Positive electrode active material, preparation method thereof and battery

Abstract

The invention discloses a positive electrode active material and a preparation method thereof, and a sodium secondary battery, the positive electrode active material comprises: a first positive electrode active material, the first positive electrode active material comprises a core, the core comprises a layered oxide and a coating layer, the coating layer is at least located on a part of the surface of the core, the coating layer comprises a polyanionic material; a second positive electrode active material, the second positive electrode active material including the polyanionic material; wherein the XRD peak intensity of the positive electrode active material is I (003) when the 2theta diffraction angle is 16.6 + / -0.5 degrees, the XRD peak intensity of the positive electrode active material is I (011) when the 2theta diffraction angle is 15.9 + / -0.3 degrees, the XRD peak intensity of the positive electrode active material is I (104) when the 2theta diffraction angle is 41.5 + / -0.5 degrees, IA is larger than or equal to 0.5 and smaller than or equal to 0.7, IB is larger than or equal to 0.02 and smaller than or equal to 0.05, IA = I (003) / (I (003) + I (104)), and IB = I (011) / (I (011) + I (104)). Therefore, the polyanionic material forms a coating layer on the surface of the layered oxide core, so that the polyanionic material and the layered oxide are tightly combined to form the first positive electrode active material, meanwhile, the polyanionic material can also fill gaps among the first positive electrode active material, and the positive electrode active material has relatively high structural stability; the synergistic effect can be exerted for a long time, so that the voltage cycling stability and safety of the battery are improved.

Description

Technical Field

[0001] The present application relates to the field of sodium ion batteries, and in particular to positive electrode active materials, preparation methods thereof, and batteries. Background Art

[0002] In recent years, sodium-ion batteries have developed rapidly in the fields of start-stop power supplies, two-wheeled vehicles, and energy storage due to their similar working principles to lithium-ion batteries, low raw material costs, and compatibility with existing lithium battery positive electrode production lines.

[0003] Sodium layered oxides, due to their high theoretical capacity, were among the first to be studied and applied as cathode active materials for sodium batteries. However, layered oxide materials are prone to structural phase transitions during charge and discharge at high voltages, resulting in poor cycling stability. Furthermore, layered oxides suffer from poor thermal stability and insufficient safety. Therefore, the current use of layered oxides as cathode active materials for sodium batteries still faces numerous challenges that need to be addressed.

[0004] It should be noted that the above statements are only used to provide background technical information related to this application and do not necessarily constitute prior art. Summary of the Invention

[0005] In a first aspect of the present application, the present application proposes a positive electrode active material for a sodium secondary battery, comprising: a first positive electrode active material, wherein the first positive electrode active material comprises a core, the core comprises a layered oxide, and a coating layer, wherein the coating layer is located at least on a portion of the surface of the core, and the coating layer comprises a polyanionic material; a second positive electrode active material, wherein the second positive electrode active material comprises the polyanionic material; wherein the positive electrode active material has an XRD diffraction peak intensity of 1 at a 2θ diffraction angle of 16.6°±0.5°. (003) The positive electrode active material has an XRD diffraction peak intensity of I within a 2θ diffraction angle of 15.9°±0.3°. (011) The positive electrode active material has an XRD diffraction peak intensity of I within a 2θ diffraction angle of 41.5°±0.5°. (104) , 0.5≤I A ≤0.7, 0.02≤I B ≤0.05, where I A =I (003) / (I (003) +I (104) ), I B =I (011) / (I (011) +I (104)As a result, the polyanionic material forms a coating layer on the surface of the layered oxide core, allowing the polyanionic material to be tightly combined with the layered oxide to form the first positive electrode active material. At the same time, the polyanionic material also fills the gaps between the first positive electrode active materials. The positive electrode active material has high structural stability and can exert a synergistic effect for a long time. During the charge and discharge cycle, the direct current internal resistance (DCR) growth rate of the battery is greatly reduced, thereby improving the high-voltage cycle stability and safety of the battery.

[0006] In some embodiments, the mass ratio of the polyanionic material to the layered oxide in the positive electrode active material is (0.01-1):1; alternatively, (0.10-0.45):1. This facilitates the polyanionic material to form a relatively uniform coating layer on the surface of the layered oxide while being embedded in the gaps around the first positive electrode active material, further improving the structural stability of the positive electrode active material.

[0007] In some embodiments, the layered oxide has a Dv50 particle size of 3 μm to 10 μm; alternatively, 4 μm to 9 μm; further, 5 μm to 8 μm; and / or the polyanionic material has a Dv50 particle size of 0.5 μm to 1.0 μm; alternatively, 0.6 μm to 0.9 μm; further, 0.7 μm to 0.8 μm. This helps increase the compaction density of the positive electrode active material, thereby improving the volumetric energy density of the battery.

[0008] In some embodiments, the layered oxide has a particle size distribution SPAN of 1.1-1.3, or alternatively, 1.2-1.4; and / or the polyanionic material has a particle size distribution SPAN of 3-4, or alternatively, 3.3-3.7. Thus, by grading particles of different sizes, the compaction density of the positive electrode active material is further increased.

[0009] In some embodiments, the Dv50 particle size of the positive electrode active material is 3 μm-10 μm, optionally 5 μm-8 μm, or further 5 μm-7 μm, thereby increasing the compaction density of the positive electrode active material.

[0010] In some embodiments, the particle size distribution SPAN of the positive electrode active material is 1.5-2.5; alternatively, 1.8-2.2. This facilitates denser packing of the positive electrode active material particles, thereby further improving the volumetric energy density of the sodium ion battery.

[0011] In some embodiments, the specific surface area of ​​the positive electrode active material is 1 m 2 / g-7m 2 Thus, the dispersion uniformity of the positive electrode active material in the positive electrode slurry can be improved, and the processing performance can be improved.

[0012] In some embodiments, the compacted density of the positive electrode active material is 3.1 g / cm 3 -3.4g / cm 3 This is beneficial to increasing the volume energy density of sodium-ion batteries and improving their capacity.

[0013] In some embodiments, the layered oxide satisfies the chemical formula: Na a1 (Ni x1 Fe y1 Mn z1 M' n1 )O2, wherein 0.90≤a1≤1.10, 0≤x1≤0.5, 0≤y1≤0.5, 0≤z1≤0.5, 0≤n1≤0.3, x1+y1+z1+n1=1, and M' comprises at least one of Sn, Ce, La, Co, Ca, Cu, Sr, Y, Ti, V, Mg, B, Cr, Sb, Al, Zn, Zr, Nb, W, and Li. This is beneficial to improving the structural stability and specific capacity of the positive electrode active material.

