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

Abstract

The present application provides a lithium manganese iron phosphate cathode material and a preparation method thereof and a lithium ion battery, which relates to the field of lithium ion batteries. The material comprises primary particles A and B; the primary particles A comprise a first core and a first coating layer disposed on the surface of the first core, and the primary particles B comprise 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 independently [Li a‑x M x ]Fe y Mn b‑y PO 4‑0.5z 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; and the particle size of primary particles A is larger than that of primary particles B. Primary particles of varying particle sizes are graded to increase compaction density, and dual-site doping with oxygen and lithium is employed to improve rate and cycle performance.

Description

Technical Field

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

[0002] Lithium iron manganese phosphate (LIMnFePO4), an upgraded version of LFP, is gaining increasing attention in the lithium-ion battery industry. Compared to LFP, LFP utilizes manganese in place of iron, forming an iron-manganese-lithium solid solution. While maintaining roughly equivalent capacity, LFP can increase its voltage platform, thereby overcoming LFP's energy density bottleneck. Ideally, LFP can increase energy density by 15-20%. Furthermore, LFP's manufacturing process is largely compatible with LFP, and its cost is primarily due to the substitution of manganese for iron, making it relatively easy for materials companies to implement large-scale applications.

[0003] However, the electronic conductivity of lithium manganese iron phosphate material is low, which will cause the electron transmission inside the electrode to be untimely and the polarization phenomenon to be serious when the battery is charged and discharged at high rates, resulting in poor rate performance; the crystal structure of lithium manganese iron phosphate has certain obstacles to the diffusion of lithium ions, and the migration speed of lithium ions is difficult to keep up with the transmission speed of electrons. In addition, the Jahn-Teller effect of lithium manganese iron phosphate makes it easier for Mn to dissolve from the bulk structure of the material. The Jahn-Teller effect is also called the Jiang-Teller effect, which will cause serious capacity decay of the material and poor rate and cycle life; the conventional preparation method of lithium manganese iron phosphate is mostly a one-time sintering process, and the particle size of the obtained lithium manganese iron phosphate material is relatively uniform and the compaction density is low, resulting in low energy density used in the corresponding battery, which cannot meet the needs of high-performance applications; these characteristics limit the practical application of lithium manganese iron phosphate.

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

[0005] The purpose of this application is to provide a lithium manganese iron phosphate positive electrode material and a preparation method thereof and a lithium ion battery to solve the above problems.

[0006] To achieve the above objectives, the present application provides, in a first aspect, a lithium manganese iron phosphate cathode material, comprising primary particles A and primary particles B; the primary particles A comprising a first core and a first coating layer disposed on the surface of the first core, and the primary particles B comprising a second core and a second coating layer disposed on the surface of the second core;

[0007] The chemical formulas of the first core and the second core are independently [Li a-x M x ]Fe y Mn b-y PO4-0.5z F z ;

[0008] wherein 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, 0.96≤b≤1;

[0009] The first coating layer and the second coating layer include carbon;

[0010] The particle size of the primary particles A is larger than the particle size of the primary particles B.

[0011] Optionally, the lithium manganese iron phosphate positive electrode material meets at least one of the following conditions:

[0012] (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;

[0013] (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 , wherein 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.

[0014] Optionally, the lithium manganese iron phosphate positive electrode material meets at least one of the following conditions:

[0015] (1) The D50 of the lithium manganese iron phosphate positive electrode material is 0.1 μm-1 μm, and D100 ≤ 2 μm;

[0016] (2) The compaction density of the lithium manganese iron phosphate positive electrode material is 2.35 g / cm 3 -2.45g / cm 3 ;

[0017] (3) The mass ratio of the primary particles A to the primary particles B is 3:7-5:5.

