Positive electrode active material, preparation method thereof and battery

Abstract

The invention discloses a positive electrode active material, a preparation method thereof and a battery, the positive electrode active material comprises lithium manganese iron phosphate, and a straight line obtained by fitting the change rate of the unit cell parameter a / c of the positive electrode active material before and after pressure applying treatment and the pressure of the pressure applying treatment meets Si = kTi, 0.005 lt; klt; 0.03 part of the mixture; wherein Ti is the pressure for pressing the positive electrode active material, the unit is MPa, Ti is more than or equal to 225 MPa and less than or equal to 750 MPa, the pressing treatment comprises pressing treatment under i different pressures, and i is more than or equal to 3 and less than or equal to 10; si is the change rate of the unit cell parameter a / c of the positive electrode active material before and after the pressure applying treatment, Si = (Si-S0) / S0 * 1000 per thousand, Si is the unit cell parameter ai / ci value of the positive electrode active material after the pressure applying treatment, Si = ai / ci, S0 is the unit cell parameter a0 / c0 value of the positive electrode active material before the pressure applying treatment, and S0 = a0 / c0. Therefore, the positive electrode active material has relatively high compressive strength and relatively small stress deformation, and is beneficial to improving the cycling stability of the battery.

Description

Technical Field

[0001] This application relates to the technical field of lithium batteries, and specifically, to a cathode active material, a preparation method thereof, and a battery. Background Art

[0002] With the accelerating pace of the development of new energy storage, the requirements for battery safety, energy density, cycle life, etc. have also been significantly improved. Among them, the lithium iron manganese phosphate energy storage battery has gradually been sought after by the market due to its advantages of low cost, high safety performance, long life, and wide temperature range adaptability, and is regarded as an iteration and upgrade of lithium iron phosphate materials. It is有望 to become one of the mainstream technologies for the新一代 low-cost and high-energy density energy storage batteries.

[0003] However, the ion and electron transport capabilities and crystal structure stability of lithium iron manganese phosphate are significantly lower than those of lithium iron phosphate, and there is a large gap in tap density and processing characteristics. Therefore, the lithium iron manganese phosphate cathode active material still有待改进.

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

[0005] In the first aspect of this application, this application proposes a cathode active material including lithium iron manganese phosphate. The change rate of the unit cell parameter a / c of the cathode active material before and after pressure application and the straight line fitted with the pressure of the pressure application satisfy: △S i =kT i , 0.005 < k < 0.03; where T i is the pressure for pressure application to the cathode active material, with the unit of MPa, 225 MPa ≤ T i ≤ 750 MPa. The pressure application includes pressure application at i different pressures, 3 ≤ i ≤ 10; △S i is the change rate of the unit cell parameter a / c of the cathode active material before and after pressure application, △S i =(S i -S0) / S0×1000‰, S i is the value of the unit cell parameter a i / c i of the cathode active material after pressure application, S i =a i / c i , S0 is the value of the unit cell parameter a0 / c0 of the cathode active material before pressure application, S0 = a0 / c0. Thus, this cathode active material has both high compressive strength and small stress deformation, which is beneficial to improving the cycle stability of the battery.

[0006] In some embodiments, 0.01 < k < 0.02. Thus, it is beneficial to further improve the particle strength of the positive electrode active material.

[0007] In some embodiments, when T i is 375 MPa, △S i is 1‰ - 3‰; and / or, when T i is 525 MPa, △S i is 1‰ - 8‰; optionally, 2‰ - 6‰. Thus, the positive electrode active material has excellent processing adaptability and strong particle size stability.

[0008] In some embodiments, the positive electrode active material satisfies: 0% < S Di ≤ 30%; optionally, 0% < S Di ≤ 20%, where S Di = (D 50（T0） - D 50（Ti） ) / D 50（T0） , D 50（T0） is the D 50 particle size of the positive electrode active material before pressure treatment, and D 50（Ti） is the D 50 particle size of the positive electrode active material after pressure treatment. Thus, the positive electrode active material is not easily broken or deformed after being pressed, has a relatively stable particle structure, and has strong compressive resistance.

[0009] In some embodiments, the D 50 particle size of the positive electrode active material is 0.3 μm - 2.0 μm; optionally, 0.5 μm - 1.5 μm. Thus, it is beneficial to improve the processing performance of the positive electrode active material and increase the tap density of the positive electrode active material.

[0010] In some embodiments, the positive electrode active material satisfies: 0% < S Ai ≤ 30%, optionally, 0% < S Ai ≤ 20%; where S Ai = (S Ai(Ti) - S A0(T0) ) / S A0(T0) , S A0(T0) is the specific surface area of the positive electrode active material before pressure treatment, and S Ai(Ti) is the specific surface area of the positive electrode active material after pressure treatment. Thus, the positive electrode active material has a high particle strength, strong compressive resistance, and is not easily cracked.

[0011] In some embodiments, the proportion of primary particles of the positive electrode active material with a size in the range of 0 μm to 0.1 μm is 10%-60%, optionally 10%-40%; the proportion of primary particles of the positive electrode active material with a size in the range of 0.1 μm to 0.2 μm is 30%-80%, optionally 20%-60%. This allows for close packing of the primary particles, which is beneficial for increasing the compaction density of the positive electrode active material.

[0012] In some embodiments, the maximum particle size of the primary particles of the positive electrode active material is 0.4 μm-0.7 μm; and / or, the minimum particle size of the primary particles of the positive electrode active material is D. min 0μm <D min The particle size is ≤0.1μm; and / or, the average particle size of the primary particles of the positive electrode active material is 0.1μm-0.4μm. Therefore, the particle size distribution of the positive electrode active material is relatively concentrated, which is beneficial for improving the compaction density.

[0013] In some embodiments, the particle size distribution curve of the positive electrode active material has a first characteristic peak and a second characteristic peak. The peak position of the first characteristic peak is less than or equal to 1 μm, and the peak position of the second characteristic peak is greater than or equal to 1 μm. The difference between the peak position of the second characteristic peak and the peak position of the first characteristic peak is 0.5 μm-3 μm. Therefore, smaller particles can effectively fill the gaps between larger particles, thereby increasing the compaction density of the positive electrode active material and helping to improve the battery's capacity.

[0014] In some embodiments, the ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the second characteristic peak is 0.5-2.5. This is beneficial for optimizing particle size distribution and achieving close packing between positive electrode active material particles.

[0015] In some embodiments, the compaction density of the positive electrode active material under 3T pressure is 2.0 g / cm³. 3 -2.4g / cm 3 Therefore, the positive electrode active material has a high capacity utilization.