[0014] In some embodiments, a1:(x1+y1+z1+n1)=(0.90-1.05):1; alternatively, (0.95-1.05):1; further, (0.97-1.03):1; and / or, 0<n1≤0.02. This helps further increase the gram capacity of the positive electrode active material.

[0015] In some embodiments, the polyanionic material satisfies the chemical formula: Na4Fe 3-x M x (PO4)2(P2O7) / C, wherein 0≤x≤0.3, and M includes at least one of Mn, Co, Ni, Ti, Cu, and V. This is beneficial to improving the safety of the positive electrode active material.

[0016] In a second aspect of the present application, a method for preparing the aforementioned positive electrode active material is proposed, comprising: mixing a sodium source, an iron source, a phosphorus source, and an M source, grinding and obtaining an intermediate; mixing the intermediate with a carbon source, and performing a first sintering treatment in a nitrogen atmosphere to obtain a polyanionic material; mixing a sodium source, a precursor, and an M' source, and performing a second sintering treatment in an oxidizing atmosphere to obtain a layered oxide; and mixing the polyanionic material with the layered oxide material, and performing a third sintering treatment in a nitrogen atmosphere, wherein the third sintering treatment temperature is less than 500°C, to obtain the positive electrode active material. Thus, through a suitable sintering treatment, the polyanionic material and the layered oxide are tightly bonded to obtain a positive electrode active material with excellent high-voltage cycling stability and safety. The process is simple and easy to implement in industrial production.

[0017] In some embodiments, the temperature of the third sintering treatment is 100°C-500°C; alternatively, 200°C-400°C; and / or the duration of the third sintering treatment is 3 hours-10 hours; alternatively, 4 hours-8 hours; and / or the heating rate of the third sintering treatment is 0.5°C / min-5°C / min, alternatively, 1°C / min-3°C / min. This helps to improve the interfacial bonding strength between the polyanionic material and the layered oxide in the positive electrode active material, thereby enhancing the efficiency of electron conduction and ion diffusion between particles and improving rate performance.

[0018] In some embodiments, the first sintering treatment is performed at a temperature of 400°C to 700°C for 8 to 12 hours, and / or the second sintering treatment is performed at a temperature of 900°C to 1100°C for 8 to 15 hours. This facilitates obtaining polyanionic materials and layered oxides that meet the aforementioned particle size requirements.

[0019] In some embodiments, the sodium source includes at least one of sodium carbonate, sodium dihydrogen phosphate, sodium hydroxide, sodium nitrate and sodium oxide; and / or, the iron source includes at least one of Fe2O3, FeC2O4, FeSO4, FeCl3, Fe(NO3)3; and / or, the phosphorus source includes at least one of (NH4)2HPO4, H3PO4, NH4H2PO4; and / or, the M source includes at least one of an oxide, carbonate, phosphate, fluoride, chloride, hydroxide and silicide of the element M; and / or, the carbon source includes at least one of glucose, sucrose, citric acid and oxalic acid; and / or, the precursor includes (Ni x1 Fe y1 Mn z1 )OOH、(Ni x1 Fe y1 Mn z1 (OH)2; and / or the M' source includes at least one of an oxide, carbonate, phosphate, fluoride, chloride, hydroxide, and silicide of the M' element. Thus, the raw material sources are extensive, facilitating large-scale promotion.

[0020] In a third aspect of the present application, a battery is provided, comprising a positive electrode sheet, wherein the positive electrode sheet comprises the aforementioned positive electrode active material, or a positive electrode active material prepared using the aforementioned method. Thus, the battery possesses all the features and advantages of the aforementioned positive electrode active material and its preparation method, which will not be further elaborated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0022] Figure 1 This is a SEM photograph of the polyanionic material prepared in Example 1 of the present application.

[0023] Figure 2 This is a SEM photograph of the layered oxide prepared in Example 1 of the present application.

[0024] Figure 3 This is a SEM photograph of the positive electrode active material prepared in Example 1 of the present application.

[0025] Figure 4 This is the XRD spectrum of the polyanionic material prepared in Example 1 of the present application.

[0026] Figure 5 This is the XRD spectrum of the layered oxide prepared in Example 1 of the present application.

[0027] Figure 6 This is the XRD spectrum of the positive electrode active material prepared in Example 1 of the present application.

[0028] Figure 7 A graph showing a half-cell assembled from the polyanionic material, layered oxide, and cathode active material prepared in Example 1 of the present application, charged to 4.15 V, and the cathode electrode piece removed for DSC testing. DETAILED DESCRIPTION

[0029] The following describes the embodiments of the present application in detail. Examples of the embodiments are shown in the accompanying drawings, but unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions of substantially identical structures may be omitted. This is to avoid unnecessary lengthiness in the following description and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0030] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by technicians in the technical field to which this application belongs; the terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application; unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0031] The terms "include" and "have" in the description and claims of this application and any variations thereof are open expressions, that is, including the contents specified in this application but not excluding other contents.

[0032] In the description of this application, regardless of whether the word "about" or "approximately" is used, all numbers disclosed herein are approximate values. The value of each number may vary by less than 10% or by a reasonable difference considered by a person skilled in the art, such as 1%, 2%, 3%, 4% or 5%.

[0033] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0034] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly specifying the quantity of the technical features indicated. "First feature" and "second feature" may include one or more of the features.

[0035] In the description of this application, "A and / or B" may include the case of A alone, the case of B alone, or any of the cases of A and B, where A and B are only used for example, and may be any technical feature connected by "and / or" in this application.

[0036] In this application, the writing order of each step does not mean a strict execution order and does not constitute any limitation on the implementation process. The specific execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of this application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, it is mentioned that the method may also include step (c), indicating that step (c) can be added to the method in any order. For example, the method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0037] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0038] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0039] In response to the problems of poor high-voltage cycle stability and insufficient safety of layered oxides, the related art can improve the cycle performance and safety of the material to a certain extent by physically mixing polyanionic materials with good thermal stability and structural stability with layered oxides in proportion. However, the interface bonding of simple physical mixing is weak, and it is difficult to achieve synergy between the two in structural changes. Problems such as particle shedding and electrode damage will occur in long cycles, affecting the cycle stability and rate performance of the battery. In the present application, while the polyanionic material is tightly combined with the surface of the layered oxide core to form the first positive electrode active material, the polyanionic material as the second positive electrode active material will also fill the gaps between the first positive electrode active material, so that the polyanionic material and the layered oxide are tightly combined, better play a synergistic role, and improve the high-voltage cycle stability and safety of the positive electrode active material.

