Positive electrode active material and preparation method thereof, positive electrode plate, sodium secondary battery and electric equipment

Abstract

The invention relates to the technical field of secondary batteries, and provides a positive active material and a preparation method thereof, a positive pole piece, a sodium secondary battery and electric equipment. The chemical formula of the positive electrode active material comprises Na Ni < 1-b-c-d > Fe Mn < c > M < d > O < 2 >; wherein 0.95 < = a < = 1.10, 0.03 < = b < = 0.50, 0.03 < = c < = 0.50, 0lt; m comprises one or more of Ta, Cr, La, Al, Ce, Y, Mg, Sr, Zr, Ca, Zn, B, W, Nb, Si, Mo, F, P, Co, Li, Ti and Cu; the crystallite boundary density of the positive electrode active material is N, N = KS / KX, KS is the mean value of the sub-grain size of the positive electrode active material obtained through an EBSD test, KX is the crystallite size of the positive electrode active material, and N is larger than or equal to 8.0 and smaller than or equal to 28.0. The battery containing the positive electrode active material has excellent cycle performance and rate capability.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly to a positive electrode active material, a preparation method thereof, a positive electrode sheet, a sodium secondary battery, and an electrical device. Background Art

[0002] With the transformation of the global energy structure and the booming development of the new energy vehicle industry, the demand for high-performance energy storage technologies has become increasingly urgent. Among many energy storage solutions, lithium-ion batteries have received extensive attention due to their advantages such as high energy density, long cycle life, and green safety. Currently, they have been widely used in many fields such as large-scale commercial energy storage, electric vehicles, and electronic digital devices, profoundly affecting all aspects of human production and life.

[0003] However, with the continuous growth of the lithium-ion battery market, problems such as the shortage and soaring prices of metal elements such as lithium and cobalt in raw materials have gradually emerged, seriously hindering its development. In this context, the sodium element in sodium-ion batteries has quickly attracted wide attention due to its advantages such as rich resources, low price, and wide distribution. Sodium-ion batteries are considered promising to become substitutes for lithium-ion batteries. Therefore, sodium-ion batteries have also迎来前所未有的发展机遇,钠离子电池产业在技术突破、市场规模扩张、竞争格局演化及政策扶持等多重因素驱动下,正逐步从实验室走向规模化应用。但是,目前钠离子电池仍存在循环寿命较差,阻抗较高,倍率性能需要提升等问题亟待需要解决。 Summary of the Invention

[0004] The present invention aims to at least partly solve one of the technical problems in the related technologies. For this reason, an object of the present invention is to provide a positive electrode active material, a preparation method thereof, a positive electrode sheet, a sodium secondary battery, and an electrical device, and the positive electrode active material has excellent cycle performance and rate performance.

[0005] For this reason, in the first aspect of the present invention, a positive electrode active material is provided, and the chemical formula of the positive electrode active material includes: Na a Ni 1-b-c-d Fe b Mn c M d O2; wherein, 0.95 ≤ a ≤ 1.10, 0.03 ≤ b ≤ 0.50, 0.03 ≤ c ≤ 0.50, 0 < d ≤ 0.10, and M includes one or more of Ta, Cr, La, Al, Ce, Y, Mg, Sr, Zr, Ca, Zn, B, W, Nb, Si, Mo, F, P, Co, Li, Ti, Cu; the microcrystalline grain boundary density of the positive electrode active material is N, and N = K S / K X , K SThe average value of the sub-grain size obtained by EBSD testing for the positive electrode active material, K X is the crystallite size of the positive electrode active material, where 8.0 ≤ N ≤ 28.0.

[0006] For the positive electrode active material according to an embodiment of the present invention, by controlling the crystallite boundary density N of the positive electrode active material, it is possible to avoid the single diffusion path of sodium ions in the grains caused by too small crystallite boundary density in the positive electrode active material, which basically completely depends on bulk diffusion. Increase the diffusion path of sodium ions in the positive electrode active material to ensure that the positive electrode active material has excellent rate performance; it is also possible to avoid a large number of defects occupying sodium sites caused by too large crystallite boundary density in the positive electrode active material, resulting in local irreversible phase transformation, loss of part of the capacity, and reduction of the initial capacity. Avoid the occurrence of voltage decay and capacity decline and poor cycle performance caused by high defect density regions in the positive electrode active material becoming the starting point of harmful phase transformation during long-term cycling. In summary, the positive electrode active material proposed in this application has excellent cycle performance and rate performance at the same time.

[0007] In some embodiments of the present invention, 10.0 ≤ N ≤ 24.5; and / or, 0.15 ≤ b ≤ 0.40; and / or, 0.15 ≤ c ≤ 0.40; and / or, 0 < d ≤ 0.06; and / or, M includes one or more of Al, Mg, Zr, Sr, Ca, Zn, Nb, Ti, Mo.

[0008] In some embodiments of the present invention, 500 nm ≤ K S ≤ 1000 nm, optionally, 650 nm ≤ K S ≤ 900 nm; and / or, 38 nm ≤ K X ≤ 59 nm, optionally, 42 nm ≤ K X ≤ 55 nm.

[0009] In some embodiments of the present invention, K X =(K (003) +K (104) ) / 2, where K (003) and K (104) are each calculated by the following formula, K (003) or K (104) =Kλ / (βcosθ), where K is the Scherrer constant, λ is the X-ray wavelength used for XRD testing of the positive electrode active material, β is the full width at half maximum of the (003) diffraction peak or (104) diffraction peak in the XRD pattern of the positive electrode active material, and θ is the Bragg angle of the (003) diffraction peak or (104) diffraction peak in the XRD pattern of the positive electrode active material, that is, the θ value corresponding to the diffraction peak.

[0010] In some embodiments of the present invention, the 2θ value of the (003) diffraction peak in the XRD pattern of the positive electrode active material is 16.1°-17.1°; and / or, the 2θ value of the (104) diffraction peak in the XRD pattern of the positive electrode active material is 41.0°-42.0°.

[0011] In some embodiments of the present invention, the positive electrode active material comprises single crystal particles, wherein the proportion of small-angle grain boundaries in the cross-section of the single crystal particles is G, 10%≤G≤22%, optionally 12%≤G≤18%; and / or, the positive electrode active material comprises single crystal particles, wherein the proportion of large-angle grain boundaries in the cross-section of the single crystal particles is L, 78%≤L≤90%, optionally 82%≤L≤88%; and / or, 3.5≤L / G≤9.0, optionally 4.5≤L / G≤7.3.

[0012] In some embodiments of the present invention, the median particle size of the positive electrode active material is D. 50 3.2μm≤D 50 ≤8.5μm, optionally, 4.0μm≤D 50 ≤7.0μm; and / or, the specific surface area of ​​the positive electrode active material is S, 0.45m². 2 / g≤S≤0.90m 2 / g, optionally, 0.50m 2 / g≤S≤0.75m 2 / g; and / or, the angle of repose of the positive electrode active material is α, α<52.0°, optionally, α≤47.5°.

