Positive electrode active material, preparation method thereof, positive electrode sheet, and sodium-ion battery

Abstract

The application discloses a positive electrode active material and a preparation method thereof, a positive electrode sheet and a sodium ion battery. The positive electrode active material comprises: an inner core, wherein the length-diameter ratio of primary particles in the inner core is L1, and the average particle size of the primary particles in the inner core is D1; and a coating layer, wherein the coating layer is located on at least part of the surface of the inner core, the length-diameter ratio of primary particles in the coating layer is L2, the average particle size of the primary particles in the coating layer is D2, L1 / L2>1, and D2 / D1>1. The average particle size of the primary particles in the inner core is small and the length-diameter ratio is large, and the average particle size of the primary particles in the coating layer is large and the length-diameter ratio is small. Therefore, the primary particles in the inner core can better exert their own capacity, and the primary particles in the coating layer can effectively improve the structural stability, so that the positive electrode active material has both excellent capacity exertion and high structural stability in the charging and discharging cycle process.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of sodium battery, in particular, to a positive electrode active material, a preparation method thereof, a positive electrode sheet and a sodium ion battery. BACKGROUND

[0002] In recent years, with the continuous progress of new energy technology and the concept of sustainable energy, the new energy industry represented by lithium ion batteries has achieved unprecedented rapid development. In the fields of energy storage, power batteries, 3C digital products, etc., the figure of lithium ion batteries can be seen, which deeply affects all aspects of our daily life. However, the lithium used in lithium ion batteries is expensive and volatile. Therefore, it is necessary to develop a new electrochemical system. Sodium ion batteries have attracted the attention of researchers due to the abundant sodium resources and low cost. Among the positive electrode active materials of sodium ion batteries, layered metal oxides have become the most promising material to meet the requirements of power batteries due to their high specific capacity and similar production process to ternary materials of lithium ion batteries. However, there are still many problems in the actual production and application of the current positive electrode active materials, which need to be further improved. SUMMARY

[0003] In a first aspect of the present application, a positive electrode active material is provided, comprising: an inner core, the aspect ratio of primary particles in the inner core is L1, and the average particle size of primary particles in the inner core is D1; a coating layer, the coating layer is located on at least part of the surface of the inner core, the aspect ratio of primary particles in the coating layer is L2, and the average particle size of primary particles in the coating layer is D2, L1 / L2>1, and D2 / D1>1. The average particle size of primary particles in the inner core is small and the aspect ratio is large, and the average particle size of primary particles in the coating layer is large and the aspect ratio is small. Therefore, the primary particles in the inner core can better play their own capacity, and the primary particles in the coating layer can effectively improve the structural stability, so that the positive electrode active material has both excellent capacity and high structural stability during the charge and discharge cycle.

[0004] In some embodiments, 1.5≤L1 / L2≤4.2. Thus, the primary particles in the inner core can provide excellent ion transmission channels, which is beneficial to the capacity of the positive electrode active material, and the primary particles in the coating layer are close to single crystal particles, which can improve the structural stability of the positive electrode active material.

[0005] In some embodiments, L1 is 3-5. Thus, the ion conductivity of the primary particles in the inner core is high.

[0006] In some embodiments, L2 is 1-3. Thus, the structural stability of the primary particles in the coating layer is excellent.

[0007] In some embodiments, 3≤D2 / D1≤13; optionally, 4≤D2 / D1≤10. In this way, the structural stability of the positive electrode active material is relatively optimal.

[0008] In some embodiments, D1 is 0.08 μm-0.3 μm. In this way, the specific surface area of the primary particles in the inner core is relatively large, and the capacity performance is relatively optimal.

[0009] In some embodiments, D2 is 0.6 μm-1.5 μm. In this way, the structural stability of the primary particles in the coating layer is relatively high.

[0010] In some embodiments, the porosity of the inner core is Q1, the porosity of the coating layer is Q2, and Q1 / Q2>1.

[0011] In some embodiments, 2≤Q1 / Q2≤7; optionally, 2≤Q1 / Q2≤5. In this way, the positive electrode active material has both relatively optimal capacity performance and relatively high structural stability.

[0012] In some embodiments, 4%≤Q1≤8%. In this way, the primary particles in the inner core can be sufficiently infiltrated with electrolyte, thereby improving the capacity performance.

[0013] In some embodiments, 1%≤Q2≤2.5%. In this way, the contact area between the primary particles in the coating layer and the electrolyte is relatively low, which can effectively improve the structural stability of the positive electrode active material.

[0014] In some embodiments, the radius of the positive electrode active material is R, and 3 μm≤R≤7 μm. In this way, the capacity performance and the structural stability of the positive electrode active material are both relatively optimal.

[0015] In some embodiments, the thickness of the coating layer is 0-0.4R, and optionally, 0-0.35R. In this way, the coating layer can effectively improve the structural stability of the positive electrode active material, and has relatively little effect on the capacity performance of the positive electrode active material.

