Lithium manganese iron phosphate positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention provides a lithium manganese iron phosphate positive electrode material, a preparation method thereof and a lithium ion battery, and relates to the field of lithium ion batteries. Comprising primary particles A and primary particles B, each primary particle A comprises a first inner core and a first coating layer arranged on the surface of the first inner core, and each primary particle B comprises a second inner core and a second coating layer arranged on the surface of the second inner core; the chemical formula of the first inner core and the chemical formula of the second inner core are respectively and independently [Li < a-x > M < x >] Fe < y > Mn < b-y > PO < 4-0.5 z > F < z >; m comprises potassium and / or sodium, 0.0001 < = x < = 0.01, 1.0 < = a < = 1.05, 0.2 < = y < = 0.6, 0.0001 < = z < = 0.01, and 0.96 < = b < = 1; the first coating layer and the second coating layer comprise carbon; the primary particles A are larger than the primary particles B in particle size. Primary particles with different particle sizes are adopted for grading, the compaction density is improved, oxygen and lithium double-site doping is adopted, and the rate and the cycle performance are improved.

Description

Technical Field

[0001] The present application relates to the field of lithium-ion batteries, and particularly to a lithium iron manganese phosphate cathode material, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] As an "upgraded version" of lithium iron phosphate, lithium iron manganese phosphate material has received increasing attention in the lithium-ion battery industry. Compared with lithium iron phosphate, substituting iron with manganese to form an iron-manganese-lithium solid solution can increase the voltage platform while ensuring basically the same capacity, thereby breaking through the bottleneck of lithium iron phosphate in terms of energy density. Ideally, the energy density can be increased by 15 - 20%. Moreover, the manufacturing process of lithium iron manganese phosphate material can be basically the same as that of lithium iron phosphate, and the cost mainly lies in the substitution of manganese for iron. Therefore, it is relatively easy to achieve large-scale application for material enterprises.

[0003] However, the electronic conductivity of lithium iron manganese phosphate material is relatively low, which will cause untimely electron transfer inside the electrode and serious polarization during high-rate charge and discharge of the battery, resulting in poor rate performance. There is a certain hindrance to the diffusion of lithium ions in the crystal structure of lithium iron manganese phosphate, and the migration speed of lithium ions is difficult to keep up with the electron transfer speed. Moreover, the Jahn-Teller effect of lithium iron manganese phosphate causes Mn to be more likely to dissolve out from the bulk phase structure of the material. The Jahn-Teller effect, also known as the Jahn-Teller effect, will cause serious capacity attenuation of the material, and poor rate and cycle life. Most of the conventional preparation methods of lithium iron manganese phosphate are one-time sintering processes. The obtained lithium iron manganese phosphate material has relatively uniform particle size and low tap density, resulting in low energy density when applied to corresponding batteries and unable to meet the requirements of high-performance applications. These characteristics limit the practical application of lithium iron manganese phosphate.

[0004] Based on this, there is an urgent need to provide a lithium iron manganese phosphate to solve the above problems. Summary of the Invention

[0005] The purpose of the present application is to provide a lithium iron manganese phosphate cathode material, a preparation method thereof, and a lithium-ion battery to solve the above problems.

[0006] To achieve the above purpose, in the first aspect of the present application, a lithium iron manganese phosphate cathode material is provided, including primary particle A and primary particle B; the primary particle A includes a first core and a first coating layer provided on the surface of the first core, and the primary particle B includes a second core and a second coating layer provided on the surface of the second core; The chemical formulas of the first core and the second core are independently [Li a-x M x Fe y Mn b-y PO 4-0.5z Fz ; Among them, M includes potassium and / or sodium, 0.0001 ≤ x ≤ 0.01, 1.0 ≤ a ≤ 1.05, 0.2 ≤ y ≤ 0.6, 0.0001 ≤ z ≤ 0.01, 0.96 ≤ b ≤ 1; The first coating layer and the second coating layer include carbon; The particle size of the primary particle A is larger than that of the primary particle B.

[0007] Optionally, the lithium iron manganese phosphate cathode material satisfies at least one of the following conditions: (1) The D50 of the first core is 0.5 μm - 1 μm, and the D50 of the second core is 0.1 μm - 0.3 μm; (2) The chemical formula of the first core is [Li a-x M x Fe y Mn b-y PO 4-0.5z F z , where 0.96 ≤ b ≤ 0.97; the chemical formula of the second core is [Li a-x M x Fe y Mn b'-y PO 4-0.5z F z , where 0.98 ≤ b' ≤ 1.

[0008] Optionally, the lithium iron manganese phosphate cathode material satisfies at least one of the following conditions: (1) The D50 of the lithium iron manganese phosphate cathode material is 0.1 μm - 1 μm, and D100 ≤ 2 μm; (2) The tap density of the lithium iron manganese phosphate cathode material is 2.35 g / cm 3 -2.45 g / cm 3 ; (3) The mass ratio of the primary particle A to the primary particle B is 3:7 - 5:5.

[0009] The second aspect of the present application provides a preparation method of the lithium iron manganese phosphate cathode material, including: Mixing and wet-milling a lithium iron manganese phosphate precursor A, a first lithium source, a first carbon source, a first fluorine source and a solvent to obtain a slurry A; mixing and wet-milling a lithium iron manganese phosphate precursor B, a second lithium source, a second carbon source, a second fluorine source and a solvent to obtain a slurry B; The slurry A and the slurry B are respectively subjected to first spray drying to obtain solid powder A and solid powder B. Under an inert atmosphere, the solid powder A and the solid powder B are respectively subjected to first sintering to obtain lithium iron manganese phosphate intermediate A and lithium iron manganese phosphate intermediate B; The lithium iron manganese phosphate intermediate A, the first alkali metal source, the third carbon source and a solvent are mixed and wet milled to obtain slurry C; the lithium iron manganese phosphate intermediate B, the second alkali metal source, the fourth carbon source and a solvent are mixed and wet milled to obtain slurry D; the slurry C and the slurry D are mixed to obtain slurry E; The slurry E is subjected to second spray drying and second sintering to obtain the lithium iron manganese phosphate cathode material; The (Fe + Mn) / P ratio of the lithium iron manganese phosphate precursor A is less than the (Fe + Mn) / P ratio of the lithium iron manganese phosphate precursor B.

[0010] Optionally, the method for preparing the lithium iron manganese phosphate cathode material satisfies at least one of the following conditions: (1) The chemical formula of the lithium iron manganese phosphate precursor A is Fe y Mn c PO4, where 0.2 ≤ y ≤ 0.6, 0.36 ≤ c ≤ 0.77, and (Fe + Mn) / P is 0.96 - 0.97; (2) The chemical formula of the lithium iron manganese phosphate precursor B is Fe y' Mn c' PO4, where 0.2 ≤ y' ≤ 0.6, 0.38 ≤ c' ≤ 0.80, and (Fe + Mn) / P is 0.98 - 1.

[0011] Optionally, the method for preparing the lithium iron manganese phosphate cathode material satisfies at least one of the following conditions: (1) Each of the first lithium source and the second lithium source independently includes one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium dihydrogen phosphate, and lithium oxalate; (2) Each of the first carbon source and the second carbon source independently includes one or more of sucrose, glucose, starch, maltodextrin, and pitch; (3) Each of the first fluorine source and the second fluorine source independently includes one or more of LiF, NH4F, and LiPF6; (4) The mass ratio of the first carbon source to the lithium iron manganese phosphate precursor A is 4 - 7:100; (5) The mass ratio of the second carbon source to the lithium iron manganese phosphate precursor B is 4 - 7:100; (6) The molar ratio of P in the lithium iron manganese phosphate precursor A, lithium in the first lithium source, and fluorine in the first fluorine source is 1:1.0 - 1.05:0.001 - 0.01; (7) The molar ratio of P in the manganese iron phosphate precursor B, lithium in the second lithium source, and fluorine in the second fluorine source is 1: 1.0 - 1.05: 0.001 - 0.01.