[0013] In a second aspect, the present invention provides a method for preparing the positive electrode active material described in the first aspect, comprising: mixing a nickel source, an iron source, a manganese source, a complexing agent, and a precipitant to perform a co-precipitation reaction to obtain a slurry; aging, filtering, washing, and drying the slurry to obtain a precursor; and mixing the precursor with a sodium source and an M source, followed by sintering to obtain the positive electrode active material. Therefore, using the above method facilitates the preparation of positive electrode active materials with a micrograin boundary density N within a desired range, enabling the positive electrode active material to simultaneously possess excellent cycle performance and rate performance.

[0014] In some embodiments of the present invention, the pH value of the coprecipitation reaction is 9.5-12.5, optionally 10-12; and / or, the temperature of the coprecipitation reaction is 40℃-65℃, optionally 45℃-60℃.

[0015] In some embodiments of the present invention, the sintering sequentially includes a constant temperature section 1 and a constant temperature section 2. The temperature of the constant temperature section 1 is T1, 650℃≤T1≤930℃, optionally 750℃≤T1≤910℃; and / or, the constant temperature time of the constant temperature section 1 is t1, 5h≤t1≤10h, optionally 6h≤t1≤8h; and / or, the heating rate to the constant temperature section 1 is V1, 2℃ / min≤V1≤8℃ / min, optionally 3℃ / min≤V1≤6℃ / min; and / or, the temperature of the constant temperature section 2 is T2, 980℃≤T2 ≤1080℃, optionally, 1000℃≤T2≤1040℃; and / or, the isothermal time of the isothermal section 2 is t2, 5h≤t2≤15h, optionally, 7h≤t2≤12h; and / or, the heating rate to the isothermal section 2 is V2, 1℃ / min≤V2≤5℃ / min, optionally, 1℃ / min≤V2≤3℃ / min; and / or, 1.1≤T2 / T1≤1.6, optionally, 1.1≤T2 / T1≤1.3; and / or, 0.5≤t2 / t1≤3.0, optionally, 1.1≤t2 / t1≤2.0.

[0016] In a third aspect of the invention, the invention provides a positive electrode sheet comprising the positive active material described in the first aspect, or a positive active material prepared by the method described in the second aspect.

[0017] In a fourth aspect, the present invention provides a sodium secondary battery comprising the positive electrode sheet described in the third aspect.

[0018] In a fifth aspect, the present invention provides an electrical device comprising the sodium secondary battery described in the fourth aspect.

[0019] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0020] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 The grain orientation diagram of the positive electrode active material prepared in Example 1 of the present invention is shown; Figure 2 The XRD pattern of the positive electrode active material prepared in Example 1 of the present invention is shown. Detailed Implementation

[0021] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0022] It should be noted that the terms "first" and "second" are only used for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of such features. Further, in the description of the present invention, unless otherwise specified, the meaning of "plural" is two or more than two.

[0023] In a first aspect of the present invention, there is provided a positive electrode active material, and the chemical formula of the positive electrode active material includes: Na a Ni 1-b-c-d Fe b Mn c M d O2; wherein, 0.95 ≤ a ≤ 1.10, 0.03 ≤ b ≤ 0.50, 0.03 ≤ c ≤ 0.50, 0 < d ≤ 0.10, and M includes one or more of Ta, Cr, La, Al, Ce, Y, Mg, Sr, Zr, Ca, Zn, B, W, Nb, Si, Mo, F, P, Co, Li, Ti, Cu; the density of the microcrystalline grain boundaries of the positive electrode active material is N, and N = K S / K X where K S is the average value of the sub-grain size obtained by EBSD testing of the positive electrode active material, and K X is the microcrystalline size of the positive electrode active material, and 8.0 ≤ N ≤ 28.0.

[0024] As an example, a can be 0.95, 1, 1.05, 1.10 or any range composed of any two of the above values; b can be 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 or any range composed of any two of the above values. According to some other embodiments of the present application, 0.15 ≤ b ≤ 0.40; c can be 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 or any range composed of any two of the above values. According to some other embodiments of the present application, 0.15 ≤ c ≤ 0.40; d can be 0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.1 or any range composed of any two of the above values. According to some other embodiments of the present application, 0 < d ≤ 0.06.

[0025] In the cathode active material of this application, a seemingly complete "single subgrain" in EBSD testing may contain numerous dislocations and fine structures, dividing the subgrain into numerous coherent scattering domains. XRD testing combined with the Scherrer equation can identify these internal differences by utilizing diffraction peak broadening, and distinguish a grain into multiple coherent diffraction domains. N can reflect the average number of actual defects contained within a single subgrain of the cathode active material, the degree of crystal integrity within the grain, and thus evaluate its impact on the cycle life, impedance, and rate performance of the cathode active material.

[0026] A smaller N value indicates that the subgrain observed by EBSD (Oxford Backscatter Diffraction) is almost a perfect single crystal, with no obvious grain boundaries or dislocation cell structures, and extremely high crystal integrity. A larger N value means that the interior of a single subgrain is divided into numerous small coherent scattering domains, and that there are many defects such as dislocations within the grain. This ratio N directly reflects the difference between the microscopic and nanoscopic structures within the sodium electrode active material grains, and the internal structure determines its macroscopic material and electrochemical properties.

[0027] In this embodiment, the micrograin boundary density of the positive electrode active material is controlled to be 8.5 ≤ N ≤ 26.3. On the one hand, by controlling the micrograin boundary density N of the positive electrode active material, the cycle stability of the material can be significantly improved. When the material defect degree N is controlled within the above range, the number of defects in the subgrains observed by EBSD is controllable, the crystal is relatively intact, and there are sufficient sodium storage sites. On the other hand, it has a high initial capacity. Furthermore, because the crystal is relatively intact, the structure is more ordered, the stability is improved, and its phase transition reversibility is better during electrochemical cycling, and the voltage decay is smaller, resulting in excellent cycle capacity retention. At the same time, the appropriate proportion of dislocations and other defects inside can act as an effective "stress buffer" during charging and discharging, preventing the accumulation of stress due to lack of release and the resulting overall grain cracking.

[0028] On the other hand, controlling the micrograin boundary density N of the positive electrode active material can significantly improve the rate performance of the material. When the micrograin boundary density N is controlled within an optimal range, it means that there are a certain number of crystal defects (such as dislocations, substructures, stacking faults, etc.) inside the grains. The irregular atomic arrangement in these defect regions provides a lower-energy diffusion path for sodium ion migration, which is equivalent to building dense defect site diffusion channels outside the bulk diffusion. Sodium ions can quickly enter or exit the grain through these short-range fast diffusion channels, greatly reducing the diffusion resistance of ions. This gives the battery excellent fast charging capability, high capacity retention, and low voltage polarization during high-current charging and discharging.

