Positive electrode active material, preparation method thereof and secondary battery

Abstract

The chemical formula of the positive electrode active material comprises LiMnx1Fey1A1z1PO4 and LiMnx2Fey2A2z2PO4, an A1 element or an A2 element comprises one or more of Ti, V, Co, Ni, Nb, Mo and Zr, and the chemical formula meets the following conditions: z1 = a * y1, 0.02% < = a < = 3.6%, z2 = b * y2, 4.5% < = b < = 9%, and 4.52% < = a + b < = 12.6%. According to the present invention, the Al element and the A2 element are introduced to be doped in the iron site of the LMFP to replace the Fe ion in the LMFP, and the lattice distortion and the cell parameter reduction can be caused at the doping position of the Al element and the A2 element so as to inhibit the growth of the LMFP crystal grain, such that the positive electrode active material with the particle size grading ratio is obtained so as to improve the compaction density of the positive electrode active material;

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly relates to a positive electrode active material, a preparation method thereof, and a secondary battery. Background Art

[0002] Lithium manganese iron phosphate (LMFP) has the same olivine structure as lithium iron phosphate (LFP), and has the advantages of high structural stability, excellent thermal safety (not easily decomposed at high temperatures), and long cycle life. By introducing manganese (Mn), both the voltage platform and energy density of lithium manganese iron phosphate can be improved. Moreover, manganese resources are abundant and the price is lower than that of cobalt and nickel, and the comprehensive cost is close to LFP and significantly lower than that of ternary materials (NCM / NCA). However, due to the addition of manganese element, the overall particle size of lithium manganese iron phosphate material is smaller and the particle size distribution range is narrower. Therefore, the tap density of lithium manganese iron phosphate is lower than that of lithium iron phosphate, and it is difficult to improve the tap density of lithium manganese iron phosphate. Therefore, how to improve the tap density of lithium manganese iron phosphate has become the key. Summary of the Invention

[0003] The purpose of the present invention is to provide a positive electrode active material, a preparation method thereof, and a secondary battery, so as to solve the problem that the tap density of the positive electrode active material of lithium manganese iron phosphate in the prior art is relatively low.

[0004] To achieve the purpose of the present invention, the present invention provides the following technical solutions:

[0005] In a first aspect, the present invention provides a positive electrode active material, and the chemical formula of the positive electrode active material includes LiMn x1 Fe y1 A1 z1 PO 4 and LiMn x2 Fe y2 A2 z2 PO 4 wherein, the A1 element or the A2 element includes one or more of Ti, V, Co, Ni, Nb, Mo, and Zr, and the chemical formula satisfies: z1 = a × y1, 0.02% ≤ a ≤ 3.6%, z2 = b × y2, 4.5% ≤ b ≤ 9%, 4.52% ≤ a + b ≤ 12.6%.

[0006] In an embodiment, the average primary particle size of the positive electrode active material is 200 nm to 450 nm. The positive electrode active material includes a first particle, a second particle, and a third particle. The primary particle size of the first particle is smaller than that of the second particle, and the primary particle size of the second particle is smaller than that of the third particle.

[0007] In one embodiment, the primary particle size D1 of the first particles satisfies 1 nm ≤ D1 < 200 nm, the primary particle size D2 of the second particles satisfies 200 nm ≤ D2 ≤ 400 nm, and the primary particle size D3 of the third particles satisfies 400 nm < D3 ≤ 1000 nm.

[0008] In one embodiment, the mass ratio of the first particles in the positive electrode active material is 20% - 50%, the mass ratio of the second particles in the positive electrode active material is 30% - 40%, and the mass ratio of the third particles in the positive electrode active material is 10% - 35%.

[0009] In one embodiment, the first particles include first particle I and first particle II. The primary particle size D11 of the first particle I satisfies 1 nm ≤ D11 < 150 nm, and the primary particle size D12 of the first particle II satisfies 150 nm ≤ D12 < 200 nm; and / or, the third particles include third particle I and third particle II. The primary particle size D31 of the third particle I satisfies 400 nm < D31 ≤ 500 nm, and the primary particle size D32 of the third particle II satisfies 500 nm < D32 ≤ 1000 nm.

[0010] In one embodiment, the mass ratio of the first particle I in the positive electrode active material is 5% - 20%, the mass ratio of the first particle II in the positive electrode active material is 15% - 30%, the mass ratio of the third particle I in the positive electrode active material is 5% - 20%, and the mass ratio of the third particle II in the positive electrode active material is 5% - 15%.

[0011] In one embodiment, the chemical formula of the positive electrode active material further satisfies: x1 + x2 = x, y1 + y2 = y, 0.2 ≤ x / (x + y) ≤ 0.8, n = x / y, 0.25 ≤ n ≤ 4.

[0012] In a second aspect, the present invention provides a method for preparing a positive electrode active material. The preparation method is used to prepare the positive electrode active material according to any one of the embodiments in the first aspect. The preparation method includes: mixing a first reaction solution with an Mn source and a second reaction solution with an Fe source to obtain two third reaction solutions, and adding an A1 source and an A2 source to the two third reaction solutions respectively; adding a Li source and a P source to the two third reaction solutions to obtain two mixed solutions, and reacting the two mixed solutions to obtain a solid precursor; calcining the solid precursor to obtain the positive electrode active material.

[0013] In one embodiment, after mixing a first reaction solution having a Mn source and a second reaction solution having an Fe source to obtain two portions of a third reaction solution, adding an A1 source and an A2 source to the two portions of the third reaction solution respectively includes: mixing the first reaction solution having the Mn source and the second reaction solution having the Fe source to obtain the third reaction solution; dividing the third reaction solution into a third reaction solution I and a third reaction solution II; adding the A1 source to the third reaction solution I to obtain a first precursor solution, and adding the A2 source to the third reaction solution II to obtain a second precursor solution.

[0014] In one embodiment, adding a Li source and a P source to the two portions of the third reaction solution to obtain two mixed solutions, and obtaining a solid precursor after reacting the two mixed solutions includes: adding the Li source and the P source to the first precursor solution and the second precursor solution respectively to obtain a first mixed solution and a second mixed solution; reacting the first mixed solution and the second mixed solution respectively to obtain a first precursor and a second precursor; mixing the first precursor and the second precursor to obtain the solid precursor.

[0015] In a third aspect, the present invention provides a secondary battery, which includes the positive electrode active material according to any one of the embodiments in the first aspect, or the secondary battery includes the positive electrode active material prepared by the preparation method of the positive electrode active material according to any one of the embodiments in the second aspect.

[0016] In the present invention, elements A1 and A2 are introduced to dope the iron sites in LMFP to replace Fe ions in LMFP. Since the ionic radii or valence states of Fe ions, A1 elements, and A2 elements are different, lattice distortion will occur at the positions doped with A1 elements and A2 elements, and the unit cell parameters will decrease, thereby inhibiting the growth of LMFP grains, and thus obtaining a cathode active material with a graded particle size ratio, thereby improving the tap density of the cathode active material. At the same time, the present invention also controls the different amounts of substances of the doped A1 elements and A2 elements, so that the particle sizes of the cathode active material can show a graded distribution, and the particle ratio in each particle size grade is appropriate. In the cathode active material, large particles serve as a skeleton support, and small particles are fully filled to reduce voids and increase the tap density. As the doping amount of transition metal elements gradually increases (from the amount of substance of A1 elements to the amount of substance of A2 elements), the average primary particle size of the particles of the cathode active material gradually decreases. Thus, by regulating the ratio of the doping amount of A1 elements to the doping amount of A2 elements, the proportion of the number of large particles and the proportion of the number of small particles reach the highest compaction grading level. In addition, the addition of Mn elements to the material may cause a decrease in its intrinsic electronic conductivity and ion diffusion rate. Therefore, by introducing A1 elements and A2 elements, and due to the differences in ionic radius or valence state from Fe ions, the diffusion path of Li ions can be optimized, more charge carriers can be introduced, and the electron transport ability can be improved. High-valence doping in LMFP leads to the formation of oxygen vacancies or lithium vacancies, promotes Li ion transport, forms a conductive network or activates the Li ion diffusion channel, and increases the Li ion migration rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0018] Figure 1 is a scanning electron microscope image (SEM) of the cathode active material in one embodiment;

[0019] Figure 2 is a preparation flow chart of the cathode active material in one embodiment;

[0020] Figure 3 is a preparation flow chart of step S10 in the preparation method in one embodiment;

[0021] Figure 4 is a preparation flow chart of step S20 in the preparation method in one embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the present invention in the specification are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. The term "and / or" used in the present invention includes any and all combinations of one or more of the related listed items.