[0040] In a first aspect of the present application, the present application proposes a positive electrode active material for a sodium secondary battery, comprising: a first positive electrode active material, wherein the first positive electrode active material comprises a core, the core comprises a layered oxide, and a coating layer, wherein the coating layer is located at least on a portion of the surface of the core, and the coating layer comprises a polyanionic material; a second positive electrode active material, wherein the second positive electrode active material comprises the polyanionic material; wherein the positive electrode active material has an XRD diffraction peak intensity of 1 at a 2θ diffraction angle of 16.6°±0.5°. (003) The positive electrode active material has an XRD diffraction peak intensity of I within a 2θ diffraction angle of 15.9°±0.3°. (011)The positive electrode active material has an XRD diffraction peak intensity of I within a 2θ diffraction angle of 41.5°±0.5°. (104) , 0.5≤I A ≤0.7, 0.02≤I B ≤0.05, where I A =I (003) / (I (003) +I (104) ), I B =I (011) / (I (011) +I (104) As a result, the polyanionic material forms a coating layer on the surface of the layered oxide core, allowing the polyanionic material to be tightly combined with the layered oxide to form the first positive electrode active material. The electron conduction and ion diffusion efficiency between the polyanionic material and the layered oxide within the first positive electrode active material are greatly improved. At the same time, the polyanionic material also fills the gaps between the first positive electrode active materials. The positive electrode active material has high structural stability and can exert a synergistic effect for a long time. During the charge and discharge cycle, the DC internal resistance growth rate of the battery is greatly reduced, thereby improving the high voltage cycle stability, rate performance and safety of the battery.

[0041] In this application, I A The physical meaning of the representation is: the relative proportion of the characteristic peak intensity of the layered oxide in the total characteristic intensity of the positive electrode active material directly reflects the proportion of the layered structure in the positive electrode active material. B The physical meaning of the representation is: the relative proportion of the characteristic peak intensity of the polyanionic material in the total characteristic intensity of the positive electrode active material directly reflects the proportion of the polyanionic material in the positive electrode active material. Specifically, the (003) crystal plane reflects the interlayer Na + The long-range ordered stacking of transition metal (TM) layers is a characteristic peak of the layered structure. The higher its intensity, the greater the proportion of typical layered phase in the positive electrode active material. The (011) crystal plane represents a plane with a specific orientation in the sodium iron pyrophosphate crystal and is a characteristic peak of the polyanionic material. The (104) peak represents the local structural order within the transition metal layer, and its intensity is related to both the layered phase and the non-layered phase. When the positive electrode active material undergoes a transition from a layered phase to a non-layered phase, the (104) peak may be enhanced due to the diffraction contribution of the new phase, or the peak may be broadened or split due to lattice distortion. Among them, the mass ratio of the polyanionic material to the layered oxide will affect the coating and mixing ratio. When the amorphous polyanionic material is coated on the layered oxide surface, the peak intensity of the (003) crystal plane diffraction peak corresponding to the c-axis direction will be reduced, the surface coating will be more uniform, and the (003) crystal plane peak intensity of the positive electrode active material will be further reduced.

[0042] As an example, I AIt can be 0.5, 0.55, 0.6, 0.65 or 0.7, etc.; B It can be 0.02, 0.03, 0.04 or 0.05, etc.

[0043] When the positive electrode active material I A and I B When the above conditions are met, the proportion of polyanionic material and layered oxide in the positive electrode active material is always the same, and the surface of the layered oxide inner core is uniformly coated with a polyanionic material coating layer, and the polyanionic material as the second positive electrode active material effectively fills the gaps between the first positive electrode active material, and the positive electrode active material has higher structural stability and safety.

[0044] In some embodiments, the polyanionic material only partially covers the surface of the layered oxide core, and the remaining polyanionic material is independently distributed in the gaps around the first positive electrode active material particles.

[0045] In some embodiments, the mass ratio of the polyanionic material to the layered oxide in the positive electrode active material is (0.01-1):1, for example, it can be 0.01:1, 0.05:1, 0.1:1, 0.3:1, 0.5:1, 0.8:1 or 1:1, etc.; optionally, (0.10-0.45):1.

[0046] The (011) peak of the positive electrode active material corresponds to the characteristic peak of the polyanionic material, and its peak intensity can reflect the content of the polyanionic material. When the mass ratio is within the above range, the polyanionic material is uniformly coated on the layered oxide surface and embedded in the gaps between the layered oxide particles, thereby improving the high-voltage cycling stability and safety of the positive electrode active material.

[0047] In this application, the polyanionic material and the layered oxide are dry-blended at a relatively low temperature. Therefore, after heat treatment, the polyanionic material only partially covers the surface of the layered oxide core, while the gaps between the layered oxides are also filled with the polyanionic material. Within the aforementioned mass ratio range, the polyanionic material can achieve both partial coating of the layered oxide and filling of the gaps between the layered oxides.

[0048] In some embodiments, the Dv50 particle size of the layered oxide is 3 μm-10 μm, for example, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm, etc.; optionally, 4 μm-9 μm; further, 5 μm-8 μm.

[0049] In some embodiments, the polyanionic material has a Dv50 particle size of 0.5 μm to 1.0 μm, such as 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or 1 μm; alternatively, 0.6 μm to 0.9 μm; further, 0.7 μm to 0.8 μm. This helps increase the compaction density of the positive electrode active material, thereby improving the volumetric energy density of the battery.

[0050] In some embodiments, the layered oxide has a particle size distribution SPAN of 1.1-1.3, such as 1.1, 1.2, or 1.3, or alternatively, 1.2-1.4; and / or the polyanionic material has a particle size distribution SPAN of 3-4, such as 3, 3.2, 3.4, 3.6, 3.8, or 4, or alternatively, 3.3-3.7. When the particle size distribution meets the above requirements, grading of particles of different sizes can further increase the compaction density of the positive electrode active material.

[0051] In some embodiments, the positive electrode active material has a Dv50 particle size of 3 μm to 10 μm, such as 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm, or alternatively, 5 μm to 8 μm, or further, 5 μm to 7 μm. This helps increase the compaction density of the positive electrode active material.