[0029] Conversely, a lower micrograin boundary density (N) in the positive electrode active material indicates that the subgrains observed by EBSD are close to perfect grains, with complete crystals, sufficient sodium storage sites, and a higher initial capacity. Simultaneously, due to the completeness of the subgrains, structural stability is increased, resulting in better phase transition reversibility during electrochemical cycling and excellent cycle retention. However, because sodium ions diffuse through a single pathway within the grains, almost entirely relying on bulk diffusion, its rate performance is poor.

[0030] If the micrograin boundary density (N) of the cathode active material is too high, it means that there are abundant defects within the subgrains. These numerous defects (such as dislocations and stacking faults) provide countless short-range diffusion paths for sodium ions, which can greatly improve the ionic conductivity of the cathode active material, thus giving it excellent rate performance. However, at the same time, a large number of defects may occupy sodium sites or cause localized irreversible phase transitions, resulting in a loss of capacity and a reduction in initial capacity. During long-term cycling, high defect density regions may become the starting point of harmful phase transitions, leading to voltage decay and capacity reduction, and deteriorating cycle performance.

[0031] In some embodiments of the present invention, 8.0 ≤ N ≤ 28.0. As an example, the value of N can be 8.0, 8.5, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28 or any range of two of the above values. In other embodiments of the present invention, 10.0 ≤ N ≤ 24.5. Thus, the positive electrode active material has both excellent cycle performance and rate performance.

[0032] It is understandable that the average subgrain size K of the positive electrode active material... S The equivalent circle diameter refers to the diameter of a circle with the same projected area as the subgrain. It can be calculated by preparing the positive electrode active material using CP (argon ion section polishing) and then measuring it under an electron microscope using EBSD testing software based on the actual projected area. As an example, the specific operating steps are as follows: An electron beam from an EBSD (Electron Back-Scattered Difference) detector mounted on a scanning electron microscope (SEM) is irradiated onto an inclined sample. Backscattered electrons undergo diffraction, and the resulting diffracted electrons are projected onto the phosphor screen of the EBSD detector, forming a diffraction pattern. The area of ​​subgrains in the cross-section of the positive electrode active material particles is analyzed, and the diameter of a circle with the same area is taken as the subgrain size K. S .

[0033] It is understandable that K mentioned above SThe value can be calculated from 10 electron microscope images with a magnification of 3K obtained from multiple test locations, representing the statistical mean of numerous particles within the observation range. It can reflect the overall actual level of the positive electrode active material.

[0034] The crystallite size K of the positive electrode active material X It can also be understood as the size of the perfect crystal region in the crystal lattice that can produce coherent diffraction, that is, the size of the region in the crystal where the atoms are arranged in a highly ordered and perfect manner without defects (such as grain boundaries, dislocations, twins). K X It can be determined by the following methods: The crystallite size K calculated using the Scherrer formula X XRD was used to obtain the broadening of the diffraction peaks of the positive electrode active material, and then the volume-weighted average of all coherent scattering domains within the grain was calculated using the Scherrer formula. Within this region, the atomic arrangement is highly ordered, the interplanar spacing is constant, and there are no defects such as dislocations or stacking faults to interrupt this coherence. X The result is obtained by formula (1), specifically: K X =(K (003) +K (104) ) / 2, formula (1) Where K (003) K represents the average coherent scattering domain thickness perpendicular to the (003) crystal plane. (104) This represents the average coherent scattering domain thickness in the direction perpendicular to the (104) crystal plane, and K (003) and K (104) Each was calculated using the Scherrer formula: D=Kλ / (βcosθ), (Scherrer formula) Where D is the thickness of the coherent diffraction domain; K is the Scherrer constant, usually taken as about 0.89; λ is the X-ray wavelength used (the wavelength of Kα radiation from the Cu target is about 0.15406 nm); β is the half-width at half maximum (FWHM) of the (003) or (104) diffraction peak, in radians; θ is the Bragg angle of the (003) or (104) diffraction peak, i.e., the θ value corresponding to the diffraction peak, in radians.

[0035] Understandable, K X The value can also be obtained by randomly sampling the volume-weighted average size of all coherent scattering domains of the positive electrode active material test sample, which can represent the overall actual level of the material.

[0036] In some embodiments of the present invention, 500nm ≤ K S ≤1000nm, as an example, K SIt can be 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, or any range of two of the above values. In some embodiments of the present invention, 650nm ≤ K S ≤900nm. Therefore, the positive electrode active material has a moderate average subgrain size, which facilitates the positive electrode active material to have the required range of micrograin boundary density, so that the positive electrode active material can take into account both excellent cycle performance and rate performance.

[0037] In some embodiments of the present invention, 38nm≤K X ≤59nm, for example, K X It can be 38nm, 39nm, 40nm, 41nm, 42nm, 43nm, 44nm, 45nm, 46nm, 47nm, 48nm, 49nm, 50nm, 51nm, 52nm, 53nm, 54nm, 55nm, 56nm, 57nm, 58nm, 59nm, or any range of two of the above values. In some other embodiments of the present invention, 42nm ≤ K X ≤55nm. Therefore, the size of the perfect crystal region in the crystal lattice of the positive electrode active material that can produce coherent diffraction is moderate, which makes it easy for the positive electrode active material to have the required range of micrograin boundary density, so that the positive electrode active material can take into account both excellent cycle performance and rate performance.

[0038] In some embodiments of the present invention, K X =(K (003) +K (104) ) / 2, K (003) and K (104) Each of the following formulas calculates to K (003) or K (104) =Kλ / (βcosθ), where K is the Scherrer constant, λ is the X-ray wavelength used for XRD testing of the positive electrode active material, β is the full width at half maximum (FWHM) of the (003) or (104) diffraction peak in the XRD pattern of the positive electrode active material, and θ is the Bragg angle of the (003) or (104) diffraction peak in the XRD pattern of the positive electrode active material, i.e., the θ value corresponding to the diffraction peak. This better reflects the size of the perfect crystalline region in the crystal lattice of the positive electrode active material that can produce coherent diffraction, facilitating the positive electrode active material to have the required range of micrograin boundary density, enabling the positive electrode active material to achieve both excellent cycle performance and rate performance.

[0039] In some embodiments of the present invention, the 2θ value of the (003) diffraction peak in the XRD pattern of the positive electrode active material is 16.1°-17.1°. As an example, the 2θ value of the (003) diffraction peak can be 16.1°, 16.2°, 16.3°, 16.4°, 16.5°, 16.6°, 16.7°, 16.8°, 16.9°, 17.0°, 17.1°, or any range of two of the above values. This facilitates the determination of the K0 value of the positive electrode active material. X The value is controlled within the required range to give the positive electrode active material the micrograin boundary density within the required range, so that the positive electrode active material can take into account both excellent cycle performance and rate performance.