[0024] It should be noted that the "range" disclosed in the present invention is defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a specific range. The range defined in this way can include the end values or not include the end values, and can be combined arbitrarily, 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, ranges of 60-110 and 80-120 are also contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present invention, unless otherwise stated, the numerical range "a-b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been fully listed herein, and "0-5" is only an abbreviated representation of these numerical combinations. In addition, when it is stated that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0025] All steps of the present invention can be carried out sequentially or randomly, and preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) carried out sequentially, or may include steps (b) and (a) carried out sequentially. For example, when it is mentioned that the method may further include step (c), it means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0026] The following will describe in detail some embodiments of the present invention with reference to the accompanying drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0027] The present invention provides a positive electrode active material, specifically a lithium iron manganese phosphate positive electrode active material doped with transition metal elements. Please refer to Figure 1 .

[0028] In one embodiment, the chemical formula of the positive electrode active material includes LiMn x1 Fe y1 A1 z1 PO 4 and LiMn x2 Fe y2 A2 z2 PO 4 , where the A1 element or the A2 element includes one or more of Ti, V, Co, Ni, Nb, Mo, Zr, and the chemical formula satisfies: z1 = a × y1, 0.02% ≤ a ≤ 3.6%, z2 = b × y2, 4.5% ≤ b ≤ 9%, 4.52% ≤ a + b ≤ 12.6%.

[0029] Specifically, lithium iron manganese phosphate (LMFP), which has the same olivine structure as lithium iron phosphate, has the characteristics of a higher average voltage, a higher capacity, and excellent low-temperature performance. Therefore, lithium iron manganese phosphate has great application prospects and markets. However, compared with lithium iron phosphate, due to the addition of manganese elements, the tap density of the lithium iron manganese phosphate material is lower than that of lithium iron phosphate, the particles are smaller, and it is difficult to improve the tap density. Therefore, in order to improve the tap density of lithium iron manganese phosphate, the present invention dopes a small amount of transition metal elements into lithium iron manganese phosphate. The above chemical formula is the chemical formula of the lithium iron manganese phosphate doped with transition metal elements, where both A1 and A2 are transition metal elements doped in lithium iron manganese phosphate.

[0030] The present invention introduces the A1 element and the A2 element to dope at the iron site in LMFP to replace the Fe ions in LMFP. Since the ionic radii or valence states of Fe ions and the A1 element and the A2 element are different. For example, the ionic radius of Fe ions (Fe 3+ ) is 0.064 nm, and the ionic radius of Ti ions (Ti 3+ ) is 0.067 nm, and the ionic radius of Co ions (Co 3+) has an ionic radius of 0.054 nm; therefore, lattice distortion will occur at the doping positions of element A1 and element A2, and the unit cell parameters will decrease, thereby inhibiting the growth of LMFP grains, obtaining a positive electrode active material with a particle size grading ratio, and improving the tap density of the positive electrode active material; at the same time, the present invention also controls the doping amounts of element A1 and element A2 within the above range based on the amount of substance of Fe element, which is more conducive to reducing the unit cell parameters and stabilizing the crystal structure, so that the positive electrode active material has the effect of multi-level particle size distribution. In a specific embodiment, by satisfying the molar ratio of element A1 and element A2 to Fe element, the tap density of the material can be controlled at 2.4 g / cm 3 ~2.6 g / cm 3 .

[0031] In a specific embodiment, the positive electrode active material includes two kinds of LMFP with different doping ratios, namely LMFP-1 and LMFP-2, wherein the chemical formula of LMFP-1 includes LiMn x1 Fe y1 A1 z1 PO 4 , and the chemical formula of LMFP-2 includes LiMn x2 Fe y2 A2 z2 PO 4 . x1 + x2 = x, y1 + y2 = y, z1 + z2 = z.

[0032] In a specific embodiment, the percentage value of the sum of the doping amounts of element A1 and element A2 (i.e., z1 + z2) and the total amount of substance of Fe element (i.e., y1 + y2) can be 4.52%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.6%. It can be understood that the above percentage values illustrate that the doping amounts of element A1 and element A2 do not exceed 4.52% - 12.6% of the amount of substance of Fe element. When the doping amounts of element A1 and element A2 are too large, the doping elements will cause more volume shrinkage of LMFP, resulting in more crystal defects, inhibiting particle growth, causing particle refinement, and the particle size is too small, thereby leading to a decrease in tap density. When the doping amounts of element A1 and element A2 are too small, the doping effect has little impact on LMFP, and the doping of element A1 and element A2 lacks significance.

[0033] In a specific embodiment, the doping amount of element A1 and the molar percentage value y1 of element Fe in LMFP-1 is a, that is, 0.02% ≤ 100% × z1 / y1 ≤ 3.6%. In a specific embodiment, a can be 0.02%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.7%, 3%, 3.3%, 3.6%. The doping amount of element A2 and the molar percentage value y2 of element Fe in LMFP-2 is b, that is, 4.5% ≤ 100% × z2 / y2 ≤ 9%. In a specific embodiment, b can be 4.5%, 4.8%, 5%, 5.2%, 5.5%, 5.7%, 6%, 6.3%, 6.5%, 6.8%, 7%, 7.2%, 7.5%, 7.7%, 8%, 8.3%, 8.5%, 8.8%, 9%.

[0034] It should be noted that in the present invention, the positive electrode active material can be composed of two different doping ratios of LMFP (LMFP-1 and LMFP-2). And because the molar amounts of element A1 and element A2 doped in LMFP-1 and LMFP-2 are different respectively, the particle size of the positive electrode active material can show a hierarchical distribution. The particle ratio in each particle size grade is appropriate. In this way, in the positive electrode active material, large particles act as a skeleton support, small particles are fully filled, voids are reduced, and the tap density is improved. As the doping amount of the transition metal element gradually increases (from the molar amount of element A1 to the molar amount of element A2), the average primary particle size of the positive electrode active material gradually decreases. In this way, by regulating the ratio of the doping amount of element A1 to the doping amount of element A2, the ratio of the number of large particles to the ratio of the number of small particles reaches the highest compaction grading level.

[0035] Therefore, controlling the doping amount of element A1 and the molar ratio a of element Fe within the above ranges, and controlling the doping amount of element A2 and the molar ratio b of element Fe within the above ranges are beneficial to reducing the unit cell parameter and stabilizing the crystal structure, thereby achieving the effect of improving the tap density. When the molar ratio a of the doping amount of element A1 to element Fe is too small, it indicates that the doping amount of element A1 is too small, and the doping effect becomes poor, and the transition metal doping is meaningless. When the molar ratio a of the doping amount of element A1 to element Fe is too large, it indicates that the doping amount of element A1 is too large, which will cause more volume shrinkage, inhibit grain growth, result in grain refinement, and the grain size is small, so large grains cannot be provided. When the molar ratio b of the doping amount of element A2 to element Fe is too small, it indicates that the doping amount of element A2 is too small, and the doping ratios of element A1 and element A2 are similar, and the purpose of adjusting the particle size range cannot be achieved. When the molar ratio b of the doping amount of element A2 to element Fe is too large, it indicates that the doping amount of element A2 is too large, which will also cause more volume shrinkage, inhibit grain growth, result in grain refinement, and the grain size is small, so large grains cannot be provided.

