] In some embodiments, see Figure 1 , Figure 1 This is a schematic diagram of the structure of an energy storage system 100 according to an embodiment of this application, and this application Figure 1 The embodiments are illustrated using a shared energy storage scenario on the generation / distribution side as an example. The energy storage device 200 of this application is not limited to its generation / distribution side energy storage scenario.

[0056] This application provides an energy storage system 100, which includes: a high-voltage cable 110, a first power conversion device 120, a second power conversion device 130, and an energy storage device 200 provided in this application. In some embodiments of the power generation scenario, the second power conversion device 130 can be a wind power conversion device. Since the electricity generated by wind power conversion is volatile, random, and intermittent, the unstable electricity output by the wind power conversion device can be stored in the energy storage device 200 through grid connection. The energy storage device 200 is connected to the high-voltage cable 110 and outputs smooth electricity to the power consumption side of the distribution network, realizing peak shaving and frequency regulation, and stable grid operation; or, wind power conversion... The power conversion device is always connected to the high-voltage cable 110. Under normal power generation conditions, the power output of the wind power conversion device is supplied to the power consumption side of the distribution network through the high-voltage cable 110. When the current power load is low and the wind power conversion device generates excess power, the excess power is first stored in the energy storage device 200 to reduce wind and solar curtailment rates and improve the problem of new energy power generation consumption. When the power load is high, the power grid issues an instruction to transmit the power stored in the energy storage device 200 together with the high-voltage cable 110 in grid-connected mode to supply power to the power consumption side. This provides the power grid with various services such as peak shaving, frequency regulation, and backup, giving full play to the peak shaving role of the power grid, promoting peak shaving and valley filling, and alleviating the power supply pressure of the power grid.

[0057] In some embodiments on the distribution network side, the first power conversion device 120 can be a photovoltaic power conversion device, and the energy storage device 200 is connected to the high-voltage cable 110 and installed downstream of the high-voltage cable 110 between the user load and the user load. The power output by the photovoltaic power conversion device is stored in the energy storage device 200, which can respond in a timely manner to act as a backup power source when the power grid / distribution network fails; or, it can provide power supply support to alleviate line congestion when the high-voltage cable 110 transmission line is blocked, and to delay the economic pressure caused by the expansion of the power grid / distribution capacity when the power grid is planned to be expanded.

[0058] Optionally, the first power conversion device 120 may include, but is not limited to, a wind power conversion device, and the second power conversion device 130 may include, but is not limited to, a photovoltaic power conversion device. The first power conversion device 120 and the second power conversion device 130 can convert at least one of solar energy, light energy, wind energy, thermal energy, tidal energy, biomass energy, and mechanical energy into electrical energy.

[0059] In some embodiments, see Figure 2 , Figure 2 This is a schematic diagram of the structure of an energy storage system 100 according to another embodiment of this application. Figure 2 The embodiments are illustrated using a home energy storage scenario in user-side energy storage as an example. The energy storage device 200 of this application is not limited to the home energy storage scenario.

[0060] This application provides an energy storage system 100, which includes a third power conversion device 140 (e.g., a photovoltaic panel), a first user load 150 (household lighting fixtures), a second user load 160 (e.g., household appliances such as air conditioners), and an energy storage device 200. The energy storage device 200 is a small energy storage box that can be wall-mounted on an outdoor wall. However, the energy storage device 200 is not limited to wall mounting and can also be placed in a user's residence in other ways. Specifically, the photovoltaic panel can convert solar energy into electrical energy during periods of low electricity prices, and the energy storage device 200 stores this electrical energy and supplies it to lighting fixtures and household appliances during peak electricity prices, or provides power during power outages / power interruptions.

[0061] Optionally, the energy storage device 200 may include, but is not limited to, energy storage applications such as energy storage power stations, hydropower / thermal / wind power generation systems, solar power generation systems, mobile power systems, smart home systems, or temporary power supply systems, and may also be applied in multiple fields such as data centers, military equipment, aerospace, charging piles, and electric vehicles.

[0062] Please see Figure 3 , Figure 3 This is a schematic diagram of the structure of an energy storage device 200 according to an embodiment of this application.

[0063] Optionally, the energy storage device 200 may include, but is not limited to, one or more individual battery cells 210.

[0064] The term "multiple" refers to two or more, such as, but not limited to, 2, 5, 10, 30, 50, 100, 200, 300, 400, 800, 1000, etc. The number of individual battery cells 210 included in the energy storage device 200 can be determined based on the rated capacity of the individual battery cells 210 and the rated capacity to be achieved by the energy storage device 200.

[0065] It should be noted that when the energy storage device 200 includes a single cell 210, the energy storage device 200 can exist in the form of a single cell 210. When the energy storage device 200 includes multiple single cells 210, the multiple single cells 210 can be stacked, arranged, assembled, and other processes to form battery integrated systems such as battery modules, battery packs, battery clusters, power banks, energy storage cabinets / energy storage containers, etc. In other words, the energy storage device 200 can exist in the form of battery integrated systems such as battery modules, battery packs, battery clusters, power banks, energy storage cabinets / energy storage containers, etc. The actual application form of the energy storage device 200 provided in this application embodiment can be, but is not limited to, the listed products, and can also be other application forms. This application embodiment does not strictly limit the application form of the energy storage device 200. This application embodiment only uses a multi-cell battery as an example for illustration.

[0066] Optionally, the single cell 210 can be, but is not limited to, at least one of cylindrical, square, prismatic, or other shaped cells.

[0067] Optionally, the single cell 210 can be a rechargeable battery, which refers to a single cell 210 that can be recharged after discharge to activate the active materials and continue to be used. The single cell 210 can be a lithium-ion battery, sodium-ion battery, sodium-lithium-ion battery, lithium metal battery, sodium metal battery, lithium-sulfur battery, magnesium-ion battery, nickel-metal hydride battery, nickel-cadmium battery, lead-acid battery, etc., and this application does not specifically limit it.

[0068] Understandably, the single cell 210 can be, but is not limited to, a sodium battery 300, a lithium battery, a magnesium battery, a nickel-metal hydride battery, a nickel-cadmium battery, a lead-acid battery, etc. In the following embodiments of this application, the single cell 210 is illustrated using a sodium battery 300 as an example.

[0069] Sodium batteries, with their advantages of abundant and widely distributed sodium resources and low cost, are considered a highly promising large-scale energy storage solution and have received widespread attention in recent years. Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7), possessing a three-dimensional sodium ion diffusion channel and a sodium superionic conductor structure, exhibits high voltage plateau, high capacity, excellent rate capability, and excellent cycle stability, making it a very promising cathode material for large-scale production in sodium batteries. However, in related technologies, sodium iron pyrophosphate has struggled to simultaneously achieve both high compaction density and high rate performance.

[0070] Therefore, embodiments of this application provide a positive electrode particle and its preparation method, a positive electrode sheet, and a sodium battery.

[0071] Figure 4 This is a schematic diagram of the structure of a sodium battery 300 according to an embodiment of this application. Figure 5 For the application of an embodiment of a sodium battery 300 along Figure 4 A schematic diagram of the cross-sectional structure along the AA direction.

[0072] Please see Figure 4 and Figure 5 This application also provides a sodium battery 300, which includes an electrolyte, a positive electrode 400, a separator 320, and a negative electrode 330.

[0073] Optionally, the sodium battery 300 of this application may be, but is not limited to, at least one of sodium-ion batteries, sodium metal batteries, lithium-sodium hybrid batteries, etc.

[0074] Understandably, the positive electrode 400 and the negative electrode 330 are located on opposite sides of the separator 320, that is, the separator 320 is located between the positive electrode 400 and the negative electrode 330, separating the positive electrode 400 and the negative electrode 330.

[0075] It should be noted that the positive electrode 400, the separator 320, and the negative electrode 330 are all at least partially immersed in the electrolyte.

[0076] Optionally, the electrolyte includes an electrolyte salt, an organic solvent, and a film-forming additive.