[0016] In some embodiments, the positive electrode active material includes a matrix and a carbon coating layer at least partially covering the surface of the matrix; wherein the matrix satisfies formula I: Li a Mn b Fe (1-b) M c (PO4) dFormula I; where 0.95≤a≤1.10, 0.3≤b≤0.7, 0<c≤0.5, 0.95≤d≤1.10; M includes at least one of Ti, Mg, Zn, Cu, Sr, Al, Zr, Y, Co, W, Ca, Nb, Sn, Sb, Mo, V, B, Si, and Na. This is beneficial for improving the structural stability and electronic conductivity of the positive electrode active material.

[0017] In some embodiments, the carbon coating content is 1wt%-3wt% based on the mass of the positive electrode active material; optionally, 1.5wt%-2wt%. This improves the conductivity of the positive electrode active material, reduces direct contact between the inside of the positive electrode active material and the electrolyte, effectively reduces interfacial side reactions, and thus significantly improves cycle life.

[0018] In a second aspect, this application proposes a method for preparing a positive electrode active material, comprising: grinding and mixing iron phosphate, a manganese source, a lithium source, a phosphorus source, a first M source, a first carbon source, and a solvent, and performing a first spray drying treatment to obtain a first spray-dried material; performing a first sintering treatment on the first spray-dried material to obtain a lithium manganese iron phosphate precursor; mixing the lithium manganese iron phosphate precursor with a second M source, a second carbon source, and a solvent, grinding and mixing, and performing a second spray drying treatment to obtain a second spray-dried material; and performing a second sintering treatment on the second spray-dried material to obtain the positive electrode active material. Thus, this application obtains a positive electrode active material with high particle strength through a two-step liquid-phase grinding and secondary coating process, and the preparation method is simple, easy to operate, and easy to industrialize.

[0019] In some embodiments, the first sintering treatment includes: heating the first spray-dried material to a first sintering temperature T1 at a first heating rate v1 and holding it at that temperature for a first time t1, and then cooling it from T1 to below 330°C at a first cooling rate v2, wherein v1 > 2°C / min, 450°C ≤ T1 ≤ 750°C, 3h ≤ t1 ≤ 12h; and / or v2 > 2°C / min. This is beneficial for increasing the reaction rate between the raw material components and for controlling the particle size and morphology of the lithium manganese iron phosphate precursor.

[0020] In some embodiments, the second sintering process includes: heating the second spray-dried material to a second sintering temperature T2 at a second heating rate v3 and holding it at that temperature for a second time t2, then cooling it from T2 to below 330°C at a second cooling rate v4, wherein v3 < 2°C / min, 600°C ≤ T2 ≤ 850°C, 4h ≤ t2 ≤ 20h; and / or v4 < 2°C / min. This is beneficial for improving the crystallinity of the positive electrode active material, reducing lattice defects and internal stress, and improving the stability of the crystal structure.

[0021] In some embodiments, the average particle size of the lithium manganese iron phosphate precursor is 0.1 μm-0.4 μm; and / or, the molar ratio of iron to phosphorus in the iron phosphate is 0.940-0.990; and / or, the carbon content in the lithium manganese iron phosphate precursor is 0.01 wt%-0.5 wt%. This is beneficial for improving the distribution of elements in the positive electrode active material and enhancing the uniformity of element distribution.

[0022] In some embodiments, the manganese source includes at least one selected from manganese sulfate, manganese carbonate, manganese nitrate, and manganese tetroxide; and / or, the lithium source includes at least one selected from lithium carbonate, lithium hydroxide, lithium phosphate, lithium nitrate, anhydrous lithium hydroxide, and lithium oxide; and / or, the phosphorus source includes at least one selected from phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, manganese phosphate, lithium phosphate, and pyrophosphate. Therefore, the raw materials are widely available, the cost is low, and large-scale promotion is convenient.

[0023] In some embodiments, the first M source and the second M source each independently include at least one of the following: oxide, hydroxide, carbonate, fluoride, sulfate, phosphate, and chloride corresponding to the M element. Therefore, the introduction of the M element can improve the structural stability of the positive electrode active material.

[0024] In some embodiments, the first carbon source and the second carbon source include at least one selected from glucose, sucrose, and organic polymers. This facilitates the formation of a carbon coating layer, thereby improving the conductivity of the positive electrode active material and reducing side reactions between the positive electrode active material and the electrolyte.

[0025] In a third aspect, this application proposes a battery comprising a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer located on at least one side of the positive current collector, the positive active material layer comprising the positive active material described in the first aspect of this application or a positive active material prepared using the method described in the second aspect of this application. It should be noted that the features and effects described for the positive active material in the first aspect of this application and the positive active material prepared using the method described in the second aspect are also applicable to the battery of the third aspect of this application, and will not be repeated here. Attached Figure Description

[0026] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, wherein, Figure 1 This is a SEM image of the positive electrode active material of Example 1 of this application; Figure 2 This is the particle size distribution curve of the positive electrode active material in Example 1 of this application; Figure 3This is a distribution diagram of the primary particle number ratio of the positive electrode active material in Example 1 of this application; Figure 4 The images show the XRD patterns of the positive electrode active material of Example 1 of this application before and after cycling. Detailed Implementation

[0027] The embodiments of this application are described in detail below, with examples of these embodiments shown in the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0028] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in this application is for the purpose of describing particular embodiments only and is not intended to limit this application; unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0029] The terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are open-ended expressions, meaning they include what is specified in this application but do not exclude other aspects.

[0030] In the description of this application, all figures disclosed herein, whether or not the words "approximately" or "about" are used, are approximate values. Each figure may vary by less than 10% or by a difference that is considered reasonable by one of the art, such as 1%, 2%, 3%, 4%, or 5%.

[0031] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0032] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. "First feature" and "second feature" may include one or more of the indicated feature.

[0033] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0034] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0035] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0036] At present, lithium manganese iron phosphate cathode active materials face problems such as low intrinsic conductivity, decreased structural stability due to the Jan Taylor effect, severe manganese ion dissolution, destruction of the material's intrinsic energy storage mechanism and bulk structure, resulting in severe capacity decay.

[0037] On the one hand, due to its intrinsic brittleness and poor crystal deformation ability, the lithium iron manganese phosphate cathode active material is prone to stress concentration under external force or volume change. During the rolling process, its particles will undergo plastic deformation due to external mechanical pressure. When the stress exceeds the fracture strength of the material, microcracks are likely to occur. On the other hand, during the charge-discharge cycle, the particles repeatedly expand and contract in volume, exacerbating the internal stress accumulation in the particles and further intensifying the generation of microcracks. A large number of microcracks not only damage the electron conduction path of the particles, cause the loss of active substances, but may also accelerate the dissolution of manganese ions,破坏负极表面SEI膜的界面稳定性,使得SEI膜反复破裂重组,继而导致电池内阻急剧增大,影响电池的输出功率,容量明显衰减,增加电池热失控风险,最终大幅缩短使用寿命。

[0038] To solve the above technical problems, the present application proposes a lithium iron manganese phosphate cathode active material with low stress, long cycle life, and high compaction, as well as a preparation method thereof, to improve the particle strength of the cathode active material, improve the cycle stability, and simultaneously enhance the compaction performance.