[0040] In some embodiments of the present invention, the 2θ value of the (104) diffraction peak in the XRD pattern of the positive electrode active material is 41.0°-42.0°. As an example, the 2θ value of the (104) diffraction peak can be 41.0°, 41.1°, 41.2°, 41.3°, 41.4°, 41.5°, 41.6°, 41.7°, 41.8°, 41.9°, 42.0°, or any range of two of the above values. This facilitates the determination of the K0 value of the positive electrode active material. X The value is controlled within the required range to give the positive electrode active material the micrograin boundary density within the required range, so that the positive electrode active material can take into account both excellent cycle performance and rate performance.

[0041] The XRD testing conditions were as follows: The XRD was performed using a Rigaku powder X-ray diffractometer (Smart Lab 9kW) with a finely tuned setup. The Cu target wavelength was 1.5418462 Å, tube voltage was 40 kV, tube current was 200 mA, scanning speed was 5 Hz / min, and the scanning range was 10–90 Å. A one-dimensional detector (1D) was used. Specific testing conditions were: operating temperature: 21 ± 5 °C, humidity: <65%; cooling water circulator: temperature: 23 ± 1 °C, water pressure: 0.36 MPa; high-pressure refrigerant: 0.8–1.8 MPa, low-pressure refrigerant: 0.4–0.7 MPa; step size: 0.0200 °C.

[0042] In some embodiments of the present invention, the positive electrode active material comprises single-crystal particles, wherein the proportion of small-angle grain boundaries in the cross-section of the single-crystal particles is G, 10%≤G≤22%. As an example, G can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, or any two of the above values. In other embodiments of the present invention, 12%≤G≤18%. If the grain orientation angle difference between adjacent subgrains is in the range of 2°-10°, then the grain boundary between these adjacent subgrains is a small-angle grain boundary; if the grain orientation angle difference between adjacent subgrains is greater than 10°, then the grain boundary between these adjacent subgrains is a large-angle grain boundary. In some embodiments of the present invention, the positive electrode active material comprises single-crystal particles, wherein the proportion of large-angle grain boundaries in the cross-section of the single-crystal particles is L, 78%≤L≤90%. For example, L can be 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, or any combination of two of the above values. In other embodiments of the present invention, 82%≤L≤88%. It is understood that the grain boundary angle of the positive electrode active material is determined by an Oxford backscatter diffractometer (EBSD), where G+L=100%. Furthermore, the aforementioned proportions can refer to length proportions; for example, the proportion of small-angle grain boundaries = (total length of small-angle grain boundaries / total length of grain boundaries) × 100%, and the proportion of large-angle grain boundaries = (total length of large-angle grain boundaries / total length of grain boundaries) × 100%.

[0043] As is understandable, in EBSD testing, "grain boundary ratio" usually does not refer to the "number" of grain boundaries, but rather the "length ratio" of the "total length" of all grain boundaries within the observation area, or the "density" of the grain boundary network. Simply put, it measures the abundance or density of these "linear defects" in the material, rather than a simple count.

[0044] It should also be noted that the grain boundary angle range of 0-2° is not considered separately in conventional analysis because it is difficult to reliably separate the real signal from noise, and the dislocation density is extremely low and the grain boundary structure is unclear within this range. Setting 2° as a practical starting point is to ensure that the analyzed grain boundary is a dislocation-type small-angle grain boundary with clear crystallographic significance.

[0045] In some embodiments of the present invention, 3.5 ≤ L / G ≤ 9.0. As an example, the value of L / G can be 3.5, 4, 5, 6, 7, 8, 9, or any two of the above values. In other embodiments of the present invention, 4.5 ≤ L / G ≤ 7.3. By controlling the ratio of large-angle grain boundaries to small-angle grain boundaries in the cross-section of the single crystal particle within the above range, the positive electrode active material can overcome the adverse effects of traditional single-size grain boundary control. This allows the positive electrode active material to simultaneously possess the advantages of both large-angle and small-angle grain boundaries, ensuring that the positive electrode active material has both sufficient large-angle grain boundaries to guarantee mechanical integrity and abundant small-angle grain boundaries to provide sufficient ion migration channels, ultimately achieving the best balance between rate performance and cycle life.

[0046] In some embodiments of the present invention, the median particle size of the positive electrode active material is D. 50 3.2μm≤D 50 ≤8.5μm, for example, D 50 It can be 3.2μm, 4μm, 5μm, 6μm, 7μm, 8μm, 8.5μm, or any range of two of the above values. In other embodiments of the present invention, 4.0μm ≤ D 50 ≤7.0μm, the median particle size of the positive electrode active material is D 50 By controlling the process within the above range, the transport path of sodium ions is shorter, and the particle size of the positive electrode active material is not too small, thus increasing the contact area with the electrolyte. This reduces the side reactions between the positive electrode active material and the electrolyte, further improving the cycle performance and rate performance of the positive electrode active material.

[0047] It is understandable that the median particle size is D. 50 The median particle size represents the particle size at which the cumulative particle size distribution percentage of the sample material reaches 50%. In some embodiments, the median particle size D... 50 It can be measured using a Malvern 3000 laser particle size analyzer.

[0048] In some embodiments of the present invention, the specific surface area of ​​the positive electrode active material is S = 0.45m². 2 / g≤S≤0.90m 2 / g. For example, S can be 0.45m 2 / g, 0.5m 2 / g, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g or a range of any two of the above values, in some other embodiments of the invention, 0.50m 2 / g≤S≤0.75m 2 / g. By controlling the specific surface area of ​​the positive electrode active material within the above range, interfacial side reactions were effectively suppressed, further improving the cycle performance of the positive electrode active material.

[0049] In some embodiments of the present invention, the angle of repose of the positive electrode active material is α, where α < 52.0°. For example, the angle of repose α can be 51.9°, 51.5°, 51°, 50°, 49°, 48°, 47°, 46°, 45°, 44°, 43°, 42°, 41°, 40°, 39°, 38°, 37°, 36°, 35°, 34°, 33°, 32°, 31°, 30°, 20°, 10°, or a range of any two of the above values. In other embodiments of the present invention, α ≤ 47.5°. The angle of repose (i.e., the angle of repose) is used to characterize the flowability of a material. The smaller the angle of repose, the better the flowability of the positive electrode active material. Therefore, by controlling the angle of repose of the positive electrode active material within the above range, the positive electrode active material exhibits better flowability and superior processing performance.