[0016] In some embodiments, the positive electrode active material satisfies the chemical formula: Na a (Ni 1-b-c-d Fe b Mn c M d )N e O 2-fWherein, M comprises at least one of V, Ta, La, Al, Ce, Y, Ba, Zr, Ca, W, Nb, Si, Mo, F, P, Ti, N comprises at least one of Cu, Cr, Mg, Sr, Fe, Ni, Mn, Sn, Zn, B, Co, 0.95a1.15, 0b0.50, 0c0.70, 0d0.15, 0e0.15, f satisfies one of the following conditions: when M and N are both cations, f=0; when M and N are both anions, f=d+e; when M is an anion and N is a cation, f=d; when N is an anion and M is a cation, f=e. Thus, by doping with different elements, a positive electrode active material meeting different performance requirements can be obtained.

[0017] In the second aspect of the present application, a method for preparing the positive electrode active material is provided, comprising: mixing a nickel source, an iron source, a manganese source, a complexing agent, and a precipitating agent to obtain a precursor slurry, and performing a drying treatment to obtain a precursor material; mixing the precursor material, a sodium source, and an M source, and performing a first sintering treatment to obtain a positive electrode active material intermediate, wherein the temperature of the first sintering treatment is T1; mixing the positive electrode active material intermediate with an N source, and performing a second sintering treatment to obtain a positive electrode active material, wherein the temperature of the second sintering treatment is T2, and T1

[0018] In some embodiments, T2-T1<200℃; optionally, 10℃T2-T1<120℃. Thus, it is helpful to promote the growth and fusion of primary particles in the coating layer during the second sintering treatment, and has less effect on the primary particles in the core.

[0019] In some embodiments, T1 is 800℃-960℃, and the holding time of the first sintering treatment is 10h-15h. Thus, it is helpful to form primary particles with smaller average particle size and larger aspect ratio in the core.

[0020] In some embodiments, T2 is 850℃-1000℃, and the holding time of the second sintering treatment is 8h-10h. Thus, it is helpful to form primary particles with larger average particle size and smaller aspect ratio in the coating layer.

[0021] In some embodiments, the nickel source, the iron source, and the manganese source each independently comprises at least one of a sulfate, a chloride, and an acetate of the corresponding element; and / or, the complexing agent comprises at least one of ammonia, sodium oxalate, sodium citrate, and ethylenediaminetetraacetic acid; and / or, the precipitant comprises at least one of sodium hydroxide and sodium carbonate. In this way, the raw materials are abundant in source, less polluting to the environment, and help to reduce manufacturing costs.

[0022] In some embodiments, the sodium source comprises at least one of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium sulfate, sodium chloride, sodium fluoride, and sodium sulfide; and / or, the M source comprises at least one of a sulfate, a carbonate, an oxalate, a phosphate, a nitrate, and a chloride of the M element; and / or, the N source comprises at least one of an oxide, a sulfide, a hydroxide, an oxyacid, a carbonate, an oxalate, a phosphide, and a hydroxyl oxide of the N element. In this way, the raw materials are abundant in source, less polluting to the environment, and help to reduce manufacturing costs.

[0023] In a third aspect of the present application, a positive electrode tab is provided, comprising a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises the aforementioned positive electrode active material or is prepared by the aforementioned method. In this way, the positive electrode tab has all the features and advantages of the aforementioned positive electrode active material and the method for preparing the same, which will not be repeated here.

[0024] In a fourth aspect of the present application, a sodium-ion battery is provided, comprising the aforementioned positive electrode tab. In this way, the sodium-ion battery has all the features and advantages of the aforementioned positive electrode tab, which will not be repeated here. BRIEF DESCRIPTION OF DRAWINGS

[0025] The above and / or additional aspects and advantages of the present application will become apparent and be readily appreciated from the following description, including the appended drawings, wherein:

[0026] Figure 1 Scanning electron microscope image of the positive electrode active material of Example 1 of the present application.

[0027] Figure 2 Scanning electron microscope image of the cross-section of the positive electrode active material of Example 1 of the present application.

[0028] Figure 3 Scanning electron microscope image of the positive electrode active material of Comparative Example 1 of the present application.

[0029] Figure 4 Scanning electron microscope image of the cross-section of the positive electrode active material of Comparative Example 1 of the present application.

[0030] BRIEF DESCRIPTION OF DRAWINGS

[0031] Core 10; cladding layer 20. DETAILED DESCRIPTION

[0032] Embodiments of the present application are described in detail below with reference to the attached drawings. Examples of the embodiments are illustrated in the drawings, but unnecessary detailed descriptions will be omitted. For example, detailed descriptions of well-known matters, repeated descriptions of substantially identical structures will be omitted. This is to avoid unnecessarily lengthy descriptions that make the following description difficult to understand, 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 recited in the claims.

[0033] Unless otherwise defined, all technical and scientific terms used in the present application have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used in the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application; unless otherwise specified, the numerical values of the parameters mentioned in the present application can be measured by various measuring methods commonly used in the art (for example, tests can be performed according to the methods given in the embodiments of the present application).

[0034] The terms "comprise" and "have" and any variations thereof used in the specification and claims of the present application are open-ended expressions, i.e., include the stated features but do not exclude other features.

[0035] In the description of the present application, all numbers disclosed herein are approximate. Each numerical value, however, has a tolerance of less than about 10% or 5%, 1%, 2%, 3%, 4%, or 5% of the stated value, unless otherwise indicated.