[0012] Optionally, the method for preparing the lithium manganese iron phosphate cathode material satisfies at least one of the following conditions: (1) The first alkali metal source and the second alkali metal source each independently include one or more of Na2CO3, CH3COOK, KH2PO4, K2CO3, CH3COONa, Na2C2O4, and NaH2PO4; (2) The third carbon source and the fourth carbon source each independently include one or more of polyethylene glycol, polyacrylic acid, polyvinylpyrrolidone, polydopamine, and phenolic resin; (3) The mass ratio of the third carbon source to the lithium manganese iron phosphate intermediate A is 1 - 3: 100; (4) The mass ratio of the fourth carbon source to the lithium manganese iron phosphate intermediate B is 1 - 3: 100; (5) The molar ratio of the P element in the lithium manganese iron phosphate intermediate A to the alkali metal in the first alkali metal source is 1: 0.001 - 0.01; (6) The molar ratio of the P element in the lithium manganese iron phosphate intermediate B to the alkali metal in the second alkali metal source is 1: 0.001 - 0.01.

[0013] Optionally, the method for preparing the lithium manganese iron phosphate cathode material satisfies at least one of the following conditions: (1) The end temperature of the first sintering is 550 °C - 650 °C, and the holding time is 6 h - 8 h; (2) The end temperature of the second sintering is 690 °C - 790 °C, and the holding time is 9 h - 12 h.

[0014] Optionally, the method for preparing the lithium manganese iron phosphate cathode material satisfies at least one of the following conditions: (1) The D50 of the solid particles in the slurry A and the slurry B are each independently 0.1 μm - 0.5 μm; (2) The D50 of the slurry C is 0.5 μm - 0.7 μm; (3) The D50 of the slurry D is 0.1 μm - 0.3 μm.

[0015] The third aspect of the present application provides a lithium ion battery, including the lithium manganese iron phosphate cathode material or the lithium manganese iron phosphate cathode material prepared by the method for preparing the lithium manganese iron phosphate cathode material.

[0016] Compared with the prior art, the beneficial effects of the present application include: The lithium iron manganese phosphate cathode material provided by the present application, firstly, uses large and small primary particles with different particle sizes for grading. The large particle size particles provide a supporting framework, and the small particle size particles can fill the gaps between the large particles, which helps the particles to achieve a more compact arrangement under pressure, thereby improving the tap density; secondly, oxygen and lithium dual-site doping is adopted. By doping alkali metal ions such as + Na + K + with an ionic radius larger than that of Li + into the Li 3+ site, it will cause a certain degree of distortion and expansion of the lattice of lithium iron manganese phosphate. This structural change is conducive to forming a more spacious ion transport channel, providing more convenient conditions for the diffusion of lithium ions, thereby increasing the migration rate of ions in the material, improving the rate and cycle performance of the material. At the same time, F with strong electronegativity is doped and substituted into the oxygen site. The F-Mn chemical bond formed by F is more stable than the Mn-O chemical bond, which can effectively inhibit the Jahn-Teller effect and reduce the dissolution of Mn

[0017] The preparation method of the lithium iron manganese phosphate cathode material provided by the present application, firstly, by using lithium iron manganese phosphate precursors with different (Fe+Mn) / P ratios, two lithium iron manganese phosphate intermediates with different primary particle sizes are formed after the first sintering. Combining with the second sintering step, the problem of grading of large and small particles of lithium iron manganese phosphate material that cannot be achieved in the prior art is realized, and the tap density of the material is improved; secondly, an oxygen site dopant (fluorine source) is added before the first sintering. The F ions replace the oxygen atoms in the phosphate group, which can enhance the structural stability of the material and is conducive to reducing the negative impact brought by lattice distortion during lithium site doping in the second high-temperature sintering process. After the first sintering and F doping, a relatively stable lattice structure has been formed in the system. Na and K ions are more likely to enter the lithium site and cooperate with the previously doped F to further regulate the lattice parameters, making the lithium ion diffusion channel more regular and unobstructed.

[0018] The lithium ion battery provided by the present application has excellent rate performance and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present application and should not be regarded as limiting the scope of the present application.

[0020] Figure 1 SEM image of the lithium iron manganese phosphate intermediate A provided for Example 1; Figure 2 SEM image of the lithium iron manganese phosphate intermediate B provided for Example 1; Figure 3 SEM image of the lithium iron manganese phosphate cathode material provided for Example 1. Detailed implementation manners

[0021] First, the solution provided by this application will be explained in more detail as follows: The first aspect of this application provides a lithium iron manganese phosphate cathode material, including primary particle A and primary particle B; the primary particle A includes a first core and a first coating layer disposed on the surface of the first core, and the primary particle B includes a second core and a second coating layer disposed on the surface of the second core; The chemical formulas of the first core and the second core are each independently [Li a-x M x Fe y Mn b-y PO 4-0.5z F z ; wherein, M includes potassium and / or sodium, 0.0001 ≤ x ≤ 0.01, 1.0 ≤ a ≤ 1.05, 0.2 ≤ y ≤ 0.6, 0.0001 ≤ z ≤ 0.01, 0.96 ≤ b ≤ 1; It should be noted that multi-site doping can simultaneously affect the charge states of multiple elements such as Li, other metals, oxygen, and phosphorus, strengthening the doping effect; Optionally, x can be 0.0001, 0.0002, 0.0004, 0.0006, 0.0008, 0.001, 0.002, 0.004, 0.006, 0.008, 0.01 or any value between 0.0001 - 0.01, a can be 1.01, 1.02, 1.03, 1.04, 1.05 or any value between 1.0 - 1.05, y can be 0.2, 0.3, 0.4, 0.5, 0.6 or any value between 0.2 - 0.6, z can be 0.0001, 0.0002, 0.0004, 0.0006, 0.0008, 0.001, 0.002, 0.004, 0.006, 0.008, 0.01 or any value between 0.0001 - 0.01, and b can be 0.96, 0.97, 0.98, 0.99, 1 or any value between 0.96 - 1; Preferably, 0.001 ≤ z ≤ 0.01; 0.001 ≤ x ≤ 0.01; It should be noted that if there is too much fluorine doping, it may cause significant changes in the crystal structure, deviating from the ideal olivine structure, thus affecting the electrochemical performance of the material. At the same time, too much fluorine doping requires the use of more fluorine sources, which not only increases the raw material cost but also may make the reaction control in the preparation process more difficult; It should also be noted that if the doping amount of Na and / or K is too small, the effect of doping on improving the migration rate of lithium ions and enhancing the structural stability of the material is not obvious, and the improvement of the electrochemical performance of the material is limited; if the doping amount of Na and / or K is too much, it may cause too large changes in the lattice parameters, affect the crystal structure stability of the material, and block the lithium ion deintercalation path; In addition, when alkali metals and fluorine are co-doped, the structure and ion transport path of the material can be more comprehensively optimized. For the doping of alkali metals at lithium sites, the ionic radius of alkali metal ions is larger than that of Li + , which can change the crystal structure of the material and expand the lithium ion migration channels; for the doping of fluorine at oxygen sites, the ionic radius of fluoride ions is smaller than that of oxygen ions. After replacing the oxygen position, the Li-O distance increases and the bond energy decreases, which can reduce the energy barrier for lithium ion migration. The synergistic effect of the two makes the lithium ion conductivity increase more significantly, thereby improving the charge-discharge performance and rate performance of the battery; In some embodiments, in the chemical formulas of the first core and the second core, preferably, 0.001 < x < 0.005, 0.001 < z < 0.005; The first coating layer and the second coating layer include carbon; The particle size of the primary particle A is larger than that of the primary particle B.