[0077] Optionally, the electrolyte salt may include, but is not limited to, sodium salts. Optionally, the sodium salt may be, but is not limited to, at least one of sodium hexafluorophosphate (NaPF6), sodium tetrafluoroborate (NaBF4), sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(oxalatoborate), sodium difluorodioxalatophosphate, sodium difluorooxalatoborate, sodium difluorophosphate (NaPO2F2), sodium trifluoromethanesulfonate (CF3SO3Na), etc.

[0078] Optionally, the organic solvent may include at least one of cyclic carbonates and chain carbonates. Optionally, the cyclic carbonate may include, but is not limited to, at least one of ethylene carbonate (EC) and propylene carbonate (PC). Ethylene carbonate has a much higher dielectric constant than propylene carbonate, and can better promote the formation of a solid electrolyte interface membrane (SEI). Optionally, the chain carbonate may include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). Optionally, the organic solvent may also include at least one of ethyl acetate, propyl acetate, ethyl propionate, propyl propionate, γ-butyrolactone, and 2,2-difluoroethyl acetate.

[0079] Optionally, the film-forming additive may include, but is not limited to, at least one of the following: propargylbenzenesulfonic acid, vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), methanedisulfonate (MMDS), butyl sulfonate lactone (BS), and 1,3-propenyl sulfonate lactone (PST).

[0080] Optionally, the diaphragm 320 can be, but is not limited to, at least one of polypropylene membrane (PP membrane), polyethylene membrane (PE membrane), ceramic diaphragm 320, and glass fiber membrane. Optionally, the thickness of the diaphragm 320 is from 10 μm to 18 μm, specifically, the thickness of the diaphragm 320 can be, but is not limited to, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 14.5 μm, 15 μm, 15.5 μm, 16 μm, 16.5 μm, 17 μm, 17.5 μm, and 18 μm.

[0081] Figure 6 This is a cross-sectional view of the negative electrode 330 according to an embodiment of the application.

[0082] Please see Figure 6 Optionally, the negative electrode 330 includes a negative current collector 331 and a negative active layer 332, wherein the negative active layer 332 is disposed on the surface of the negative current collector 331.

[0083] It should be noted that the negative electrode active layer 332 can be disposed on one or more surfaces (greater than or equal to two surfaces) of the negative electrode current collector 331. In the schematic diagram of the accompanying drawings of this application, the negative electrode active layer 332 is disposed on two opposite surfaces of the negative electrode current collector 331 as an example, which should not be construed as a limitation on the negative electrode active layer 332 and the negative electrode sheet 330 of the embodiments of this application.

[0084] Optionally, the negative electrode current collector 331 can be, but is not limited to, at least one of copper foil, copper sheet, aluminum foil, and aluminum sheet.

[0085] Optionally, the negative electrode active layer 332 includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a negative electrode thickener.

[0086] Optionally, the negative electrode active material can be, but is not limited to, hard carbon.

[0087] Optionally, the negative electrode conductive agent may be, but is not limited to, at least one of conductive carbon black (SP), acetylene black, carbon nanotubes, carbon fibers, graphene, etc.

[0088] Optionally, the negative electrode binder may be, but is not limited to, at least one of polyvinylidene fluoride, polyamide, polyacrylonitrile, polyacrylate, polyethylene ether, polymethyl methacrylate, polyhexafluoropropylene, sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber, etc.

[0089] Optionally, the negative electrode thickener may be, but is not limited to, at least one of polyacrylamide (PAM) and polymethyl methacrylate (PMA).

[0090] Please see again Figure 4 and Figure 5Optionally, the sodium battery 300 further includes a housing 340 and an end cap assembly 350, the housing 340 and the end cap assembly 350 forming a closed receiving cavity (not shown) for housing the electrolyte, the positive electrode 400, the separator 320, and the negative electrode 330. Understandably, the end cap assembly 350 electrically connects the positive electrode 400 and the negative electrode 330 respectively, leading out the positive electrode 400 and the negative electrode 330 for electrical connection to external devices or other sodium batteries 300.

[0091] Figure 7 This is a cross-sectional view of the positive electrode 400 according to an embodiment of the application.

[0092] Please see Figure 7 Optionally, the positive electrode 400 includes a positive current collector 410 and a positive active layer 420, the positive active layer 420 being disposed on the surface of the positive current collector 410, and the positive active layer 420 including positive electrode particles 500.

[0093] It should be noted that the positive electrode active layer 420 can be disposed on one or more surfaces (greater than or equal to two surfaces) of the positive electrode current collector 410. In the schematic diagram of the accompanying drawings of this application, the positive electrode active layer 420 is disposed on two opposite surfaces of the positive electrode current collector 410 as an example, which should not be construed as a limitation on the positive electrode active layer 420 and the positive electrode sheet 400 of the embodiments of this application.

[0094] Understandably, the positive electrode particle 500 serves as the positive electrode active layer 420 material of the positive electrode active layer 420.

[0095] Optionally, the positive current collector 410 can be, but is not limited to, an aluminum sheet, aluminum foil, etc.

[0096] Optionally, the positive electrode active layer 420 may also include a positive electrode conductive agent and a positive electrode binder.

[0097] Optionally, the positive electrode conductive agent may be, but is not limited to, at least one of conductive carbon black (SP), acetylene black, carbon nanotubes, carbon fibers, graphene, etc.

[0098] Optionally, the positive electrode binder may be, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), polyhexanefluoropropylene, and polymerized styrene-butadiene rubber (SBR).

[0099] Figure 8 This is a schematic diagram of the structure of a positive electrode particle 500 according to an embodiment of this application.

[0100] Please see Figure 8 This application provides a positive electrode particle 500, wherein the positive electrode particle 500 has a maximum cross-sectional area within 1 / 2 radius (e.g., Figure 8 The area within the dashed line O) has a first porosity P1, and the positive electrode particle 500 has a maximum cross-sectional area between 1 / 2 and 3 / 4 of its radius (e.g., the range within the dashed line O). Figure 8 The area between the dashed lines O and P has a second porosity P2, and the radius of the positive electrode particle 500 extends from 3 / 4 of its maximum cross-section to the surface of the positive electrode particle 500 (e.g., ...). Figure 8 There is a third porosity P3 between the dashed lines P and Q, where P1 < P2 < P3. The maximum cross-section of the positive electrode particle 500 refers to the cross-section along the maximum length direction of the positive electrode particle 500.

[0101] In the embodiments of this application, when the numerical range a to b is involved, unless otherwise specified, the numerical value can be any value between a and b, including the endpoint value a and the endpoint value b.

[0102] Understandably, the porosity of the positive electrode particles 500 increases in a gradient from the inside to the outside.

[0103] Optionally, the positive electrode particle 500 includes a plurality of pores 510. The term "a plurality of" means two or more, such as, but not limited to, 2, 5, 10, 30, 50, 100, 200, etc.

[0104] It should be noted that "maximum length direction" refers to the direction of the line connecting the two points that are furthest apart from each other in the positive electrode 500.

[0105] It should be noted that the first porosity P1 refers to the average porosity within 1 / 2 radius of the maximum cross-section of the positive electrode particle 500, that is, the average porosity from the center to 1 / 2 radius of the maximum cross-section of the positive electrode particle 500.

[0106] It should be noted that the second porosity P2 refers to the average porosity between 1 / 2 and 3 / 4 of the radius on the maximum cross-section of the positive electrode particle 500.

[0107] It should be noted that the third porosity P3 refers to the average porosity between 3 / 4 radius of the maximum cross-section of the positive electrode particle 500 and the surface of the positive electrode particle 500.

[0108] Understandably, the positive electrode particle 500 can be divided into inner layers from the inside out. Figure 8 The part within the dashed line O), the intermediate layer ( Figure 8 The portion between the middle dashed line O and the dashed line P) and the outer layer ( Figure 8 (The portion between the dashed lines P and Q). The porosity of the inner layer is less than that of the middle layer, and the porosity of the middle layer is less than that of the outer layer. It should be noted that the positive electrode particles 500 do not have clear boundaries; the inner, middle, and outer layers are merely artificial divisions for descriptive convenience.