[0036] The ranges disclosed herein are intended to be "open" ranges, i.e., the upper and lower limits of the range are not included. The ranges are also intended to include any and all sub-ranges of the range, i.e., all combinations of any two of the range limits, unless the context clearly dictates otherwise. For example, a range of "0-10" is intended to include any and all sub-ranges, e.g., 1-10, 2-10, 3-10, etc. of the same integers within the specified ranges and any other subset thereof. Unless otherwise stated, the "a-b" range format means "greater than or equal to a, and less than or equal to b". The range format of "a-b" means "greater than a, and less than b". The terms "comprises", "comprising", "includes", "including", "has", "having" and the like can be used in the above description and claims. Each of these terms shall not be construed as setting forth a complete description of the application before the application is ready to be examined and prior art is searched by the patent office. The terms "comprises", "comprising", "includes", "including", "has", "having" and the like are not used in reference to the terms "consisting of" and / or "consisting essentially of" to permit the inclusion of additional integers or components thereof without

[0037] In the description of the application, it should be understood that the terms "thickness", "radial", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, only for the purpose of facilitating the description of the application and simplifying the description, and do not indicate or imply that the indicated structure must have a particular orientation, therefore it cannot be understood as a limitation of the application.

[0038] In the description of the application, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. "First feature", "second feature" can include one or more of the features.

[0039] In the description of the application, "A and / or B" can include any one of the case of A alone, the case of B alone, the case of A and B, where A, B are only for example, which can be any technical feature connected by "and / or" in the application.

[0040] In the present application, the order of writing 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. If not specifically stated, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method comprises steps (a) and (b), which means that the method can comprise sequentially performed steps (a) and (b), or sequentially performed steps (b) and (a). For example, the method also comprises step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0041] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0042] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0043] In order to fully exert the capacity of the positive active material and meet the demand of high energy density as much as possible, sodium ion batteries need to be charged and discharged at a high upper limit voltage, but the stability of the positive active material is usually poor at high voltage. The positive active material in the related art is usually a secondary particle formed by the accumulation of primary particles with consistent particle size, and the primary particles on the surface and inside the secondary particle have no obvious difference in particle size, aspect ratio, etc. When the particle size of the primary particles forming the secondary particle is small, the specific surface area of the positive active material is large, the contact area with the electrolyte is larger, and the transport path of sodium ions is shorter, which will exhibit higher capacity release under the same charge and discharge conditions. However, the larger specific surface area will increase the area of side reactions between the positive active material and the electrolyte, resulting in poor cycle performance of the positive active material at high voltage. When the particle size of the primary particles forming the secondary particle is large, the specific surface area of the positive active material is small, and the structural stability at high voltage can be improved, but it will also lead to poor capacity release of the positive active material. That is, when the particle size of the primary particles in the positive active material is large or small, it will seriously restrict the application of the positive active material in the field of high energy density.

[0044] In the present application, the ion transmission path of the primary particles with smaller average particle size and larger aspect ratio in the core is shorter, and the ion transmission channels are more, which can effectively improve the capacity of the positive active material. At the same time, the primary particles with larger average particle size and smaller aspect ratio in the coating layer have similar structure to single crystal materials, and the structure stability is higher and the contact area with the electrolyte is small, which can effectively improve the stability of the primary particles in the core during the charge and discharge cycle process. By combining the core with smaller average particle size and larger aspect ratio of the primary particles with the coating layer with larger average particle size and smaller aspect ratio of the primary particles, the problems of structure instability of the positive active material in the related art during the high voltage cycle process, easy particle rupture, poor cycle performance and the like can be effectively overcome under the premise of meeting the demand of high gram capacity.

[0045] In the first aspect of the present application, the present application provides a positive active material, referring to Figure 1 and Figure 2 , comprising: a core 10, the aspect ratio of the primary particles in the core 10 is L1, and the average particle size of the primary particles in the core 10 is D1; a coating layer 20, the coating layer 20 is located on at least part of the surface of the core 10, the aspect ratio of the primary particles in the coating layer 20 is L2, and the average particle size of the primary particles in the coating layer 20 is D2, L1 / L2>1, D2 / D1>1. The average particle size of the primary particles in the core 10 is smaller and the aspect ratio is larger, and the average particle size of the primary particles in the coating layer 20 is larger and the aspect ratio is smaller, so that the primary particles in the core 10 can better exert their own capacity, and the primary particles in the coating layer 20 can effectively improve the structural stability, so that the positive active material has better capacity and higher structural stability during the charge and discharge cycle process.

[0046] As an example, referring to Figure 1 and Figure 2 , the positive active material can be a secondary spherical particle with core-shell structure, the core 10 of which is composed of primary particles with smaller average particle size, and the coating layer 20 is formed by surrounding arrangement of primary particles with larger average particle size.