[0022] In some embodiments, the lithium iron manganese phosphate cathode material satisfies at least one of the following conditions: (1) The D50 of the first core is 0.5 μm - 1 μm, and the D50 of the second core is 0.1 μm - 0.3 μm; Optionally, the D50 of the first core can be 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm or any value between 0.5 μm - 1 μm, and the D50 of the second core can be 0.1 μm, 0.2 μm, 0.3 μm or any value between 0.1 μm - 0.3 μm; (2) The chemical formula of the first core is [Li a-x M x Fe y Mn b-y PO 4-0.5z F z , where 0.96 ≤ b ≤ 0.97; the chemical formula of the second core is [Li a-x M xFe y Mn b'-y PO 4-0.5z F z , wherein, 0.98 ≤ b' ≤ 1.

[0023] Optionally, b in the first core chemical formula can be 0.96, 0.965, 0.97 or any value between 0.96 and 0.97; b' in the second core chemical formula can be 0.98, 0.99, 1 or any value between 0.98 and 1.

[0024] In some embodiments, the lithium iron manganese phosphate cathode material satisfies at least one of the following conditions: (1) The D50 of the lithium iron manganese phosphate cathode material is 0.1 μm - 1 μm, and D100 ≤ 2 μm; Optionally, the D50 of the lithium iron manganese phosphate cathode material can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm or any value between 0.1 μm and 1 μm, and D100 can be 1 μm, 1.5 μm, 2 μm or any value ≤ 2 μm; (2) The tap density of the lithium iron manganese phosphate cathode material is 2.35 g / cm 3 -2.45 g / cm 3 ; Optionally, the tap density of the lithium iron manganese phosphate cathode material can be 2.35 g / cm 3 , 2.4 g / cm 3 , 2.45 g / cm 3 or any value between 2.35 g / cm 3 -2.45 g / cm 3 ; (3) The mass ratio of the primary particle A to the primary particle B is 3:7 - 5:5.

[0025] Optionally, the mass ratio of the primary particle A to the primary particle B can be 3:7, 4:6, 5:5 or any value between 3:7 and 5:5.

[0026] The second aspect of the present application provides a preparation method of the lithium iron manganese phosphate cathode material, including: Mixing and wet-milling a lithium iron manganese phosphate precursor A, a first lithium source, a first carbon source, a first fluorine source and a solvent to obtain a slurry A; mixing and wet-milling a lithium iron manganese phosphate precursor B, a second lithium source, a second carbon source, a second fluorine source and a solvent to obtain a slurry B; It should be noted that the wet-milling for preparing the slurry A and the slurry B is to reduce the particle size of the precursor and make various raw materials mix evenly; The slurry A and the slurry B are respectively subjected to first spray drying to obtain solid powder A and solid powder B. Under an inert atmosphere, the solid powder A and the solid powder B are respectively subjected to first sintering to obtain lithium iron manganese phosphate intermediate A and lithium iron manganese phosphate intermediate B; The lithium iron manganese phosphate intermediate A, the first alkali metal source, the third carbon source and the solvent are mixed and wet-milled to obtain slurry C; the lithium iron manganese phosphate intermediate B, the second alkali metal source, the fourth carbon source and the solvent are mixed and wet-milled to obtain slurry D; the slurry C and the slurry D are mixed to obtain slurry E; It should be noted that the beneficial effects of stepwise adding the oxygen site dopant (fluorine source) and the lithium site dopant B (alkali metal source) are as follows: ① The element distribution and lattice optimization are more precise. Adding the fluorine source before the first sintering, the F ions replace the oxygen atoms in the phosphate group, which can enhance the structural stability of the material and is beneficial to reducing the negative impact brought by lattice distortion during lithium site doping in the second high-temperature sintering process; adding the alkali metal source before the second sintering, at this time, after the first sintering and F doping, a relatively stable lattice structure has been formed in the system, and the alkali metal ions are more likely to enter the lithium site and cooperate with the previously doped F to further regulate the lattice parameters, making the lithium ion diffusion channels more regular and unobstructed; ② It is more conducive to performance regulation. According to the characteristics and requirements of different sintering stages, the addition amount of the dopant and the sintering conditions can be precisely controlled, so as to more flexibly regulate the performance of lithium iron phosphate. Exemplarily, when adding the fluorine source in the first sintering, the doping concentration of oxygen sites in the lattice can be precisely controlled by adjusting the sintering temperature, time and the addition amount of the fluorine source to optimize the electronic conductivity of the material. When adding the alkali metal in the second sintering, the addition amount of the alkali metal and the sintering conditions are adjusted according to the needs to further improve the diffusion rate of lithium ions and the structural stability of the material, so as to achieve fine regulation of the comprehensive performance of the material; It should be noted that the wet milling for preparing slurry C and slurry D dissociates the secondary particles of the precursor after spray drying and primary sintering and makes various raw materials mix evenly.

[0027] The slurry E is subjected to second spray drying and second sintering to obtain the lithium iron manganese phosphate cathode material; The (Fe + Mn) / P ratio of the lithium iron manganese phosphate precursor A is less than the (Fe + Mn) / P ratio of the lithium iron manganese phosphate precursor B.

[0028] It should be noted that in a single sintering step, the reaction rates between the iron source, manganese source, and phosphorus source, as well as their reaction rates with the lithium source, are affected by (Fe + Mn) / P; when (Fe + Mn) / P is high, the nucleation rate is greater than the crystal growth rate, resulting in a large number of lithium iron manganese phosphate crystal nuclei. These crystal nuclei compete for limited growth space and material resources during subsequent growth, thereby restricting the growth size of each crystal particle and ultimately leading to the preparation of lithium iron phosphate materials with smaller particles; when (Fe + Mn) / P is low, the more abundant phosphorus source allows the generated lithium iron manganese phosphate crystal nuclei to have more time and space for growth, thus facilitating the formation of larger particles; the lithium iron manganese phosphate precursor with a high (Fe + Mn) / P ratio (lithium iron manganese phosphate precursor B) generates a lithium iron manganese phosphate secondary sphere intermediate (lithium iron manganese phosphate intermediate B) composed of primary particles with smaller particle sizes during a single sintering, which is dissociated into smaller lithium iron manganese phosphate primary particles through grinding; the lithium iron manganese phosphate precursor with a low (Fe + Mn) / P ratio (lithium iron manganese phosphate precursor A) generates a lithium iron manganese phosphate secondary sphere intermediate (lithium iron manganese phosphate intermediate A) composed of primary particles with larger particle sizes, which is dissociated into larger lithium iron manganese phosphate primary particles through grinding; the lithium iron manganese phosphate primary particles of different sizes further form a lithium iron manganese phosphate cathode material with a particle size grading.