[0109] The positive electrode particle 500 of this application embodiment has a first porosity P1 within half the radius of its maximum cross-section, a second porosity P2 between half and three-quarters of the radius of its maximum cross-section, and a third porosity P3 between three-quarters of the radius of its maximum cross-section and the surface of its surface, wherein P1 < P2 < P3, and the maximum cross-section of the positive electrode particle 500 refers to the cross-section along its maximum length direction. This application achieves a gradient increase in the pore size 510 of the positive electrode particle 500 from the inside out, resulting in a lower overall porosity and higher compaction density, thereby enabling the sodium battery 300 to have a higher energy density. Furthermore, the pores 510 are distributed in a gradient increasing pattern from the inside to the outside. The surface of the positive electrode particle 500 has more pores 510, a larger specific surface area, and a larger roughness. This is beneficial for the wetting of the electrolyte on the surface of the positive electrode particle 500, thus improving the electrolyte wettability of the positive electrode particle 500. Moreover, the pore structure with a decreasing porosity from the outside to the inside allows the electrolyte to gradually penetrate into the interior of the positive electrode particle 500 under capillary action after wetting the surface of the positive electrode particle 500. This ensures that even the lower porosity parts inside the positive electrode particle 500 can be well wetted by the electrolyte, resulting in good electrolyte wettability. This, in turn, can better improve the dynamic performance and rate performance of the sodium battery 300. In summary, this application, through the gradient distribution design of the porosity of the positive electrode particles 500 from the inside to the outside, enables the positive electrode particles 500 to have a high powder compaction density, thereby enabling the sodium battery 300 to have a high energy density, as well as high kinetic performance and rate performance.

[0110] In some embodiments, the porosity of the positive electrode particle 500 gradually increases from the inside to the outside in the radial direction.

[0111] Understandably, the porosity of the positive electrode particle 500 is lowest near the center and highest near the surface.

[0112] In this embodiment, by making the pores 510 of the positive electrode particles 500 gradually increase from the inside to the outside, the positive electrode particles 500 can have a lower overall porosity and a higher compaction density, thereby enabling the sodium battery 300 to have a higher energy density. Furthermore, the pores 510 are distributed in a gradually increasing manner from the inside to the outside. The surface of the positive electrode particle 500 has more pores 510, a larger specific surface area, and a larger roughness. This is beneficial for the wetting of the electrolyte on the surface of the positive electrode particle 500, thus improving the electrolyte wettability of the positive electrode particle 500. Moreover, the pore structure with gradually decreasing porosity from the outside to the inside allows the electrolyte to gradually penetrate into the interior of the positive electrode particle 500 under capillary action after wetting the surface of the positive electrode particle 500. This ensures that even the parts of the positive electrode particle 500 with lower porosity can be well wetted by the electrolyte, resulting in good electrolyte wettability. This, in turn, can better improve the dynamic performance and rate performance of the sodium battery 300. In summary, this embodiment, through the distribution design of gradually increasing porosity from the inside to the outside of the positive electrode particles 500, enables the positive electrode particles 500 to have a high powder compaction density, thereby enabling the sodium battery 300 to have a high energy density, as well as high kinetic performance and rate performance.

[0113] In some embodiments, the average porosity P of the maximum cross-section of the positive electrode particle 500 ranges from 5% to 30%.

[0114] Specifically, the average porosity P of the maximum cross-section of the positive electrode particle 500 can be, but is not limited to, 5%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, etc.

[0115] In this embodiment, if the average porosity P of the maximum cross-section of the positive electrode particle 500 is too small, the overall electrolyte wettability of the positive electrode particle 500 is reduced, thus reducing the kinetic performance and rate performance of the sodium battery 300; if the average porosity P of the maximum cross-section of the positive electrode particle 500 is too large, the powder compaction density of the positive electrode particle 500 is reduced, thus reducing the energy density of the sodium battery 300.

[0116] Furthermore, in some embodiments, the average porosity P of the maximum cross-section of the positive electrode particles 500 is in the range of 7% ≤ P ≤ 15%. This allows the positive electrode particles 500 to have good electrolyte wettability, the sodium battery 300 to have high kinetic performance and rate performance, and also allows the positive electrode particles 500 to have high powder compaction density, resulting in high energy density for the sodium battery 300.

[0117] In some embodiments, the first porosity P1 ranges from 3% to 15%.

[0118] Specifically, the first porosity P1 can be, but is not limited to, 3%, 5%, 7%, 9%, 11%, 13%, 15%, etc.

[0119] In this embodiment, if the first porosity P1 is too small, the electrolyte wettability of the inner layer of the positive electrode particle 500 is reduced, thereby reducing the kinetic performance and rate performance of the sodium battery 300; if the first porosity P1 is too large, the powder compaction density of the positive electrode particle 500 is reduced, thereby reducing the energy density of the sodium battery 300.

[0120] Furthermore, in some embodiments, the first porosity P1 ranges from 5% to 10%. This allows the positive electrode particles 500 to have good electrolyte wettability, the sodium battery 300 to have high kinetic and rate performance, and the positive electrode particles 500 to have high powder compaction density, resulting in a high energy density for the sodium battery 300.

[0121] In some embodiments, the second porosity P2 ranges from 7% to 18%.

[0122] Specifically, the second porosity P2 can be, but is not limited to, 7%, 8%, 10%, 12%, 14%, 16%, 18%, etc.

[0123] In this embodiment, if the second porosity P2 is too small, the electrolyte wettability of the inner and middle layers of the positive electrode particles 500 is reduced, thereby reducing the kinetic performance and rate performance of the sodium battery 300; if the second porosity P2 is too large, the powder compaction density of the positive electrode particles 500 is reduced, thereby reducing the energy density of the sodium battery 300.

[0124] Furthermore, in some embodiments, the second porosity P2 ranges from 8% to 15%. This allows the positive electrode particles 500 to have good electrolyte wettability, the sodium battery 300 to have high kinetic and rate performance, and the positive electrode particles 500 to have high powder compaction density, resulting in a high energy density for the sodium battery 300.

[0125] In some embodiments, the third porosity P3 ranges from 10% to 28%.

[0126] Specifically, the third porosity P3 can be, but is not limited to, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, etc.

[0127] In this embodiment, if the third porosity P3 is too small, the overall electrolyte wettability of the positive electrode particles 500 is reduced, thus reducing the kinetic performance and rate performance of the sodium battery 300; if the third porosity P3 is too large, the powder compaction density of the positive electrode particles 500 is reduced, thus reducing the energy density of the sodium battery 300.

[0128] Furthermore, in some embodiments, the third porosity P3 ranges from 12% to 20%. This allows the positive electrode particles 500 to have good electrolyte wettability, the sodium battery 300 to have high kinetic and rate performance, and the positive electrode particles 500 to have high powder compaction density, resulting in a high energy density for the sodium battery 300.

[0129] In some embodiments, the average porosity P of the maximum cross-section of the positive electrode particles 500 ranges from 7% ≤ P1 ≤ 15%; the first porosity P1 ranges from 5% ≤ P1 ≤ 10%; the second porosity P2 ranges from 7% ≤ P2 ≤ 18%; and the third porosity P3 ranges from 10% ≤ P3 ≤ 28%. This allows the positive electrode particles 500 to have good electrolyte wettability, the sodium battery 300 to have high kinetic performance and rate performance, and also allows the positive electrode particles 500 to have high powder compaction density, resulting in a high energy density for the sodium battery 300.

[0130] In some embodiments, the maximum cross-section of the positive electrode particle 500 satisfies at least one of the following relationships:

[0131] (5P2+7P3) / P1>12; and

[0132] (4P1+5P2) / P3<9.