[0018] The second aspect of the present application provides a method for preparing the lithium manganese iron phosphate positive electrode material, comprising:

[0019] The ferromanganese phosphate precursor A, the first lithium source, the first carbon source, the first fluorine source and the solvent are mixed and wet-ground to obtain slurry A; the ferromanganese phosphate precursor B, the second lithium source, the second carbon source, the second fluorine source and the solvent are mixed and wet-ground to obtain slurry B;

[0020] The slurry A and the slurry B are respectively subjected to a first spray drying to obtain solid powder A and solid powder B, and the solid powder A and the solid powder B are respectively subjected to a first sintering under an inert atmosphere to obtain lithium iron manganese phosphate intermediate A and lithium iron manganese phosphate intermediate B;

[0021] Mixing lithium manganese iron phosphate intermediate A, a first alkali metal source, a third carbon source and a solvent, and wet-grinding to obtain slurry C; mixing lithium manganese iron phosphate intermediate B, a second alkali metal source, a fourth carbon source and a solvent, and wet-grinding to obtain slurry D; mixing the slurry C and the slurry D to obtain slurry E;

[0022] The slurry E is subjected to a second spray drying and a second sintering to obtain the lithium manganese iron phosphate positive electrode material;

[0023] The (Fe+Mn) / P ratio of the ferromanganese phosphate precursor A is smaller than the (Fe+Mn) / P ratio of the ferromanganese phosphate precursor B.

[0024] Optionally, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0025] (1) The chemical formula of the ferromanganese 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;

[0026] (2) The chemical formula of the ferromanganese 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.

[0027] Optionally, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0028] (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, and lithium oxalate;

[0029] (2) the first carbon source and the second carbon source each independently include one or more of sucrose, glucose, starch, maltodextrin, and asphalt;

[0030] (3) the first fluorine source and the second fluorine source each independently include one or more of LiF, NH4F, and LiPF6;

[0031] (4) The mass ratio of the first carbon source to the ferromanganese phosphate precursor A is 4-7:100;

[0032] (5) The mass ratio of the second carbon source to the ferromanganese phosphate precursor B is 4-7:100;

[0033] (6) The molar ratio of P in the manganese iron phosphate precursor A, the lithium in the first lithium source, and the fluorine in the first fluorine source is 1:1.0-1.05:0.001-0.01;

[0034] (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.

[0035] Optionally, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0036] (1) The first alkali metal source and the second alkali metal source independently include one or more of Na2CO3, CH3COOK, KH2PO4, K2CO3, CH3COONa, Na2C2O4, and NaH2PO4;

[0037] (2) the third carbon source and the fourth carbon source each independently include one or more of polyethylene glycol, polyacrylic acid, polyvinyl pyrrolidone, polydopamine and phenolic resin;

[0038] (3) The mass ratio of the third carbon source to the lithium manganese iron phosphate intermediate A is 1-3:100;

[0039] (4) The mass ratio of the fourth carbon source to the lithium manganese iron phosphate intermediate B is 1-3:100;

[0040] (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;

[0041] (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.

[0042] Optionally, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0043] (1) The end temperature of the first sintering is 550°C-650°C, and the constant temperature time is 6h-8h;

[0044] (2) The final temperature of the second sintering is 690°C-790°C, and the constant temperature time is 9h-12h.

[0045] Optionally, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0046] (1) The solid particles D50 in the slurry A and the slurry B are each independently 0.1 μm to 0.5 μm;

[0047] (2) D50 of the slurry C is 0.5 μm-0.7 μm;

[0048] (3) The D50 of the slurry D is 0.1 μm-0.3 μm.

[0049] A third aspect of the present application provides a lithium-ion battery, comprising the lithium iron manganese phosphate positive electrode material or the lithium iron manganese phosphate positive electrode material prepared by the method for preparing the lithium iron manganese phosphate positive electrode material.

[0050] Compared with the prior art, the advantages of this application include:

[0051] The lithium manganese iron phosphate cathode material provided in this application is firstly graded by using large and small primary particles of different particle sizes. The large particle size particles provide a supporting skeleton, and the small particle size particles can fill the gaps between the large particles, which helps the particles to be arranged more closely under pressure, thereby increasing the compaction density. Secondly, oxygen and lithium dual-site doping is used to utilize the ion radius larger than Li + Na + , K + Alkali metal ions are doped into Li + The addition of F to the oxygen site will cause the lattice of lithium manganese iron phosphate to undergo a certain degree of distortion and expansion. This structural change is conducive to the formation of a more spacious ion transmission 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, and also using F with strong electronegativity to replace 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 Jiang-Taylor effect and reduce the Mn 3+ At the same time, the radius of the fluoride ion is smaller than that of the oxygen ion. After replacing the oxygen position, the Li-O distance increases and the bond energy decreases, which is beneficial to the diffusion of lithium ions and improves the rate performance and cycle performance of the material.