[0029] In some embodiments, the method for preparing the lithium iron manganese phosphate cathode material satisfies at least one of the following conditions: (1) The chemical formula of the lithium iron manganese phosphate precursor A is Fe y Mn c PO4, where 0.2 ≤ y ≤ 0.6, 0.36 ≤ c ≤ 0.77, and (Fe + Mn) / P is 0.96 - 0.97; Optionally, in the chemical formula of the lithium iron manganese phosphate precursor A, y can be 0.2, 0.3, 0.4, 0.5, 0.6, or any value between 0.2 - 0.6, c can be 0.36, 0.4, 0.5, 0.6, 0.7, 0.77, or any value between 0.36 - 0.77, and (Fe + Mn) / P can be 0.96, 0.965, 0.97, or any value between 0.96 - 0.97; (2) The chemical formula of the lithium iron manganese phosphate precursor B is Fe y' Mn c' PO4, where 0.2 ≤ y' ≤ 0.6, 0.38 ≤ c' ≤ 0.80, and (Fe + Mn) / P is 0.98 - 1.

[0030] Optionally, in the chemical formula of the manganese iron phosphate precursor B, y' can be 0.2, 0.3, 0.4, 0.5, 0.6 or any value between 0.2 and 0.6, c' can be 0.38, 0.4, 0.5, 0.6, 0.7, 0.8 or any value between 0.38 and 0.8, and (Fe + Mn) / P can be 0.98, 0.98, 1 or any value between 0.98 and 1; In some embodiments, the method for preparing the lithium manganese iron phosphate cathode material satisfies at least one of the following conditions: (3) The first lithium source and the second lithium source each independently include one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium dihydrogen phosphate, and lithium oxalate; (4) The first carbon source and the second carbon source each independently include one or more of sucrose, glucose, starch, maltodextrin, and pitch; It should be noted that the first carbon source and the second carbon source have a reducing effect. During the first sintering process, the carbon source undergoes thermal decomposition and carbonization. These carbons exhibit reducing properties in an inert atmosphere and can reduce Fe 3+ / Mn 3+ to Fe 2+ / Mn 2+ ; C. The first fluorine source and the second fluorine source each independently include one or more of LiF, NH4F, and LiPF6; (5) The mass ratio of the first carbon source to the manganese iron phosphate precursor A is 4 - 7:100; Optionally, the mass ratio of the first carbon source to the manganese iron phosphate precursor A can be 4:100, 5:100, 6:100, 7:100 or any value between 4 and 7:100; (6) The mass ratio of the second carbon source to the manganese iron phosphate precursor B is 4 - 7:100; Optionally, the mass ratio of the second carbon source to the manganese iron phosphate precursor B can be 4:100, 5:100, 6:100, 7:100 or any value between 4 and 7:100; (7) The molar ratio of P in the manganese iron phosphate precursor A, lithium in the first lithium source, and fluorine in the first fluorine source is 1:1.0 - 1.05:0.001 - 0.01; Optionally, the molar ratio of P in the iron manganese phosphate precursor A, Li in the first lithium source, and F in the first fluorine source can be 1:1.0:0.001, 1:1.01:0.001, 1:1.02:0.001, 1:1.03:0.001, 1:1.04:0.001, 1:1.05:0.001, 1:1.0:0.005, 1:1.0:0.01, 1:1.05:0.01, or any value between 1:1.0 - 1.05:0.001 - 0.01; (8) The molar ratio of P in the iron manganese phosphate precursor B, Li in the second lithium source, and F in the second fluorine source is 1:1.0 - 1.05:0.001 - 0.01.

[0031] Optionally, the molar ratio of P in the iron manganese phosphate precursor B, Li in the second lithium source, and F in the second fluorine source can be 1:1.0:0.001, 1:1.01:0.001, 1:1.02:0.001, 1:1.03:0.001, 1:1.04:0.001, 1:1.05:0.001, 1:1.0:0.005, 1:1.0:0.01, 1:1.05:0.01, or any value between 1:1.0 - 1.05:0.001 - 0.01; In some embodiments, the method for preparing the lithium iron manganese phosphate cathode material satisfies at least one of the following conditions: (1) The first alkali metal source and the second alkali metal source each independently include one or more of Na2CO3, CH3COOK, KH2PO4, K2CO3, CH3COONa, Na2C2O4, NaH2PO4; (2) The third carbon source and the fourth carbon source each independently include one or more of polyethylene glycol, polyacrylic acid, polyvinylpyrrolidone, polydopamine, and phenolic resin; It should be noted that the third carbon source and the fourth carbon source play a coating role; (3) The mass ratio of the third carbon source to the lithium iron manganese phosphate intermediate A is 1 - 3:100; Optionally, the mass ratio of the third carbon source to the lithium iron manganese phosphate intermediate A can be 1:100, 2:100, 3:100, or any value between 1 - 3:100; (4) The mass ratio of the fourth carbon source to the lithium iron manganese phosphate intermediate B is 1 - 3:100; Optionally, the mass ratio of the fourth carbon source to the lithium iron manganese phosphate intermediate B can be 1:100, 2:100, 3:100, or any value between 1 - 3:100; (5) The molar ratio of P element in the lithium iron manganese phosphate intermediate A to the alkali metal in the first alkali metal source is 1:0.001 - 0.01; Optionally, the molar ratio of P element in the lithium iron manganese phosphate intermediate A to the alkali metal in the first alkali metal source can be 1:0.001, 1:0.005, 1:0.01 or any value between 1:0.001 - 0.01; (6) The molar ratio of P element in the lithium iron manganese phosphate intermediate B to the alkali metal in the second alkali metal source is 1:0.001 - 0.01.

[0032] Optionally, the molar ratio of P element in the lithium iron manganese phosphate intermediate B to the alkali metal in the second alkali metal source can be 1:0.001, 1:0.005, 1:0.01 or any value between 1:0.001 - 0.01.

[0033] In some embodiments, the method for preparing the lithium iron manganese phosphate cathode material satisfies at least one of the following conditions: (1) The final temperature of the first sintering is 550°C - 650°C, and the holding time is 6h - 8h; Optionally, the final temperature of the first sintering can be 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C or any value between 550°C - 650°C, and the holding time can be 6h, 7h, 8h or any value between 6h - 8h; It should be noted that when the final temperature of the first sintering is in the range of 550°C - 650°C, the reactants can react fully, enabling the gradual conversion of lithium iron manganese phosphate into lithium iron manganese phosphate, while ensuring that the product has good crystallinity and electrochemical performance; when the final temperature of the first sintering is too low, the reaction may be incomplete, and unreacted lithium iron manganese phosphate may remain in the product, resulting in poor performance of the final material; when the final temperature of the first sintering is too high, it may cause the growth of material particles, a decrease in specific surface area, which is not conducive to the dissociation of particles during the second grinding process and will also have an adverse impact on the electrochemical performance of the material; (2) The final temperature of the second sintering is 690°C - 790°C, and the holding time is 9h - 12h.

[0034] Optionally, the final temperature of the second sintering can be 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C or any value between 690°C - 790°C, and the holding time can be 9h, 10h, 11h, 12h or any value between 9h - 12h.

[0035] It should be noted that when the final temperature of the second sintering is 690°C - 790°C, the carbon source decomposes to form carbon coating on the surface of lithium iron phosphate manganese, and the alkali metal source provides sodium and / or potassium ions; within this temperature range, the carbon source can decompose well and form a uniform carbon coating layer on the surface of lithium iron phosphate manganese particles, improving the conductivity of the lithium iron phosphate manganese material; the higher temperature is also conducive to the entry of sodium / potassium ions with larger radii into the lattice of lithium iron phosphate manganese, achieving uniform doping, forming a wider ion transport channel, and improving the rate performance and cycling performance of the material; at the same time, the second sintering can further promote the growth and development of crystals, make the lattice more regular, reduce defects, thereby improving the crystallinity of the material and providing an internal structural basis for improving the tap density; when the final temperature of the second sintering is too low, the carbon source decomposes insufficiently, unable to form a complete and dense carbon coating layer, resulting in poor carbon coating effect, unable to effectively improve the electronic conductivity and electrochemical performance of lithium iron phosphate, and the diffusion rate of sodium and / or potassium ions is slow, making it difficult to fully enter the lattice, resulting in uneven doping and unable to effectively play the improvement role of sodium / potassium doping on the material structure and performance; when the final temperature of the second sintering is too high, it may lead to excessive growth of the carbon layer, reducing the bonding force between the carbon layer and the lithium iron phosphate manganese particles, and may even cause problems such as over-sintering of the crystal structure of the material and grain growth, affecting the specific surface area and pore structure of the material, and thus reducing the electrochemical performance of the material.