[0052] In some embodiments, the particle size distribution SPAN of the positive electrode active material is 1.5-2.5, for example, 1.5, 1.7, 1.9, 2.1, 2.3, or 2.5; alternatively, 1.8-2.2. This facilitates denser packing of the positive electrode active material particles, thereby further improving the volumetric energy density of the sodium-ion battery.

[0053] The Dv50 particle size is also called the median particle size, which means that 50% of the volume of the particles is less than or equal to this value. The specific test can be measured using the Mastersizer 3000 laser particle size analyzer from Malvern.

[0054] Particle size distribution SPAN is a key parameter that describes the degree of dispersion of particle size distribution. The smaller the SPAN value, the more concentrated the particle size distribution, and the larger the SPAN value, the wider the particle size distribution. The specific test of particle size distribution SPAN can be calculated based on the particle size data measured by Malvern's Mastersizer 3000 laser particle size analyzer, and the particle size distribution SPAN is calculated as follows: SPAN = (D 90 -D 10 ) / D 50 .

[0055] When the Dv50 particle size and particle size distribution SPAN of the polyanionic material, layered oxide and positive electrode active material meet the above conditions, the positive electrode active material has a higher compaction density and gram capacity.

[0056] In some embodiments, the specific surface area of ​​the positive electrode active material is 1 m 2 / g-7m 2 / g, for example, 1m 2 / g, 2m 2 / g、3m 2 / g、4m 2 / g、5m 2 / g、6m 2 / g or 7m 2 / g, etc.

[0057] The specific surface area refers to the total surface area per unit mass of the positive electrode active material. The specific test can be measured by the Tristar II3020 specific surface area tester of Micromertics Company of the United States.

[0058] When the specific surface area is within the above range, the dispersion uniformity of the positive electrode active material in the positive electrode slurry can be improved, the amount of dispersant and binder used can be reduced, and the processing performance of the positive electrode active material can be improved.

[0059] In some embodiments, the compacted density of the positive electrode active material is 3.1 g / cm 3 -3.4g / cm 3 , for example, it can be 3.1 g / cm 3 、3.2g / cm 3 , 3.3g / cm 3 or 3.4g / cm 3 wait.

[0060] The compaction density refers to the mass per unit volume of the electrode coating after rolling, which is measured by the weighing-thickness method. Specifically, the compaction density is calculated by measuring the mass, area and thickness of the electrode.

[0061] A compaction density within the above range is beneficial to increasing the volume energy density of the battery and improving the capacity performance during the charge and discharge cycle.

[0062] In some embodiments, the layered oxide satisfies the chemical formula: Na a1 (Ni x1 Fe y1 Mn z1 M' n1)O2, wherein 0.90≤a1≤1.10, 0≤x1≤0.5, 0≤y1≤0.5, 0≤z1≤0.5, 0≤n1≤0.3, x1+y1+z1+n1=1, and M' comprises at least one of Sn, Ce, La, Co, Ca, Cu, Sr, Y, Ti, V, Mg, B, Cr, Sb, Al, Zn, Zr, Nb, W, and Li. This is beneficial to improving the structural stability and specific capacity of the positive electrode active material.

[0063] As an example, a1 can be 0.90, 0.95, 1, 1.05 or 1.10, etc., x1 can be 0, 0.1, 0.2, 0.3, 0.4 or 0.5, etc., y1 can be 0, 0.1, 0.2, 0.3, 0.4 or 0.5, etc., z1 can be 0, 0.1, 0.2, 0.3, 0.4 or 0.5, etc., and n1 can be 0, 0.1, 0.2 or 0.3, etc.

[0064] In some embodiments, a1:(x1+y1+z1+n1)=(0.90-1.05):1, for example, 0.90:1, 0.93:1, 0.95:1, 0.98:1, 1:1, 1.03:1, or 1.05:1; alternatively, (0.95-1.05):1; further, (0.97-1.03):1; and / or, 0<n1≤0.02. This is beneficial to further increase the gram capacity of the positive electrode active material.

[0065] In some embodiments, the polyanionic material satisfies the chemical formula: Na4Fe 3-x M x (PO4)2(P2O7) / C, wherein 0≤x≤0.3, for example, 0, 0.1, 0.2 or 0.3, and M includes at least one of Mn, Co, Ni, Ti, Cu, and V. This is beneficial to improving the safety of the positive electrode active material.

[0066] It should be noted that “ / C” indicates that a carbon coating layer is present on at least a portion of the surface of the polyanionic material, thereby improving the electronic conductivity of the polyanionic material.

[0067] In a second aspect of the present application, the present application proposes a method for preparing the aforementioned positive electrode active material, comprising:

[0068] S1: Mix and grind the sodium source, iron source, phosphorus source, and M source to obtain an intermediate, mix the intermediate with a carbon source, and perform a first sintering treatment in a nitrogen atmosphere to obtain a polyanionic material.

[0069] In this step, the specific method of mixing the sodium source, iron source, phosphorus source, and M source is not particularly limited, and for example, a mixer can be used for mixing. It is understood that in this step, whether to add the M source can be flexibly selected as needed.

[0070] In some embodiments, the sodium source includes at least one of sodium carbonate, sodium dihydrogen phosphate, sodium hydroxide, sodium nitrate, and sodium oxide; the iron source includes at least one of Fe2O3, FeC2O4, FeSO4, FeCl3, and Fe(NO3)3; the phosphorus source includes at least one of (NH4)2HPO4, H3PO4, and NH4H2PO4; the M source includes at least one of an oxide, carbonate, phosphate, fluoride, chloride, hydroxide, and silicide of the element M; and the carbon source includes at least one of glucose, sucrose, citric acid, and oxalic acid. Thus, the raw materials are widely available, facilitating large-scale promotion.

[0071] Specifically, the M source may include at least one of Mn(CH3COO)2, MnO2, MnSO4, Co(OH)2, Co(NO3)2, CoO, CoCl2, NiCl2, NiO, NiSO4, TiO2, TiCl4, CuO, CuSO4, V2O5, and NH4VO3.

[0072] In some embodiments, the first sintering treatment is performed at a temperature of 400° C. to 700° C. (e.g., 400° C., 500° C., 600° C., or 700° C.) and for a time of 8 hours to 12 hours (e.g., 8 hours, 9 hours, 10 hours, 11 hours, or 12 hours). This helps control the particle size of the polyanionic material and improves its crystallinity and structural stability.