[0133] Specifically, (5P2+7P3) / P1 can be, but is not limited to, 12.9, 13, 15, 17, 19, 21, 23, 25, 27, 29, 30, etc. If (5P2+7P3) / P1 is too small, the porosity of the outer layer will be lower than that of the inner layer (i.e., P3 < P1), which reduces the electrolyte wettability of the positive electrode particles 500 and reduces the kinetic performance of the sodium battery 300.

[0134] Specifically, (4P1+5P2) / P3<9 can be, but is not limited to, 8.9, 8, 7, 6, 5, 4, 3, 2, 1, etc. If (4P1+5P2) / P3<9 is too large, the porosity of the middle layer of the positive electrode particle 500 will be higher than that of the outer layer (i.e., P2>P3), which will reduce the electrolyte wettability of the positive electrode particle 500 and reduce the kinetic performance of the sodium battery 300. In addition, it will also reduce the powder compaction density of the positive electrode particle 500 and the compaction density of the positive electrode sheet 400.

[0135] In some embodiments, the volume median particle size Dv501 of the positive electrode particle 500 is in the range of 5μm≤Dv501≤15μm.

[0136] "Dv501" refers to the particle size corresponding to the cumulative particle size volume distribution percentage of positive electrode particles 500 reaching 50%; in other words, in the particle sample, 50% of the particle volume is composed of particles smaller than this particle size, and the other 50% of the particle volume is composed of particles larger than this particle size.

[0137] Specifically, the volume median particle size Dv501 of the positive electrode particle 500 can be, but is not limited to, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, etc.

[0138] If the median volumetric particle size Dv501 of the positive electrode particles 500 is too small, the viscosity of the positive electrode slurry will be too high when the positive electrode particles 500 are made into positive electrode slurry, making the slurry coating uneven or even impossible; this reduces the processing performance of the positive electrode slurry and affects the rate performance of the positive electrode particles 500. If the median volumetric particle size Dv501 of the positive electrode particles 500 is too large, the deposition effect of the positive electrode particles 500 on the positive electrode current collector 410 will be poor when the positive electrode particles 500 are made into slurry and coated on the positive electrode current collector 410. This will generate excessive pressure on the positive electrode current collector 410 during the rolling process of the positive electrode sheet 400, reducing the ion conductivity of the positive electrode sheet 400, reducing the discharge capacity of the sodium battery 300 under high current, and reducing the kinetic performance of the sodium battery 300.

[0139] Furthermore, in some embodiments, the volume median particle size Dv501 of the positive electrode particles 500 is in the range of 9 μm ≤ Dv501 ≤ 9 μm. This allows the positive electrode particles 500 to have better processing performance and higher kinetic performance.

[0140] In some embodiments, the particle size distribution of the positive electrode particles 500 satisfies: 1≤(Dv901-Dv101) / Dv501≤4.

[0141] Specifically, (Dv901-Dv101) / Dv501 can be, but is not limited to, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, etc.

[0142] In this embodiment, if (Dv901-Dv101) / Dv501 is too small, the particle size distribution of the positive electrode particles 500 will be too narrow, reducing the compaction density of the positive electrode particles 500; if (Dv901-Dv101) / Dv501 is too large, Dv901 will be too large, or Dv101 and Dv501 will be too small, reducing the processing performance of the positive electrode particles 500 in the process of making positive electrode slurry.

[0143] In some embodiments, the specific surface area S of the positive electrode particles 500 ranges from 5m². 2 / g≤S≤15m 2 / g.

[0144] Specifically, the specific surface area S of the positive electrode particle 500 can be, but is not limited to, 5m². 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g, 10m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g, 15m 2 / g etc.

[0145] In this embodiment, if the specific surface area of ​​the positive electrode particles 500 is too small, the porosity of the positive electrode particles 500 will be too small, which is not conducive to electrolyte wetting and reduces the rate performance of the sodium battery 300. If the specific surface area of ​​the positive electrode particles 500 is too large, the pores 510 of the positive electrode particles 500 will be too numerous. Under the same formulation conditions, the flowability of the positive electrode slurry will be worse, the processing of the positive electrode sheet 400 will be worse, and it will be easy to cause uneven film surface and material shedding, etc. In addition, the number of side reaction sites increases, which reduces the cycle performance of the sodium battery 300, but the rate performance of the sodium battery 300 will be better.

[0146] Furthermore, in some embodiments, the specific surface area S of the positive electrode particle 500 ranges from 6m². 2 / g≤S≤9m 2 / g. This allows the cathode particles 500 to have high kinetic and cycle performance, as well as good processing performance.

[0147] In some embodiments, the positive electrode particle 500 is a secondary particle of sodium iron pyrophosphate, which is composed of multiple primary particles of sodium iron pyrophosphate; the chemical formula of the sodium iron pyrophosphate is: Na x Fe y Me z (PO4) α(P2O7) β Me is at least one of Mg, Ca, Al, Ti, V, Mn, Cr, Cu, Zn, Zr, Nb, W, Mo, Sc, and Y, with 3.7≤x≤4.3, 2.8≤y+z≤3.0, 0≤z≤1.0, and 3.9≤α+2β≤4.1.

[0148] Understandably, secondary sodium ferric phosphate pyrophosphate particles are formed by the aggregation of multiple primary sodium ferric phosphate pyrophosphate particles. The number of primary sodium ferric phosphate pyrophosphate particles in the secondary sodium ferric phosphate pyrophosphate particles can be, but is not limited to, 2, 5, 10, 20, 30, 50, 80, 100, 130, 150, etc.

[0149] Specifically, x can be, but is not limited to, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, etc. If x is too small, the sodium content in sodium ferric pyrophosphate will be too low, which will increase the proportion of sodium ferric pyrophosphate impurities in sodium ferric pyrophosphate and reduce the specific capacity of sodium ferric pyrophosphate. If x is too large, the sodium content in sodium ferric pyrophosphate will be too high, which will increase the proportion of sodium ferric phosphate impurities in sodium ferric pyrophosphate and reduce the specific capacity of sodium ferric pyrophosphate.

[0150] Specifically, y+z can be, but is not limited to, 2.8, 2.9, 3.0, etc. If y+z is too small, it will increase the proportion of sodium iron pyrophosphate impurities in sodium iron pyrophosphate, thus reducing the specific capacity of sodium iron pyrophosphate. If y+z is too large, it will also increase the proportion of sodium iron phosphate impurities in sodium iron pyrophosphate, thus reducing the specific capacity of sodium iron pyrophosphate.

[0151] Specifically, z can be, but is not limited to, 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.00, etc. If z is too small, the effect of the Me metal element on improving the kinetic performance of the cathode particle 500 is not obvious; if z is too large, it will affect the stability of the sodium iron pyrophosphate structure.

[0152] Specifically, α+2β can be, but is not limited to, 3.9, 4.0, 4.1, etc. If α+2β is too small, it will increase the proportion of sodium iron phosphate impurities in sodium iron phosphate, thus reducing the specific capacity of sodium iron phosphate; if α+2β is too large, it will increase the proportion of sodium iron phosphate impurities in sodium iron phosphate, thus reducing the specific capacity of sodium iron phosphate.

[0153] In this embodiment, at least one metal element selected from Mg, Ca, Al, Ti, V, Mn, Cr, Cu, Zn, Zr, Nb, W, Mo, Sc, and Y is used to modify sodium iron pyrophosphate, which can improve the sodium ion mobility of the cathode particles 500 and enhance their kinetic performance. Furthermore, this application designs the proportions of each element in the sodium iron pyrophosphate to ensure that the cathode particles 500 all have low impurity phase content and high specific capacity.

[0154] Figure 9 This is a schematic diagram of the structure of the positive electrode particle 500 according to another embodiment of this application.