[0047] The aspect ratio of the primary particles in the selected region can be obtained by counting the aspect ratios of all primary particles of a single positive electrode active material particle in the selected region and calculating the average value. As an example, the aspect ratio of the primary particles in the present application can be tested by the following method: cutting the positive electrode active material using an ion milling device to obtain a cross-sectional structure of the positive electrode active material; and testing the cross-sectional structure of the positive electrode active material using a scanning electron microscope (e.g., Hitachi S-4800) to obtain an electron microscope image. Subsequently, the particle aspect ratio analysis module in the lithium battery material intelligent image analysis system software (software vendor: Oupotong (China) Co., Ltd., version number: Metis Vision 1.8326) can be used to obtain the aspect ratio.

[0048] In the present application, the aspect ratio refers to the ratio of the maximum dimension of the primary particle in the length direction to the maximum dimension of the primary particle in the width direction, wherein the maximum dimension of the primary particle in the length direction is greater than the maximum dimension of the primary particle in the width direction, and the length direction of the primary particle is perpendicular to the width direction.

[0049] Average particle size D p The average particle size D of the primary particles in the selected region can be obtained by calculating the average value of the longest diagonal D1 and the shortest diagonal D2 of the primary particles in the selected region in the electron microscope image, i.e. n is the total number of primary particles in the selected region in the electron microscope image. As an example, the average particle size of the primary particles in the present application can be tested by the following method: after obtaining the electron microscope image of the cross-sectional structure of the positive electrode active material by scanning electron microscopy, the particle size analysis module in the lithium battery material intelligent image analysis system software (software vendor: Oupotong (China) Co., Ltd., version number: Metis Vision 1.8326) is used to obtain the average particle size.

[0050] In some embodiments, 1.5≤L1 / L2≤4.2.

[0051] As an example, L1 / L2 can be 1.5, 2, 2.5, 3, 3.5, 4, or 4.2.

[0052] When L1 / L2 is within the aforementioned range, the aspect ratio of the primary particles in the core is large, which can provide an excellent ion transport channel and is beneficial to the capacity development of the positive electrode active material.

[0053] In some embodiments, L1 is 3-5. In this way, the ionic conductivity of the primary particles in the core is high.

[0054] As an example, L1 can be 3, 3.5, 4, 4.5, or 5.

[0055] When L1 is within the foregoing range, the ion transport path of sodium ions between the center and the surface of the primary particles is short, the transport channels are more, and the embedding and extraction of sodium ions in the charging and discharging process are facilitated, thereby improving the capacity of the core.

[0056] In some embodiments, L2 is 1-3. In this way, the structural stability of the primary particles in the coating layer is better.

[0057] For example, L2 can be 1, 1.5, 2, 2.5 or 3.

[0058] In the charging and discharging process of the battery, the positive active material will change in volume. When L2 is within the foregoing range, the size of the primary particles in the coating layer in each direction is close, which can effectively alleviate the stress of the positive active material in the charging and discharging process due to the volume change, reduce the cracks caused by stress concentration, and thus improve the structural stability of the positive active material in the charging and discharging process.

[0059] In some embodiments, 3≤D2 / D1≤13; optionally, 4≤D2 / D1≤10. In this way, the structural stability of the positive active material is better.

[0060] For example, D2 / D1 can be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or 13.

[0061] When D2 / D1 is within the foregoing range, the average particle size of the primary particles in the coating layer is large, and the primary particles in the coating layer are close to single crystal particles, which can improve the structural stability of the positive active material.

[0062] In some embodiments, D1 is 0.08-0.3 μm. In this way, the specific surface area of the primary particles in the core is large, and the capacity is better.

[0063] For example, D1 can be 0.08 μm, 0.12 μm, 0.16 μm, 0.20 μm, 0.24 μm, 0.28 μm or 0.3 μm.

[0064] When D1 is within the foregoing range, the specific surface area of the primary particles is relatively large, and the area in contact with the electrolyte is more, which is helpful for the rapid transport of sodium ions at the two-phase interface.

[0065] In some embodiments, D2 is 0.6-1.5 μm. In this way, the structural stability of the primary particles in the coating layer is higher.

[0066] For example, D2 can be 0.6 μm, 0.8 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm or 1.5 μm.

[0067] When D2 is within the foregoing range, the specific surface area of the primary particles is relatively small, and the surface area in contact with the electrolyte is correspondingly reduced, which helps to reduce the occurrence of unnecessary interface side reactions and improve the structural stability of the positive electrode active material.

[0068] In some embodiments, the porosity of the core is Q1, the porosity of the coating layer is Q2, and Q1 / Q2>1.

[0069] When the porosity of the core is relatively large, the primary particles in the core are fully infiltrated by the electrolyte, which improves the capacity performance; when the porosity of the coating layer is relatively small, the contact area between the outer surface of the positive electrode active material and the electrolyte can be effectively reduced, which improves the structural stability of the outer surface of the positive electrode active material.

[0070] The porosity of the selected area can be obtained by counting the ratio of the pore area between the primary particles of a single positive electrode active material particle in the selected area to the area of the selected area. As an example, the porosity in the present application can be tested by the following method: after obtaining the cross-sectional structure electron microscope photo of the positive electrode active material by scanning electron microscope, the cross-sectional pore analysis module in the lithium battery material intelligent image analysis system software (software vendor: Oupotong (China) Co., Ltd., version number: Metis Vision1.8326) is used to obtain the porosity.