[0073] In some embodiments, natural cooling may be performed after the first sintering process, and then the material may be crushed and sieved to obtain a submicron-sized polyanionic material.

[0074] S2: mixing the sodium source, the precursor, and the M' source and performing a second sintering treatment in an oxidizing atmosphere to obtain a layered oxide.

[0075] In this step, the specific method of mixing the sodium source, precursor, and M' source is not particularly limited, and for example, a mixer can be used for mixing. It is understood that in this step, whether to add the M' source can be flexibly selected as needed.

[0076] In some embodiments, the precursor comprises (Ni x1 Fe y1 Mn z1 )OOH、(Ni x1 Fe y1 Mn z1 )(OH)2.

[0077] In some embodiments, the M' source includes at least one of oxides, carbonates, phosphates, fluorides, chlorides, hydroxides, and silicides of the M' element. Therefore, the raw material sources are extensive, facilitating large-scale promotion.

[0078] Specifically, the M' source may include at least one of CaO, CaCO3, Ca3(PO4)2, CaF2, CaCl2, Ca(OH)2, CaSi2, CuO, CuSO4, Sr(OH)2, SrCO3, Sn(OH)4, Y2O3, La2O3, Ce2O3, CeO2, Li2CO3, LiOH, Co(OH)2, V2O5, Cr2O3, ZrO, Zr(HPO4)2, ZrSi2, Al2O3, AlPO4, AlCl3, ZnO, TiO2, MgO, MgCO3, Mg2Si, Mg3(PO4)2, MgF2, MgCl2, Nb2O5, WO3, B2O3, Sb2O5, and Sb2O3.

[0079] In some embodiments, the second sintering treatment is performed at a temperature of 900° C. to 1100° C. (e.g., 900° C., 950° C., 1000° C., 1050° C., or 1100° C.) and for a time of 8 hours to 15 hours (e.g., 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, or 15 hours). This helps to increase the crystallinity of the layered oxide and control the particle size.

[0080] In some embodiments, the second sintering process may further be subjected to natural cooling, followed by crushing and screening to obtain micron-sized layered oxides.

[0081] S3: mixing the polyanionic material and the layered oxide material and performing a third sintering treatment under a nitrogen atmosphere, wherein the temperature of the third sintering treatment is less than 500° C., to obtain the positive electrode active material.

[0082] Specifically, the polyanionic material and the layered oxide can be weighed and mixed uniformly according to the mass ratio to obtain a mixed material, and the mixed material is subjected to a third sintering treatment under a nitrogen atmosphere. The sintered material is naturally cooled and then crushed and sieved to obtain the positive electrode active material.

[0083] In some embodiments, the temperature of the third sintering treatment is 100°C-500°C, for example, 100°C, 200°C, 300°C, 400°C or 500°C, etc.; optionally, 200°C-400°C.

[0084] In some embodiments, the third sintering treatment lasts for 3 hours to 10 hours, for example, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours, etc.; optionally, 4 hours to 8 hours.

[0085] In some embodiments, the heating rate of the third sintering treatment is 0.5°C / min-5°C / min, for example, it can be 0.5°C / min, 1°C / min, 2°C / min, 3°C / min, 4°C / min or 5°C / min, etc.; optionally, 1°C / min-3°C / min.

[0086] The lower temperature of the third sintering treatment is conducive to obtaining a positive electrode active material in which the amorphous polyanionic material is partially coated on the surface of the layered oxide. The uniformity of the surface coating is related to the temperature of the third sintering treatment. When the temperature of the third sintering treatment is within the above range, the diffraction peak intensity of a specific crystal plane in the positive electrode active material can be controlled and can meet the requirements of I A , I B As a result, it is beneficial to improve the interfacial bonding strength between the polyanionic material and the layered oxide in the positive electrode active material, greatly enhance the efficiency of electron conduction and ion diffusion between particles, improve rate performance, significantly reduce the growth of direct current internal resistance (DCR) during the cycle, and improve high voltage cycle performance.

[0087] In some embodiments, the layered oxide and the polyanionic material each independently comprise single crystalline particles.

[0088] In some embodiments, the crystalline phase of the layered oxide includes an O3 phase.

[0089] Compared to simple single-crystal layered oxides, this application achieves a high-voltage cycling-stable and highly safe positive electrode active material by regulating the mass ratio of the polyanionic material to the layered oxide and selecting an appropriate sintering process. The two particles in the sintered positive electrode active material are more tightly bonded at the interface, significantly increasing the efficiency of electron conduction and ion diffusion between the particles, improving rate performance, significantly reducing the DCR growth during cycling, and improving high-voltage cycling performance. Furthermore, because an appropriate amount of thermally stable polyanionic material is uniformly compounded around the layered oxide, the positive electrode active material exhibits high safety. The process is also simple, making it easy to implement industrial production.

[0090] In a third aspect of the present application, a battery is provided, comprising a positive electrode sheet, wherein the positive electrode sheet comprises the aforementioned positive electrode active material, or a positive electrode active material prepared using the aforementioned method. Thus, the battery possesses all the features and advantages of the aforementioned positive electrode active material and its preparation method, which will not be further elaborated here.

[0091] It can be understood that there is no special restriction on the specific type of the battery, which can be a primary battery or a secondary battery; the shape of the sodium ion battery can be a cylindrical battery, a square battery or a battery of any other shape, etc., and according to the outer packaging classification, the sodium ion battery can be a hard shell battery, a soft pack battery, etc.

[0092] Taking sodium-ion batteries as an example, sodium-ion batteries usually include positive electrode sheets, negative electrode sheets, electrolytes and separators, wherein the positive electrode sheets, negative electrode sheets and separators can be made into electrode assemblies through winding or lamination processes, and the electrode assemblies and electrolytes can be contained in outer packaging. During the charge and discharge process of sodium-ion batteries, sodium ions are embedded and extracted back and forth between the positive electrode sheets and the negative electrode sheets. The electrolyte plays the role of conducting ions between the positive electrode sheets and the negative electrode sheets. The separator is arranged between the positive electrode sheets and the negative electrode sheets, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing active sodium ions to pass through.

[0093] In some embodiments, the positive electrode sheet may include a positive electrode current collector and a positive electrode active material layer provided on at least one side of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material, a conductive agent and a binder, and the positive electrode current collector may include a metal foil, for example, the metal foil may be aluminum foil. The positive electrode active material may be the positive electrode active material of the first aspect of the present application or the positive electrode active material prepared by the method described in the second aspect of the present application. The conductive agent may include acetylene black, single-walled carbon nanotubes and conventional materials in the art. The binder may be polyvinylidene fluoride (PVDF) and conventional materials in the art.