[0036] It should be noted that low-temperature sintering is used in the first sintering, and F ions with a smaller doping radius at the oxygen site are doped. F ions can start to diffuse into the lattice at a relatively low temperature, replace the oxygen atoms in the phosphate group, form stronger M-F bonds (M is a transition metal), enhance the structural stability of the material, and are beneficial to reducing the negative impact of lattice distortion during lithium site doping in the secondary sintering process; high-temperature sintering is used in the second sintering. The Na / K ions doped at the lithium site have a larger radius. Utilizing the high diffusion activity of atoms at high temperature, they can better enter the lattice stabilized by F, and at the same time, the secondary high-temperature sintering is beneficial to further improving the crystal structure of lithium iron phosphate manganese, reducing lattice defects and voids, thereby providing an internal structural basis for improving the tap density.

[0037] In some embodiments, the preparation method of the lithium iron phosphate manganese cathode material satisfies at least one of the following conditions: (1) The D50 of the solid particles in the slurry A and the slurry B are each independently 0.1 μm - 0.5 μm; Optionally, the D50 of the solid particles in the slurry A and the slurry B can be each independently 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm or any value between 0.1 μm - 0.5 μm; It should be noted that the main components of slurry A and slurry B are manganese iron phosphate precursors. If the particle sizes of slurry A and slurry B are too large, it is likely to affect the reaction uniformity and rate. A larger D50 means that the manganese iron phosphate particles are larger. When mixed with other reactants such as lithium sources, the contact area between the particles is relatively small, resulting in insufficient contact between the reactants. This causes the reaction to occur only on the particle surface or in local areas, affecting the reaction uniformity and making it difficult to obtain lithium manganese iron phosphate products with uniform composition and performance. At the same time, in the solid-phase reaction, ions need to be transported between particles through diffusion to complete the reaction. If the D50 of the manganese iron phosphate precursor particles is too large, the ion diffusion path will become longer, the diffusion resistance will increase, and the reaction rate will slow down. When the wet-milling particle size is too small, more energy and longer time are required, which will not only increase the production cost but also reduce the production efficiency. Prolonged grinding may also lead to increased equipment wear; Manganese iron phosphate precursors with too small D50 have a high surface energy and are in an energy-unstable state, prone to agglomeration. After agglomeration, the particles will affect the mixing uniformity when mixed with substances such as lithium sources in the subsequent process, resulting in uneven reactions. During the high-temperature sintering process, too high surface energy will also cause the crystal to grow too fast or the crystal structure to be unstable, resulting in defects in the lithium manganese iron phosphate crystal structure and affecting its electrochemical performance; (2) The D50 of the said slurry C is 0.5 μm - 0.7 μm; Optionally, the D50 of slurry C can be 0.5 μm, 0.6 μm, 0.7 μm or any value between 0.5 μm - 0.7 μm; It should be noted that the main component of slurry C is lithium manganese iron phosphate intermediate A. The particle size of slurry C is affected by the primary particle size of lithium manganese iron phosphate intermediate A itself and also by the grinding parameters; The manganese iron phosphate precursor with a lower (Fe + Mn) / P ratio reacts to form lithium manganese iron phosphate intermediate A in the first sintering step. Since there is a more abundant phosphorus source when (Fe + Mn) / P is low, the formed lithium manganese iron phosphate crystal nuclei have more time and space to grow, which is conducive to the formation of larger particles. Therefore, lithium manganese iron phosphate intermediate A consists of larger primary particles; By controlling the grinding parameters, slurry C with Dv50 = 0.5 - 0.7 μm is obtained; The primary particles in slurry C form large particles in the lithium manganese iron phosphate finished product with a particle size grading in the subsequent second sintering and pulverization steps, providing a supporting framework; It should also be noted that if the particle size of the solid particles in slurry C is too large, the resulting large particles of lithium iron manganese phosphate product will be too large, which is not conducive to the diffusion of lithium ions during the charge and discharge process, resulting in a decrease in the charge and discharge rate performance of the battery. The contact resistance between large-sized lithium iron manganese phosphate particles is relatively large, the conduction path of electrons between particles becomes longer, which hinders the transmission of electrons and leads to an increase in the internal resistance of the battery; if the particle size of the solid particles in slurry C is too small, the specific surface area of the lithium iron manganese phosphate product will be too large, affecting the electrode preparation process. (3) The D50 of the said slurry D is 0.1μm - 0.3μm.

[0038] Optionally, the D50 of slurry D can be 0.1μm, 0.2μm, 0.3μm or any value between 0.1μm - 0.3μm.

[0039] It should be noted that the main component of slurry D is lithium iron manganese phosphate intermediate B. The particle size of slurry D is affected by the size of the primary particles of lithium iron manganese phosphate intermediate B itself and also by the grinding parameters; the higher (Fe + Mn) / P lithium iron manganese phosphate precursor reacts to form lithium iron manganese phosphate intermediate B in the first sintering step. Since the nucleation rate is greater than the crystal growth rate when (Fe + Mn) / P is high, a large number of lithium iron manganese phosphate crystal nuclei will be formed. These crystal nuclei compete for limited growth space and material resources during the subsequent growth process, thus limiting the growth size of each crystal particle. Eventually, the prepared lithium iron phosphate material particles are relatively small. Therefore, lithium iron manganese phosphate intermediate B is composed of primary particles with a smaller particle size; by controlling the grinding parameters, slurry D with Dv50 = 0.1 - 0.3μm is obtained; the primary particles in slurry D form small particles in the lithium iron manganese phosphate product with a size particle size distribution in the second sintering and pulverization steps, filling the gaps between large particles to improve the tap density. It should also be noted that if the particle size of the solid particles in slurry D is too large, the resulting small particles of lithium iron manganese phosphate product will be too large and cannot effectively fill the gaps between large particles, resulting in a decrease in the overall packing density and further reducing the energy density of the battery; if the particle size of the solid particles in slurry D is too small, the resulting small particles of lithium iron manganese phosphate product will be too small. When the small particles are mixed with large particles, due to the large difference in particle size, separation is likely to occur, making it difficult to achieve a uniform particle size distribution. During vibration or compaction, it is difficult to form a tight packing with large particles, resulting in a decrease in the tapped density of the material and thus affecting the energy density of the battery.

[0040] In some embodiments, the solid content of the said slurry A, slurry B, slurry C, slurry D and slurry E are each independently 25% - 55%; The third aspect of the present application provides a lithium-ion battery, including the lithium iron manganese phosphate cathode material described above or the lithium iron manganese phosphate cathode material prepared by the preparation method of the lithium iron manganese phosphate cathode material described above.

[0041] The following will describe the implementation solutions of the present application in detail with reference to specific embodiments. However, those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. For those not specified in the embodiments, they are carried out under conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified in the manufacturer are all conventional products that can be obtained through commercial purchase.