[0050] It is understandable that the angle of repose of the positive electrode active material can be determined according to the standard GB / T 6609.24-2004, which specifies the physical properties of the angle of repose. The specific test method is as follows: In a drying room, 50g of the positive electrode active material is added to a funnel (with a stainless steel screen at the inlet), allowing the material to fall naturally onto a smooth base plate. The angle of repose is then calculated by measuring the height of the accumulated material.

[0051] In a second aspect, the present invention provides a method for preparing the positive electrode active material described in the first aspect, comprising: S1. A nickel source, an iron source, a manganese source, a complexing agent, and a precipitant are mixed and subjected to a co-precipitation reaction to obtain a slurry. The slurry is then aged, filtered, washed, and dried to obtain a precursor.

[0052] According to some embodiments of the present invention, the nickel source may be at least one of nickel sulfate, nickel chloride, and nickel acetate; the iron source may be at least one of ferric sulfate, ferric chloride, and ferric acetate; the manganese source may be at least one of manganese sulfate, manganese chloride, and manganese acetate; the complexing agent includes at least one of ammonia, sodium oxalate, sodium citrate, and ethylenediaminetetraacetic acid; preferably, the nickel source, iron source, and manganese source are all sulfates, or all nitrates, or all acetates; the precipitant is a sodium-containing alkaline compound and / or a potassium-containing alkaline compound, each independently selected from at least one of sodium and potassium hydroxides, carbonates, bicarbonates, and oxalates.

[0053] The feeding ratio of nickel, iron, and manganese sources can be calculated based on the stoichiometric ratio of the corresponding elements in the target sodium-ion single-crystal cathode material. Furthermore, there are no specific requirements for the amount of complexing agent and precipitant; those skilled in the art can flexibly select them according to actual needs such as the amount of nickel, iron, and manganese sources.

[0054] In some embodiments of the present invention, the specific methods and conditions for steps such as aging, filtering, washing, and drying are not limited. Those skilled in the art can flexibly select and design according to actual needs and existing technical means, and no limitations are imposed here.

[0055] In some embodiments of the present invention, the pH value of the coprecipitation reaction is 9.5-12.5, for example, it can be 9.5, 10, 10.5, 11, 11.5, 12, 12.5 or any two of the above values. In other embodiments of the present invention, the pH value of the coprecipitation reaction is 10-12. Thus, during the growth stage of the precursor in the coprecipitation reaction, by controlling the pH value of the reaction, the crystallinity of the precursor and the primary fiber stacking mode can be regulated, thereby affecting the defect density and substructure development of the final positive electrode active material grains, i.e., the micrograin boundary density N.

[0056] In some embodiments of the present invention, the temperature of the coprecipitation reaction is 40℃-65℃. For example, the temperature of the coprecipitation reaction can be 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, or any combination of two of these values. In other embodiments of the present invention, the temperature of the coprecipitation reaction is 45℃-60℃. By controlling the temperature of the coprecipitation reaction, the crystallinity of the precursor and the primary fiber stacking method can be adjusted, thereby affecting the defect density and substructure development within the grains of the final positive electrode active material, i.e., the micrograin boundary density N.

[0057] S2. The precursor is mixed with sodium source and M source and sintered to obtain positive electrode active material.

[0058] The precursor is mixed with sodium source and M source dopants and sintered. The M source tends to migrate to the grain boundary position during the subsequent sintering process and eventually accumulate at the grain boundary. The resulting pinning effect inhibits the migration of subgrain boundaries, thereby controlling the micrograin boundary density within a single crystal particle of the positive electrode active material.

[0059] In some embodiments of the present invention, the sintering process includes a constant temperature section 1 and a constant temperature section 2 in sequence, wherein the temperature of the constant temperature section 1 is lower than the temperature of the constant temperature section 2.

[0060] In some embodiments of the present invention, the temperature of the isothermal section 1 is T1, where 650℃≤T1≤930℃. For example, T1 can be 650℃, 700℃, 750℃, 800℃, 850℃, 900℃, 930℃, or any range of two of these values. In other embodiments of the present invention, 750℃≤T1≤910℃. Therefore, the aforementioned low-temperature sodiumization temperature can further enable control over the micrograin boundary density N of the positive electrode active material.

[0061] In some embodiments of the present invention, the isothermal time of the isothermal section 1 is t1, where 5h ≤ t1 ≤ 10h. For example, the isothermal time of the isothermal section 1 can be 5h, 6h, 7h, 8h, 9h, 10h, or any range of two of the above values. In other embodiments of the present invention, 6h ≤ t1 ≤ 8h, thereby, within the aforementioned low-temperature sodiumization time range, the micrograin boundary density N of the positive electrode active material can be further controlled.

[0062] In some embodiments of the present invention, the heating rate to the isothermal section 1 is V1, where 2℃ / min ≤ V1 ≤ 8℃ / min. For example, the heating rate can be 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, or any combination of two of the above values. In other embodiments of the present invention, 3℃ / min ≤ V1 ≤ 6℃ / min. Thus, within the above heating rate range, it is convenient to further control the micrograin boundary density N of the positive electrode active material.

[0063] In some embodiments of the present invention, the temperature of the isothermal section 2 is T2, where 980℃≤T2≤1080℃. As an example, T2 can be 980℃, 990℃, 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, 1050℃, 1060℃, 1070℃, 1080℃, or any range of two of the above values. In other embodiments of the present invention, 1000℃≤T2≤1040℃. Thus, within the above-mentioned high-temperature single crystallization temperature range, it is convenient to further control the micrograin boundary density N of the positive electrode active material.

[0064] In some embodiments of the present invention, the isothermal time of the isothermal section 2 is t2, where 5h ≤ t2 ≤ 15h. For example, t2 can be 5h, 7h, 9h, 10h, 12h, 14h, 15h, or any two of the above values. In other embodiments of the present invention, 7h ≤ t2 ≤ 12h. Thus, within the above-mentioned high-temperature single crystallization time range, it is convenient to further control the micrograin boundary density N of the positive electrode active material.

[0065] In some embodiments of the present invention, the heating rate to the isothermal section 2 is V2, where 1℃ / min ≤ V2 ≤ 5℃ / min. For example, V2 can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, or any two of these ranges. In other embodiments of the present invention, 1℃ / min ≤ V2 ≤ 3℃ / min. Therefore, within the above heating rate range, it is convenient to further control the micrograin boundary density N of the positive electrode active material.

[0066] In some embodiments of the present invention, 1.1≤T2 / T1≤1.6. As an example, T2 / T1 can be 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 or any two of the above values. In other embodiments of the present invention, 1.1≤T2 / T1≤1.3. Controlling the ratio of the two temperatures within the above range facilitates further control of the micrograin boundary density N of the positive electrode active material.

[0067] In some embodiments of the present invention, 0.5≤t2 / t1≤3.0. As an example, t2 / t1 can be 0.5, 1, 1.5, 2, 2.5, 3.0 or any two of the above values. In other embodiments of the present invention, 1.1≤t2 / t1≤2.0 controls the time of the two isothermal periods within the above range, which facilitates further control of the micrograin boundary density N of the positive electrode active material.