[0071] In some embodiments, 2≤Q1 / Q2≤7; optionally, 2≤Q1 / Q2≤5. In this way, the positive electrode active material has relatively good capacity performance and high structural stability.

[0072] As an example, Q1 / Q2 can be 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, or 7.

[0073] In some embodiments, 4%≤Q1≤8%. In this way, the primary particles in the core can be fully infiltrated by the electrolyte, which improves the capacity performance.

[0074] As an example, Q1 can be 4%, 5%, 6%, 7%, or 8%.

[0075] In some embodiments, 1%≤Q2≤2.5%. In this way, the contact area between the primary particles in the coating layer and the electrolyte is relatively low, which can effectively improve the structural stability of the positive electrode active material.

[0076] As an example, Q2 can be 1%, 1.5%, 2%, or 2.5%.

[0077] In some embodiments, the radius of the positive electrode active material is R, and 3μm≤R≤7μm. In this way, the capacity performance and structural stability of the positive electrode active material are both relatively good.

[0078] As an example, R can be 3 pm, 3.5 pm, 4 pm, 4.5 pm, 5 pm, 5.5 pm, 6 pm, 6.5 pm, or 7 pm.

[0079] When the radius of the positive electrode active material is within the aforementioned range, the radius of the positive electrode active material is moderate, and the transport path of sodium ions is short. The positive electrode active material is in sufficient contact with the electrolyte, and has less side reactions with the electrolyte, and is less likely to have structural rupture at high voltage, and has better cycle performance.

[0080] It should be noted that the radius of the positive electrode active material is the average of the maximum distance from the center of the positive electrode active material radially outward and the minimum distance from the center of the positive electrode active material radially outward.

[0081] As an example, the radius R of the positive electrode active material in this application can be tested by the following method: After obtaining the cross-sectional structure electron microscope photo of the positive electrode active material by scanning electron microscope, the distance measurement module in the lithium battery material intelligent image analysis system software (software vendor: Oupotong (China) Co., Ltd., version number: Metis Vision 1.8326) is used to obtain the distance measurement module.

[0082] In some embodiments, the thickness d of the coating layer is 0-0.4R, and optionally, 0-0.35R. In this way, the coating layer can effectively improve the structural stability of the positive electrode active material, and has less impact on the capacity of the positive electrode active material.

[0083] As an example, the thickness d of the coating layer can be 0.05R, 0.1R, 0.15R, 0.2R, 0.25R, 0.3R, 0.35R, or 0.4R.

[0084] When the thickness of the coating layer is within the aforementioned range, the thickness of the coating layer is moderate, which can effectively improve the structural stability of the positive electrode active material at high voltage, and has less impact on the capacity of the positive electrode active material, and helps the inner core with a large proportion to fully play the capacity.

[0085] As an example, the coating layer can be Figure 2 the annular area shown by the dashed line.

[0086] As an example, with reference to Figure 2 The inner boundary position of the coating layer can be determined according to the difference in the aspect ratio, and then the thickness d of the coating layer is determined by the distance from the outer surface of the secondary particle radially inward to the inner boundary of the coating layer.

[0087] As an example, the thickness d of the coating layer in this application can be obtained by the following method: after obtaining the cross-sectional structure electron microscope image of the positive electrode active material by scanning electron microscopy, the distance measurement module in the intelligent image analysis system software for lithium battery materials (software vendor: Opto-Tech (China) Co., Ltd., version number: Metis Vision 1.8326) is used.

[0088] In some embodiments, the positive electrode active material satisfies the chemical formula: Na a (Ni 1-b-c-d Fe b Mn c M d )N e O 2-f Wherein, M includes at least one of V, Ta, La, Al, Ce, Y, Ba, Zr, Ca, W, Nb, Si, Mo, F, P, and Ti; N includes at least one of Cu, Cr, Mg, Sr, Fe, Ni, Mn, Sn, Zn, B, and Co; 0.95≤a≤1.15, 0<b≤0.50, 0<c≤0.70, 0<d≤0.15, 0<e≤0.15; and f satisfies one of the following conditions: when both M and N are cations, f=0; when both M and N are anions, f=d+e; when M is an anion and N is a cation, f=d; and when N is an anion and M is a cation, f=e. Therefore, by using different elements for doping, positive electrode active materials that meet different performance requirements can be obtained.

[0089] Doping with element M can stabilize the structure of the transition metal layer in layered metal oxides and improve the structural stability of the positive electrode active material under high voltage.

[0090] By doping with nitrogen, a stable protective layer can be formed on the surface of the positive electrode active material, which can effectively suppress the side reactions between the positive electrode active material and the electrolyte and improve cycle performance.

[0091] In a second aspect, this application proposes a method for preparing the aforementioned positive electrode active material. By adding raw materials in stages and employing a strategy of using a second sintering temperature slightly higher than the first sintering temperature, the growth process of primary particles in the core and coating layer of the positive electrode active material is precisely controlled. This results in a smaller average particle size and a larger aspect ratio for the primary particles in the core, and a larger average particle size and a smaller aspect ratio for the primary particles in the coating layer. The method for preparing the positive electrode active material in this application is simple, efficient, and suitable for large-scale production. Specifically, the method includes:

[0092] S100: Nickel source, iron source, manganese source, complexing agent, and precipitant are mixed to obtain a precursor slurry, which is then dried to obtain the precursor material.