[0039] In the first aspect of the present application, a cathode active material is proposed, which includes lithium iron manganese phosphate. The change rate of the unit cell parameter a / c of the cathode active material before and after the pressure application treatment and the straight line fitted with the pressure of the pressure application treatment satisfy: △S i =kT i , 0.005 < k < 0.03; where T i is the pressure for the pressure application treatment of the cathode active material, with the unit of MPa, 225 MPa ≤ T i ≤ 750 MPa. The pressure application treatment includes pressure application treatments at i different pressures, 3 ≤ i ≤ 10; △S i is the change rate of the unit cell parameter a / c of the cathode active material before and after the pressure application treatment, △S i =(S i -S0) / S0×1000‰, S i is the value of the unit cell parameter a i / c i of the cathode active material after the pressure application treatment, S i =a i / c i , S0 is the value of the unit cell parameter a0 / c0 of the cathode active material before the pressure application treatment, S0 = a0 / c0. Thus, this cathode active material has both high compressive strength and small stress deformation, which is beneficial to improving the cycle stability of the battery.

[0040] It should be noted that there is an unclear part in the original text "破坏负极表面SEI膜的界面稳定性,使得SEI膜反复破裂重组,继而导致电池内阻急剧增大,影响电池的输出功率,容量明显衰减,增加电池热失控风险,最终大幅缩短使用寿命." in the English translation, which needs to be further clarified in the original text for a more accurate translation.As an example, k can be 0.006, 0.008, 0.01, 0.012, 0.015, 0.018, 0.02, 0.022, 0.025, 0.027, 0.029, etc. Among them, the magnitude of the k value reflects the degree of change in the change rate of the unit cell parameters a / c of the positive electrode active material under different pressures. When the k value is within the aforementioned range, it indicates that the change in the unit cell parameters of the positive electrode active material is small, the structural stability is high, and the cycle stability is excellent.

[0041] As an example, the minimum difference between different test pressures T i can be 45 MPa - 90 MPa, such as 45 MPa, 60 MPa, 75 MPa, 90 MPa, etc.; in some embodiments, the minimum difference can be 75 MPa.

[0042] As an example, i can be 3, 4, 5, 6, 7, 8, 9, or 10, etc.

[0043] In this application, the structure of the positive electrode active material is analyzed by an X-ray diffractometer, the grain size is obtained through refined calculation, and the unit cell parameters under different pressures are tested to complete the material characterization. When 0.005 < k < 0.03 is satisfied, the positive electrode active material has a high compressive strength, excellent processing adaptability, strong particle size stability after pole piece rolling, and the cycle performance is significantly optimized.

[0044] In this application, the method of fitting the change rate of the unit cell parameters a / c of the positive electrode active material before and after pressure application treatment with the pressure of the pressure application treatment includes: (1) The positive electrode active material is dried, usually using a blast drying oven or a vacuum drying oven, and dried at 100°C - 150°C for 1 - 2 hours; (2) Weigh 3 g - 5 g of the positive electrode active material dried in step (1), add it to a compaction mold, and vibrate the mold to make the powder surface flat; (3) Place the mold processed in step (2) in the equipment and slowly pressurize it to the specified pressure T i (The reference pressures are 225 MPa, 300 MPa, 375 MPa, 450 MPa, 525 MPa, 600 MPa, 675 MPa, or 750 MPa, etc.), leave it standing for 30 s and then remove it. Demold the positive electrode active material. Only one pressure point can be measured for one sample preparation. After measurement, re-prepare the sample for testing the next pressure point; (4) Put the demolded positive electrode tablet into a mortar, gently dissociate the tablet and perform particle screening using a 300 - 400 mesh standard sieve to make the tablet completely pulverized without flaky particles, and obtain a powder sample; (5) Perform XRD refinement tests on the obtained powder samples, and calculate the lattice parameters a and c of the cathode active material at different pressures according to the results, that is, a i and c i ; (6) According to S i = a i / c i , the lattice parameter a i / c i value of the cathode active material at different pressures can be obtained; (7) According to △S i = (S i - S0) / S0×1000‰, the change rate of the lattice parameter a i / c i value of the cathode active material at different pressures can be obtained.

[0045] (8) Make a scatter plot of the change rate △S i of the lattice parameter a / c of the cathode active material before and after pressure application and T i , and fit a straight line. Judge whether the structure of the cathode active material is stable according to the slope k of the fitted straight line.

[0046] In some embodiments, 0.01 < k < 0.02. Thus, it is beneficial to further improve the particle strength of the cathode active material.

[0047] In some embodiments, when T i is 375 MPa, △S i is 1‰ - 3‰; and / or, when T i is 525 MPa, △S i is 2‰ - 8‰; optionally, 3‰ - 6‰.

[0048] As an example, when T i is 375 MPa, △S i can be 1‰, 1.5‰, 2‰, 2.5‰ or 3‰, etc.

[0049] As an example, when T i is 525 MPa, △S i can be 2‰, 3‰, 4‰, 5‰, 6‰, 7‰ or 8‰, etc.

[0050] When △S i is within the above range, the cathode active material has excellent processing adaptability and strong particle size stability. <000​​​​(1) The positive electrode active material is dried, usually by using a forced-air oven or a vacuum oven, at 100℃-150℃ for 1-2 hours; (2) Weigh 3g-5g of the positive electrode active material dried in step (1), add it into the compaction mold, and vibrate the mold to make the powder smooth. (3) Place the mold processed in step (2) into the equipment and slowly pressurize it to the specified pressure T. i (When the pressure is 375MPa, the corresponding T is obtained from subsequent calculations) i ΔS at 375MPa i When the pressure is 525 MPa, the corresponding T is obtained from subsequent calculations. i ΔS at 525MPa i After standing for 30 seconds, remove the positive electrode active material and demold it. Only one pressure point can be measured at a time. After the test is completed, prepare a new sample and test the next pressure point. (4) Place the demolded positive electrode tablet into a mortar, gently separate the tablet and sieve it with a 300-400 mesh standard sieve to make the tablet completely pulverized without any flaky particles, and obtain a powder sample. (5) The obtained powder sample was subjected to XRD fine-tuning test, and the cell parameters a and c of the positive electrode active material under different pressures were calculated based on the results, i.e., a i c i ; (6) According to S i =a i / c i The cell parameters a of the positive electrode active material under different pressures can be obtained. i / c i value; (7) According to △S i =(S i -S0) / S0×1000‰, the cell parameter a of the positive electrode active material under different pressures can be obtained. i / c i The rate of change of the value.