[0036] In addition, the addition of Mn element to the material may lead to a decrease in its electronic conductivity and ion diffusion rate. Therefore, by introducing elements A1 and A2, and due to the different ionic radii or valence states of Fe ions, the diffusion path of Li ions can be optimized, more charge carriers can be introduced, and the electron transport ability can be improved. High-valence doping in LMFP leads to the formation of oxygen vacancies or lithium vacancies, promotes Li ion transport, forms a conductive network or activates the Li ion diffusion channel, and increases the Li ion migration rate.

[0037] In one implementation, the cathode active material also satisfies the following relationship: 0.2 ≤ x / (x + y) ≤ 0.8, n = x / y, 0.25 ≤ n ≤ 4. The above relational expressions define the proportion of Mn element and Fe element in LMFP, which can be understood as the manganese-iron ratio being 0.2:0.8 to 0.8:0.2. When the value of n is too small, that is, when the proportion of Fe element is too large, it may cause changes in the lattice structure, reduce the lattice stability, thereby affecting the interaction and arrangement of atoms, and further leading to lattice distortion or twist, affecting the Li ion diffusion channel and electron conduction path; at the same time, when the proportion of Fe element is too large and the proportion of Mn element is too small, it will also affect the discharge performance of the finished product, that is, there is almost no manganese plateau during discharge, and the discharge specific energy of the material cannot be effectively improved. When the value of n is too large, that is, when the proportion of Mn element is too large, it will cause lattice distortion of the material, and the Jahn-Teller effect will intensify, resulting in unstable microstructure of the material, further affecting the performance and life of the material, and will also cause serious manganese dissolution, thus causing safety problems of the battery.

[0038] In one embodiment, the A1 element and the A2 element are the same or different. Specifically, the A1 element includes one or more of Ti, V, Co, Ni, Nb, Mo, and Zr, and the A2 element includes one or more of Ti, V, Co, Ni, Nb, Mo, and Zr. In a specific embodiment, the A1 element and the A2 element can be the same element. For example, the A1 element is Ti and the A2 element is also Ti. In other specific embodiments, the A1 element and the A2 element can be two different elements or more than two elements. For example, the A1 element is Ti and the A2 element is Co; or the A1 element includes Ti and V, and the A2 element is Co.

[0039] In a specific embodiment, the A1 element and the A2 element are the same, and both are doped with Co element. The chemical formula of LMFP-1 can be LiMn x1 Fe y1 Co z1 PO 4 , and the chemical formula of LMFP-2 can be LiMn x2 Fe y2 Co z2 PO 4 ; In other embodiments, the A1 element and the A2 element are different. The chemical formula of LMFP-1 can be LiMn x1 Fe y1 Co z1 PO 4 , and the chemical formula of LMFP-2 can be LiMn x2 Fe y2 Ni z2 PO 4 ; where 4.52% ≤ (z1 / y1 + z2 / y2) × 100% ≤ 12.6%.

[0040] In one embodiment, the average primary particle size of the positive electrode active material is 200 nm to 450 nm. Herein, the average primary particle size refers to the average value of the primary particle sizes of all the particles in the positive electrode active material. In a specific embodiment, the average primary particle size of the positive electrode active material can be 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, or 450 nm. It can be understood that the actual primary particle size of the particles in the positive electrode active material can be greater than or less than this range. The above average primary particle size is only the average value of all the particle sizes, and not all the particles are within this range.

[0041] Satisfying that the average primary particle size of the positive electrode active material is within the above range can control the specific surface area of the particles within a suitable range, which is beneficial to the infiltration of the electrolyte and the diffusion of lithium ions, thereby improving the rate performance of the battery. When the average primary particle size of the positive electrode active material is less than the above range, it indicates that there are too many small-sized particles in the positive electrode active material, which may lead to an increase in the contact resistance between particles or generate more heat under high current, affecting the cycle life and safety of the battery; when the average primary particle size of the positive electrode active material is greater than the above range, it indicates that there are too many large-sized particles in the positive electrode active material, which is not conducive to improving the tap density of the material nor to the diffusion of lithium ions.

[0042] In one embodiment, the positive electrode active material includes a first particle, a second particle, and a third particle. The primary particle size D1 of the first particle satisfies 1 nm ≤ D1 < 200 nm, the primary particle size D2 of the second particle satisfies 200 nm ≤ D2 ≤ 400 nm, and the primary particle size D3 of the third particle satisfies 400 nm < D3 ≤ 1000 nm. Specifically, in the present invention, different amounts of element A1 and element A2 are doped into LMFP, and the doping amount of element A1 is less than that of element A2. Since the doping amount of element A1 is relatively low, it can provide large particles of LMFP, and since the doping amount of element A2 is relatively high, it can provide small particles of LMFP. The total doping amount of element A1 and element A2 is also controlled within a suitable range, thereby ensuring that the positive electrode active material has a high capacity. In a specific embodiment, because the doping amount of element A2 is relatively large, the first particle is mainly composed of LiMn x2 Fe y2 A2 z2 PO 4 provided; while the doping amount of element A1 is relatively small, the third particle is mainly composed of LiMn x1 Fe y1 A1 z1 PO 4 provided, and the second particle is provided by both.

[0043] In a specific embodiment, the primary particle size D1 of the first particle can be 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, 100 nm, 150 nm, 199 nm. The primary particle size D2 of the second particle can be 200 nm, 250 nm, 300 nm, 350 nm, 400 nm. The primary particle size D3 of the third particle can be 401 nm, 450 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm.

[0044] The particle size distributions of the first particles, the second particles, and the third particles meet the above ranges, so that the overall positive electrode active material forms a multi-level particle size distribution. The multiple distribution intervals can not only meet the average primary particle size range in the above embodiments, but also improve the tap density and the wettability of the electrolyte by matching the particle sizes. When the particles of the positive electrode active material are too small, smaller than the particle size range of the first particles, it not only increases the difficulty of material preparation, but also makes it easier for the small particles to agglomerate, and it is not easy to make a uniform positive electrode sheet. When the particles of the positive electrode active material are too large, larger than the particle size range of the third particles, the diffusion difficulty of lithium ions in the particles increases, resulting in difficulty in fully releasing the capacity.

[0045] In one embodiment, the mass ratio of the first particles in the positive electrode active material is 20% to 50%, the mass ratio of the second particles in the positive electrode active material is 30% to 40%, and the mass ratio of the third particles in the positive electrode active material is 10% to 35%. Specifically, the present invention also controls the mass ratios of the first particles, the second particles, and the third particles to be within the above ranges to ensure that the relatively large-sized third particles serve as a framework support, gaps are formed between the third particles, or gaps are formed between the third particles and the second particles, and the second particles and the first particles are fully filled in the above gaps, thereby reducing the voids in the material and improving the tap density. When the mass ratios of the first particles, the second particles, and the third particles are unbalanced, and at least two of the first particles, the second particles, and the third particles have excessive or insufficient ratios, it will lead to an increase in voids in the material or an increase in the ratio of agglomerated secondary particles, both of which are not conducive to improving the tap density and releasing lithium ions.

[0046] In a specific embodiment, the mass ratio of the first particles in the positive electrode active material can be 20%, 25%, 30%, 35%, 40%, 45%, 50%. The mass ratio of the second particles in the positive electrode active material can be 30%, 32%, 34%, 36%, 38%, 40%. The mass ratio of the third particles in the positive electrode active material can be 10%, 15%, 20%, 25%, 30%, 35%. Of course, it can be understood that the sum of the mass ratios of the first particles, the second particles, and the third particles is 100%.