[0155] Please see Figure 9 In some embodiments, the positive electrode particle 500 is a secondary particle of sodium iron pyrophosphate, which is composed of a plurality of primary particles of sodium iron pyrophosphate 520. The primary particles of sodium iron pyrophosphate 520 include an active core 521 and a carbon layer 522. The active core 521 is sodium iron pyrophosphate, and the carbon layer 522 is wrapped around the surface of the active core 521. The mass fraction of the carbon layer 522 in the positive electrode particle 500 ranges from 1.0 wt% to 3.0 wt%.

[0156] Specifically, the mass fraction of the carbon layer 522 in the positive electrode particle 500 can be, but is not limited to, 1.0 wt%, 1.2 wt%, 1.4 wt%, 1.6 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.4 wt%, 2.6 wt%, 2.8 wt%, 3.0 wt%, etc.

[0157] If the mass fraction of the carbon layer 522 in the positive electrode particle 500 is too low, the electron transport rate of the positive electrode particle 500 is reduced, the powder resistivity of the positive electrode particle 500 is increased, and the kinetic performance of the positive electrode particle 500 is reduced. If the mass fraction of the carbon layer 522 in the positive electrode particle 500 is too high, the content of the carbon layer 522 is too high, which reduces the specific capacity, powder compaction density, and electrode layer compaction density of the positive electrode particle 500.

[0158] In some embodiments, the compacted density of the positive electrode particles 500 is ≥2.15 g / cm³. 3 .

[0159] Specifically, the compacted density of the positive electrode particles 500 can be, but is not limited to, 2.15 g / cm³. 3 2.18 g / cm 3 2.20g / cm 3 2.22 g / cm 3 2.24 g / cm 32.26 g / cm 3 2.28g / cm 3 2.30g / cm 3 2.32 g / cm 3 2.34 g / cm 3 2.36 g / cm 3 wait.

[0160] In this embodiment, the powder compaction density of the positive electrode particles 500 is determined by the mass fraction and porosity of the carbon layer 522 of the positive electrode particles 500. If the powder compaction density of the positive electrode particles 500 is too low, it indicates that the mass fraction and porosity of the carbon layer 522 of the positive electrode particles 500 are too high. This will result in a low compaction density of the positive electrode active layer 420 of the positive electrode sheet 400, and a low energy density of the sodium battery 300. If the powder compaction density of the positive electrode particles 500 is too high, it indicates that the carbon content and porosity of the positive electrode particles 500 are too low. This will reduce the wettability of the electrolyte to the positive electrode active layer 420, which is not conducive to shortening the solid-phase transport distance, reducing the specific capacity of the sodium battery 300, and lowering the rate performance and low-temperature performance of the sodium battery 300.

[0161] The test method for the powder compaction density of the positive electrode particles 500 is as follows: take 2g to 3g of Na4Fe3(PO4)2(P2O7) powder and add it into the mold of a powder resistance instrument with a diameter of 13mm. After applying pressure to 3T, measure its volume and mass, and calculate the powder compaction density of the positive electrode particles 500.

[0162] The positive electrode particles 500 of this application embodiment can be prepared by the methods described in the following embodiments of this application. In addition, they can also be prepared by other methods. The preparation methods of this application embodiment are merely one or more preparation methods of the positive electrode particles 500 of this application and should not be construed as limiting the positive electrode particles 500 provided in the embodiments of this application.

[0163] Figure 10 This is a schematic flowchart illustrating a method for preparing positive electrode particles 500 according to an embodiment of this application.

[0164] Please see Figure 10 This application provides a method for preparing positive electrode particles 500, the preparation method comprising:

[0165] S601 provides sodium, phosphorus, iron and carbon sources;

[0166] Understandably, the raw material components of the positive electrode particles 500 include sodium source, phosphorus source, iron source and carbon source.

[0167] Optionally, the sodium source may include, but is not limited to, at least one of these compounds, such as sodium dihydrogen phosphate, sodium pyrophosphate, sodium carbonate, sodium acetate, and compounds containing water of crystallization.

[0168] Optionally, the phosphorus source may include, but is not limited to, at least one of these compounds, such as sodium dihydrogen phosphate, sodium pyrophosphate, ammonium dihydrogen phosphate, and those containing water of crystallization.

[0169] Optionally, the iron source may include, but is not limited to, at least one of these compounds, including ferrous oxalate, ferric nitrate, ferrous sulfate, and compounds containing water of crystallization. For example, ferrous oxalate dihydrate.

[0170] Optionally, the carbon source is at least one of glucose, sucrose, starch, polyethylene glycol, ascorbic acid, citric acid, etc.

[0171] S602 involves placing sodium, phosphorus, iron, and carbon sources in a solvent and milling them to obtain a slurry.

[0172] S603, the slurry is spray-dried to obtain precursor powder; and

[0173] S604, the precursor powder is pre-sintered and sintered sequentially to obtain positive electrode particles 500, wherein the positive electrode particles 500 have a first porosity P1 within 1 / 2 radius of the maximum cross-section, a second porosity P2 between 1 / 2 radius and 3 / 4 radius of the maximum cross-section, and a third porosity P3 between 3 / 4 radius of the maximum cross-section and the surface of the positive electrode particles 500, wherein P1 < P2 < P3, and the maximum cross-section of the positive electrode particles 500 refers to the cross-section in the direction of the maximum length of the positive electrode particles 500.

[0174] The method for preparing the positive electrode particles 500 in this application embodiment involves first pre-sintering the precursor powder and then sintering it. This pre-sintering process allows for better removal of moisture (including free and bound water) from the precursor powder, preventing residual moisture from increasing the impurity content of the resulting positive electrode particles 500 and improving the specific capacity of the positive electrode particles 500. Furthermore, the resulting positive electrode particles 500 have a first porosity P1 within half the radius of their maximum cross-section, a second porosity P2 between half and three-quarters of the radius of their maximum cross-section, and a third porosity P3 between three-quarters of the radius of their maximum cross-section and the surface of the positive electrode particles 500, where P1 < P2 < P3. The maximum cross-section of the positive electrode particles 500 refers to the cross-section along the maximum length direction of the positive electrode particles 500. This application achieves a gradient distribution of the pores 510 of the positive electrode particles 500 from the inside to the outside, which results in a lower overall porosity and a higher compaction density for the positive electrode particles 500, thereby enabling the sodium battery 300 to have a higher energy density. Furthermore, the pores 510 are distributed in a gradient increasing pattern from the inside to the outside. The surface of the positive electrode particle 500 has more pores 510, a larger specific surface area, and a larger roughness. This is beneficial for the wetting of the electrolyte on the surface of the positive electrode particle 500, thus improving the electrolyte wettability of the positive electrode particle 500. Moreover, the pore structure with a decreasing porosity from the outside to the inside allows the electrolyte to gradually penetrate into the interior of the positive electrode particle 500 under capillary action after wetting the surface of the positive electrode particle 500. This ensures that even the lower porosity parts inside the positive electrode particle 500 can be well wetted by the electrolyte, resulting in good electrolyte wettability. This, in turn, can better improve the dynamic performance and rate performance of the sodium battery 300. In summary, this application, through the gradient distribution design of the porosity of the positive electrode particles 500 from the inside to the outside, enables the positive electrode particles 500 to have a high powder compaction density, thereby enabling the sodium battery 300 to have a high energy density, as well as high kinetic performance and rate performance.

[0175] In some embodiments, S602, the step of placing the sodium source, phosphorus source, iron source and carbon source in a solvent and milling them to obtain a slurry includes: mixing the sodium source, phosphorus source, iron source and carbon source in water and performing a milling process to obtain a slurry, wherein the milling time is 5h to 15h and the milling speed is 100rpm to 400rpm.

[0176] Understandably, the solvent may be, but is not limited to, water.

[0177] In this embodiment, before spray drying, the raw materials (i.e., sodium source, phosphorus source, iron source and carbon source) are first mixed by sand milling. Sand milling can reduce the particle size of the raw materials, providing a prerequisite for the formation of sodium iron pyrophosphate. In addition, sand milling can fully mix the soluble and insoluble substances in the raw materials, avoiding uneven mixing of the raw materials when there are multiple insoluble raw materials, which would increase the probability of impurity phases in the final sodium iron pyrophosphate.