[0068] In some embodiments of the present invention, the sintering atmosphere may include one or more of air and oxygen.

[0069] In summary, the method for preparing the positive electrode active material proposed in this invention, combining precursor crystallization regulation and morphology design with elemental grain boundary oriented doping and multi-platform sintering curves, allows for effective control over the formation and growth of defects such as dislocations and substructures within the positive electrode active material grains. This method enables simple and rapid control of the micrograin boundary density N of the positive electrode active material grains, ultimately achieving "genetic regulation" of the material. Specifically, it allows for the synthesis of high-performance sodium-ion battery positive electrode active materials through micro / nano structure design, thereby meeting the market requirements for high-rate and long-cycle performance in sodium-ion batteries. Furthermore, the method is simple and convenient to operate, making it suitable for large-scale production.

[0070] In a third aspect, the present invention provides a positive electrode sheet. According to embodiments of the present invention, the positive electrode sheet comprises the positive active material described in the first aspect of the present invention, or the positive active material prepared using the method described in the second aspect.

[0071] According to an embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on the positive current collector. The positive active material layer includes the aforementioned positive active material. The positive current collector can be a metal foil or a composite current collector (a metal material can be disposed on a polymer substrate to form a composite current collector). For example, the positive current collector can be an aluminum foil.

[0072] According to some embodiments of the present invention, the positive electrode active material layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin.

[0073] According to some embodiments of the present invention, the positive electrode active material layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0074] According to some embodiments of the present invention, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto a positive current collector, and obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0075] It should be noted that the features and advantages described above for the positive electrode active material and its preparation method also apply to this positive electrode sheet, and will not be repeated here.

[0076] In a fourth aspect, the present invention provides a sodium secondary battery. According to an embodiment of the invention, the battery includes at least one of the positive electrode plates described in the third aspect.

[0077] As an example, a sodium secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, with the separator located between the positive and negative electrodes. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0078] According to an embodiment of the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode current collector can be a metal foil or a composite current collector (a metal material can be disposed on a polymer substrate to form a composite current collector). For example, the positive electrode current collector can be a copper foil.

[0079] According to some embodiments of the present invention, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and sodium titanate, etc.

[0080] According to some embodiments of the present invention, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0081] According to some embodiments of the present invention, the negative electrode active material layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0082] According to some embodiments of the present invention, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, and binder, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing, and other processes.

[0083] According to further embodiments of the present invention, the type of separator is not particularly limited, and any known porous separator with good chemical and mechanical stability can be selected. As an example, the material of the separator may include at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, or polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0084] According to further embodiments of the present invention, there is no specific limitation on the type of electrolyte, which can be selected according to requirements. For example, the electrolyte can be in a gel state or a completely solid state. According to some specific embodiments of the present invention, the electrolyte is an electrolyte solution comprising a sodium salt and a solvent.

[0085] According to some specific embodiments of the present invention, the sodium salt may include at least one of sodium hexafluorophosphate, sodium difluorooxalate borate, sodium tetrafluoroborate, sodium dioxalate borate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, or sodium bis(trifluoromethanesulfonyl)imide.

[0086] According to some specific embodiments of the present invention, the solvent may include at least one selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, ethylene glycol dimethyl ether, methyl ethyl sulfone, or diethyl sulfone.

[0087] In some embodiments of this application, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0088] It should be noted that the features and advantages described above for the positive electrode also apply to this sodium secondary battery, and will not be repeated here.

[0089] In a fifth aspect, the present invention provides an electrical device. According to an embodiment of the invention, the electrical device includes the aforementioned sodium secondary battery. According to an embodiment of the invention, the electrical device may include, but is not limited to, mobile phones, laptops, electric vehicles, etc.

[0090] It should be noted that the features and advantages described above for sodium secondary batteries also apply to this electrical device, and will not be repeated here.

[0091] The present disclosure will be explained below with reference to embodiments. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the disclosure. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0092] Example 1 (1) Nickel sulfate, ferric sulfate, manganese sulfate, complexing agent ammonia water and precipitant sodium hydroxide solution are mixed in proportion to carry out coprecipitation reaction to obtain precursor slurry. The pH value of the coprecipitation reaction solution is 11.0 and the reaction temperature TF is 55℃. Then the above slurry is aged, filtered, washed and dried to obtain sodium-ionized precursor. The molar ratio of each element in the precursor satisfies Ni / Fe / Mn=1 / 1 / 1. (2) The sodium-ion battery precursor obtained above is mixed uniformly with sodium source and dopants Al2O3, ZrO2 and CaCO3, wherein the molar ratio of the mixture satisfies Na / Me(Ni+Fe+Mn+Al+Zr+Ca)=1.0, and the molar ratio of the other elements satisfies Ni / Fe / Mn / Al / Zr / Ca=0.320 / 0.320 / 0.320 / 0.010 / 0.010 / 0.020; then the mixture is sintered in an atmosphere, wherein T1=850℃, t1=7.0h, V1=4.0℃ / min; T2=1015℃, t2=10.0h, V2=2.0℃ / min; then crushed and sieved in sequence to obtain single crystal sodium-ion battery positive electrode active material; wherein the sodium source is sodium carbonate, and the sintering atmosphere is air.

[0093] The chemical formula, characteristic parameters, and electrochemical performance data of the finished product in Example 1 are shown in the table below. Figure 1 The image shows the grain orientation of the positive electrode active material prepared in Example 1, obtained by SEM-EBSD testing. Figure 2 The image shows the XRD pattern of the positive electrode active material prepared in Example 1.

[0094] The remaining embodiments and comparative examples are consistent with Embodiment 1, with the differences shown in the table below.

[0095] Table 1

[0096] Table 1-2

[0097] The positive electrode active materials obtained in each embodiment and comparative example were measured as follows, and the test results are shown in Table 2.

[0098] 1. K of the positive electrode active material in each embodiment and comparative example S and K X Tests for N: An electron beam from an EBSD (Electron Back-Scattered Difference) detector mounted on a scanning electron microscope (SEM) is irradiated onto a tilted sample. Backscattered electrons undergo diffraction, and the resulting diffracted electrons are projected onto the phosphor screen of the EBSD detector, forming a diffraction pattern. The area of ​​subgrains in the cross-section of the positive electrode active material particles is analyzed, and the diameter of a circle with the same area is taken as the subgrain size. The equivalent circle diameters of the subgrains in the positive electrode active material particles are randomly counted from 10 SEM images at 3K magnification (e.g., all clear primary particles in a cross-sectional image can be counted), and the average value is calculated as the mean K of the subgrain size of the positive electrode active material. S .