[0094] In some embodiments, the negative electrode plate may include a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector. The negative electrode active material layer may include a negative electrode active material, a thickener, a conductive agent, and a binder. The negative electrode current collector may be a metal foil, for example, a copper foil. The negative electrode active material may include artificial graphite, natural graphite, a silicon-containing carbon-based composite material, a metal sodium sheet, and a negative electrode active material commonly used in the art. The thickener may be a thickener sodium carboxymethyl cellulose (CMC-Na) and a conventional material in the art. The conductive agent may be acetylene black and a conventional material in the art. The binder may be styrene-butadiene rubber and a conventional material in the art.

[0095] In some embodiments, the separator can be a separator known in the art that can be used in sodium ion batteries and is stable to the electrolyte used, such as a polyethylene separator, a polypropylene separator, a polyethylene / polypropylene composite separator, etc.

[0096] The present invention will be described below by way of specific examples. It should be noted that the following examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. Where specific techniques or conditions are not specified in the examples, they are determined according to the techniques or conditions described in the literature in this area or according to the product specifications. Reagents or instruments not specified by manufacturer are all commercially available conventional products.

[0097] Example 1

[0098] S1: Sodium carbonate, ferric oxalate, ammonium dihydrogen phosphate, and manganese oxalate are uniformly mixed in a molar ratio of Na, Fe, P, and Mn of 4:2.7:4:0.3 to obtain a first mixed material.

[0099] S2: Place the first mixed material in S1 into the grinding chamber of a sand mill, add an appropriate amount of deionized water as a grinding medium, and control the solid-to-liquid ratio to 1:3 (mass ratio). Set the sand mill grinding speed to 2000 r / min and grind for 2 hours to obtain a slurry of appropriate particle size. Spray dry the intermediate to obtain the intermediate, and mix it with an appropriate amount of glucose to obtain the second mixed material.

[0100] S3: The second mixture of S2 was heated to 600°C in a nitrogen atmosphere at a heating rate of 1.5°C / min and kept at that temperature for 9 hours. The calcined material was naturally cooled and then crushed and sieved to obtain a single crystalline polyanionic material with a Dv50 of 0.7 μm and a particle size distribution SPAN of 3.5;

[0101] S4: Nickel sulfate, iron sulfate, and manganese sulfate were dissolved in a molar ratio of 1:1:1 to obtain a 2 mol / L mixed salt solution. Sodium hydroxide was dissolved to form a precipitant solution with a concentration of 10 mol / L. Ammonia was dissolved to form a complexing agent solution with a concentration of 8 mol / L. 100 L of the mixed salt solution, the precipitant solution, and the complexing agent solution were introduced into the reactor in parallel at a temperature of 60°C and a pH of 11.50. Then, under a nitrogen atmosphere, the precipitate crystallized and grew continuously in an overflow device until the average particle size reached 3.5 μm. The precursor slurry was filtered and washed, and the filter cake was dried at 120°C and sieved to obtain a precursor with a particle size distribution (SPAN) of 1.3.

[0102] S5: The multi-component precursor described in S4 is mixed with Na2CO3, calcium oxide and strontium hydroxide in a molar ratio of Na / (Ni+Fe+Mn+Ca+Sr)=1.01:1, wherein Na / Ca / Sr=1.01:0.01:0.01. The temperature is first raised to 600°C at a heating rate of 6°C / min in an oxidizing atmosphere, and then raised to 1000°C at a heating rate of 1.5°C / min. The mixture is kept warm for 12 hours. After cooling, crushing and screening, a single-crystalline layered oxide with Dv50=6.5 microns and a particle size distribution SPAN of 1.3 is obtained.

[0103] S6: The single-crystalline polyanionic material described in S3 and the single-crystalline layered oxide described in S5 are uniformly mixed in a mass ratio of 0.25:1. The temperature is first raised to 300°C at a heating rate of 3°C / min in a nitrogen atmosphere and kept warm for 6 hours. The calcined material is naturally cooled and then crushed and sieved to obtain a positive electrode active material with Dv50 = 5.3 microns and a particle size distribution SPAN of 1.9.

[0104] The differences between the remaining embodiments and comparative examples and embodiment 1 are shown in Table 1. In comparative example 2, only layered oxide is used as the positive electrode active material, and in comparative example 7, the polyanionic material and layered oxide are physically mixed as the positive electrode active material.

[0105]

[0106]

[0107]

[0108]

[0109]

[0110]

[0111] 1. Powder testing methods and results:

[0112] (1) Morphology test

[0113] The present application tests the scanning electron microscope images of the single crystal polyanionic material and the single crystal layered oxide, as well as the positive electrode active material prepared in the above examples and comparative examples, and provides the SEM images of the single crystal polyanionic material, the single crystal layered oxide, and the positive electrode active material prepared in Example 1 as an example. The results are as follows: Figure 1-3 As shown. Figure 1 It can be seen that the morphology of the single-crystal polyanionic material is submicron small particles with a wide particle size distribution; Figure 2It can be seen that the morphology of the single-crystal layered oxide is micron-sized particles with a smooth surface and a wide particle size distribution. Figure 3 It can be seen that a small part of the single-crystalline polyanionic material in the positive electrode active material covers the surface of the single-crystalline layered oxide, and most of the single-crystalline polyanionic material is independently distributed in the gaps around its particles, playing a filling role, which is beneficial to improving the compaction density of the positive electrode active material.

[0114] (2) Physical property testing

[0115] The present application tests the XRD patterns of the single-crystal polyanionic materials, single-crystal layered oxide materials and positive electrode active materials in the above-mentioned embodiments and comparative examples, and provides the XRD spectra of the single-crystal polyanionic materials, single-crystal layered oxides and positive electrode active materials prepared in Example 1 as an example. The results are as follows: Figure 4-Figure 6 As shown, from Figure 4 It can be seen that the phase of the single crystal polyanionic material is Na4Fe 2.7 Mn 0.3 (PO4)2(P2O7) / C; from Figure 5 It can be seen that the phase of the single crystal layered oxide of the present application is O3 type layered oxide (Na 1.01 Ni 0.327 Fe 0.327 Mn 0.327 Ca 0.01 Sr 0.01 O2), Figure 6 It can be seen that the phase of the positive electrode active material of the present application is a composite phase of sodium polyanion material and O3 type layered oxide material (Na4Fe 2.7 Mn 0.3 (PO4)2(P2O7) / C@Na 1.01 Ni 0.327 Fe 0.327 Mn 0.327 Ca 0.01 Sr 0.01 O2).