[0052] The preparation method of the lithium iron manganese phosphate positive electrode material provided in the present application, first, by using a manganese iron phosphate precursor with different (Fe+Mn) / P ratios, two lithium iron manganese phosphate intermediates with different primary particle sizes are formed after one sintering, and combined with a secondary sintering step, the problem of large and small particle grading of the lithium iron manganese phosphate material that cannot be achieved in the prior art is achieved, and the compaction density of the material is improved; secondly, an oxygen site dopant (fluorine source) is added before the first sintering, and F ions replace oxygen atoms in phosphate groups, which can enhance the structural stability of the material and is beneficial to reducing the negative impact of lattice distortion during lithium site doping during the second high-temperature sintering process. After the first sintering and F doping, a relatively stable lattice structure has been formed in the system, and Na and K ions are more likely to enter the lithium site, and synergistically with the previously doped F to further regulate the lattice parameters, making the lithium ion diffusion channel more regular and unobstructed.

[0053] The lithium-ion battery provided in this application has excellent rate performance and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope of the present application.

[0055] Figure 1 This is a SEM image of the lithium iron manganese phosphate intermediate A provided in Example 1;

[0056] Figure 2 This is a SEM image of the lithium iron manganese phosphate intermediate B provided in Example 1;

[0057] Figure 3 This is an SEM image of the lithium manganese iron phosphate positive electrode material provided in Example 1. DETAILED DESCRIPTION

[0058] First, the solution provided by this application is explained in more detail as follows:

[0059] In a first aspect, the present application provides a lithium manganese iron phosphate positive electrode material, comprising primary particles A and primary particles B; the primary particles A comprise a first core and a first coating layer disposed on the surface of the first core, and the primary particles B comprise a second core and a second coating layer disposed on the surface of the second core;

[0060] 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 F z ;

[0061] wherein 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, 0.96≤b≤1;

[0062] It should be noted that multi-site doping can simultaneously affect the charge state of multiple elements such as Li, other metals, oxygen, and phosphorus, thereby enhancing the doping effect;

[0063] 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 and 0.01, a can be 1.01, 1.02, 1.03, 1.04, 1.05 or any value between 1.0 and 1.05, and y can be 0.2, 0.3, 0.4 , 0.5, 0.6 or any value between 0.2 and 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 and 0.01, and b can be 0.96, 0.97, 0.98, 0.99, 1 or any value between 0.96 and 1;

[0064] Preferably, 0.001≤z≤0.01; 0.001≤x≤0.01;

[0065] It should be noted that excessive fluorine doping may cause significant changes in the crystal structure, deviating from the ideal olivine structure, thereby affecting the electrochemical properties of the material. Excessive fluorine doping also requires the use of more fluorine sources, which not only increases the cost of raw materials but also may make reaction control during the preparation process more difficult.

[0066] It should also be noted that if Na and / or K doping is too little, the effect of doping on improving the migration rate of lithium ions and enhancing the structural stability of the material will not be obvious, and the electrochemical performance of the material will be limited. If Na and / or K doping is too much, it may cause the lattice parameters to change too much, affecting the crystal structure stability of the material and blocking the lithium ion deintercalation path.

[0067] In addition, when alkali metals and fluorine are co-doped, the structure and ion transport path of the material can be more comprehensively optimized. The alkali metal doping at the lithium site has an alkali metal ion radius larger than that of Li +, which can change the crystal structure of the material and expand the lithium ion migration channel; and the fluorine doping of the oxygen site, the radius of the fluorine ion is smaller than that of the oxygen ion. After replacing the oxygen site, the Li-O distance increases and the bond energy decreases, which can reduce the energy barrier of lithium ion migration. The synergistic effect of the two makes the lithium ion conductivity more significantly improved, thereby improving the charge and discharge performance and rate performance of the battery;

[0068] 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;

[0069] The first coating layer and the second coating layer include carbon;

[0070] The particle size of the primary particles A is larger than the particle size of the primary particles B.

[0071] In some embodiments, the lithium manganese iron phosphate cathode material satisfies at least one of the following conditions:

[0072] (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;

[0073] Optionally, the D50 of the first core may 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 and 1 μm, and the D50 of the second core may be 0.1 μm, 0.2 μm, 0.3 μm, or any value between 0.1 μm and 0.3 μm;

[0074] (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 , wherein 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.