[0178] Specifically, the milling time can be, but is not limited to, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, etc. If the milling time is too short, the particle size of the insoluble raw materials in the sodium, phosphorus, iron, and carbon sources will be too large, resulting in localized growth and crystallization during the subsequent sintering process, thus reducing the sphericity of the obtained cathode particles. If the milling time is too long, it will reduce production efficiency.

[0179] Specifically, the milling speed can be, but is not limited to, 100 rpm, 150 rpm, 200 rpm, 250 rpm, 300 rpm, 350 rpm, 400 rpm, etc. If the milling speed is too low, the particle size of the insoluble raw materials in the sodium, phosphorus, iron, and carbon sources will be too large, resulting in localized protruding crystal growth during the subsequent sintering process, which reduces the sphericity of the obtained cathode particles. If the milling speed is too high, the slurry is prone to splashing.

[0180] In some embodiments, the solid content of the slurry ranges from 20% to 50%. Specifically, the solid content of the slurry can be, but is not limited to, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, etc. In this embodiment, if the solid content of the slurry is too low, the material cannot quickly and effectively shrink into a spherical shape during the spray drying stage, affecting the sphericity of the final cathode particles 500. If the solid content of the slurry is too high, it will cause great difficulties in the sand milling stage. In addition, if the material forms spheres too quickly during the spray drying stage, there is not enough time to form spheres with high sphericity, thereby reducing the sphericity of the obtained precursor powder, which in turn leads to a reduction in the sphericity of the final cathode particles 500.

[0181] In some embodiments, the slurry comprises solid particles, wherein the volume median particle size Dv50 of the solid particles is in the range of 100nm≤Dv50≤300nm.

[0182] "Dv50" refers to the particle size at which the cumulative particle size distribution percentage of solid particles reaches 50%; in other words, in a particle sample, 50% of the particle volume is composed of particles smaller than this particle size, and the other 50% of the particle volume is composed of particles larger than this particle size.

[0183] Specifically, the volume median particle size Dv50 of the solid particles can be, but is not limited to, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, 220nm, 240nm, 260nm, 280nm, 300nm, etc.

[0184] In this embodiment, the particle size of the solid particles in the milled slurry is controlled to a small value. This allows the solid particles to fuse and react with each other during sintering of the precursor powder, forming larger primary particles, resulting in a cathode particle 500 with low porosity. However, if the median volume diameter (Dv50) of the solid particles is too small, the porosity of the cathode particle 500 will be too low, reducing its electrolyte wettability and thus its kinetic performance. Furthermore, a small median volume diameter requires longer milling times, increasing the preparation cost of the cathode particle 500. If the median volume diameter (Dv50) is too large, the raw materials are prone to uneven mixing, resulting in a high proportion of impurities in the cathode particle 500, reducing its specific capacity. Additionally, it hinders the fusion and growth of the cathode particle 500 during the subsequent sintering stage, easily leading to large pores and excessive porosity. In addition, if the volume median particle size Dv50 of the solid particles is too small or too large, it will affect the gradient distribution of porosity from the inside to the outside of the obtained cathode particles 500, making it difficult to achieve both high powder compaction density and kinetic performance.

[0185] In some embodiments, in S603, spray drying the slurry to obtain precursor powder includes: spray drying the slurry at an inlet air temperature of 180℃≤T1≤250℃ and an outlet air temperature of 80℃≤T2≤120℃ to obtain precursor powder.

[0186] Specifically, the inlet air temperature for the spray drying can be, but is not limited to, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, etc.

[0187] In this embodiment, if the inlet air temperature for spray drying the slurry is too high, the solvent (such as water) will evaporate too quickly, easily forming hollow or collapsed spherical precursor powder, affecting the processing performance and compaction density of the final cathode particles 500. If the inlet air temperature is too low, the solvent (such as water) will evaporate too slowly, affecting the equipment's production capacity.

[0188] Specifically, the outlet air temperature of the spray dryer can be, but is not limited to, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, etc. If the outlet air temperature of the spray dryer is too low, the slurry is prone to sticking to the filter bag, resulting in a reduced yield. If the outlet air temperature of the spray dryer is too high, the organic carbon source is easily carbonized.

[0189] Optionally, the volume median particle size Dv502 of the precursor powder is in the range of 4μm≤Dv502≤20μm.

[0190] "Dv502" refers to the particle size corresponding to a cumulative particle size volume distribution percentage of 50% for the precursor powder; in other words, in the particle sample, 50% of the particle volume consists of particles smaller than this particle size, and the other 50% of the particle volume consists of particles larger than this particle size.

[0191] Specifically, the volume median particle size Dv502 of the precursor powder can be, but is not limited to, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, etc.

[0192] If the median volumetric particle size Dv502 of the precursor powder is too small, the Dv501 of the resulting cathode particles 500 will be too small. When the cathode particles 500 are made into cathode slurry, the viscosity of the slurry will be too high, making the slurry coating uneven or even impossible. This reduces the processing performance of the cathode slurry and affects the rate performance of the cathode particles 500. If the median volumetric particle size Dv502 of the precursor powder is too large, the Dv501 of the resulting cathode particles 500 will be too large. This makes it easy for the cathode particles 500 to have poor deposition effect on the cathode current collector 410 when the slurry is coated. During the rolling process of the cathode electrode 400, excessive pressure is generated on the cathode current collector 410, reducing the ion conductivity of the cathode electrode 400, reducing the discharge capacity of the sodium battery 300 under high current, and reducing the kinetic performance of the sodium battery 300.

[0193] In some embodiments, the particle size distribution of the precursor powder satisfies: 1≤(Dv902-Dv102) / Dv502≤4.

[0194] Specifically, (Dv902-Dv102) / Dv502 can be, but is not limited to, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, etc.

[0195] In this embodiment, if (Dv902-Dv102) / Dv502 is too small, the particle size distribution of the obtained positive electrode particles 500 will be too narrow, reducing the compaction density of the positive electrode particles 500; if (Dv902-Dv102) / Dv502 is too large, Dv902 will be too large, or Dv102 and Dv502 will be too small, resulting in a too wide particle size distribution of the obtained positive electrode particles 500, reducing the processing performance of the positive electrode particles 500 in the process of making positive electrode slurry.

[0196] Please see Figure 11 In some embodiments, in S604, the step of sequentially pre-sintering and sintering the precursor powder to obtain positive electrode particles 500 includes:

[0197] S6041, the precursor powder is heated to 250°C to 400°C at a first heating rate to undergo pre-sintering, thereby obtaining an intermediate; and

[0198] Optionally, the pre-sintering is carried out under the protection of an inert atmosphere. Optionally, the inert atmosphere includes at least one of helium, nitrogen, and argon.

[0199] Specifically, the pre-sintering temperature can be, but is not limited to, 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, and 400℃. If the pre-sintering temperature is too low, the moisture in the precursor powder cannot be completely removed during the pre-sintering stage, resulting in an excessive amount of impurities in the sintered cathode particles 500, thus reducing the specific capacity of the cathode particles 500. If the pre-sintering temperature is too high, it will also increase the impurity content of the obtained cathode particles 500, reducing the specific capacity of the cathode particles 500.

[0200] Furthermore, the pre-sintering temperature ranges from 275°C to 375°C. This results in a lower content of impurities in the cathode particles 500, thereby increasing the specific capacity of the cathode particles 500.

[0201] Optionally, the first heating rate ranges from 2℃ / min to 10℃ / min. Specifically, the first heating rate can be, but is not limited to, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, etc. If the first heating rate is too slow, the porosity and specific surface area of ​​the obtained positive electrode particles 500 will be too large, reducing the compaction density of the positive electrode particles 500. If the first heating rate is too fast, the porosity and specific surface area of ​​the obtained positive electrode particles 500 will be too small, reducing the electrolyte wettability of the positive electrode particles 500 and reducing the kinetic performance of the positive electrode particles 500. In addition, both too slow and too fast first heating rates will affect the gradient distribution of porosity from the inside to the outside of the obtained positive electrode particles 500, making it difficult to achieve both high powder compaction density and kinetic performance.