[0093] In some embodiments, the step of mixing the nickel source, the iron source, the manganese source, the complexing agent and the precipitant to perform a co-precipitation reaction to obtain a precursor slurry, and then aging, filtering, washing and drying the precursor slurry to obtain the precursor material.

[0094] In some embodiments, the nickel source, the iron source and the manganese source each independently comprises at least one of a sulfate, a chloride, an acetate of the corresponding element. In this way, the raw materials are abundant in source, less polluting to the environment, and help to reduce manufacturing costs.

[0095] In some embodiments, the complexing agent comprises at least one of ammonia, sodium oxalate, sodium citrate, ethylenediaminetetraacetic acid. In this way, the raw materials are abundant in source, less polluting to the environment, and can be adapted to conventional process lines, helping to reduce manufacturing costs.

[0096] In some embodiments, the precipitant comprises at least one of sodium hydroxide, sodium carbonate. In this way, the raw materials are abundant in source, less polluting to the environment, and can be adapted to conventional process lines, helping to reduce manufacturing costs.

[0097] As an example, the feeding ratio of the nickel source, the iron source and the manganese source can be calculated according to the stoichiometric ratio of the corresponding elements in the target product. The complexing agent and the precipitant can be reasonably proportioned according to the amount of the nickel source, the iron source and the manganese source.

[0098] S200: mixing the precursor material, a sodium source and an M source, and performing a first sintering treatment to obtain an intermediate positive electrode active material, wherein the temperature of the first sintering treatment is T1

[0099] In some embodiments, the step of mixing the precursor material prepared in the foregoing with a sodium source and an M source, and then performing a first sintering treatment, and crushing and sieving to obtain an intermediate positive electrode active material.

[0100] By adding the M source as a dopant in this step, in addition to achieving the effects of uniform bulk doping and stabilizing the structure of the positive electrode active material, it also has the effect of inhibiting the continued growth of primary grains in the inner core, helping the primary particles in the inner core to have a smaller grain size.

[0101] In some embodiments, T1 is 800-960°C, and the holding time of the first sintering treatment is 10-15h. In this way, it helps to form primary particles with a smaller average particle size and a larger aspect ratio in the inner core.

[0102] As an example, T1 can be 800°C, 840°C, 880°C, 920°C or 960°C.

[0103] As an example, the holding time of the first sintering treatment can be 10h, 11h, 12h, 13h, 14h or 15h.

[0104] In some embodiments, the sodium source comprises at least one of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium sulfate, sodium chloride, sodium fluoride, sodium sulfide.

[0105] In some embodiments, the M source comprises at least one of sulfate, carbonate, oxalate, phosphate, nitrate, chloride of the M element.

[0106] As an example, the feeding ratio of the precursor material, the sodium source, and the M source can be calculated according to the stoichiometric ratio of the corresponding elements of the target product.

[0107] S300: mixing the positive electrode active material intermediate with an N source, and performing a second sintering treatment to obtain a positive electrode active material, wherein the temperature of the second sintering treatment is T2, and T1

[0108] In some embodiments, the positive electrode active material intermediate prepared in the foregoing step is mixed with an N source, and after a second sintering treatment, crushing and sieving, a positive electrode active material is obtained.

[0109] During the second sintering treatment, the N source acts as a coating agent to promote the growth and fusion of primary particles. The N source mainly exists on the surface of the particles and has a limited content, which can promote the melting and growth of the surface primary particles during the second sintering treatment, while having little effect on the growth of the primary particles in the core. As a result, the primary particles on the surface layer can further grow due to the promoting effect, forming a primary particle coating layer with a larger size, which has a morphology similar to a single crystal, a more stable structure, and effectively reduces the side reaction with the electrolyte, providing structural support for the positive electrode active material at high voltage. The primary particles in the core are inhibited by the M source, and even at the temperature of the second sintering treatment, they still grow slowly and can still maintain a small particle size. In addition, when the second sintering temperature is slightly higher than the first sintering temperature, it also helps to repair the defects of the crystal grains inside the positive electrode active material, further improving the performance.

[0110] In some embodiments, T2-T1<200℃; optionally, 10℃

[0111] As an example, T2-T1may be 10℃, 30℃, 50℃, 70℃, 90℃, 110℃, 120℃, 130℃, 150℃, 170℃, 190℃ or 200℃.

[0112] In some embodiments, T2 is 850℃-1000℃, and the holding time of the second sintering treatment is 8h-10h. In this way, it is helpful to form primary particles with a larger average particle size and a smaller aspect ratio in the coating layer.

[0113] As an example, T2 can be 850℃, 900℃, 950℃, or 1000℃.

[0114] As an example, the holding time of the second sintering treatment can be 8h, 8.5h, 9h, 9.5h, or 10h.