[0047] In one embodiment, the first particles include first particle I and first particle II. Among them, the primary particle size D11 of the first particle I satisfies 1 nm ≤ D11 < 150 nm, and the primary particle size D12 of the first particle II satisfies 150 nm ≤ D12 < 200 nm. The mass ratio of the first particle I in the positive electrode active material is 5% - 20%, and the mass ratio of the first particle II in the positive electrode active material is 15% - 30%. By controlling the first particles to also include the first particle I and the first particle II in the above two particle size ranges, the purpose of further expanding the particle size distribution range of the positive electrode active material can be achieved, and the particles form a multi-level distribution, which is more conducive to improving the tap density of the material. In addition, by controlling the mass ratio of the first particle I and the first particle II, it can be ensured that more voids can be formed between the first particle II, so that some of the first particle I can fill the voids in the first particle II.

[0048] In a specific embodiment, the primary particle size D11 of the first particle I can be 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 149 nm. The primary particle size D12 of the first particle II can be 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 199 nm.

[0049] In one embodiment, the third particles include third particle I and third particle II. Among them, the primary particle size D31 of the third particle I satisfies 400 nm < D31 ≤ 500 nm, and the primary particle size D32 of the third particle II satisfies 500 nm < D32 ≤ 1000 nm. The mass ratio of the third particle I in the positive electrode active material is 5% - 20%, and the mass ratio of the third particle II in the positive electrode active material is 5% - 15%. By controlling the third particles to also include the third particle I and the third particle II in the above two particle size ranges, the purpose of further expanding the particle size distribution range of the positive electrode active material can be achieved, and the particles form a multi-level distribution, which is more conducive to improving the tap density of the material. In addition, by controlling the mass ratio of the third particle I and the third particle II, it can be ensured that more large voids can be formed between the third particle II, so that the first particles, the second particles, and the third particle II can all fill the voids in the third particle II, and some of the second particles can fill the voids in the third particle I, while some of the first particles can fill the voids in the second particles, finally forming the effect of hierarchical proportioning and filling, thereby improving the overall tap density of the material.

[0050] In a specific embodiment, the primary particle size D31 of the third particle I may be 401 nm, 410 nm, 420 nm, 430 nm, 440 nm, 450 nm, 460 nm, 470 nm, 480 nm, 490 nm, 500 nm. The primary particle size D32 of the third particle II may be 501 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nm.

[0051] In a specific embodiment, the present invention defines particles with a primary particle size less than 200 nm as small particles (i.e., the first particle I), and particles with a primary particle size greater than 400 nm as large particles (i.e., the third particle). By further adjusting the particle size and particle proportion in the positive electrode active material, the present invention enables the particles in the positive electrode active material to achieve a multi-level distribution ratio, whereby the tap density of the lithium iron phosphate manganese positive electrode active material can reach 2.4 g / cm 3 ~2.6 g / cm 3 .

[0052] In one embodiment, the positive electrode active material further includes a carbon material, which is mixed in the lithium iron phosphate manganese or coated on the outer surface of the lithium iron phosphate manganese to form a carbon coating layer.

[0053] The present invention also provides a preparation method for the positive electrode active material. The preparation method is used to prepare the positive electrode active material provided in the above embodiment. Please refer to Figures 2 - 4 .

[0054] In one embodiment, the preparation method of the positive electrode active material specifically includes the following steps:

[0055] Step S10: Mix a first reaction solution with an Mn source and a second reaction solution with an Fe source to obtain two third reaction solutions, and add an A1 source and an A2 source to the two third reaction solutions respectively.

[0056] Step S20: Add an Li source and a P source to the two third reaction solutions to obtain two mixed solutions, and the two mixed solutions react to obtain a solid precursor.

[0057] Step S30: Calcinate the solid precursor to obtain the positive electrode active material.

[0058] In the present invention, a first reaction solution with an Mn source and a second reaction solution with an Fe source are made into a third reaction solution, and element A1 and element A2 are added to the third reaction solution so that element A1 and element A2 can be doped at the iron site. By regulating the doping amounts of element A1 and element A2, and obtaining a positive electrode active material after a first sintering of the solid precursor, the positive electrode active material has a relatively wide particle size distribution range. The positions where element A1 and element A2 are doped will cause lattice distortion and a decrease in the unit cell parameter, thereby inhibiting the growth of LMFP grains, and thus obtaining a positive electrode active material with a particle size grading ratio, thereby improving the tap density of the positive electrode active material, and the tap density can be controlled at 2.4 g / cm 3 ~2.6 g / cm 3 .

[0059] In one embodiment, in step S10, the first reaction solution with an Mn source and the second reaction solution with an Fe source are mixed to obtain a third reaction solution, and an A source is added to the third reaction solution, which specifically includes: dissolving the Mn source and the Fe source in a solvent respectively to obtain the first reaction solution and the second reaction solution, then mixing the first reaction solution and the second reaction solution to obtain two portions of the third reaction solution, and then adding the reaction solution with element A1 source and element A2 source to the third reaction solution.

[0060] In a specific embodiment, the Mn source includes one or more of manganese sulfate, manganese phosphate, manganese nitrate, manganese carbonate, manganese acetate, and manganese chloride. The Fe source includes one or more of iron sulfate, iron phosphate, iron nitrate, ferrous chloride, ferrous oxalate, iron chloride, iron acetate, and iron(III) oxide. The element A1 source and the element A2 source include one or more of titanium source (Ti), vanadium source (V), cobalt source (Co), nickel source (Ni), niobium source (Nb), molybdenum source (Mo), and zirconium source (Zr).

[0061] In one embodiment, in step S20, a Li source and a P source are further added to the two portions of the third reaction solution to obtain a mixed solution, and the solid precursor is obtained after the mixed solution reacts, which specifically includes: independently mixing the two portions of the third reaction solution with a Li source solution, a P source solution, and a surfactant to obtain a mixed solution, then subjecting the mixed solution to a reaction treatment, filtering, drying, and grinding the product after hydrothermal reaction treatment to obtain the solid precursor.

[0062] The present invention uses the hydrothermal method to prepare the solid precursor of the positive electrode active material, which can make the crystal grains grow completely, and also has the advantages of uniform distribution of doped metal elements and good particle dispersibility, providing good preconditions for the subsequent self-generation grading of the material. In other embodiments, the method for preparing the solid precursor of the present invention also includes one of high-temperature solid phase, sol-gel, and co-precipitation methods.

[0063] In a specific embodiment, the Li source includes one or more of lithium phosphate, lithium carbonate, lithium hydroxide, lithium nitrate, and lithium sulfate. The P source includes one or more of ammonium dihydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate. The surfactant includes one or more of ascorbic acid, polyvinylpyrrolidone, potassium bromide, ammonium bromide, potassium iodide, sodium iodide, tetramethylammonium bromide, and tetrapropylammonium bromide.

[0064] In one embodiment, in step S20, the amount of substance of Mn element in the third reaction solution is N1, the amount of substance of Fe element is N2, the amount of substance of Li element in the Li source solution is N3, and the amount of substance of P element in the P source solution is N4. The ratio of the third reaction solution, the Li source solution, and the P source solution satisfies (N1 + N2):N3:N4 = 1:(1 to 1.03):(1 to 1.03). The ratio of the amount of substance of the surfactant to the amount of substance of the Fe element is (0.1 to 9):1.

[0065] In one embodiment, in step S20, the temperature of the hydrothermal reaction treatment is 60°C to 180°C, and the heat preservation time is 5h to 24h. In a specific embodiment, the temperature of the hydrothermal reaction treatment can be 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C. The heat preservation time can be 5h, 6h, 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h.