[0202] Furthermore, the first heating rate ranges from 2°C / min to 5°C / min. This allows the obtained cathode particles 500 to have a high compaction density.

[0203] Optionally, the pre-sintering time can range from 3 hours to 12 hours. Specifically, the pre-sintering time can be, but is not limited to, 3 hours, 4 hours, 5 hours, 6 hours, 8 hours, 10 hours, 12 hours, etc. If the pre-sintering time is too short, the bound water in the precursor powder cannot be completely removed, affecting the phase formation of the material during subsequent sintering; if the pre-sintering time is too long, the production energy consumption will increase, increasing the preparation cost of the cathode particles 500.

[0204] S6042, the intermediate is heated to 500°C to 650°C at a second heating rate and sintered to obtain positive electrode particles 500.

[0205] Optionally, sintering is carried out under the protection of an inert atmosphere. Optionally, the inert atmosphere includes at least one of helium, nitrogen, and argon.

[0206] Specifically, the sintering temperature can be, but is not limited to, 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, and 650℃. If the sintering temperature of the intermediate is too low, the carbonization degree of the carbon layer 522 in the resulting cathode particles 500 will be too low, and the powder resistivity of the cathode particles 500 will be too high. When applied to the sodium battery 300, this will result in excessive internal resistance of the sodium battery 300, reducing its cycle life. If the sintering temperature of the intermediate is too high, the sodium iron pyrophosphate in the cathode particles 500 will easily decompose to form sodium iron phosphate impurities and sodium iron pyrophosphate impurities, thereby reducing the specific capacity of the cathode particles 500.

[0207] Optionally, the first heating rate is greater than the second heating rate. During the pre-sintering stage, the precursor powder mainly produces water vapor. The higher first heating rate during the pre-sintering stage allows for better degassing and pore formation. During the sintering stage, the intermediate mainly produces carbon dioxide. The lower second heating rate allows the gas to be released from the inside to the outside, thereby achieving gradient pore expansion and pore formation, forming a gradient pore distribution.

[0208] Optionally, the second heating rate ranges from 1°C / min to 5°C / min. Specifically, the second heating rate can be, but is not limited to, 1°C / min, 1.5°C / min, 2°C / min, 2.5°C / min, 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, 5°C / min, etc. If the second heating rate is too slow, the porosity and specific surface area of ​​the obtained cathode particles 500 will be too small, reducing the electrolyte wettability of the cathode particles 500 and decreasing the kinetic performance of the cathode particles 500. If the second heating rate is too fast, the porosity and specific surface area of ​​the obtained cathode particles 500 will be too large, reducing the compaction density of the cathode particles 500. In addition, both excessively slow and excessively fast second heating rates will affect the gradient distribution of porosity from the inside to the outside of the obtained cathode particles 500, making it difficult to simultaneously achieve high powder compaction density and kinetic performance.

[0209] Furthermore, the second heating rate ranges from 1°C / min to 3°C / min. This allows the prepared cathode particles 500 to possess both high compaction density and high kinetic performance.

[0210] Optionally, the sintering time can be from 6 hours to 24 hours. Specifically, the sintering time of the precursor powder can be, but is not limited to, 6 hours, 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, 24 hours, etc. If the sintering time is too short, the carbonization degree of the carbon layer 522 of the obtained cathode particle 500 will be too low, increasing the powder resistivity of the cathode particle 500; if the sintering time is too long, the probability of decomposition of sodium iron pyrophosphate will increase, increasing the content of sodium iron phosphate impurities and sodium iron pyrophosphate impurities in the cathode particle 500, and reducing the specific capacity of the cathode particle 500.

[0211] The following specific embodiments further describe the positive electrode particle 500 and the sodium battery 300 of this application.

[0212] Examples 1 to 13, Comparative Examples 1 to 6

[0213] The positive electrode particles 500 of each embodiment and comparative example were prepared by the following steps:

[0214] (1) Sodium pyrophosphate (sodium source), ammonium dihydrogen phosphate (phosphorus source), ferrous oxalate dihydrate (iron source), and glucose (carbon source) are weighed according to a preset ratio and dispersed in water (solvent). The mixture is then milled to obtain a slurry. The solid content of the slurry is 35%. The median volume particle size Dv50 of the solid particles in the milled slurry in each embodiment and comparative example is shown in Table 1 below.

[0215] (2) The slurry was spray-dried at 105°C to obtain precursor powder;

[0216] (3) The precursor powder was placed in a sintering furnace and heated to the pre-sintering temperature at the first heating rate under nitrogen protection. Pre-sintering was carried out for 5 hours, and then heated to the sintering temperature at the second heating rate for 12 hours. After cooling, 500 positive electrode particles were obtained. The first heating rate, pre-sintering temperature, second heating rate and sintering temperature of each embodiment and comparative example are shown in Table 1 below.

[0217] Sodium battery 300 assembly: (1) Mix positive electrode particles 500, polyvinylidene fluoride (binder), and conductive carbon black (conductive agent, Super P) in a mass ratio of 96:2:2 and disperse them in N-methylpyrrolidone to make a positive electrode slurry. Coat the positive electrode slurry onto the positive electrode current collector 410 and dry it to make a positive electrode sheet 400; (2) Assemble the positive electrode sheet 400, negative electrode sheet 330, separator 320 and electrolyte into a soft pack battery. The negative electrode sheet 330 is composed of hard carbon negative electrode, conductive agent and binder. The separator 320 is a polypropylene separator 320. The electrolyte is a carbonate electrolyte. The electrolyte includes a mixed solvent of sodium hexafluoroborate (sodium salt), ethylene carbonate and dimethyl carbonate and fluoroethylene carbonate additive. The molar concentration of sodium hexafluorophosphate in the electrolyte is 1 mol / L.

[0218] Various performance tests were conducted on the positive electrode particles 500 and pouch cells of each embodiment and comparative example:

[0219] (1) P, P1, P2 and P3 test: After the positive electrode particles 500 are cut by argon ion beam, the cross-sectional photos of the material are taken under an electron microscope. Then, the porosity data of different ranges are obtained by selecting areas in the obtained cross-sectional photos through image recognition software.

[0220] (2) Specific surface area of ​​cathode particles 500: The specific surface area of ​​cathode particles 500 was tested according to the method for determining the specific surface area of ​​solid materials by gas adsorption BET method (GB / T19587-2017).

[0221] (3) Test method for the compaction density of positive electrode particles 500: The thickness of aluminum foil and the mass of aluminum foil cut into 12mm round pieces are measured in advance. Then, the thickness of positive electrode sheet 400 made from positive electrode particles 500 is measured, and the positive electrode sheet 400 is cut into 12mm round pieces and weighed. The mass and volume of positive electrode particles 500 on positive electrode sheet 400 are calculated, and then the compaction density of positive electrode particles 500 on positive electrode sheet 400 is calculated.

[0222] (4) Rate performance test: The assembled soft pack battery was tested using the Blue Battery Test System in the voltage range of 1.5V-3.5V with 1.0P charge / 1.0P discharge, 1.0P charge / 2.0P discharge, 1.0P charge / 3.0P discharge, and 0.5P charge / 4.0P discharge steps. The rate performance was determined as the ratio of 4.0P discharge capacity to 1.0 discharge capacity.