[0042] Example 1 This example provides a lithium iron manganese phosphate cathode material and its preparation method. The specific preparation steps are as follows: S1: Add lithium iron manganese phosphate precursor A (Fe 0.39 Mn 0.58 PO4, (Fe + Mn) / P is 0.97), lithium source (lithium carbonate), carbon source (sucrose), and fluorine source (NH4F) into water, and perform homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the mass ratio of lithium iron manganese phosphate precursor A to the carbon source is 100:5.5, the molar ratio of Li in the lithium source to P in the lithium iron manganese phosphate precursor A is 1.025:1, and the molar ratio of P in the lithium iron manganese phosphate precursor A, lithium in the lithium source to element F in the fluorine source is 1:1.025:0.005. Transfer the slurry to sand grinding, and keep the average solid particle size of the slurry after sand grinding at 0.3 μm to obtain slurry A; S2: Add lithium iron manganese phosphate precursor B (Fe 0.4 Mn 0.59 PO4, (Fe + Mn) / P is 0.99), lithium source (lithium carbonate), carbon source (sucrose), and fluorine source (NH4F) into water, and perform homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the mass ratio of lithium iron manganese phosphate precursor B to the carbon source is 100:5.5, the molar ratio of P in the lithium iron manganese phosphate precursor B, lithium in the lithium source to element F in the fluorine source is 1:1.025:0.005. Transfer the slurry to sand grinding, and keep the average solid particle size of the slurry after sand grinding at 0.3 μm to obtain slurry B; S3: Perform spray drying treatment on slurry A and slurry B respectively (inlet air temperature is 240 °C, outlet air temperature is 100 °C) to obtain solid powder A and solid powder B. Under a nitrogen atmosphere, sinter solid powder A and solid powder B respectively (sintering temperature is 600 °C, holding time is 7 h) to obtain lithium iron manganese phosphate intermediate A and lithium iron manganese phosphate intermediate B respectively. The SEM of lithium iron manganese phosphate intermediate A is as Figure 1 , and the SEM of lithium iron manganese phosphate intermediate B is as Figure 2 ; S4: Add lithium iron manganese phosphate intermediate A, carbon source (polyethylene glycol), and alkali metal source (Na2CO3) into water, and perform homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the molar ratio of element Na in the alkali metal source to P in lithium iron manganese phosphate intermediate A is 0.005:1, and the mass ratio of lithium iron manganese phosphate intermediate A to the carbon source is 100:2. Transfer the slurry to a sand mill and keep the average solid particle size of the slurry after sanding at 0.6 μm to obtain slurry C; S5: Add lithium iron manganese phosphate intermediate B, carbon source (polyethylene glycol), and alkali metal source (Na2CO3) into water, and perform homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the molar ratio of element Na in the alkali metal source to P in lithium iron manganese phosphate intermediate B is 0.005:1, and the mass ratio of lithium iron manganese phosphate intermediate B to the carbon source is 100:2. Transfer the slurry to a sand mill and keep the average solid particle size of the slurry after sanding at 0.2 μm to obtain slurry D; S6: According to the mass ratio of 4:6 of lithium iron manganese phosphate intermediate A in slurry C to lithium iron manganese phosphate intermediate B in slurry D, pour slurry C and slurry D into a dispersion tank and stir and mix them to obtain slurry E; S7: Spray-dry slurry E (inlet air temperature is 240 °C, outlet air temperature is 100 °C) to obtain a solid product. Then, under an inert atmosphere, perform secondary sintering on the solid product (temperature is 740 °C, constant temperature time is 10.5 h). When it is cooled to room temperature after sintering, dissociate the sintered secondary spheres into primary particles through an air classifier to obtain a lithium iron manganese phosphate cathode material with a D50 of 0.45 μm and a D100 of 1.28 μm.

[0043] The lithium iron manganese phosphate cathode material includes primary particle A and primary particle B with a mass ratio of 4:6. Primary particle A includes a core (chemical formula is Li 1.02 Na 0.005 Fe 0.39 Mn 0.58 PO 3.9975 F 0.005 , the D50 of the core is 0.7 μm, ) and a coating layer provided on the surface of the core. Primary particle B includes a core (chemical formula is Li 1.02 Na 0.005 Fe 0.4 Mn 0.59 PO 3.9975 F 0.005 , the D50 of the core is 0.2 μm) and a coating layer provided on the surface of the core. The SEM of the lithium iron manganese phosphate cathode material is as shown in Figure 3 shown.

[0044] Example 2 This embodiment provides a lithium iron manganese phosphate cathode material and a preparation method thereof. The specific preparation steps are as follows: S1: Add the manganese iron phosphate precursor A (Fe 0.2 Mn 0.77 PO4, (Fe + Mn) / P is 0.97), a lithium source (lithium hydroxide), a carbon source (glucose), and a fluorine source (LiF) into water, and perform homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the mass ratio of the manganese iron phosphate precursor A to the carbon source is 100:4, and the molar ratio of P in the manganese iron phosphate precursor A, lithium in the lithium source, and element F in the fluorine source is 1:1:0.001. Transfer the slurry to sand grinding, and keep the average solid particle size of the slurry after sand grinding at 0.1 μm to obtain slurry A; S2: Add the manganese iron phosphate precursor B (Fe 0.2 Mn 0.8 PO4, (Fe + Mn) / P is 1), a lithium source (lithium dihydrogen phosphate), a carbon source (pitch), and a fluorine source (LiPF6) into water, and perform homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the mass ratio of the manganese iron phosphate precursor B to the carbon source is 100:4, and the molar ratio of P in the manganese iron phosphate precursor B, lithium in the lithium source, and element F in the fluorine source is 1:1:0.001. Transfer the slurry to sand grinding, and keep the average solid particle size of the slurry after sand grinding at 0.1 μm to obtain slurry B; S3: Perform spray drying treatment on slurry A and slurry B respectively (inlet air temperature is 240 °C, outlet air temperature is 100 °C) to obtain solid powder A and solid powder B. Under a nitrogen atmosphere, sinter solid powder A and solid powder B respectively (sintering temperature is 550 °C, holding time is 6 h) to obtain lithium iron manganese phosphate intermediate A and lithium iron manganese phosphate intermediate B respectively; S4: Add the lithium iron manganese phosphate intermediate A, a carbon source (polyvinylpyrrolidone), and an alkali metal source (CH3COOK) into water, and perform homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the molar ratio of element Na in the alkali metal source to P in the lithium iron manganese phosphate intermediate A is 0.001:1, and the mass ratio of the lithium iron manganese phosphate intermediate A to the carbon source is 100:1. Transfer the slurry to sand grinding, and keep the average solid particle size of the slurry after sand grinding at 0.5 μm to obtain slurry C; S5: Add the lithium iron manganese phosphate intermediate B, a carbon source (polyethylene glycol), and an alkali metal source (Na2CO3) into water, and perform homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the molar ratio of element Na in the alkali metal source to P in the lithium iron manganese phosphate intermediate B is 0.001:1, and the mass ratio of the lithium iron manganese phosphate intermediate B to the carbon source is 100:1. Transfer the slurry to sand grinding, and keep the average solid particle size of the slurry after sand grinding at 0.1 μm to obtain slurry D; S6: Pour slurry C and slurry D into a dispersion tank according to the mass ratio of 3:7 of lithium iron manganese phosphate intermediate A in slurry C to lithium iron manganese phosphate intermediate B in slurry D, and stir and mix them to obtain slurry E; S7: Spray-dry slurry E (inlet air temperature is 240 °C, outlet air temperature is 100 °C) to obtain a solid product. Then, under an inert atmosphere, conduct a second sintering on the solid product (temperature is 690 °C, holding time is 9 h). When it is cooled to room temperature after sintering, dissociate the sintered secondary spheres into primary particles through an air classifier to obtain a lithium iron manganese phosphate cathode material.