[0066] In one embodiment, in step S30, the solid precursor is calcined, which specifically includes: mixing the solid precursor with a carbon source, placing the mixed material under the protection of an inert atmosphere for calcination treatment, and the product obtained by calcination is the positive electrode active material of lithium iron manganese phosphate.

[0067] In a specific embodiment, the carbon source includes one or more of glucose, sucrose, starch, citric acid, polyethylene glycol, polyvinyl alcohol, and carboxymethyl cellulose. The inert atmosphere includes one of nitrogen, argon, and a mixture of nitrogen and argon.

[0068] In one embodiment, the calcination process is a two-stage temperature-raising calcination. The first stage is to heat to 300°C to 500°C and keep the temperature in this range for 2h to 5h. The second stage is to continue heating to 600°C to 800°C and keep the temperature in this range for 6h to 12h. In a specific embodiment, the calcination temperature in the first stage can be 300°C, 350°C, 400°C, 450°C, 500°C, and the heat preservation time is 2h, 3h, 4h, 5h; the calcination temperature in the second stage can be 600°C, 650°C, 700°C, 750°C, 800°C, and the heat preservation time is 6h, 7h, 8h, 9h, 10h, 11h, 12h.

[0069] In one embodiment, please refer to Figure 3 , in step S10, after mixing the first reaction solution with an Mn source and the second reaction solution with an Fe source, two portions of a third reaction solution are obtained. Adding an A1 source and an A2 source to the two portions of the third reaction solution respectively specifically includes:

[0070] Step S11, mixing the first reaction solution with an Mn source and the second reaction solution with an Fe source to obtain a third reaction solution.

[0071] Step S12, dividing the third reaction solution equally into a third reaction solution I and a third reaction solution II.

[0072] Step S13, adding an A1 source to the third reaction solution I to obtain a first precursor solution, and adding an A2 source to the third reaction solution II to obtain a second precursor solution.

[0073] In one embodiment, in step S11, the amount of substance of Mn element in the first reaction solution is N1, and the amount of substance of Fe element in the second reaction solution is N2. Among them, 0.2 ≤ N1 / (N1 + N2) ≤ 0.8, N = N1 / N2, and 0.25 ≤ N ≤ 4. Specifically, in step S11, the first reaction solution with an Mn source and the second reaction solution with an Fe source are mixed according to a variable ratio N to obtain a third reaction solution.

[0074] In one embodiment, in step S12, dividing the third reaction solution equally into a third reaction solution I and a third reaction solution II is to facilitate the addition of A1 element and A2 element in subsequent steps. At the same time, by configuring the third reaction solution I and the third reaction solution II in an equal - division manner, the process efficiency can be improved, and the configuration error between the third reaction solution I and the third reaction solution II can be reduced. Of course, in other embodiments, the same third reaction solution I and third reaction solution II can also be configured using the first reaction solution and the second reaction solution simultaneously.

[0075] In one embodiment, in step S13, the amount of substance of A1 element in the first precursor solution is M1, and the amount of substance of A2 element in the second precursor solution is M2. Among them, M1 = (a×N2) / 2, 0.02% ≤ a ≤ 3.6%, M2 = (b×N2) / 2, 4.5% ≤ b ≤ 9%, and 4.52% ≤ a + b ≤ 12.6%.

[0076] In one embodiment, please refer to Figure 4 , in step S20, adding a Li source and a P source to the two portions of the third reaction solution to obtain two mixed solutions, and obtaining a solid precursor after the two mixed solutions react, including:

[0077] Step S21, adding a Li source and a P source to the first precursor solution and the second precursor solution respectively to obtain a first mixed solution and a second mixed solution.

[0078] Step S22: React the first mixture and the second mixture respectively to obtain a first precursor and a second precursor.

[0079] Step S23: Mix the first precursor and the second precursor to obtain a solid precursor.

[0080] In one embodiment, in Step S21 and Step S22, adding a Li source and a P source to the first precursor solution and the second precursor solution respectively may refer to the method provided in the above embodiment, and the amounts of the Li source and the P source added to the first precursor solution and the second precursor solution satisfy the ratio provided in the above embodiment, which will not be elaborated here.

[0081] In one embodiment, in Step S23, mixing the first precursor and the second precursor to obtain a solid precursor specifically includes: mixing the filtered, dried and ground first precursor and second precursor to obtain a solid precursor. In a specific embodiment, the mixing method may include ball milling mixing, mechanical stirring mixing, etc.

[0082] The present invention also provides a positive electrode sheet.

[0083] In one embodiment, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer located on at least one surface of the positive electrode current collector. The positive electrode film layer includes the positive electrode active material obtained by the above preparation method or the above positive electrode active material. Optionally, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is provided on any one or both of the two opposite surfaces of the positive electrode current collector.

[0084] In one embodiment, the positive electrode film layer may further optionally include a positive electrode conductive agent. The present invention does not particularly limit the type of the positive electrode conductive agent. As an example, the positive electrode conductive agent includes at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene oxide and carbon nanofibers. In some embodiments, based on the total mass of the positive electrode film layer, the mass percentage content of the positive electrode conductive agent is ≤5%.

[0085] In one embodiment, the positive electrode film layer may further optionally include a positive electrode binder. The present invention does not particularly limit the type of the positive electrode binder. As an example, the positive electrode binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride - tetrafluoroethylene - propylene terpolymer, vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene terpolymer, tetrafluoroethylene - hexafluoropropylene copolymer and fluorinated acrylate resin. In some embodiments, based on the total mass of the positive electrode film layer, the mass percentage content of the positive electrode binder is ≤5%.

[0086] In one embodiment, the positive electrode current collector can be a metal foil or a composite current collector. As an example of the metal foil, aluminum foil can be used. The composite current collector can include a polymer material substrate and a metal material layer formed on at least one surface of the polymer material substrate. As an example, the metal material can include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material substrate can include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0087] In one embodiment, the positive electrode film layer is generally formed by coating a positive electrode slurry on a positive electrode current collector and then drying and cold pressing. The positive electrode slurry is generally formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring evenly. The solvent can be N-methylpyrrolidone (NMP), but is not limited thereto.

[0088] The present invention also provides a secondary battery.

[0089] In one embodiment, the secondary battery includes a positive electrode and a negative electrode, where the positive electrode is the positive electrode provided in the above embodiment; or, the secondary battery includes the positive electrode active material provided in the above embodiment; or the secondary battery includes the positive electrode active material obtained by the above preparation method.

[0090] The technical solutions of the present invention will be described in detail below through specific examples.

[0091] Example 1

[0092] This example provides a lithium iron manganese phosphate positive electrode active material doped with transition metal elements. The chemical formula of the positive electrode active material includes: LiMn 0.5964 Fe 0.4 Ti 0.0036 PO 4 and LiMn 0.564 Fe 0.4 Ti 0.036 PO 4 . Among them, the average primary particle size of the positive electrode active material is 245.9 nm. The proportion of the primary particle size of the positive electrode active material that satisfies 1 nm to 200 nm is 46.15%, the proportion of the primary particle size that satisfies 200 nm to 400 nm is 32.68%, and the proportion of the primary particle size that satisfies 400 nm to 1000 nm is 21.17%.

[0093] This example also provides a preparation method for a lithium iron manganese phosphate positive electrode active material doped with transition metal elements:

[0094] (1) Weigh 133.39 g of manganese carbonate (1.1604 mol) and 120.65 g of iron phosphate (0.8 mol) respectively to prepare solutions, mix them evenly, and divide them into two equal parts.

[0095] (2) Weigh 0.288 g of titanium dioxide (0.0036 mol) to prepare the first doping amount solution, weigh 2.875 g of titanium dioxide (0.036 mol) to prepare the second doping amount solution, and mix the two equal parts of the mixed solution in (1) with the first doping amount solution and the second doping amount solution respectively to obtain reaction solution A and reaction solution B.