[0052] In some embodiments, the positive electrode active material satisfies: 0% Di ≤30%; optional, 0% Di ≤20%, where S Di =(D 50（T0） -D 50（Ti） ) / D 50（T0） D 50（T0） D of the positive electrode active material before pressure treatment 50 Particle size, D 50（Ti） D of the positive electrode active material after pressure treatment 50 Particle size.​​

[0053] As an example, S Di It can be 1%, 5%, 8%, 10%, 15%, 20%, 25%, or 30%, etc.

[0054] S Di Within the above range, the positive electrode active material is not easily broken or deformed after being subjected to pressure, the particle structure is relatively stable, and the compressive strength is strong.

[0055] In some embodiments, the D of the positive electrode active material 50 The particle size is 0.3μm-2.0μm; optionally, 0.5μm-1.5μm.

[0056] As an example, the D of the positive electrode active material 50 The particle size can be 0.3μm, 0.5μm, 0.8μm, 1.0μm, 1.3μm, 1.5μm, 1.8μm or 2.0μm, etc.

[0057] D of positive electrode active material 50 When the particle size is within the above range, it is beneficial to improve the processing performance of the positive electrode active material and increase the compaction density of the positive electrode active material.

[0058] In particle size distribution, D 50 Also known as the median particle size, it means that 50% of the volume of particles are smaller than or equal to this value. In this application, D 50（Ti） and D 50 It can be measured using a Malvern particle size analyzer.

[0059] In some embodiments, the positive electrode active material satisfies: 0% Ai ≤30%, optional, 0% Ai ≤20%; where S Ai =(S Ai(Ti) -S A0(T0) ) / S A0(T0) S A0(T0) S represents the specific surface area of ​​the positive electrode active material before pressure treatment. Ai(Ti) The specific surface area of ​​the positive electrode active material after pressure treatment.

[0060] As an example, S Ai It can be 1%, 5%, 8%, 10%, 15%, 20%, 25%, or 30%, etc. S Ai Within the above range, the positive electrode active material particles have high strength, strong compressive strength, and are not prone to cracking.

[0061] Among them, the specific surface area of ​​the positive electrode active material refers to the surface area per unit mass of the positive electrode active material, S​​A0(T0) and S Ai(Ti) It can be measured using a surface area analyzer.

[0062] In some embodiments, the proportion of primary particles of the positive electrode active material with a size in the range of 0 μm to 0.1 μm is 10%-60%, optionally 10%-40%; the proportion of primary particles of the positive electrode active material with a size in the range of 0.1 μm to 0.2 μm is 30%-80%, optionally 20%-60%.

[0063] As an example, the proportion of primary particles with a size in the range of 0μm-0.1μm in the positive electrode active material can be 10%, 20%, 30%, 40%, 50% or 60%, etc., and the proportion of particles with a size in the range of 0.1μm-0.2μm can be 30%, 40%, 50%, 60%, 70% or 80%, etc.

[0064] Meeting the above particle size distribution allows for close packing of primary particles, which is beneficial for increasing the compaction density of the positive electrode active material.

[0065] In some embodiments, the positive electrode active material includes secondary particles, which are formed by the aggregation of primary particles.

[0066] In some embodiments, the maximum particle size of the primary particles of the positive electrode active material is 0.4 μm-0.7 μm; and / or, the minimum particle size of the primary particles of the positive electrode active material is D. min 0μm <D min ≤0.1μm; and / or, the average particle size of the primary particles of the positive electrode active material is 0.1μm-0.4μm.

[0067] As an example, the maximum particle size of the primary particles in the positive electrode active material can be 0.4 μm, 0.5 μm, 0.6 μm or 0.7 μm, etc., the minimum particle size of the primary particles can be 0.01 μm, 0.02 μm, 0.04 μm, 0.06 μm, 0.08 μm or 0.1 μm, etc., and the average particle size of the primary particles can be 0.1 μm, 0.2 μm, 0.3 μm or 0.4 μm, etc.

[0068] Therefore, the particle size distribution of the positive electrode active material is relatively concentrated, which is beneficial to improving the compaction density.

[0069] As an example, by testing the positive electrode active material using scanning electron microscopy and employing intelligent analysis software for lithium-ion battery material microscopic images, a distribution map of the primary particle count can be obtained, which can be used as a reference. Figure 3 As can be seen, the distribution of particle size and quantity follows a normal distribution.

[0070] In some embodiments, the particle size distribution curve of the positive electrode active material has a first characteristic peak and a second characteristic peak, wherein the peak position of the first characteristic peak is less than or equal to 1 μm, the peak position of the second characteristic peak is greater than or equal to 1 μm, and the difference between the peak position of the second characteristic peak and the peak position of the first characteristic peak is 0.5 μm-3 μm.

[0071] As an example, the peak position of the first characteristic peak can be 0.1μm, 0.2μm, 0.4μm, 0.6μm, 0.8μm or 1μm, etc., and the peak position of the second characteristic peak can be 1.5μm, 1.8μm, 2μm, 2.3μm, 2.5μm, 2.8μm or 3μm, etc.

[0072] With the particle size distribution curve described above, smaller particles can effectively fill the gaps between larger particles, thereby increasing the compaction density of the positive electrode active material and helping to improve the battery's capacity.

[0073] As an example, the particle size distribution curve of the positive electrode active material can be obtained by testing with a Malvern particle size analyzer, which can be referred to... Figure 2 .

[0074] In some embodiments, the ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the second characteristic peak is 0.5-2.5, for example, it can be 0.5, 0.8, 1, 1.3, 1.5, 1.8, 2, 2.3, or 2.5. This is beneficial for optimizing particle size distribution and achieving close packing between the positive electrode active material particles.

[0075] In some embodiments, the compaction density of the positive electrode active material under 3T pressure is 2.0 g / cm³. 3 -2.4g / cm 3 For example, it can be 2.0 g / cm³. 3 2.1g / cm 3 2.2g / cm 3 2.3g / cm 3 Or 2.4g / cm 3 Therefore, the positive electrode active material has a high capacity utilization.

[0076] In some embodiments, the positive electrode active material includes a matrix and a carbon coating layer at least partially covering the surface of the matrix; wherein the matrix satisfies formula I: Li a Mn b Fe (1-b) M c (PO4) dFormula I; where 0.95≤a≤1.10, 0.3≤b≤0.7, 0<c≤0.5, 0.95≤d≤1.10; M includes at least one of Ti, Mg, Zn, Cu, Sr, Al, Zr, Y, Co, W, Ca, Nb, Sn, Sb, Mo, V, B, Si, and Na. Therefore, introducing element M helps reduce lattice distortion, improve ionic conductivity, and reduce polarization stress; simultaneously, forming a carbon coating layer helps improve the structural stability and electronic conductivity of the positive electrode active material.