[0099] The crystallite size K calculated using the Scherrer formula X XRD was used to obtain the broadening of the diffraction peaks of the positive electrode active material, and then the volume-weighted average of all coherent scattering domains within the grain was calculated using the Scherrer formula. Within this region, the atomic arrangement is highly ordered, the interplanar spacing is constant, and there are no defects such as dislocations or stacking faults to interrupt this coherence. X The result is obtained by formula (1), specifically: K X =(K (003) +K (104) ) / 2, formula (1) Where K (003) K represents the average coherent scattering domain thickness perpendicular to the (003) crystal plane. (104) This represents the average coherent scattering domain thickness in the direction perpendicular to the (104) crystal plane, and K (003) and K (104) Each was calculated using the Scherrer formula: D=Kλ / (βcosθ), (Scherrer formula) Where D is the thickness of the coherent diffraction domain (K) (003) or K (104) K is the Scherrer constant, usually taken as about 0.89; λ is the X-ray wavelength used (the wavelength of Kα radiation from the Cu target is about 0.15406 nm); β is the half-width at half maximum (FWHM) of the (003) or (104) diffraction peak, in radians; θ is the Bragg angle of the (003) or (104) diffraction peak, i.e., the θ value corresponding to the diffraction peak, in radians.

[0100] The specific test conditions were as follows: The Cu target wavelength was 1.5418462 Å, tube voltage 40 kV, tube current 200 mA, scanning speed 5 Hz / min, and scanning range 10–90 Å, obtained using a one-dimensional detector (1D). Specific test conditions were: operating temperature: 21 ± 5 °C, humidity: <65%; cooling water circulator: temperature: 23 ± 1 °C, water pressure: 0.36 MPa; high-pressure refrigerant: 0.8–1.8 MPa, low-pressure refrigerant: 0.4–0.7 MPa; step size: 0.0200 °C.

[0101] According to N=K S / K X The micrograin boundary density N of the positive electrode active material was obtained.

[0102] 2. The proportion of small-angle grain boundaries (G), the proportion of large-angle grain boundaries (L), and the L / G of the positive electrode active materials in each embodiment and comparative example were determined by Oxford backscatter diffractometer (EBSD).

[0103] 3. The median particle size of the positive electrode active material in each embodiment and comparative example is D. 50 Measurement: The results were obtained using a Malvern 3000 laser particle size analyzer.

[0104] 4. Determination of the specific surface area S of the positive electrode active materials in each embodiment and comparative example: The surface area S was determined using the nitrogen adsorption method and measured with a surface analyzer, such as the Tristar 3020 surface analyzer from Micromeritics. The specific operation may include: gradually adding N2 to the test material (after which physically adsorbed components have been removed) under vacuum conditions in the testing apparatus; calculating the pressure change caused by N2 adsorption using the constant volume method; and determining the amount of N2 adsorbed according to the gas equation. This yields the amount of N2 adsorbed from 0 atm to 0.3 atm at liquid nitrogen temperature, which is then converted into specific surface area per unit weight.

[0105] 5. Determination of the angle of repose α of the positive electrode active materials in each embodiment and comparative example: The determination of the angle of repose according to standard GB / T 6609.24-2004 (Physical Properties) was conducted using the following method: 50g of positive electrode active material was added to a funnel (with a stainless steel screen at the inlet) in a drying room, allowing the material to fall naturally onto a smooth base plate. The angle of repose was obtained by measuring the height of the accumulated material and then calculating it.

[0106] Table 2

[0107] The examples and comparative examples were made into batteries according to the following method: The test was conducted using a CR2032 coin cell. Electrode preparation: The prepared positive electrode active material, conductive carbon, and binder were thoroughly mixed in N-methylpyrrolidone (NMP) at a mass ratio of 95:3:2 to form a uniform slurry. This slurry was coated onto aluminum foil and dried in an oven at 120°C for 12 hours. The foil was then pressed into a positive electrode sheet with a diameter of 12 mm and a thickness of 120 mm using a pressure of 100 MPa. The loading of the positive electrode active material was 15 mg / cm³. 2 Battery Assembly: In an argon-filled glove box, the aforementioned positive electrode, separator, negative electrode, and electrolyte are assembled into a CR2032 coin cell. The negative electrode uses a sodium metal sheet; the separator is a polypropylene membrane; the electrolyte solvent is an equal volume mixture of ethylene carbonate, diethyl carbonate, and vinylene carbonate, and the electrolyte salt is NaPF6 at a concentration of 1 mol / L.

[0108] The following measurements were performed on the batteries prepared in the examples and comparative examples: (1) Capacity retention test: At 25℃, the charge-discharge range is 2.0V-4.1V, 0.1C cycle for 2 weeks, and then cycle once each at 0.2C, 0.33C, 0.5C and 1C. The specific capacity of the first discharge at 0.1C is the discharge specific capacity of the button cell in the first cycle. Battery rate performance = 1C discharge specific capacity / first week 0.1C discharge specific capacity × 100%; 80-cycle capacity retention test: After the material has undergone the above rate test, continue to conduct 80-cycle charge-discharge test at 1C rate. The capacity retention of the battery material is evaluated by comparing the discharge capacity of the last cycle with the discharge capacity of the first cycle.

[0109] (2) Electrochemical Impedance Spectroscopy (EIS) Test: Using the EIS test method, the previously prepared unactivated half-cell was taken, allowed to stand for 6 hours, and then charged at a constant current and constant voltage of 0.1C to 4.1V (cutoff current 0.05C). It was then discharged at a current of 0.1C to 2.0V; subsequently, it was charged again at a constant current and constant voltage of 0.1C to 4.1V (cutoff current 0.05C). The fully charged half-cell was then removed, and EIS tests were performed in the frequency range of 1MHz to 0.01Hz, with an amplitude of 10mV. According to the formula: Z re =R s +R ct +σω -1 / 2 , and ω=2πf, where Z re R is the real part of the impedance spectrum obtained from the test. s R is the resistance of the solution. ct Here, ω is the charge transfer resistor, f is the angular frequency, and σ is the Warburg factor; the EIS impedance written in this paper is R = R s +R ct .

[0110] (3) Sodium ion diffusion coefficient test: To explain the reason for the performance improvement of the characteristic material described in this invention, the sodium ion diffusion coefficient of the powder material was tested using the EIS test method. The specific EIS test method is the same as described above. Furthermore, the Warburg factor σ obtained from the test was substituted into the sodium ion diffusion coefficient calculation formula: D Na+ =R 2 T 2 / (2A 2 n 4 F 4 C 2 σ 2The diffusion coefficient of Na ions in the material was calculated using the following method: R is the ideal gas constant, T is the absolute temperature, A is the cross-sectional area of ​​the electrode, n is the number of electrons transferred, F is the Faraday constant, and C is the concentration of sodium ions in the electrode. The measurement results are shown in Table 3 below.