[0096] (3) Weigh 74.63 g of lithium carbonate (1.01 mol) and 234.65 g of ammonium dihydrogen phosphate (2.04 mol) respectively to prepare solutions, weigh 177.83 g of polyvinylpyrrolidone (1.6 mol), and divide the above materials into two equal parts. Mix them with reaction solution A and reaction solution B respectively and stir evenly, then transfer them to a reaction kettle.

[0097] (4) Seal the reaction kettle in (3) and carry out hydrothermal reaction. The hydrothermal temperature is 100 °C, keep warm for 5 hours, and cool naturally to obtain the hydrothermal product.

[0098] (5) Filter the hydrothermal product by suction, wash it three times with anhydrous ethanol, place the washed solid product in a vacuum drying oven, dry at a temperature of 80 °C for 12 hours. Pour the solid substances obtained with two different doping amounts into an agate mortar and grind them together. The obtained powder is the lithium iron manganese phosphate precursor material.

[0099] (6) Mix the precursor obtained in (5) with 20 g of sucrose, heat it to 400 °C under a nitrogen atmosphere, keep warm for 4 hours, continue to heat up to 800 °C, and keep warm for 8 hours. The mixture obtained by calcination is the lithium iron manganese phosphate cathode active material.

[0100] Example 2

[0101] This example provides a lithium iron manganese phosphate cathode active material doped with transition metal elements. The chemical formula of the cathode active material includes: LiMn 0.5928 Fe 0.4 Ti 0.0072 PO 4 and LiMn 0.572 Fe 0.4 Ti 0.028 PO 4 . Among them, the average primary particle size of the cathode active material is 219.61 nm. The proportion of the primary particle size in the cathode active material that satisfies 1 nm to 200 nm is 41.306%, the proportion of the primary particle size that satisfies 200 nm to 400 nm is 39.049%, and the proportion of the primary particle size that satisfies 400 nm to 1000 nm is 19.646%.

[0102] This embodiment also provides a preparation method of a lithium iron manganese phosphate cathode active material doped with transition metal elements:

[0103] (1) Weigh 133.89 g of manganese carbonate (1.165 mol) and 120.65 g of iron phosphate (0.8 mol) and dissolve them in solutions respectively. After mixing evenly, divide them into two equal parts.

[0104] (2) Weigh 0.575 g of titanium dioxide (0.0072 mol) to prepare a first doping amount solution, and weigh 2.236 g of titanium dioxide (0.028 mol) to prepare a second doping amount solution. Mix the two equal parts of the mixed solution in (1) with the first doping amount solution and the second doping amount solution respectively to obtain reaction solution A and reaction solution B.

[0105] (3) Weigh 75.74 g of lithium carbonate (1.025 mol) and 233.50 g of ammonium dihydrogen phosphate (2.03 mol) and dissolve them in solutions respectively. Weigh 22.23 g of polyvinylpyrrolidone (0.2 mol). Divide the above materials into two equal parts. Mix them with reaction solution A and reaction solution B respectively and stir evenly, then transfer them to a reaction kettle.

[0106] (4) Seal the reaction kettle in (3) and carry out a hydrothermal reaction. The hydrothermal temperature is 100 °C, keep warm for 5 hours, and cool naturally to obtain a hydrothermal product.

[0107] (5) Filter the hydrothermal product by suction filtration, and wash it three times with absolute ethanol. Place the washed solid product in a vacuum drying oven, with a drying temperature of 80 °C and dry for 12 hours. Pour the solid substances obtained with two different doping amounts into an agate mortar and grind them together. The obtained powder is the lithium iron manganese phosphate precursor material.

[0108] (6) Mix the precursor obtained in (5) with 20 g of sucrose. Under a nitrogen atmosphere, heat it to 400 °C, keep warm for 4 hours, then continue to heat up to 800 °C and keep warm for 8 hours. The mixture obtained by calcination is the lithium iron manganese phosphate cathode active material.

[0109] Example 3

[0110] This embodiment provides a lithium iron manganese phosphate cathode active material doped with transition metal elements. The chemical formula of the cathode active material includes: LiMn 0.5982 Fe 0.4 Ti 0.0018 PO 4 and LiMn 0.5676 Fe 0.4 Ti 0.0324 PO 4Among them, the average primary particle size of the positive electrode active material is 278.7 nm. The proportion of the primary particle size in the positive electrode active material that satisfies 1 nm to 150 nm is 7.125%, the proportion that satisfies 150 nm to 200 nm is 21.625%, the proportion that satisfies 200 nm to 400 nm is 39.5%, the proportion that satisfies 400 nm to 500 nm is 15.75%, and the proportion that satisfies 500 nm to 1000 nm is 16%.

[0111] This embodiment also provides a method for preparing a lithium iron manganese phosphate positive electrode active material doped with transition metal elements:

[0112] (1) Weigh 134.01 g of manganese carbonate (1.1658 mol) and 120.65 g of iron phosphate (0.8 mol) respectively to prepare solutions, mix them evenly and divide them into two equal parts.

[0113] (2) Weigh 0.144 g of titanium dioxide (0.0018 mol) to prepare a first doping amount solution, weigh 2.588 g of titanium dioxide (0.0324 mol) to prepare a second doping amount solution, and mix the two equal parts of the mixed solution in (1) with the first doping amount solution and the second doping amount solution respectively to obtain a reaction solution A and a reaction solution B.

[0114] (3) Weigh 75.37 g of lithium carbonate (1.02 mol) and 234.08 g of ammonium dihydrogen phosphate (2.035 mol) respectively to prepare solutions, weigh 133.37 g of polyvinylpyrrolidone (1.2 mol), and divide the above materials into two equal parts. Mix them with the reaction solution A and the reaction solution B respectively and stir evenly, then transfer them to a reaction kettle.

[0115] (4) Seal the reaction kettle in (3) and carry out a hydrothermal reaction. The hydrothermal temperature is 100 °C, keep warm for 5 hours, and cool naturally to obtain a hydrothermal product.

[0116] (5) Filter the hydrothermal product by suction, wash it three times with absolute ethanol, place the washed solid product in a vacuum drying oven, dry it at a temperature of 80 °C for 12 hours. Pour the solid substances obtained with two different doping amounts into an agate mortar and grind them together. The obtained powder is the lithium iron manganese phosphate precursor material.

[0117] (6) Mix the precursor obtained in (5) with 20 g of sucrose, heat it to 400 °C in a nitrogen atmosphere, keep warm for 4 hours, continue to heat up to 800 °C, and keep warm for 8 hours. The mixture obtained by calcination is the lithium iron manganese phosphate positive electrode active material.

[0118] Example 4

[0119] This embodiment provides a lithium iron manganese phosphate cathode active material doped with transition metal elements. The chemical formula of the cathode active material includes: LiMn 0.5964 Fe 0.4 V 0.0036 PO 4 and LiMn 0.5712 Fe 0.4 Ti 0.0288 PO 4 . Among them, the average primary particle size of the cathode active material is 254.06 nm. The proportion of the primary particle size of the cathode active material that satisfies 1 nm to 200 nm is 40.35%, the proportion of the primary particle size that satisfies 200 nm to 400 nm is 39.76%, and the proportion of the primary particle size that satisfies 400 nm to 1000 nm is 19.89%.

[0120] This embodiment also provides a preparation method for a lithium iron manganese phosphate cathode active material doped with transition metal elements:

[0121] (1) Weigh 134.22 g of manganese carbonate (1.1676 mol) and 120.65 g of iron phosphate (0.8 mol) respectively to prepare solutions. After mixing them evenly, divide them into two equal parts.

[0122] (2) Weigh 0.655 g of vanadium pentoxide (0.0036 mol) to prepare a first doping amount solution, and weigh 2.300 g of titanium dioxide (0.0288 mol) to prepare a second doping amount solution. Mix the two equal parts of the mixed solution in (1) with the first doping amount solution and the second doping amount solution respectively to obtain a reaction solution A and a reaction solution B.