[0115] In some embodiments, the N source includes at least one of an oxide, a sulfide, a hydroxide, an oxyacid, a carbonate, an oxalate, a phosphide, and a hydroxyl oxide of the N element. In this way, the raw material source is abundant, the environmental pollution is less, and it is helpful to reduce the manufacturing cost.

[0116] As an example, the feeding ratio of the positive electrode active material intermediate to the N source can be calculated according to the stoichiometric ratio of the corresponding elements of the target product.

[0117] In a third aspect of the present application, the present application provides a positive electrode tab, which includes a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector, and the positive electrode active material layer includes the aforementioned positive electrode active material or the positive electrode active material prepared by the aforementioned method. In this way, the positive electrode tab has all the characteristics and advantages of the aforementioned positive electrode active material and the preparation method thereof, which will not be repeated here.

[0118] In a fourth aspect of the present application, the present application provides a sodium ion battery, which includes the aforementioned positive electrode tab. In this way, the sodium ion battery has all the characteristics and advantages of the aforementioned positive electrode tab, which will not be repeated here.

[0119] The scheme of the present application will be described below through specific examples. It should be noted that the following examples are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application. If the specific technology or condition is not specified in the examples, it is performed according to the technology or condition described in the literature in the art or according to the product instruction. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0120] Example 1

[0121] The nickel source, the iron source, the manganese source, the complexing agent, and the precipitating agent are mixed to perform a co-precipitation reaction to obtain a precursor slurry, and then the precursor slurry is aged, filtered, washed, and dried to obtain a precursor material.

[0122] The precursor material prepared above is mixed with a sodium source and an M source, and then subjected to a first sintering treatment (sintering atmosphere: air), and crushing and sieving to obtain an intermediate positive electrode active material. The sodium source is sodium carbonate, the type of M source, the molar ratio of each raw material, the sintering temperature and the holding time are shown in Table 1.

[0123] The intermediate positive electrode active material prepared above is mixed with an N source, and then subjected to a second sintering treatment (sintering atmosphere: air), and crushing and sieving to obtain a positive electrode active material. The type of N source, the molar ratio of each raw material, the sintering temperature and the holding time are shown in Table 1.

[0124] The differences between the remaining examples, comparative examples and Example 1 are shown in Table 1.

[0125] Table 1

[0126]

[0127] The material composition and structure parameters of the positive electrode active materials prepared in the above examples and comparative examples are shown in Table 2. The structure parameters are obtained by using a lithium battery material intelligent image analysis system software (software vendor: Oupotong (China) Co., Ltd., version number: Metis Vision 1.8326).

[0128] Table 2

[0129]

[0130] The positive electrode active materials in the above examples and comparative examples are assembled into CR2032 button cells, and the assembly method is as follows:

[0131] The above positive electrode active material, conductive agent acetylene black and binder PVDF are mixed in a mass ratio of 95:3:2 in N-methyl pyrrolidone to form a uniform slurry, the slurry is coated on an aluminum foil and dried in an oven at 120°C for 12h, and then stamped into a positive electrode sheet with a diameter of 12mm and a thickness of 120mm using a pressure of 100MPa, wherein the loading amount of the positive electrode active material is 15mg / cm 2 .

[0132] The aforementioned positive electrode sheet, the separator, the negative electrode sheet and the electrolyte were assembled into a CR2032 button cell in an argon-filled glove box. The negative electrode sheet used a metal sodium sheet; the separator used a polypropylene film; the solvent of the electrolyte was an equal volume mixture of ethylene carbonate, diethyl carbonate and vinylene carbonate; the electrolyte salt in the electrolyte was NaPF6; and the concentration of NaPF6 in the electrolyte was 1 mol / L.

[0133] The button cell assembled as described above was subjected to the following tests, and the test results are shown in Table 3.

[0134] Capacity rate test: 25℃, 2.0V-4.1V, 0.1C cycle for 2 weeks, and then 0.2C, 0.33C, 0.5C and 1C respectively for 1 cycle, wherein the 0.1C first discharge specific capacity was the discharge specific capacity of the first week cycle of the button cell.

[0135] 80 cycle capacity retention rate test: after the material was subjected to the above rate test, 80 weeks of cycle charge-discharge test was continued at a rate of 1C, and the capacity retention rate was obtained by dividing the discharge capacity of the last cycle by the discharge capacity of the first cycle.

[0136] Table 3 Electrochemical performance of positive electrode active material

[0137]

[0138] Figure 1 FIG. 1 is a scanning electron microscope photograph of the positive electrode active material in Example 1, Figure 2 FIG. 2 is a scanning electron microscope photograph of the positive electrode active material in Example 2, Figure 1 FIG. 3 is a cross-sectional electron microscope photograph of the particles in Example 2. Figure 3 FIG. 4 is a scanning electron microscope photograph of the positive electrode active material in Comparative Example 1, Figure 4 FIG. 5 is a scanning electron microscope photograph of the positive electrode active material in Comparative Example 2, Figure 3 FIG. 6 is a cross-sectional electron microscope photograph of the particles in Comparative Example 2.