[0111] Table 3

[0112] As shown in the table above, this application, through precursor crystallinity regulation, elemental doping, and multi-platform sintering curve technology, effectively controls the formation and growth of defects such as dislocations and substructures within the cathode active material grains, achieving simple and rapid control of the grain boundary density N. When the grain boundary density N of the cathode active material is within the range defined in this application, the battery exhibits excellent cycle performance and rate performance.

[0113] Among them, the micrograin boundary density N of the positive electrode active material prepared in Example 6 is slightly small, and the diffusion path of sodium ions in the grains is relatively simple, resulting in slightly poor rate performance; the micrograin boundary density N of the positive electrode active material prepared in Example 7 is slightly large, and although the sodium ion diffusion coefficient is high, the capacity retention rate is low; the proportion of large-angle grain boundaries in the positive electrode active material prepared in Example 10 is low, resulting in low mechanical integrity and poor cycle stability; the proportion of large-angle grain boundaries in the positive electrode active material prepared in Example 11 is high, resulting in insufficient ion migration channels and poor rate performance.

[0114] Comparative Example 1 did not involve elemental doping during the preparation process, resulting in a cathode active material with low crystallinity and excessively high micrograin boundary density (N), leading to poor battery cycle performance. Comparative Example 2 employed a sintering method of low-speed heating + high-temperature sintering + long holding time in the isothermal section 2 of the preparation process, resulting in a cathode active material with high crystallinity and excessively low micrograin boundary density (N), leading to poor battery rate performance. Comparative Example 3 employed a sintering method of rapid heating + medium-low temperature sintering + short holding time in the isothermal section 2 of the preparation process, resulting in a cathode active material with low crystallinity and excessively high micrograin boundary density (N), leading to poor battery cycle performance.

[0115] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," "some implementations," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0116] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A positive electrode active material, characterized in that, The chemical formula of the positive electrode active material includes: So a Ni 1-b-c-d Feb b Mr c M d O2; where 0.95 ≤ a ≤ 1.10, 0.03 ≤ b ≤ 0.50, 0.03 ≤ c ≤ 0.50, 0 < d ≤ 0.10, and M includes one or more of Ta, Cr, La, Al, Ce, Y, Mg, Sr, Zr, Ca, Zn, B, W, Nb, Si, Mo, F, P, Co, Li, Ti, Cu; The micrograin boundary density of the positive electrode active material is N, where N = K. S / K X K S The average subgrain size, K, of the positive electrode active material was obtained by EBSD testing. X The crystallite size of the positive electrode active material is 8.0 ≤ N ≤ 28.

0.

2. The positive electrode active material according to claim 1, characterized in that, 10.0 ≤ N ≤ 24.5; and / or, 0.15 ≤ b ≤ 0.40; and / or, 0.15 ≤ c ≤ 0.40; and / or, 0 < d ≤ 0.06; and / or, M includes one or more of Al, Mg, Zr, Sr, Ca, Zn, Nb, Ti, Mo.

3. The positive electrode active material according to claim 1, characterized in that, 500nm≤K S ≤1000nm, optionally, 650nm≤K S ≤900nm; and / or, 38nm≤K X ≤59nm, optionally, 42nm≤K X ≤55nm.

4. The positive electrode active material according to any one of claims 1-3, characterized in that, The positive electrode active material includes single crystal particles. In the cross-section of the single crystal particles, the proportion of small-angle grain boundaries is G, 10% ≤ G ≤ 22%, optionally, 12% ≤ G ≤ 18%; and / or, The positive electrode active material includes single crystal particles. In the cross-section of the single crystal particles, the proportion of large-angle grain boundaries is L, 78% ≤ L ≤ 90%, optionally, 82% ≤ L ≤ 88%; and / or, 3.5 ≤ L / G ≤ 9.0, optionally, 4.5 ≤ L / G ≤ 7.

3.

5. The positive electrode active material according to any one of claims 1-3, characterized in that, The median particle size of the positive electrode active material is D. 50 3.2μm≤D 50 ≤8.5μm, optionally, 4.0μm≤D 50 ≤7.0μm; and / or, The specific surface area of ​​the positive electrode active material is S, 0.45m². 2 / g≤S≤0.90m 2 / g, optionally, 0.50m 2 / g≤S≤0.75m 2 / g; and / or, The tapering angle of the positive electrode active material is α, α < 52.0°, optionally, α ≤ 47.5°.

6. A method for preparing the positive electrode active material according to any one of claims 1-5, characterized in that, It includes: Mixing a nickel source, an iron source, a manganese source, a complexing agent and a precipitating agent for coprecipitation reaction to obtain a slurry, aging, filtering, washing and drying the slurry to obtain a precursor; Mixing the precursor with a sodium source and an M source and sintering to obtain the positive electrode active material.

7. The method according to claim 6, characterized in that, The pH value of the coprecipitation reaction is 9.5 - 12.5, optionally 10 - 12; and / or, The temperature of the coprecipitation reaction is 40°C - 65°C, optionally 45°C - 60°C.

8. The method according to claim 6 or 7, characterized in that, The sintering sequentially includes a constant temperature section 1 and a constant temperature section 2, The temperature of the constant temperature section 1 is T1, 650°C ≤ T1 ≤ 930°C, optionally, 750°C ≤ T1 ≤ 910°C; and / or, The constant temperature time of the constant temperature section 1 is t1, 5h ≤ t1 ≤ 10h, optionally, 6h ≤ t1 ≤ 8h; and / or, The heating rate to the constant temperature section 1 is V1, 2°C / min ≤ V1 ≤ 8°C / min, optionally, 3°C / min ≤ V1 ≤ 6°C / min; and / or, The temperature of the constant temperature section 2 is T2, 980°C ≤ T2 ≤ 1080°C, optionally, 1000°C ≤ T2 ≤ 1040°C; and / or, The constant temperature time of the constant temperature section 2 is t2, 5h ≤ t2 ≤ 15h, optionally, 7h ≤ t2 ≤ 12h; and / or, The heating rate to the constant temperature section 2 is V2, 1°C / min ≤ V2 ≤ 5°C / min, optionally, 1°C / min ≤ V2 ≤ 3°C / min; and / or, 1.1 ≤ T2 / T1 ≤ 1.6, optionally, 1.1 ≤ T2 / T1 ≤ 1.3; and / or, 0.5 ≤ t2 / t1 ≤ 3.0, optionally, 1.1 ≤ t2 / t1 ≤ 2.

0.

9. A positive electrode sheet, characterized in that, It includes the positive electrode active material according to any one of claims 1-5, or the positive electrode active material prepared by the method according to any one of claims 6-8.

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

11. An electrical appliance, characterized in that, Including the sodium secondary battery as described in claim 10.

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