[0123] (3) Weigh 75.37 g of lithium carbonate (1.02 mol) and 234.65 g of ammonium dihydrogen phosphate (2.04 mol) respectively to prepare solutions. Weigh 88.91 g of polyvinylpyrrolidone (0.8 mol). Divide the above materials into two equal parts. Mix them with the reaction solution A and the reaction solution B respectively and stir evenly, then transfer them to a reaction kettle.

[0124] (4) Seal the reaction kettle in (3) and carry out a hydrothermal reaction. The hydrothermal temperature is 100 °C, keep it warm for 5 hours, and then cool it naturally to obtain a hydrothermal product.

[0125] (5) Filter the hydrothermal product by suction filtration, and wash it three times with absolute ethanol. Place the washed solid product in a vacuum drying oven, dry it at a temperature of 80 °C for 12 hours. Pour the solid substances obtained with two different doping amounts into an agate mortar and grind them together. The obtained powder is the lithium iron manganese phosphate precursor material.

[0126] (6) Mix the precursor obtained in (5) with 20 g of sucrose, heat it to 400 °C under a nitrogen atmosphere, keep it at this temperature for 4 hours, then continue to heat it to 800 °C and keep it at this temperature for 8 hours. The mixture obtained by calcination is the lithium iron manganese phosphate cathode active material.

[0127] Comparative Example 1

[0128] This example provides a lithium iron manganese phosphate cathode active material doped with transition metal elements. The chemical formula of the cathode active material includes: LiMn 0.582 Fe 0.4 Ti 0.018 PO 4 and LiMn 0.564 Fe 0.4 Ti 0.036 PO 4 . Among them, the average primary particle size of the cathode active material is 123.65 nm. The proportion of the primary particle size of the cathode active material that satisfies 1 nm to 200 nm is 79.55%, the proportion that satisfies 200 to 400 nm is 18.29%, and the proportion that satisfies 300 nm to 400 nm is 2.16%.

[0129] This example also provides a preparation method of a lithium iron manganese phosphate cathode active material doped with transition metal elements:

[0130] (1) Weigh 131.73 g of manganese carbonate (1.146 mol) and 120.65 g of iron phosphate (0.8 mol) respectively to prepare solutions, mix them evenly and divide them into two equal parts.

[0131] (2) Weigh 1.438 g of titanium dioxide (0.018 mol) to prepare a first doping amount solution, weigh 2.875 g of titanium dioxide (0.036 mol) to prepare a second doping amount solution, and mix the two equal parts of the mixed solution in (1) with the first doping amount solution and the second doping amount solution respectively to obtain reaction solution A and reaction solution B.

[0132] (3) Weigh 74.63 g of lithium carbonate (1.01 mol) and 231.20 g of ammonium dihydrogen phosphate (2.01 mol) respectively to prepare solutions, weigh 177.83 g of polyvinylpyrrolidone (1.6 mol), and divide the above materials into two equal parts. Mix them with reaction solution A and reaction solution B respectively and stir evenly, then transfer them to a reaction kettle.

[0133] (4) Seal the reaction kettle in (3) and carry out a hydrothermal reaction. The hydrothermal temperature is 100 °C, keep it at this temperature for 5 hours, and then cool it naturally to obtain a hydrothermal product.

[0134] (5) Filter the hydrothermal product by suction filtration, and wash it three times with absolute ethanol. Place the washed solid product in a vacuum drying oven, with a drying temperature of 80 °C and dry for 12 hours. Pour the solid substances obtained with two different doping amounts into an agate mortar and grind them together. The resulting powder is the lithium iron manganese phosphate precursor material.

[0135] (6) Mix the precursor obtained in (5) with 20 g of sucrose. Under a nitrogen atmosphere, heat it to 400 °C, hold for 4 hours, continue to heat up to 800 °C, and hold for 8 hours. The mixture obtained by calcination is the lithium iron manganese phosphate positive electrode active material.

[0136] Comparative Example 2

[0137] This example provides a lithium iron manganese phosphate positive electrode active material doped with transition metal elements. The chemical formula of the positive electrode active material includes: LiMn 0.582 Fe 0.4 Ti 0.018 PO 4 and LiMn 0.564 Fe 0.4 Ti 0.0324 PO 4 . Among them, the average primary particle size of the positive electrode active material is 149.62 nm. The proportion of the primary particle size in the positive electrode active material that satisfies 1 nm to 200 nm is 70.93%, the proportion that satisfies 200 nm to 400 nm is 22.422%, and the proportion that satisfies 400 nm to 1000 nm is 6.648%.

[0138] This example also provides a preparation method of a lithium iron manganese phosphate positive electrode active material doped with transition metal elements:

[0139] (1) Weigh 131.73 g of manganese carbonate (1.146 mol) and 120.65 g of iron phosphate (0.8 mol) respectively to prepare solutions, mix them evenly and divide them into two equal parts.

[0140] (2) Weigh 1.438 g of titanium dioxide (0.018 mol) to prepare a first doping amount solution, weigh 2.556 g of titanium dioxide (0.0324 mol) to prepare a second doping amount solution, and mix the two equal parts of the mixed solution in (1) with the first doping amount solution and the second doping amount solution respectively to obtain a reaction solution A and a reaction solution B.

[0141] (3) Weigh 74.26 g of lithium carbonate (1.005 mol) and 231.20 g of ammonium dihydrogen phosphate (2.01 mol) respectively to prepare solutions, weigh 44.46 g of polyvinylpyrrolidone (0.4 mol), and divide the above materials into two equal parts. Mix them with the reaction solution A and the reaction solution B respectively and stir evenly, and transfer them to a reaction kettle.

[0142] (4) Seal the reactor in (3) and conduct a hydrothermal reaction at a hydrothermal temperature of 100 °C, keep the temperature for 5 hours, and then cool naturally to obtain a hydrothermal product.

[0143] (5) Filter the hydrothermal product by suction and wash it three times with absolute ethanol. Place the washed solid product in a vacuum drying oven, dry it at a temperature of 80 °C for 12 hours. Pour the solid substances obtained with two different doping amounts into an agate mortar and grind them together. The obtained powder is the lithium iron manganese phosphate precursor material.

[0144] (6) Mix the precursor obtained in (5) with 20 g of sucrose, heat it to 400 °C under a nitrogen atmosphere, keep the temperature for 4 hours, and then continue to heat it to 800 °C and keep the temperature for 8 hours. The mixture obtained by calcination is the lithium iron manganese phosphate positive electrode active material.

[0145] The parameters of the positive electrode active materials provided in Examples 1 - 4 and Comparative Examples 1 - 2 are shown in Table 1:

[0146] Table 1 Parameters of the positive electrode active materials provided in Examples and Comparative Examples

[0147]

[0148] It can be seen from the parameters of the positive electrode active materials in Examples 1 - 4 and Comparative Example 1 in Table 1 that by controlling the ratio of the doped metals (A1 and A2), the average primary particle size of the positive electrode material can be increased, so that the proportion of large particles and the proportion of small particles reach the highest compaction grading level. The large particles act as a skeleton support, and the small particles are fully filled, reducing voids and improving the compaction density. The compaction densities of the positive electrode materials in Examples 1 - 4 are all greater than that of Comparative Example 1.

[0149] It can be seen from the parameters of the positive electrode active materials in Examples 1 - 4 and Comparative Example 2 in Table 1 that in the solution provided by the present invention, not only the total amount of the doped metals needs to be regulated, but also the ratio of different doping elements (A1 or A2) needs to be regulated. By further controlling the doping ratio of A1 or A2, the average primary particle size of the positive electrode material can be increased, and the compaction density can be further improved.