[0116] 2. Battery electrochemical performance test and results

[0117] 1. Battery preparation method:

[0118] The positive electrode active material, acetylene black and polyvinylidene fluoride (PVDF) were mixed with an appropriate amount of N-methylpyrrolidone (NMP) in a mass ratio of 95:3:2 to form a uniform slurry. The slurry was coated on aluminum foil and dried at 120°C for 12 hours. The slurry was then pressed with a pressure of 100 MPa to form a positive electrode sheet with a diameter of 12 mm and a thickness of 120 μm. The loading of the positive electrode active material was 15 mg / cm 2 .

[0119] 2. Battery Assembly: In an Ar glove box with water and oxygen contents less than 5 ppm, assemble the positive electrode, separator, negative electrode, and electrolyte into a 2025 button cell and let it rest for 6 hours. The negative electrode is a 17 mm diameter, 1 mm thick sodium metal sheet; the separator is a 25 μm thick Celgard 2325 porous membrane; and the electrolyte is a mixture of 1 mol / L NaPF6 and equal volumes of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0120] 3. Electrical performance test method

[0121] The button cell was electrochemically tested at 25°C using a Xinwei battery testing system, with a charge and discharge current density of 140 mA / g at 0.1C.

[0122] The positive electrode active materials prepared in the aforementioned examples and comparative examples were assembled into button-type batteries and subjected to electrical performance tests. The test method is as follows. The results are shown in Table 3.

[0123] (1) 2.0V-4.15V, 0.1C first charge and discharge specific capacity test method

[0124] The prepared button battery was subjected to charge and discharge tests at 25°C, 2.0V-4.15V, and 0.1C to evaluate the 0.1C first charge and discharge specific capacity of the material.

[0125] (2) Average voltage test method for positive electrode active materials

[0126] The prepared button battery was subjected to charge and discharge tests at 25°C, 2.0V-4.15V, and 0.1C to evaluate the material's 0.1C first discharge specific energy and discharge specific capacity, and the average voltage was calculated as: discharge specific energy / discharge specific capacity

[0127] (3) Rate performance test method

[0128] The prepared button battery was cycled twice at 25°C, 2.0V-4.15V, and 0.1C, and then cycled once at 0.2C, 0.33C, 0.5C, and 1C, respectively. The rate performance of the positive electrode active material was evaluated by the ratio of the 0.1C first discharge specific capacity to the 1C discharge specific capacity; the 0.1C first discharge specific capacity is the discharge specific capacity of the button battery in the first cycle, and the 1C discharge specific capacity is the discharge specific capacity of the button battery in the sixth cycle.

[0129] (4) Cyclic performance test method

[0130] The prepared button battery was cycled twice at 25°C, 2.0V-4.15V, and 0.1C, and then cycled at 1C for 80 weeks to evaluate the capacity retention of the material.

[0131] (5) Volume energy density test method

[0132] The volume energy density can be obtained by multiplying the 0.1C discharge specific capacity, average voltage and corresponding compaction density obtained by the above methods (1) and (2).

[0133] (6) HPPC DCR test method

[0134] Test process: First, charge the button battery to 4.15V at 0.3C constant current and constant voltage, with a cutoff current of 0.05C, and let it sit for 10 minutes. Then, discharge it at constant current to a cutoff voltage of 2.0V and let it sit for 10 minutes. Then, charge it to 4.15V at 0.3C constant current and constant voltage, with a cutoff current of 0.05C and let it sit for 10 minutes. Then, discharge it to 90% SOC at 0.3C constant current, let it sit for 10 minutes. Then, discharge it at 3C for 18 seconds, let it sit for 10 minutes, charge it at 3C for 10 seconds, let it sit for 10 minutes, and calculate the DCR at this SOC according to the HPPC test method. Then, discharge it to 80% SOC at 0.3C constant current, let it sit for 10 minutes, discharge it at 3C for 18 seconds, let it sit for 10 minutes, charge it at 3C for 10 seconds, let it sit for 10 minutes, and calculate the DCR at this SOC according to the HPPC test method. Then, calculate the DCR values ​​at different SOCs (90% to 10% SOC) in this way.

[0135] The button cell was then cycled twice at 2.0-4.15V and 0.1C, and then cycled at 1C for 80 cycles.

[0136] The HPPC DCR test was then repeated on the cycled button cell to calculate the DCR values ​​at different SOCs after cycling. The DCR growth rate of the battery was calculated using the discharge DCR values ​​at 60% SOC before and after cycling (DCR growth rate = (DCR value at 60% SOC before cycling - DCR value at 60% SOC after cycling) / DCR value at 60% SOC before cycling).

[0137] Table 3

[0138]

[0139]

[0140] As shown in Table 3, by comparing the examples and comparative examples, it can be seen that the heat treatment process of the positive electrode active material is related to the coating conditions of the single crystal polyanion material and the single crystal layered oxide and the filling effect, thereby affecting IA and I B The value of . Does not satisfy I A and I B The restricted cathode active materials exhibit higher initial DCR and faster DCR growth after cycling during charge and discharge, while having worse rate and cycling performance.

[0141] When the types of doping elements of the single-crystalline polyanionic material and the single-crystalline layered oxide are within the preferred range, at the same doping amount, after a heat treatment process, the composite positive electrodes with different doping elements can also show comparable improved performance.

[0142] Compared with simple single-crystal layered oxides, the positive electrode active material obtained by mixing single-crystal polyanionic materials with single-crystal layered oxides in proportion and then performing a heat treatment process has improved DCR, rate performance and cycle performance.

[0143] Compared with simple physical mixing, the positive electrode active material prepared under the heat treatment process in this application has improved DCR, rate performance and cycle performance.

[0144] The single crystal morphology of single-crystalline polyanionic materials and single-crystalline layered oxides exhibits a wide distribution of large and small particles. When the particle size distribution (SPAN) of the single-crystalline polyanionic materials and single-crystalline layered oxides is within the aforementioned range, and their mass ratio is also within the aforementioned range, the positive electrode active material has a higher compaction density, and the battery has a higher volumetric energy density.