[0077] As an example, a can be 0.95, 0.98, 1.00, 1.02, 1.04, 1.06, 1.08, or 1.10, etc.; b can be 0.3, 0.4, 0.5, 0.6, or 0.7, etc.; c can be 0.1, 0.2, 0.3, 0.4, or 0.5, etc.; d can be 0.95, 0.98, 1.00, 1.02, 1.04, 1.06, 1.08, or 1.10, etc.

[0078] In some embodiments, the positive electrode active material is a single crystal.

[0079] In some embodiments, based on the mass of the positive electrode active material, the content of the carbon coating layer is 1wt%-3wt%, for example, it can be 1wt%, 1.5wt%, 2wt%, 2.5wt%, or 3wt%, etc.; optionally, it is 1.5wt%-2wt%. In some embodiments, the carbon content in the positive electrode active material is 1wt%-2.5wt%, for example, it can be 1wt%, 1.5wt%, 2wt%, or 2.5wt%, etc. This is beneficial for improving the conductivity of the positive electrode active material, reducing the direct contact between the inside of the positive electrode active material and the electrolyte, effectively reducing interfacial side reactions, and thus achieving a significant improvement in cycle life.

[0080] In a second aspect, this application proposes a method for preparing positive electrode active materials. This method involves a two-step liquid-phase grinding and secondary coating process to obtain positive electrode active materials with high particle strength. The preparation method is simple, easy to operate, and readily applicable to industrial production. Specifically, the method includes: S1: Iron phosphate, manganese source, lithium source, phosphorus source, first M source, first carbon source and solvent are ground and mixed, and then subjected to first spray drying treatment to obtain first spray-dried material.

[0081] In some embodiments, the manganese source includes at least one selected from manganese sulfate, manganese carbonate, manganese nitrate, and manganese tetroxide; and / or, the lithium source includes at least one selected from lithium carbonate, lithium hydroxide, lithium phosphate, lithium nitrate, anhydrous lithium hydroxide, and lithium oxide; and / or, the phosphorus source includes at least one selected from phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, manganese phosphate, lithium phosphate, and pyrophosphate. Therefore, the raw materials are widely available, the cost is low, and large-scale promotion is convenient.

[0082] In some embodiments, the manganese source may be manganese tetroxide, and the manganese content of manganese tetroxide is 0.68-0.75%.

[0083] In some embodiments, the first M source includes at least one of the following: oxide, hydroxide, carbonate, fluoride, sulfate, phosphate, and chloride corresponding to the M element. Thus, the introduction of the M element can improve the structural stability of the positive electrode active material.

[0084] In some embodiments, the first carbon source includes at least one selected from glucose, sucrose, and organic polymers. This facilitates the formation of a carbon coating layer, thereby improving the conductivity of the positive electrode active material and reducing side reactions between the positive electrode active material and the electrolyte.

[0085] In some embodiments, the molar ratio of iron to phosphorus in the ferric phosphate is 0.940-0.990, for example, it can be 0.940, 0.950, 0.960, 0.970, 0.980 or 0.990.

[0086] As an example, iron phosphate, manganese source, lithium source, phosphorus source, first M source and first carbon source are weighed and mixed according to the molar ratio of Li, Mn, Fe, M and P in Formula I, and dispersed in a solvent with a certain solid content. Then, they are ground to the target particle size and subjected to the first spray drying treatment to obtain the first spray-dried material.

[0087] S2: Perform a first sintering treatment on the first spray-dried material to obtain the lithium manganese iron phosphate precursor.

[0088] In some embodiments, the first sintering process includes: heating the first spray-dried material to a first sintering temperature T1 at a first heating rate v1 and holding it at that temperature for a first time t1, then cooling it from T1 to below 330°C at a first cooling rate v2, and then naturally cooling it to room temperature; wherein v1 > 2°C / min, 450°C ≤ T1 ≤ 750°C, 3h ≤ t1 ≤ 12h; and / or v2 > 2°C / min. This is beneficial for improving the reaction rate between the raw material components and controlling the particle size and morphology of the lithium manganese iron phosphate precursor.

[0089] As an example, v1 can be 2.5℃ / min, 2.8℃ / min, 3℃ / min, 3.3℃ / min, 3.5℃ / min, 3.8℃ / min, or 4℃ / min, etc.; T1 can be 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, or 750℃, etc.; t1 can be 3h, 5h, 7h, 9h, 10h, or 12h, etc.; v2 can be 2.5℃ / min, 2.8℃ / min, 3℃ / min, 3.3℃ / min, 3.5℃ / min, 3.8℃ / min, or 4℃ / min, etc.

[0090] In some embodiments, the average particle size of the lithium manganese iron phosphate precursor is 0.1 μm-0.4 μm, for example, it can be 0.1 μm, 0.2 μm, 0.3 μm or 0.4 μm.

[0091] In some embodiments, the carbon content in the lithium manganese iron phosphate precursor is 0.01wt%-0.5wt%, for example, it can be 0.01wt%, 0.05wt%, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, or 0.5wt%.

[0092] S3: The lithium manganese iron phosphate precursor is mixed with the second M source, the second carbon source, and the solvent, and then ground and mixed, and subjected to a second spray drying treatment to obtain the second spray-dried material.

[0093] In some embodiments, each of the second M sources independently includes at least one of the following: oxide, hydroxide, carbonate, fluoride, sulfate, phosphate, and chloride corresponding to the M element. Thus, the introduction of the M element can improve the structural stability of the positive electrode active material.

[0094] In some embodiments, the second carbon source includes at least one of glucose, sucrose, and an organic polymer (such as polyethylene glycol PEG1500). This facilitates the formation of a carbon coating layer, thereby improving the conductivity of the positive electrode active material and reducing side reactions between the positive electrode active material and the electrolyte.

[0095] This application does not limit the solvent and solid content used in the two grinding processes in principle, as long as they can be sufficiently dispersed and do not chemically react with iron phosphate, manganese source, phosphorus source, lithium source, M source, carbon source, and lithium manganese iron phosphate precursor. For manufacturing cost considerations, the solvent mentioned in this application can be deionized water with a solid content ≥35%.

[0096] S4: Perform a second sintering treatment on the second spray-dried material to obtain the positive electrode active material.

[0097] In some embodiments, the second sintering process includes: heating the second spray-dried material to a second sintering temperature T2 at a second heating rate v3 and holding it at that temperature for a second time t2, then cooling it from T2 to below 330°C at a second cooling rate v4, and then naturally cooling it to room temperature; wherein v3 < 2°C / min, 600°C ≤ T2 ≤ 850°C, 4h ≤ t2 ≤ 20h; and / or v4 < 2°C / min. This is beneficial for improving the crystallinity of the positive electrode active material, reducing lattice defects and internal stress, and improving the stability of the crystal structure.