[0150] It can be seen from the parameters of the positive electrode active materials in Examples 1 - 4 in Table 1 that by regulating the ratio of the doped A1 or A2, the ratio of large particles and small particles can be adjusted, thereby controlling the average primary particle size of the positive electrode material to obtain the required compaction density of the positive electrode material.

[0151] Test Example

[0152] Battery Preparation:

[0153] The lithium iron manganese phosphate positive electrode active materials provided in the above-mentioned Examples 1 to 4 and Comparative Examples 1 to 2 are prepared as positive electrode sheets. The preparation method of the positive electrode sheet includes: mixing the lithium iron manganese phosphate positive electrode active material, polyvinylidene fluoride and carbon black in a ratio of 96.5%, 2.2% and 1.3% by mass, respectively, and mixing and fully stirring with a certain amount of N-methylpyrrolidone to obtain a positive electrode slurry, drying the positive electrode slurry on the surface of the positive electrode collector at a temperature of 120° C. on a coating machine to form a film, and then rolling on a roller press to obtain a positive electrode sheet.

[0154] The positive electrode sheets provided in the above-mentioned Examples 1 to 4 and Comparative Examples 1 to 2 are prepared as secondary batteries. The preparation method of the secondary battery includes: cutting the above positive electrode sheets, fixing the positive electrode sheets on the positive electrode shell, and moving the fixed positive electrode shell into an oven for drying. The prepared positive electrode shell, gasket, spring, diaphragm, electrolyte, negative electrode sheet and negative electrode shell are put into a glove box for assembly. The assembled button half-cell is placed in the center of the tablet press with the negative electrode facing up and the positive electrode facing down for tableting. When the tableting is completed, the battery assembly is completed. The selection of the electrolyte can be based on any electrolyte suitable for lithium-ion batteries in the industry, and the type of electrolyte is not limited here.

[0155] The secondary batteries provided in the above-mentioned Examples 1 to 4 and Comparative Examples 1 to 2 were subjected to electrochemical performance tests, and the test conditions were as follows:

[0156] Conventional charge and discharge test:

[0157] The electrochemical performance was tested on an electrochemical test cabinet at room temperature of 25°C, and the charge and discharge tests were performed using 0.1C and 1C constant currents.

[0158] The secondary battery results are shown in Table 2 below:

[0159] Table 2. Test results of secondary batteries assembled in Examples and Comparative Examples

[0160]

[0161] From the test results of the secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 2 in Table 2, it can be seen that by controlling and increasing the average primary particle size of the positive electrode material to obtain a larger compaction density, the discharge specific capacity of the secondary battery can be effectively increased, that is, a secondary battery with better electrical performance can be obtained. The discharge specific capacity of the secondary batteries of Examples 1 to 4 is greater than that of Comparative Example 1.

[0162] From the test results of the secondary batteries in Examples 1 to 4 in Table 2, it can be seen that by controlling the average primary particle size of the positive electrode material within the range provided in this application, not only can a positive electrode material with a larger tap density be obtained to improve the energy density of the secondary battery, but also the doping elements can optimize the path of Li-ion diffusion, introduce more charge carriers, and improve the electron transport ability.

[0163] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship indicated by terms such as "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention.

[0164] The above-disclosed is only a preferred embodiment of the present invention. Of course, it cannot be used to limit the scope of the rights of the present invention. Those of ordinary skill in the art can understand all or part of the processes of implementing the above embodiments, and the equivalent changes made according to the claims of the present invention still fall within the scope covered by the present invention.

Claims

1. A positive electrode active material, characterized in that: The chemical formula of the positive electrode active material includes LiMn x1 Fe y1 A1 z1 PO4 and LiMn x2 Fe y2 A2 z2 PO4, wherein the A1 element or the A2 element includes one or more of Ti, V, Co, Ni, Nb, Mo, and Zr, and the chemical formula satisfies: z1=a×y1, 0.02%≤a≤3.6%, z2=b×y2, 4.5%≤b≤9%, 4.52%≤a+b≤12.6%.

2. The positive electrode active material according to claim 1, characterized in that The average primary particle size of the positive electrode active material is 200nm to 450nm. The positive electrode active material includes first particles, second particles and third particles. The primary particle size of the first particles is smaller than that of the second particles, and the primary particle size of the second particles is smaller than that of the third particles.

3. The positive electrode active material according to claim 2, characterized in that The primary particle size D1 of the first particle satisfies 1nm≤D1<200nm, the primary particle size D2 of the second particle satisfies 200nm≤D2≤400nm, and the primary particle size D3 of the third particle satisfies 400nm<D3≤1000nm.

4. The positive electrode active material according to claim 3, characterized in that The mass proportion of the first particles in the positive electrode active material is 20% to 50%, the mass proportion of the second particles in the positive electrode active material is 30% to 40%, and the mass proportion of the third particles in the positive electrode active material is 10% to 35%.

5. The positive electrode active material according to claim 3, characterized in that The first particles include first particles I and first particles II, the primary particle size D11 of the first particles I satisfies 1nm≤D11<150nm, and the primary particle size D12 of the first particles II satisfies 150nm≤D12<200nm; and / or, the third particles include third particles I and third particles II, the primary particle size D31 of the third particles I satisfies 400nm<D31≤500nm, and the primary particle size D32 of the third particles II satisfies 500nm<D32≤1000nm.

6. The positive electrode active material according to claim 5, characterized in that The mass proportion of the first particle I in the positive electrode active material is 5% to 20%, the mass proportion of the first particle II in the positive electrode active material is 15% to 30%, the mass proportion of the third particle I in the positive electrode active material is 5% to 20%, and the mass proportion of the third particle II in the positive electrode active material is 5% to 15%.

7. The positive electrode active material according to any one of claims 1 to 6, characterized in that The chemical formula of the positive electrode active material also satisfies: x1+x2=x, y1+y2=y, 0.2≤x / (x+y)≤0.8, n=x / y, 0.25≤n≤4.

8. A method for preparing a positive electrode active material, characterized in that: The preparation method is used to prepare the positive electrode active material according to any one of claims 1 to 7, and the preparation method comprises: The first reaction solution containing the Mn source and the second reaction solution containing the Fe source are mixed to obtain two third reaction solutions, and the A1 source and the A2 source are added to the two third reaction solutions respectively; Adding a Li source and a P source to two portions of the third reaction solution to obtain two portions of mixed solution, and reacting the two portions of the mixed solution to obtain a solid precursor; The solid precursor is calcined to obtain a positive electrode active material.

9. The preparation method according to claim 8, characterized in that: The first reaction solution containing the Mn source and the second reaction solution containing the Fe source are mixed to obtain two third reaction solutions, and the A1 source and the A2 source are respectively added to the two third reaction solutions, comprising: Mixing the first reaction solution containing the Mn source and the second reaction solution containing the Fe source to obtain the third reaction solution; The third reaction liquid is evenly divided into a third reaction liquid I and a third reaction liquid II; A1 source is added to the third reaction liquid I to obtain a first precursor solution, and A2 source is added to the third reaction liquid II to obtain a second precursor solution.

10. The preparation method according to claim 9, characterized in that: The Li source and the P source are added to the two third reaction solutions to obtain two mixed solutions, and the two mixed solutions are reacted to obtain a solid precursor, including: Adding a Li source and a P source to the first precursor solution and the second precursor solution respectively to obtain a first mixed solution and a second mixed solution; Respectively reacting the first mixed liquid and the second mixed liquid to obtain a first precursor and a second precursor; The first precursor and the second precursor are mixed to obtain the solid precursor.

11. A secondary battery, characterized in that: The secondary battery comprises the positive electrode active material according to any one of claims 1 to 7, or the secondary battery comprises the positive electrode active material prepared by the method for preparing a positive electrode active material according to any one of claims 8 to 10.

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