[0139] The test results show that when L1 / L2>1 and D2 / D1>1, the primary particles in the core 10 can better exert their own capacity, and the primary particles in the coating layer 20 can effectively improve the structural stability, so that the positive electrode active material has both better capacity exertion and higher structural stability during the charge-discharge cycle. Under the premise of meeting the demand for high gram capacity, the problems of structural instability of the positive electrode active material in the related art during high-voltage cycle, easy particle breakage and poor cycle performance are effectively overcome, and the battery performance is improved.

[0140] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. Furthermore, other modes constructed by applying various modifications to the embodiments, or by combining part of the configurations of the embodiments, which can be conceived by those skilled in the art, without departing from the spirit of the present application, are also included in the scope of the present application.

Claims

1. A positive electrode active material, characterized in that, include: The core, wherein the aspect ratio of the primary particles in the core is L1, and the average particle size of the primary particles in the core is D1; A coating layer is located on at least a portion of the surface of the core. The aspect ratio of the primary particles in the coating layer is L2, and the average particle size of the primary particles in the coating layer is D2, where L1 / L2 > 1, D2 / D1 > 1, L1 is 3-5, L2 is 1-3, D1 is 0.08 μm-0.3 μm, and D2 is 0.6 μm-1.5 μm. The radius of the positive electrode active material is R, where 3μm ≤ R ≤ 7μm, and the thickness of the coating layer is 0.05R-0.4R. The positive electrode active material satisfies the chemical formula: Na a (Ni 1-b-c-d Fe b Mn c M d )N e O 2-f ,in, M includes at least one of V, Ta, La, Al, Ce, Y, Ba, Zr, Ca, W, Nb, Si, Mo, F, P, and Ti; N includes at least one of Cu, Cr, Mg, Sr, Fe, Ni, Mn, Sn, Zn, B, and Co; 0.95 ≤ a ≤ 1.15, 0 < b ≤ 0.50, 0 < c ≤ 0.70, 0 < d ≤ 0.15, 0 < e ≤ 0.15; f satisfies one of the following conditions: When both M and N are cations, f = 0; When both M and N are anions, f = d + e; When M is an anion and N is a cation, f = d; When N is an anion and M is a cation, f = e.

2. The positive electrode active material according to claim 1, characterized in that, 1.5≤L1 / L2≤4.

2.

3. The positive electrode active material according to claim 1, characterized in that, 3≤D2 / D1≤13.

4. The positive electrode active material according to claim 3, characterized in that, 4≤D2 / D1≤10.

5. The positive electrode active material according to claim 1, characterized in that, The porosity of the core is Q1, and the porosity of the coating layer is Q2, where Q1 / Q2 > 1.

6. The positive electrode active material according to claim 5, characterized in that, 2≤Q1 / Q2≤7.

7. The positive electrode active material according to claim 6, characterized in that, 2≤Q1 / Q2≤5.

8. The positive electrode active material according to any one of claims 5-7, characterized in that, 4%≤Q1≤8%。 9. The positive electrode active material according to claim 8, characterized in that, 1%≤Q2≤2.5%。 10. The positive electrode active material according to claim 1, characterized in that, The thickness of the coating layer is 0.05R-0.35R.

11. A method for preparing the positive electrode active material according to any one of claims 1-10, characterized in that, include: Nickel source, iron source, manganese source, complexing agent and precipitant are mixed to obtain precursor slurry, and then dried to obtain precursor material; The precursor material, sodium source, and M source are mixed and subjected to a first sintering treatment to obtain a positive electrode active material intermediate, wherein the temperature of the first sintering treatment is T1. The positive electrode active material intermediate is mixed with an N source and subjected to a second sintering treatment to obtain a positive electrode active material, wherein the temperature of the second sintering treatment is T2, and T1 < T2.

12. The method according to claim 11, characterized in that, T2-T1 < 200℃.

13. The method according to claim 12, characterized in that, 10℃<T2-T1<120℃.

14. The method according to claim 13, characterized in that, T1 is 800℃-960℃, and the holding time for the first sintering treatment is 10h-15h.

15. The method according to claim 13 or 14, characterized in that, T2 is 850℃-1000℃, and the holding time for the second sintering treatment is 8h-10h.

16. The method according to claim 11, characterized in that, The nickel source, iron source, and manganese source each independently include at least one of the sulfate, chloride, and acetate of the corresponding element; and / or, The complexing agent includes at least one of ammonia, sodium oxalate, sodium citrate, and ethylenediaminetetraacetic acid; and / or, The precipitant includes at least one of sodium hydroxide and sodium carbonate.

17. The method according to claim 11 or 16, characterized in that, The sodium source includes at least one selected from sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium sulfate, sodium chloride, sodium fluoride, and sodium sulfide; and / or, The M source includes at least one of the following: sulfate, carbonate, oxalate, phosphate, nitrate, and chloride corresponding to element M; and / or, The N source includes at least one of the following: oxides, sulfides, hydroxides, oxyacids, carbonates, oxalates, phosphides, and hydroxyoxides corresponding to the element N.

18. A positive electrode plate, characterized in that, It includes a positive current collector and a positive active material layer located on at least one side of the positive current collector, wherein the positive active material layer includes the positive active material according to any one of claims 1-10 or the positive active material prepared by the method according to any one of claims 11-17.

19. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 18.

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