[0098] As an example, v3 can be 0.5℃ / min, 0.8℃ / min, 1℃ / min, 1.2℃ / min, 1.5℃ / min, 1.7℃ / min, or 1.9℃ / min, etc.; T2 can be 600℃, 650℃, 700℃, 750℃, 800℃, or 850℃, etc.; t2 can be 4h, 6h, 8h, 10h, 12h, 14h, 16h, 18h, or 20h, etc.; v4 can be 0.5℃ / min, 0.8℃ / min, 1℃ / min, 1.2℃ / min, 1.5℃ / min, 1.7℃ / min, or 1.9℃ / min, etc.

[0099] In some embodiments, the sintered material can be crushed after the second sintering treatment, for example, by using an air jet mill, to obtain the positive electrode active material.

[0100] This application utilizes the aforementioned two-step liquid-phase grinding and secondary coating process, combined with precise control of the key sintering temperature during the solid-phase reaction, and controlled heating and cooling rates, to produce a lithium manganese iron phosphate cathode active material that possesses high compressive strength, low stress deformation, and high compaction density. This significantly suppresses particle cracking during electrode rolling. This process reduces direct contact between the cathode active material and the electrolyte, effectively minimizing interfacial side reactions, thereby significantly improving cycle life and effectively meeting battery application requirements.

[0101] In a third aspect, this application proposes a battery comprising a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer located on at least one side of the positive current collector, the positive active material layer comprising the positive active material described in the first aspect of this application or a positive active material prepared using the method described in the second aspect of this application. It should be noted that the features and effects described for the positive active material in the first aspect of this application and the positive active material prepared using the method described in the second aspect are also applicable to the battery of the third aspect of this application, and will not be repeated here.

[0102] The description of the various embodiments above tends to emphasize the differences between the various embodiments. The similarities or similarities between them can be referred to, and for the sake of brevity, they will not be repeated here.

[0103] The following specific embodiments illustrate the solution of this application. It should be noted that these embodiments are for illustrative purposes only and should not be considered as limiting the scope of this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0104] Example 1 Step S1: Weigh and mix ferric phosphate, manganese tetroxide, lithium carbonate, lithium dihydrogen phosphate, titanium dioxide, ammonium metavanadate, tungsten trioxide, and cobalt carbonate according to the molar ratio of Li, Mn, Fe, Ti, V, W, Co, and P of 1.035:0.6:0.4:0.04:0.03:0.02:0.025:1.020. The first carbon source (glucose) accounts for 5.0% of the theoretical yield of lithium manganese iron phosphate. Add it to deionized water as a solvent, control the solid content at 35wt%, and grind it using a ball mill (2000 rpm). 50 The machine is stopped when the particle size reaches 0.15 μm, and the first grinding material is obtained. A first grinding material with a solid content of 35% is fed into the feed inlet of an atomizing disc atomizing dryer (inlet temperature set at 245±5℃, outlet temperature controlled at 100±5℃) for the first spray drying process to obtain the first spray-dried material.

[0105] Step S2: The first spray-dried material is heated to 600°C at a heating rate of 2.6°C / min under N2 atmosphere, held at that temperature for 6 hours, then cooled to 300°C at a rate of 3.3°C / min, and then naturally cooled to room temperature. After cooling in the furnace, lithium manganese iron phosphate precursor is obtained.

[0106] Step S3: Weigh and mix the lithium manganese iron phosphate precursor and alumina according to a molar ratio of lithium manganese iron phosphate to Al of 1:0.004. The second carbon source (glucose) accounts for 5.5% of the lithium manganese iron phosphate precursor, and the second carbon source (polyethylene glycol PEG1500) accounts for 5.5% of the lithium manganese iron phosphate precursor. Add them to deionized water as solvent, controlling the solid content to 40wt%. Grind using a ball mill (2000 rpm). 50 The machine is stopped when the particle size reaches 0.30 μm, and the second grinding material is obtained. A second grinding material with a solid content of 40% is fed into the feed inlet of a two-fluid spray dryer (inlet temperature set at 245±5℃, outlet temperature controlled at 100±5℃) for a second spray drying process to obtain a second spray-dried material. Step S4: The second spray-dried material is heated to 760°C at a heating rate of 1°C / min under N2 atmosphere, held at that temperature for 10h, then cooled to 300°C at a rate of 1.8°C / min, and then naturally cooled to room temperature. After cooling in the furnace, the sintered material is obtained. The sintered material was crushed using an air jet mill. 50 The process was stopped when the thickness reached 0.8 μm, and lithium manganese iron phosphate cathode active material was obtained.

[0107] The differences between other embodiments and comparative examples and embodiment 1 are shown in Tables 1-1 and 1-2.

[0108] Table 1-1

[0109] Table 1-2

[0110] " / " indicates that the substance was not added or that the step was not performed.

[0111] The positive electrode active materials prepared in the above embodiments and comparative examples were tested as follows, and the results are shown in Tables 2-1 and 2-2.

[0112] 1. The rate of change of the cell parameter a / c of ​​the positive electrode active material before and after pressure treatment is fitted to the pressure of the pressure treatment as a straight line: (1) The positive electrode active material is dried, usually by using a forced-air oven or a vacuum oven, at 100℃-150℃ for 1-2 hours; (2) Weigh 3g-5g of the positive electrode active material dried in step (1), add it into the compaction mold, and vibrate the mold to make the powder smooth. (3) Place the mold processed in step (2) into the equipment and slowly pressurize it to the specified pressure T. i (Reference pressures are 225MPa, 375MPa and 525MPa respectively). After standing for 30 seconds, remove the sample and demold the positive electrode active material. Only one pressure point can be measured at a time. After the test, prepare a new sample and test the next pressure point. (4) Place the demolded positive electrode tablet into a mortar, gently separate the tablet and sieve it with a 300-400 mesh standard sieve to make the tablet completely pulverized without any flaky particles, and obtain a powder sample. (5) The obtained powder sample was subjected to XRD fine-tuning test, and the cell parameters a and c of the positive electrode active material under different pressures were calculated based on the results, i.e., a i c i ; (6) According to S i =a i / c iThe cell parameters a of the positive electrode active material under different pressures can be obtained. i / c i value; (7) According to △S i =(S i -S0) / S0×1000‰, the cell parameter a of the positive electrode active material under different pressures can be obtained. i / c i The rate of change of the value.

[0113] (8) The rate of change of the cell parameter a / c of ​​the positive electrode active material before and after pressure treatment, ΔS i With T i Perform a scatter plot and fit a straight line to obtain the slope k of the fitted line.