[0223] Table 1. Preparation process parameters of positive electrode particles 500

[0224]

[0225] Table 2 Performance parameters of cathode particles 500 and sodium battery 300

[0226]

[0227]

[0228] The test results from Examples 1 to 6, Comparative Examples 1 and 2 show that when the Dv50 of the solid particles in the slurry is small (as in Comparative Example 1), the porosity of the obtained cathode particles 500 is small, the specific surface area of ​​the cathode particles 500 is small, and the powder compaction density of the cathode particles 500 is large. However, the rate performance of the sodium battery 300 is low. As the Dv50 of the solid particles in the slurry increases, the porosity and specific surface area of ​​the cathode particles 500 gradually increase, and the porosity shows a gradual increasing trend from the inside to the outside. The powder compaction density of the cathode particles 500 gradually decreases, and the rate performance of the sodium battery 300 gradually increases. When the Dv50 of the solid particles in the slurry is too large (as in Comparative Example 2), the porosity of the obtained cathode particles 500 is relatively uniform at various positions. The porosity of the cathode particles 500 does not show an increasing trend from the inside to the outside, the specific surface area is large, and the powder compaction density is significantly reduced.

[0229] The test results from Examples 4, 7 to 10, Comparative Examples 3 and 4 show that when the first heating rate during pre-sintering is slow (as in Comparative Example 3), the porosity of the obtained cathode particles 500 is too large, and the porosity regions in each part of the cathode particles 500 are uniform. The cathode particles 500 have a large specific surface area and rate performance; however, the compaction density of the cathode particles 500 is greatly reduced. As the first heating rate during pre-sintering increases, the porosity of the obtained cathode particles 500 gradually decreases, and the porosity of the cathode particles 500 shows a gradient increase from the inside to the outside. The powder compaction density of the cathode particles 500 gradually increases, and the rate performance of the sodium battery 300 gradually decreases. When the first heating rate of pre-sintering is too fast (as in Comparative Example 4), the porosity of the positive electrode particles 500 is small, especially the porosity inside the positive electrode particles 500 is too small. The porosity of the positive electrode particles 500 does not have an increasing distribution from the inside to the outside, and the rate performance of the sodium battery 300 is greatly reduced.

[0230] The test results from Examples 8, 11 to 13, Comparative Examples 5 and 6 show that when the second heating rate during sintering is slow (as in Comparative Example 5), the porosity of the obtained cathode particles 500 is small, and the porosity regions of each part of the cathode particles 500 are uniform. The cathode particles 500 have a small specific surface area, but the compaction density of the cathode particles 500 is high, and the rate performance of the sodium battery 300 is greatly reduced. As the second heating rate during sintering increases, the porosity of the obtained cathode particles 500 gradually increases, and the porosity of the cathode particles 500 shows a gradient increase from the inside to the outside. The powder compaction density of the cathode particles 500 decreases slightly, and the rate performance of the sodium battery 300 gradually increases. When the second heating rate of sintering is too fast (as in Comparative Example 6), the porosity of the positive electrode particles 500 is large. However, the porosity of the positive electrode particles 500 is relatively uniform at various positions, and there is no increasing distribution from the inside to the outside. The sodium battery 300 has a large rate performance, but the compaction density of the positive electrode particles 500 is greatly reduced.

[0231] In this application, the terms "embodiment" and "implementation" mean that a specific feature, structure, or characteristic described in connection with an embodiment can be included in at least one embodiment of this application. The appearance of these phrases in various locations throughout the specification does not necessarily refer to the same embodiment, nor are they independent or alternative embodiments mutually exclusive with other embodiments. Those skilled in the art will understand, explicitly and implicitly, that the embodiments described in this application can be combined with other embodiments. Furthermore, it should be understood that the features, structures, or characteristics described in the various embodiments of this application can be arbitrarily combined to form another embodiment that does not depart from the spirit and scope of the technical solution of this application, provided there is no contradiction between them.

[0232] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. A positive electrode particle, characterized in that, The positive electrode particle has a first porosity P1 within half the radius of its maximum cross-section, a second porosity P2 between half and three-quarters of the radius of its maximum cross-section, and a third porosity P3 between three-quarters of the radius of its maximum cross-section and the surface of the positive electrode particle, wherein P1 < P2 < P3. The maximum cross-section of the positive electrode particle refers to the cross-section along the maximum length direction of the positive electrode particle, and the average porosity P of the maximum cross-section of the positive electrode particle is in the range of 5% ≤ P ≤ 30%.

2. The positive electrode particle according to claim 1, characterized in that, Along the radial direction from the inside to the outside, the porosity of the maximum cross-section of the positive electrode particle gradually increases.

3. The positive electrode particle according to claim 1, characterized in that, The positive electrode particle satisfies at least one of the following conditions: The first porosity P1 is in the range of 3% ≤ P1 ≤ 15%; The range of the second porosity P2 is 7% ≤ P2 ≤ 18%; The range of the third porosity P3 is 10% ≤ P3 ≤ 28%.

4. The positive electrode particle according to claim 1, characterized in that, The maximum cross-section of the positive electrode particle satisfies at least one of the following relationships: (5P2+7P3) / P1>12; and (4P1+5P2) / P3<9.

5. The positive electrode particles according to any one of claims 1-4, characterized in that, The positive electrode particles are secondary particles of sodium iron pyrophosphate, which are composed of multiple primary sodium iron pyrophosphate particles; the chemical formula of the sodium iron pyrophosphate is: Na. x Fe y Me z (PO4) α (P2O7) β , wherein Me is at least one of Mg, Ca, Al, Ti, V, Mn, Cr, Cu, Zn, Zr, Nb, W, Mo, Sc, Y, 3.7≤x≤4.3, 2.8≤y+z≤3.0, 0≤z≤1.0, and 3.9≤α+2β≤4.

1.

6. The positive electrode particles according to any one of claims 1-4, characterized in that, The positive electrode particles are secondary sodium iron pyrophosphate particles, which are composed of multiple primary sodium iron pyrophosphate particles. The primary sodium iron pyrophosphate particles include an active core and a carbon layer. The active core is sodium iron pyrophosphate, and the carbon layer is wrapped around the surface of the active core. The mass fraction of the carbon layer in the positive electrode particles ranges from 1.0 wt% to 3.0 wt%.

7. A method for preparing positive electrode particles, characterized in that, The preparation method includes: It provides sodium, phosphorus, iron, and carbon sources; Sodium, phosphorus, iron, and carbon sources are placed in a solvent and milled to obtain a slurry; The slurry was spray-dried to obtain precursor powder; and The precursor powder is pre-sintered and sintered sequentially to obtain positive electrode particles. The positive electrode particles have a first porosity P1 within half the radius of their maximum cross-section, a second porosity P2 between half and three-quarters of the radius of their maximum cross-section, and a third porosity P3 between three-quarters of the radius of their maximum cross-section and the surface of the positive electrode particles. P1 < P2 < P3. The maximum cross-section of the positive electrode particles refers to the cross-section along the maximum length direction of the positive electrode particles. The average porosity P of the maximum cross-section of the positive electrode particles is in the range of 5% ≤ P ≤ 30%.

8. The method for preparing positive electrode particles according to claim 7, characterized in that, The slurry comprises solid particles, and the volume median particle size Dv50 of the solid particles is in the range of 100nm≤Dv50≤300nm.

9. The method for preparing positive electrode particles according to claim 7 or 8, characterized in that, The step of sequentially pre-sintering and sintering the precursor powder to obtain positive electrode particles includes: The precursor powder is heated to 250°C to 400°C at a first heating rate for pre-sintering to obtain an intermediate; and The intermediate is heated to 500°C to 650°C at a second heating rate and sintered to obtain positive electrode particles.

10. The method for preparing positive electrode particles according to claim 9, characterized in that, The first heating rate ranges from 2°C / min to 10°C / min; the second heating rate ranges from 1°C / min to 5°C / min.

11. A positive electrode plate, characterized in that, The positive electrode sheet includes: Positive current collector; and A positive electrode active layer, wherein the positive electrode active layer comprises the positive electrode particles as described in any one of claims 1-6 or the positive electrode particles prepared by the method described in any one of claims 7-10.

12. A sodium battery, characterized in that, The sodium battery comprises: an electrolyte, a positive electrode as described in claim 11, a separator, and a negative electrode.

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