[0045] This lithium iron manganese phosphate cathode material includes primary particle A and primary particle B with a mass ratio of 3:7.

[0046] Example 3 This example provides a lithium iron manganese phosphate cathode material and its preparation method. The specific preparation steps are as follows: S1: Add lithium iron manganese phosphate precursor A (Fe 0.6 Mn 0.36 PO4, (Fe + Mn) / P is 0.96), a lithium source (lithium dihydrogen phosphate), a carbon source (maltodextrin), and a fluorine source (NH4F) into water, and conduct homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the mass ratio of lithium iron manganese phosphate precursor A to the carbon source is 100:7, and the molar ratio of P in lithium iron manganese phosphate precursor A, lithium in the lithium source, and element F in the fluorine source is 1:1.05:0.01. Transfer the slurry to a sand mill and keep the average solid particle size of the slurry after sand milling at 0.5 μm to obtain slurry A; S2: Add lithium iron manganese phosphate precursor B (Fe 0.8 Mn 0.2 PO4, (Fe + Mn) / P is 1), a lithium source (lithium oxalate), a carbon source (starch), and a fluorine source (LiPF6) into water, and conduct homogeneous stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the mass ratio of lithium iron manganese phosphate precursor B to the carbon source is 100:7, and the molar ratio of P in lithium iron manganese phosphate precursor B, lithium in the lithium source, and element F in the fluorine source is 1:1.05:0.01. Transfer the slurry to a sand mill and keep the average solid particle size of the slurry after sand milling at 0.5 μm to obtain slurry B; S3: Conduct spray-drying treatment on slurry A and slurry B respectively (inlet air temperature is 240 °C, outlet air temperature is 100 °C) to obtain solid powder A and solid powder B. Under a nitrogen atmosphere, conduct sintering on solid powder A and solid powder B respectively (sintering temperature is 650 °C, holding time is 8 h) to obtain lithium iron manganese phosphate intermediate A and lithium iron manganese phosphate intermediate B respectively; S4: Add lithium iron manganese phosphate intermediate A, carbon source (polydopamine), and alkali metal source (CH3COOK) into water, and perform homogenizing stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the molar ratio of element K in the alkali metal source to P in lithium iron manganese phosphate intermediate A is 0.01:1, and the mass ratio of lithium iron manganese phosphate intermediate A to the carbon source is 100:3. Transfer the slurry to a sand mill and keep the average solid particle size of the slurry after sanding at 0.7 μm to obtain slurry C; S5: Add lithium iron manganese phosphate intermediate B, carbon source (polydopamine), and alkali metal source (K2CO3) into water, and perform homogenizing stirring in a dispersion tank to obtain a slurry with a solid content of 40%. Among them, the molar ratio of element K in the alkali metal source to P in lithium iron manganese phosphate intermediate B is 0.01:1, and the mass ratio of lithium iron manganese phosphate intermediate B to the carbon source is 100:3. Transfer the slurry to a sand mill and keep the average solid particle size of the slurry after sanding at 0.3 μm to obtain slurry D; S6: According to the mass ratio of 5:5 of lithium iron manganese phosphate intermediate A in slurry C and lithium iron manganese phosphate intermediate B in slurry D, pour slurry C and slurry D into a dispersion tank and stir and mix them to obtain slurry E; S7: Spray-dry slurry E (inlet air temperature is 240 °C, outlet air temperature is 100 °C) to obtain a solid product. Then, under an inert atmosphere, perform a second sintering on the solid product (temperature is 790 °C, holding time is 12 h). When cooling to room temperature after sintering, dissociate the sintered secondary spheres into primary particles through a jet mill to obtain a lithium iron manganese phosphate cathode material.

[0047] This lithium iron manganese phosphate cathode material includes primary particle A and primary particle B with a mass ratio of 5:5.

[0048] Example 4 The difference from Example 1 is that in step S1, the molar ratio of P in lithium iron manganese phosphate precursor A to element F in the fluorine source is 1:0.001, and in step S5, the molar ratio of element Na in the alkali metal source to P in lithium iron manganese phosphate intermediate B is 0.01:1.

[0049] Example 5 The difference from Example 1 is that in step S1, the molar ratio of P in lithium iron manganese phosphate precursor A to element F in the fluorine source is 1:0.01, and in step S5, the molar ratio of element Na in the alkali metal source to P in lithium iron manganese phosphate intermediate B is 0.001:1.

[0050] Example 6 The difference from Example 1 is that in step S4, the average solid particle size of slurry C is 0.5 μm, and in step S5, the average solid particle size of slurry D is 0.3 μm.

[0051] Example 7 The differences from Example 1 are as follows: in step S4, the average solid particle size of slurry C is 0.7 μm; in step S5, the average solid particle size of slurry D is 0.1 μm.

[0052] Comparative Example 1 The differences from Example 1 are as follows: in steps S1 and S2, no fluorine source is added.

[0053] Comparative Example 2 The differences from Example 1 are as follows: in steps S4 and S5, no alkali metal source is added.

[0054] Comparative Example 3 The differences from Example 1 are as follows: in steps S1 and S2, no fluorine source is added, and in steps S4 and S5, no alkali metal source is added.

[0055] Comparative Example 4 The differences from Example 1 are as follows: the lithium iron manganese phosphate precursor B in step S2 is replaced with the lithium iron manganese phosphate precursor A.

[0056] Comparative Example 5 The differences from Example 1 are as follows: in steps S1 and S2, no carbon source is added.

[0057] Comparative Example 6 The differences from Example 1 are as follows: in steps S4 and S5, no carbon source is added.

[0058] Comparative Example 7 The differences from Example 1 are as follows: in steps S4 and S5, no alkali metal source is added, and the alkali metal sources in steps S4 and S5 are added in steps S1 and S2 respectively, that is, the fluorine source and the alkali metal source are added in the same step.

[0059] Comparative Example 8 The differences from Example 1 are as follows: the sintering temperature in step S3 is 740 °C, and the constant temperature time is 10.5.

[0060] Comparative Example 9 The differences from Example 1 are as follows: step S3 is not sintered.

[0061] Comparative Example 10 The differences from Example 1 are as follows: the mass ratio of primary particle A to primary particle B is 1:9.

[0062] Comparative Example 11 The differences from Example 1 are as follows: the mass ratio of primary particle A to primary particle B is 9:1.

[0063] The relevant parameters of the lithium iron manganese phosphate cathode material prepared in the above Examples 1-7 are shown in Table 1, where the tap density refers to the tap density under 150 MPa.

[0064]

[0065] The relevant parameters of the lithium iron manganese phosphate cathode materials prepared in the above Comparative Examples 1-11 are shown in Table 2.

[0066]

[0067] Button cells were respectively prepared from the lithium iron manganese phosphate cathode materials prepared in the above Examples and Comparative Examples. Among them, the mass ratio of the lithium iron manganese phosphate active material, the conductive agent (carbon black KS-6), and the binder (polyvinylidene fluoride (PVDF)) was 90:5:5. And the relevant physical and chemical properties and electrochemical properties were tested with reference to the national standard "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium Ion Batteries" GB / T 30835 2014. Specifically, the discharge specific capacity at 0.1C and 1C, and the capacity retention rate after 100 cycles of 1C discharge were tested. The relevant test results are shown in Table 3.