[0114] 2. D of the positive electrode active material before and after pressure treatment 50 Particle size change rate S Di The test: Particle size was tested using a Malvern 3000 instrument; the instrument's light-blocking limits were 10%-20%; the stirring speed in the sample cell was 2000 rpm / min; 0.03-0.04 g of sample was weighed, 1 g of 10% NP40 was added, and then 9 g of pure water was added (approximately 10 ml in total). The mixture was sonicated for 3 min (40 kHz). After dispersion, all liquid was transferred to the sample cell, and the sample was measured to determine the particle size distribution. Specifically, the test samples included the directly obtained positive electrode active material (before pressing) and the positive electrode active material obtained after applying 525 MPa pressure to the positive electrode active material using an MCP-PD51 tester, followed by manual grinding in a mortar for 20 min, and passing through a 300-mesh sieve (after pressing). Then, according to S... Di =(D 50压前 -D 50压后 ) / D 50压前 Calculate D 50 Particle size change rate S Di .

[0115] 3. Change rate S of specific surface area of ​​positive electrode active material before and after pressure treatment Ai The test: The test was conducted using a MCP-3030 instrument: After weighing the empty tube, 2g ± 0.01g of sample was weighed into a 1 / 4 specific surface area tube, placed in a degassing station, and purged with nitrogen at 100℃ for two hours. After degassing, the tube was allowed to cool naturally to room temperature, and the total weight after degassing was recorded as the sample mass. The degassed sample tube was then tested using a MCP-3030 specific surface area analyzer. The instrument was set to the nitrogen adsorption method, with relative pressures ranging from 0.05 to 0.2 at 10 points. After the tests, the specific surface area of ​​the material was calculated using the multi-point BET equation. The test samples included the directly obtained positive electrode active material (i.e., pre-pressure) and the positive electrode active material obtained after applying a pressure of 525MPa to the positive electrode material using an MCP-PD51 analyzer, followed by manual grinding in a mortar for 20 minutes and passing through a 300-mesh sieve (i.e., post-pressure). The specific surface area was then calculated according to S... Ai =(S A压后 -S A压前 ) / S A压前 Calculate the increase rate of specific surface area S Ai .

[0116] Other testing methods have been described above and will not be repeated here.

[0117] Table 2-1

[0118] Table 2-2

[0119] The positive electrode active materials from the aforementioned embodiments and comparative examples are assembled into batteries, and the assembly method is as follows: Positive electrode preparation: The prepared positive electrode active material, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 90:5:5, and then mixed with an appropriate amount of solvent N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. The obtained positive electrode slurry was coated onto an aluminum foil (20 μm thick) as a current collector and dried at 120°C for 5 minutes. The single-sided coating weight of the positive electrode active material layer was 80 mg / cm². 2 The positive electrode is then compressed and shaped using a roller press, with the compaction controlled at 2.0 g / cm³. 3 The rolled electrode sheets are punched and weighed, and then punched into a disc shape with a diameter of 14mm to ensure that the edges of the electrode sheets are free of gaps and wrinkles.

[0120] Button cell fabrication: In a glove box under an argon atmosphere, the obtained positive electrode, separator, negative electrode, and electrolyte are assembled into a button cell. The negative electrode uses a 15mm diameter lithium metal sheet; the separator uses a 23μm thick polypropylene membrane; and the electrolyte is a 1mol / L mixture of equal parts LiPF6, ethylene carbonate (EC), and diethyl carbonate (DEC).

[0121] The batteries in the aforementioned embodiments and comparative examples were tested as follows, and the test results are shown in Table 3.

[0122] Capacity retention: High-temperature test at 45℃ was conducted in a high-temperature chamber, with the charge / discharge voltage range set to 2.5V-4.35V; the charging step was set to CC-CV with a cutoff voltage of 4.35V, and the discharging step was set to CV discharge with a cutoff current of 0.05C; the nominal capacity at 1C was 140mAh / g; after 80 charge / discharge cycles, the capacity retention of the battery was calculated and used as an evaluation of cycle durability.

[0123] Table 3

[0124] Figure 1 The microstructure of the lithium manganese iron phosphate cathode active material is shown, which exhibits a relatively uniform particle distribution and no large-area abnormal agglomeration or structural defect regions are observed.

[0125] Figure 2 The particle size distribution curve of the positive electrode active material in Example 1 of this application shows that the positive electrode active material in Example 1 exhibits a bimodal distribution. The first characteristic peak is located at 0.43 μm, and the second characteristic peak is located at 2.0 μm. The difference between the peak positions of the second and first characteristic peaks is 1.57 μm, and the ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the second characteristic peak is 1.39. Therefore, smaller particles can effectively fill the gaps between larger particles, thereby increasing the compaction density of the positive electrode active material and contributing to improved battery capacity.

[0126] Figure 3 The diagram shows the distribution of primary particle size in the positive electrode active material of Example 1 of this application. It indicates that the distribution of primary particle size in the positive electrode active material of Example 1 follows a normal distribution, with 28.9% of the particles being in the range of 0μm-0.1μm and 53.9% being in the range of 0.1μm-0.2μm. Therefore, the particle size distribution of the positive electrode active material is relatively concentrated, which is beneficial to improving the compaction density.

[0127] Figure 4XRD patterns of the lithium iron manganese phosphate cathode active material prepared in Example 1 before and after cycling. It can be seen that the peak shape remains basically unchanged, the peak position has no obvious shift, and no new impurity phase peaks are generated, indicating that the cathode active material has excellent structural stability.

[0128] As can be seen from Table 3, the cathode active materials of Examples 1-22 satisfy 0.005 < k < 0.03, indicating that they have relatively high compressive strength and small stress deformation, which is beneficial to improving the high-temperature cycling stability of the battery.

[0129] The k value of the cathode active material in Comparative Example 1 is too small, resulting in slow lithium-ion diffusion kinetics, which is not conducive to the performance of the battery's electrical properties.

[0130] The cathode active material in Comparative Example 2 was not subjected to the second sintering treatment during the preparation process, resulting in an excessively large k value and poor battery cycling performance.

[0131] It should be noted that this application is not limited to the above embodiments. The above embodiments are only examples, and embodiments with the same structure and the same effect as the technical idea within the scope of the technical solution of this application are included in the technical scope of this application. In addition, within the scope of not departing from the gist of this application, various modifications that can be conceived by those skilled in the art to the embodiments, and other embodiments constructed by combining some constituent elements of the embodiments are also included in the scope of this application.