[0075] Optionally, b in the first core chemical formula may be 0.96, 0.965, 0.97 or any value between 0.96 and 0.97; b' in the second core chemical formula may be 0.98, 0.99, 1 or any value between 0.98 and 1.

[0076] In some embodiments, the lithium manganese iron phosphate cathode material satisfies at least one of the following conditions:

[0077] (1) The D50 of the lithium manganese iron phosphate positive electrode material is 0.1 μm-1 μm, and D100 ≤ 2 μm;

[0078] Optionally, the D50 of the lithium manganese iron phosphate positive electrode material may 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 the D100 may be 1 μm, 1.5 μm, 2 μm or any value ≤2 μm;

[0079] (2) The compaction density of the lithium manganese iron phosphate positive electrode material is 2.35 g / cm 3 -2.45g / cm 3 ;

[0080] Optionally, the compaction density of the lithium manganese iron phosphate cathode material can be 2.35 g / cm 3 , 2.4g / cm 3 , 2.45g / cm 3 or 2.35g / cm 3 -2.45g / cm 3 Any value between

[0081] (3) The mass ratio of the primary particles A to the primary particles B is 3:7-5:5.

[0082] Optionally, the mass ratio of the primary particles A to the primary particles B may be 3:7, 4:6, 5:5, or any value between 3:7 and 5:5.

[0083] The second aspect of the present application provides a method for preparing the lithium manganese iron phosphate positive electrode material, comprising:

[0084] The ferromanganese phosphate precursor A, the first lithium source, the first carbon source, the first fluorine source and the solvent are mixed and wet-ground to obtain slurry A; the ferromanganese phosphate precursor B, the second lithium source, the second carbon source, the second fluorine source and the solvent are mixed and wet-ground to obtain slurry B;

[0085] It should be noted that the wet grinding to prepare slurry A and slurry B is to reduce the particle size of the precursor and make the various raw materials mix evenly;

[0086] The slurry A and the slurry B are respectively subjected to a first spray drying to obtain solid powder A and solid powder B, and the solid powder A and the solid powder B are respectively subjected to a first sintering under an inert atmosphere to obtain lithium iron manganese phosphate intermediate A and lithium iron manganese phosphate intermediate B;

[0087] Mixing lithium manganese iron phosphate intermediate A, a first alkali metal source, a third carbon source and a solvent, and wet-grinding to obtain slurry C; mixing lithium manganese iron phosphate intermediate B, a second alkali metal source, a fourth carbon source and a solvent, and wet-grinding to obtain slurry D; mixing the slurry C and the slurry D to obtain slurry E;

[0088] It should be noted that the beneficial effects of adding oxygen site dopant (fluorine source) and lithium site dopant B (alkali metal source) in steps are: ① The element distribution and lattice optimization are more accurate. The fluorine source is added before the first sintering, and the F ions replace the oxygen atoms in the phosphate, which can enhance the structural stability of the material and help reduce the negative impact of lattice distortion during lithium site doping during the second high-temperature sintering process; the alkali metal source is added 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 synergistically with the previously doped F to further regulate the lattice parameters, so that the lithium The ion diffusion channel is more regular and unobstructed; ② It is more conducive to performance regulation. The amount of dopant added and the sintering conditions can be precisely controlled according to the characteristics and requirements of different sintering stages, so as to more flexibly regulate the performance of lithium iron phosphate. For example, when adding a 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 amount of the fluorine source added to optimize the electronic conductivity of the material. When adding an alkali metal in the second sintering, the amount of alkali metal added and the sintering conditions can be adjusted as needed to further improve the diffusion rate of lithium ions and the structural stability of the material, thereby achieving fine regulation of the comprehensive performance of the material;

[0089] It should be noted that the wet grinding to prepare slurry C and slurry D is to dissociate the secondary particles of the precursor after spray drying and primary sintering, and to mix the various raw materials uniformly.

[0090] The slurry E is subjected to a second spray drying and a second sintering to obtain the lithium manganese iron phosphate positive electrode material;

[0091] The (Fe+Mn) / P ratio of the ferromanganese phosphate precursor A is smaller than the (Fe+Mn) / P ratio of the ferromanganese phosphate precursor B.