[0145] The polyanionic material, layered oxide and positive electrode active material in Example 1 were assembled into half-cells, charged to 4.15V, and the positive electrode was taken out for DSC testing. The test result curve is as follows: Figure 7 As shown. Figure 7 It can be seen that the heat release of the positive electrode active material (Example 1) is significantly lower than that of the single crystal layered oxide, and the heat release peak is shifted later, indicating that the safety of the positive electrode active material is greatly improved compared to the simple single crystal layered oxide.

[0146] In Comparative Example 7, the amount of polyanionic material used is too small, and only a very small amount of coating on the surface of the layered oxide can be achieved, resulting in poor cycle performance and rate performance of the positive electrode active material.

[0147] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode active material for a sodium secondary battery, characterized in that include: a first positive electrode active material, the first positive electrode active material comprising a core, the core comprising a layered oxide, and a coating layer, the coating layer being located at least on a portion of a surface of the core, the coating layer comprising a polyanionic material; a second positive electrode active material, wherein the second positive electrode active material comprises the polyanionic material; The positive electrode active material has an XRD diffraction peak intensity of I within a 2θ diffraction angle of 16.6°±0.5°. (003) The positive electrode active material has an XRD diffraction peak intensity of I within a 2θ diffraction angle of 15.9°±0.3°. (011) The positive electrode active material has an XRD diffraction peak intensity of I within a 2θ diffraction angle of 41.5°±0.5°. (104) , 0.5 ≤ I A ≤ 0.7, 0.02 ≤ I B ≤ 0.05, where, I A = I (003) / (I (003) + I (104) ), I B = I (011) / (I (011) + I (104) ).

2. The positive electrode active material according to claim 1, characterized in that The mass ratio of the polyanionic material to the layered oxide in the positive electrode active material is (0.01-1):1; optionally, (0.10-0.45):

1.

3. The positive electrode active material according to claim 1, characterized in that The layered oxide has a Dv50 particle size of 3 μm-10 μm; optionally, 4 μm-9 μm; further, 5 μm-8 μm; and / or, The Dv50 particle size of the polyanionic material is 0.5 μm-1.0 μm; optionally, 0.6 μm-0.9 μm; further, 0.7 μm-0.8 μm.

4. The positive electrode active material according to claim 3, characterized in that The particle size distribution SPAN of the layered oxide is 1.1-1.3; optionally, 1.2-1.4; and / or, The particle size distribution SPAN of the polyanionic material is 3-4; optionally, 3.3-3.

7.

5. The positive electrode active material according to any one of claims 1 to 4, characterized in that The Dv50 particle size of the positive electrode active material is 3 μm-10 μm; optionally, 5 μm-8 μm; further, 5 μm-7 μm.

6. The positive electrode active material according to claim 5, wherein the particle size distribution SPAN of the positive electrode active material is 1.5-2.5; optionally, 1.8-2.2; and / or The specific surface area of ​​the positive electrode active material is 1 m 2 / g-7m 2 / g; and / or, The compaction density of the positive electrode active material is 3.1 g / cm 3 -3.4g / cm 3 .

7. The positive electrode active material according to any one of claims 1 to 4, characterized in that The layered oxide satisfies the chemical formula: Na a1 (Ni x1 Fe y1 Mn z1 M' n1 )O2, among which, 0.90≤a1≤1.10,0≤x1≤0.5,0≤y1≤0.5,0≤z1≤0.5,0≤n1≤0.3, x1+y1+z1+n1=1, M' includes Sn, Ce, La, Co, Ca, Cu, Sr, Y, Ti, V, Mg, B, Cr, Sb, Al, At least one of Zn, Zr, Nb, W, and Li.

8. The positive electrode active material according to claim 7, characterized in that a1:(x1+y1+z1+n1)=(0.90-1.05):1; optionally, (0.95-1.05):1; further, (0.97-1.03):1; and / or, 0＜n1≤0.02。 9. The positive electrode active material according to any one of claims 1 to 4, characterized in that The polyanionic material satisfies the chemical formula: Na4Fe 3-x M x (PO4)2(P2O7) / C, wherein 0≤x≤0.3, and M includes at least one of Mn, Co, Ni, Ti, Cu, and V.

10. A method for preparing the positive electrode active material according to any one of claims 1 to 9, characterized in that: A sodium source, an iron source, a phosphorus source, and an M source are mixed and ground to obtain an intermediate, and the intermediate is mixed with a carbon source and then subjected to a first sintering treatment in a nitrogen atmosphere to obtain a polyanionic material; The sodium source, the precursor, and the M' source are mixed and then subjected to a second sintering treatment in an oxidizing atmosphere to obtain a layered oxide; The polyanionic material and the layered oxide material are mixed and then subjected to a third sintering treatment in a nitrogen atmosphere at a temperature of less than 500° C. to obtain the positive electrode active material.

11. The method according to claim 10, characterized in that The temperature of the third sintering treatment is 100°C-500°C; optionally, 200°C-400°C; and / or, The third sintering treatment time is 3h-10h; optionally, 4h-8h; and / or, The heating rate of the third sintering treatment is 0.5°C / min-5°C / min, optionally, 1°C / min-3°C / min.

12. The method according to claim 10 or 11, characterized in that The temperature of the first sintering treatment is 400° C.-700° C., and the time is 8 hours-12 hours; and / or the temperature of the second sintering treatment is 900° C.-1100° C., and the time is 8 hours-15 hours.

13. The method according to claim 12, characterized in that The sodium source includes at least one of sodium carbonate, sodium dihydrogen phosphate, sodium hydroxide, sodium nitrate and sodium oxide; and / or, The iron source includes at least one of Fe2O3, FeC2O4, FeSO4, FeCl3, and Fe(NO3)3; and / or, The phosphorus source includes at least one of (NH4)2HPO4, H3PO4, and NH4H2PO4; and / or, The M source includes at least one of an oxide, carbonate, phosphate, fluoride, chloride, hydroxide and silicide of the M element; and / or, The carbon source includes at least one of glucose, sucrose, citric acid, and oxalic acid; and / or, The precursor includes (Ni x1 Fe y1 Mn z1 )OOH、(Ni x1 Fe y1 Mn z1 )(OH)2; and / or, The M' source includes at least one of oxides, carbonates, phosphates, fluorides, chlorides, hydroxides and silicides of the M' element.

14. A battery, characterized in that: include: A positive electrode plate, comprising the positive electrode active material according to any one of claims 1 to 9, or the positive electrode active material prepared by the method according to any one of claims 10 to 13.

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