Claims

1. A positive electrode active material, characterized in that, Including lithium manganese iron phosphate, The rate of change of the cell parameter a / c of ​​the positive electrode active material before and after the pressure treatment is fitted to the straight line obtained by the pressure treatment, which satisfies: △S i =kT i 0.005 <k<0.03; Among them, T i The pressure applied to the positive electrode active material is measured in MPa, where 225 MPa ≤ T. i ≤750MPa, the pressure treatment includes i pressure treatments at different pressures, 3≤i≤10; △S i ΔS represents the rate of change of the cell parameter a / c of ​​the positive electrode active material before and after pressure treatment. i =(S i -S0) / S0×1000‰,S i The cell parameter a of the positive electrode active material after pressure treatment. i / c i Value, S i =a i / c i S0 is the cell parameter a0 / c0 value of the positive electrode active material before pressure treatment, S0=a0 / c0.

2. The positive electrode active material according to claim 1, characterized in that, 0.01<k<0.02。 3. The positive electrode active material according to claim 1, characterized in that, T i When the pressure is 375 MPa, △S i For 1‰-3‰; and / or, T i When the pressure is 525 MPa, △S i The range is 2‰-8‰; in any location, it is 3‰-6‰.

4. The positive electrode active material according to claim 1, characterized in that, The positive electrode active material satisfies: 0% Di ≤30%; optional, 0% Di ≤20%, where S Di =(D 50（T0） -D 50（Ti） ) / D 50（T0） ,​​ D 50（T0） D of the positive electrode active material before pressure treatment 50 Particle size, D 50（Ti） D of the positive electrode active material after pressure treatment 50 Particle size.

5. The positive electrode active material according to claim 4, characterized in that, The positive electrode active material D 50 The particle size is 0.3μm-2.0μm; optionally, 0.5μm-1.5μm.

6. The positive electrode active material according to claim 1, characterized in that, The positive electrode active material satisfies: 0% Ai ≤30%, optional, 0% Ai ≤10%;​​ Among them, S Ai =(S Ai(Ti) -S A0(T0) ) / S A0(T0) , S A0(T0) S represents the specific surface area of ​​the positive electrode active material before pressure treatment. Ai(Ti) The specific surface area of ​​the positive electrode active material after pressure treatment.

7. The positive electrode active material according to claim 1, characterized in that, The proportion of primary particles of the positive electrode active material with a size in the range of 0 μm to 0.1 μm is 10%-60%, optionally 10%-40%; The proportion of primary particles of the positive electrode active material with a size in the range of 0.1μm-0.2μm is 30%-80%, optionally 20%-60%.

8. The positive electrode active material according to claim 7, characterized in that, The maximum particle size of the primary particles of the positive electrode active material is 0.4 μm-0.7 μm; and / or, The minimum particle size of the primary particles of the positive electrode active material is D. min 0μm <D min ≤0.1μm; and / or, The average particle size of the primary particles of the positive electrode active material is 0.1 μm-0.4 μm.

9. The positive electrode active material according to claim 1, characterized in that, The particle size distribution curve of the positive electrode active material has a first characteristic peak and a second characteristic peak. The peak position of the first characteristic peak is less than or equal to 1 μm, the peak position of the second characteristic peak is greater than or equal to 1 μm, and the difference between the peak position of the second characteristic peak and the peak position of the first characteristic peak is 0.5 μm-3 μm.

10. The positive electrode active material according to claim 9, characterized in that, The ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the second characteristic peak is 0.5-2.

5.

11. The positive electrode active material according to claim 1, characterized in that, The compaction density of the positive electrode active material under 3T pressure is 2.0 g / cm³. 3 -2.4g / cm 3 .

12. The positive electrode active material according to any one of claims 1-11, characterized in that, The positive electrode active material includes a matrix and a carbon coating layer that is at least partially coated on the surface of the matrix; Wherein, the matrix satisfies Equation I: Li a Mn b Fe (1-b) M c (PO4) d Formula I; Wherein, 0.95≤a≤1.10, 0.3≤b≤0.7, 0<c≤0.5, and 0.95≤d≤1.10; M includes at least one of Ti, Mg, Zn, Cu, Sr, Al, Zr, Y, Co, W, Ca, Nb, Sn, Sb, Mo, V, B, Si, and Na.

13. The positive electrode active material according to claim 12, characterized in that, Based on the mass of the positive electrode active material, the content of the carbon coating layer is 1wt%-3wt%; optionally, 1.5wt%-2wt%.

14. A method for preparing the positive electrode active material according to any one of claims 1-13, characterized in that, include: Iron phosphate, manganese source, lithium source, phosphorus source, first M source, first carbon source and solvent are ground and mixed, and then subjected to first spray drying treatment to obtain first spray-dried material; The first spray-dried material is subjected to a first sintering treatment to obtain lithium manganese iron phosphate precursor; The lithium manganese iron phosphate precursor is mixed with a second M source, a second carbon source, and a solvent, then ground and mixed, and subjected to a second spray drying process to obtain a second spray-dried material. The second spray-dried material is subjected to a second sintering treatment to obtain the positive electrode active material.

15. The method according to claim 14, characterized in that, The first sintering process includes: heating the first spray-dried material to a first sintering temperature T1 at a first heating rate v1 and holding it at that temperature for a first time t1, and then cooling it from T1 to below 330°C at a first cooling rate v2, wherein v1 > 2°C / min, 450°C ≤ T1 ≤ 750°C, 3h ≤ t1 ≤ 12h; and / or v2 > 2°C / min.

16. The method according to claim 15, characterized in that, The second sintering process includes: heating the second spray-dried material to a second sintering temperature T2 at a second heating rate v3 and holding it at that temperature for a second time t2, and then cooling it from T2 to below 330°C at a second cooling rate v4, wherein v3 < 2°C / min, 600°C ≤ T2 ≤ 850°C, 4h ≤ t2 ≤ 20h; and / or, v4 < 2°C / min.

17. The method according to claim 14, characterized in that, The average particle size of the lithium manganese iron phosphate precursor is 0.1 μm-0.4 μm; and / or, The molar ratio of iron to phosphorus in the ferric phosphate is 0.940-0.990; and / or, The carbon content in the lithium manganese iron phosphate precursor is 0.01wt%-0.5wt%.

18. The method according to claim 14, characterized in that, The manganese source includes at least one of manganese sulfate, manganese carbonate, manganese nitrate, and manganese tetroxide; and / or, The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium nitrate, anhydrous lithium hydroxide, and lithium oxide; and / or, The phosphorus source includes at least one selected from phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, manganese phosphate, lithium phosphate, and pyrophosphate; and / or, The first M source and the second M source each independently include at least one of the following: oxides, hydroxides, carbonates, fluorides, sulfates, phosphates, and chlorides corresponding to the M element; and / or, The first carbon source and the second carbon source include at least one of glucose, sucrose, and organic polymers.

19. A battery, characterized in that, The invention includes a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer located on at least one side of the positive current collector, the positive active material layer comprising the positive active material according to any one of claims 1-13 or the positive active material prepared by the method according to any one of claims 14-18.

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