[0092] It should be noted that in a single sintering step, the reaction rates among the iron source, manganese source and phosphorus source, as well as the reaction rates between them and the lithium source, will be affected by (Fe+Mn) / P. When (Fe+Mn) / P is high, the nucleation rate is greater than the crystal growth rate, and a large number of lithium iron manganese phosphate nuclei will be formed. These nuclei compete for limited growth space and material resources in the subsequent growth process, thereby limiting the growth size of each crystal particle, and ultimately resulting in smaller particles of the prepared lithium iron phosphate material. When (Fe+Mn) / P is low, the more abundant phosphorus source gives the generated lithium iron manganese phosphate nuclei more time and space to grow, which is conducive to the formation of larger particles. ; A manganese iron phosphate precursor with a high (Fe+Mn) / P ratio (manganese iron phosphate precursor B) generates a manganese iron phosphate lithium iron secondary sphere intermediate (manganese iron phosphate lithium iron intermediate B) composed of primary particles with smaller particle size during the first sintering, which is dissociated into smaller manganese iron phosphate lithium iron primary particles by grinding; a manganese iron phosphate precursor with a low (Fe+Mn) / P ratio (manganese iron phosphate precursor A) generates a manganese iron phosphate lithium iron secondary sphere intermediate (manganese iron phosphate lithium iron intermediate A) composed of primary particles with larger particle size, which is dissociated into larger manganese iron phosphate lithium iron primary particles by grinding; the manganese iron phosphate lithium iron primary particles of different sizes further form a manganese iron phosphate lithium iron positive electrode material with a large and small particle size distribution.

[0093] In some embodiments, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0094] (1) The chemical formula of the ferromanganese 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;

[0095] Optionally, in the chemical formula of the ferromanganese phosphate precursor A, y may be 0.2, 0.3, 0.4, 0.5, 0.6, or any value between 0.2 and 0.6, c may be 0.36, 0.4, 0.5, 0.6, 0.7, 0.77, or any value between 0.36 and 0.77, and (Fe + Mn) / P may be 0.96, 0.965, 0.97, or any value between 0.96 and 0.97;

[0096] (2) The chemical formula of the ferromanganese 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.

[0097] Optionally, in the chemical formula of the ferromanganese phosphate precursor B, y' may be 0.2, 0.3, 0.4, 0.5, 0.6, or any value between 0.2 and 0.6, c' may 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 may be 0.98, 0.98, 1, or any value between 0.98 and 1;

[0098] In some embodiments, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0099] (3) The first lithium source and the second lithium source independently include one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium dihydrogen phosphate, and lithium oxalate;

[0100] (4) the first carbon source and the second carbon source each independently include one or more of sucrose, glucose, starch, maltodextrin, and asphalt;

[0101] 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 show reducing properties in an inert atmosphere and can reduce the Fe 3+ / Mn 3+ Reduction to Fe 2+ / Mn 2+ ; C. the first fluorine source and the second fluorine source each independently include one or more of LiF, NH4F, LiPF6;

[0102] (5) The mass ratio of the first carbon source to the ferromanganese phosphate precursor A is 4-7:100;

[0103] Optionally, the mass ratio of the first carbon source to the ferromanganese phosphate precursor A may be 4:100, 5:100, 6:100, 7:100 or any value between 4 and 7:100;

[0104] (6) The mass ratio of the second carbon source to the ferromanganese phosphate precursor B is 4-7:100;

[0105] Optionally, the mass ratio of the second carbon source to the ferromanganese phosphate precursor B may be 4:100, 5:100, 6:100, 7:100 or any value between 4 and 7:100;

[0106] (7) The molar ratio of P in the manganese iron phosphate precursor A, the lithium in the first lithium source, and the fluorine in the first fluorine source is 1:1.0-1.05:0.001-0.01;

[0107] Optionally, the molar ratio of P in the ferromanganese phosphate precursor A, the lithium in the first lithium source, and the fluorine 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;

[0108] (8) 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.

[0109] Optionally, the molar ratio of P in the ferromanganese phosphate precursor B, the lithium in the second lithium source, and the fluorine 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;

[0110] In some embodiments, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0111] (1) The first alkali metal source and the second alkali metal source independently include one or more of Na2CO3, CH3COOK, KH2PO4, K2CO3, CH3COONa, Na2C2O4, and NaH2PO4;

[0112] (2) the third carbon source and the fourth carbon source each independently include one or more of polyethylene glycol, polyacrylic acid, polyvinyl pyrrolidone, polydopamine and phenolic resin;

[0113] It should be noted that the third and fourth carbon sources play a coating role;

[0114] (3) The mass ratio of the third carbon source to the lithium manganese iron phosphate intermediate A is 1-3:100;

[0115] 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 and 3:100;

[0116] (4) The mass ratio of the fourth carbon source to the lithium manganese iron phosphate intermediate B is 1-3:100;

[0117] 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 and 3:100;

[0118] (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;

[0119] Optionally, 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 can be 1:0.001, 1:0.005, 1:0.01 or any value between 1:0.001 and 0.01;

[0120] (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.