[0068] Analysis: As can be seen from the above results, in this application, by using lithium iron manganese phosphate precursors with different (Fe+Mn) / P ratios, two lithium iron manganese phosphate intermediates with different primary particle sizes were formed by one-time sintering. After the two intermediates were subjected to secondary grinding, mixing, secondary spraying, secondary sintering, and pulverization steps, a lithium iron manganese phosphate finished product with a size particle size distribution was formed, effectively improving the tap density of the material; through double doping of lithium sites and oxygen sites, the rate performance and cycling performance of the material were improved.

[0069] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

[0070] In addition, those skilled in the art can understand that although some of the embodiments herein include certain features included in other embodiments but not other features, the combination of the features of different embodiments means that it is within the scope of the present application and forms different embodiments. For example, in the above claims, any one of the claimed embodiments can be used in any combination. The information disclosed in this background art section is only intended to deepen the understanding of the overall background art of the present application, and should not be regarded as an admission or any form of suggestion that this information constitutes the prior art known to those skilled in the art.

Claims

1. A lithium iron manganese phosphate cathode material, characterized in that, It includes primary particles A and primary particles B; the primary particles A include a first core and a first coating layer provided on the surface of the first core, and the primary particles B include a second core and a second coating layer provided on the surface of the second core; The chemical formula of the first core and the second core are each independently [Li a-x M x Fe y Mn b-y PO 4-0.5z F z ; wherein, M includes potassium and / or sodium, 0.0001 ≤ x ≤ 0.01, 1.0 ≤ a ≤ 1.05, 0.2 ≤ y ≤ 0.6, 0.0001 ≤ z ≤ 0.01, 0.96 ≤ b ≤ 1; the first coating layer and the second coating layer include carbon; the particle size of the primary particles A is larger than that of the primary particles B.

2. The lithium iron manganese phosphate cathode material according to claim 1, wherein Meet at least one of the following conditions: (1) The D50 of the first core is 0.5 μm - 1 μm, and the D50 of the second core is 0.1 μm - 0.3 μm; (2) The chemical formula of the first core is [Li a-x M x Fe y Mn b-y PO 4-0.5z F z , where 0.96 ≤ b ≤ 0.97; the chemical formula of the second core is [Li a-x M x Fe y Mn b'-y PO 4-0.5z F z , where 0.98 ≤ b' ≤ 1.

3. The lithium iron manganese phosphate cathode material according to claim 1 or 2, wherein Meet at least one of the following conditions: (1) The D50 of the lithium iron manganese phosphate cathode material is 0.1 μm - 1 μm, and D100 ≤ 2 μm; (2) The tap density of the lithium iron manganese phosphate cathode material is 2.35 g / cm 3 -2.45 g / cm 3 ; (3) The mass ratio of the primary particles A to the primary particles B is 3:7 - 5:

5.

4. A method for preparing a lithium iron manganese phosphate cathode material according to any one of claims 1-3, characterized in that, It includes: Mix the lithium iron manganese phosphate precursor A, the first lithium source, the first carbon source, the first fluorine source and the solvent, and wet grind to obtain slurry A; Mix the lithium iron manganese phosphate precursor B, the second lithium source, the second carbon source, the second fluorine source and the solvent, and wet grind to obtain slurry B; Perform first spray drying on the slurry A and the slurry B respectively to obtain solid powder A and solid powder B, and under an inert atmosphere, perform first sintering on the solid powder A and the solid powder B respectively to obtain lithium iron manganese phosphate intermediate A and lithium iron manganese phosphate intermediate B; Mix the lithium iron manganese phosphate intermediate A, the first alkali metal source, the third carbon source and the solvent, and wet grind to obtain slurry C; mix the lithium iron manganese phosphate intermediate B, the second alkali metal source, the fourth carbon source and the solvent, and wet grind to obtain slurry D; mix the slurry C and the slurry D to obtain slurry E; Perform second spray drying and second sintering on the slurry E to obtain the lithium iron manganese phosphate cathode material; The (Fe + Mn) / P ratio of the lithium iron manganese phosphate precursor A is less than the (Fe + Mn) / P ratio of the lithium iron manganese phosphate precursor B.

5. The preparation method of the lithium iron manganese phosphate cathode material according to claim 4, wherein Meet at least one of the following conditions: (1) The chemical formula of the manganese iron phosphate precursor A is Fe y Mn c PO4, where 0.2 ≤ y ≤ 0.6, 0.36 ≤ c ≤ 0.77, and (Fe + Mn) / P is 0.96 - 0.97; (2) The chemical formula of the manganese iron phosphate precursor B is Fe y' Mn c' PO4, where 0.2 ≤ y' ≤ 0.6, 0.38 ≤ c' ≤ 0.80, and (Fe + Mn) / P is 0.98 - 1.

6. The preparation method of the lithium iron manganese phosphate cathode material according to claim 4, wherein, Meet at least one of the following conditions: (1) The first lithium source and the second lithium source each independently include one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium dihydrogen phosphate, lithium oxalate; (2) The first carbon source and the second carbon source each independently include one or more of sucrose, glucose, starch, maltodextrin, pitch; (3) The first fluorine source and the second fluorine source each independently include one or more of LiF, NH4F, LiPF6; (4) The mass ratio of the first carbon source to the lithium iron manganese phosphate precursor A is 4 - 7:100; (5) The mass ratio of the second carbon source to the lithium iron manganese phosphate precursor B is 4 - 7:100; (6) The molar ratio of P in the lithium iron manganese phosphate precursor A, lithium in the first lithium source and fluorine in the first fluorine source is 1:1.0 - 1.05:0.001 - 0.01; (7) The molar ratio of P in the manganese iron phosphate precursor B, lithium in the second lithium source, and fluorine in the second fluorine source is 1: 1.0 - 1.05: 0.001 - 0.

01.

7. The preparation method of the lithium iron manganese phosphate cathode material according to claim 4, characterized in that, Meet at least one of the following conditions: (1) The first alkali metal source and the second alkali metal source each independently include one or more of Na2CO3, CH3COOK, KH2PO4, K2CO3, CH3COONa, Na2C2O4, NaH2PO4; (2) The third carbon source and the fourth carbon source each independently include one or more of polyethylene glycol, polyacrylic acid, polyvinylpyrrolidone, polydopamine, and phenolic resin; (3) The mass ratio of the third carbon source to the lithium manganese iron phosphate intermediate A is 1 - 3: 100; (4) The mass ratio of the fourth carbon source to the lithium manganese iron phosphate intermediate B is 1 - 3: 100; (5) The molar ratio of the P element in the lithium manganese iron phosphate intermediate A to the alkali metal in the first alkali metal source is 1: 0.001 - 0.01; (6) The molar ratio of the P element in the lithium manganese iron phosphate intermediate B to the alkali metal in the second alkali metal source is 1: 0.001 - 0.

01.

8. The preparation method of the lithium iron manganese phosphate cathode material according to claim 4, characterized in that, Meet at least one of the following conditions: (1) The end temperature of the first sintering is 550 °C - 650 °C, and the holding time is 6 h - 8 h; (2) The end temperature of the second sintering is 690 °C - 790 °C, and the holding time is 9 h - 12 h.

9. The preparation method of the lithium iron manganese phosphate cathode material according to any one of claims 4-8, characterized in that, Meet at least one of the following conditions: (1) The D50 of the solid particles in the slurry A and the slurry B are each independently 0.1 μm - 0.5 μm; (2) The D50 of the slurry C is 0.5 μm - 0.7 μm; (3) The D50 of the slurry D is 0.1 μm - 0.3 μm.

10. A lithium-ion battery, characterized in that, Include the lithium manganese iron phosphate cathode material according to any one of claims 1 - 3, or the lithium manganese iron phosphate cathode material prepared by the preparation method of the lithium manganese iron phosphate cathode material according to any one of claims 4 - 9.

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