[0121] Optionally, 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 can be 1:0.001, 1:0.005, 1:0.01 or any value between 1:0.001 and 0.01.

[0122] In some embodiments, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0123] (1) The end temperature of the first sintering is 550°C-650°C, and the constant temperature time is 6h-8h;

[0124] Optionally, the endpoint temperature of the first sintering may 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 and 650°C, and the constant temperature time may be 6h, 7h, 8h, or any value between 6h and 8h;

[0125] It should be noted that when the endpoint temperature of the first sintering is in the range of 550℃-650℃, the reactants can fully react, so that the manganese iron phosphate is gradually converted into lithium manganese iron phosphate, while ensuring that the product has good crystallinity and electrochemical properties; when the temperature of the first sintering is too low, the reaction may be incomplete, and unreacted manganese iron phosphate may remain in the product, resulting in poor performance of the final material; when the temperature of the first sintering is too high, it may cause the particles of the material to grow and the specific surface area to decrease, which is not conducive to the dissociation of the particles in the second grinding process and will also have an adverse effect on the electrochemical properties of the material.

[0126] (2) The final temperature of the second sintering is 690°C-790°C, and the constant temperature time is 9h-12h.

[0127] Optionally, the endpoint temperature of the second sintering can be 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃ or any value between 690℃-790℃, and the constant temperature time can be 9h, 10h, 11h, 12h or any value between 9h-12h.

[0128] It should be noted that when the terminal temperature of the second sintering is 690℃-790℃, the carbon source decomposes to form a carbon coating on the surface of the lithium iron manganese phosphate, 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 on the surface of the lithium iron manganese phosphate particles, thereby improving the conductivity of the lithium iron manganese phosphate material; higher temperatures are also conducive to the entry of sodium / potassium ions with larger radii into the lattice of the lithium iron manganese phosphate, achieving uniform doping, forming a more spacious ion transmission channel, and improving the rate and cycle performance of the material; at the same time, the second sintering can further promote the growth and development of the crystal, making the lattice more regular and reducing defects, thereby improving the crystallinity of the material and providing internal heat for increasing the compaction density. On the structural basis of the second sintering; when the endpoint temperature of the second sintering is too low, the carbon source is not fully decomposed, and a complete and dense carbon coating layer cannot be formed, resulting in poor carbon coating effect, and the electronic conductivity and electrochemical properties of lithium iron phosphate cannot be effectively improved. In addition, the diffusion rate of sodium and / or potassium ions is slow, and it is difficult for them to fully enter the crystal lattice, resulting in uneven doping, and the improvement effect of sodium / potassium doping on the material structure and performance cannot be effectively exerted; when the endpoint temperature of the second sintering is too high, it may cause excessive growth of the carbon layer, reduce the bonding force between the carbon layer and the lithium iron phosphate particles, and may even cause the crystal structure of the material to have problems such as over-sintering and grain growth, affecting the specific surface area and pore structure of the material, and thus reducing the electrochemical properties of the material.

[0129] It should be noted that low-temperature sintering is used during the first sintering, and F ions with a smaller radius are doped at the oxygen sites. The F ions can begin to diffuse into the lattice at a relatively low temperature, replacing the oxygen atoms in the phosphate group to form a stronger MF bond (M is a transition metal), thereby enhancing the structural stability of the material and reducing the negative impact of lattice distortion caused by lithium site doping during the secondary sintering process. High-temperature sintering is used for the second sintering, and the Na / K ions doped at the lithium site have a larger radius. By utilizing the high diffusion activity of atoms at high temperature, they can better enter the lattice stabilized by F. At the same time, the secondary high-temperature sintering is beneficial to further improve the crystal structure of lithium manganese iron phosphate, reduce lattice defects and voids, and thus provide an intrinsic structural basis for improving the compaction density.

[0130] In some embodiments, the method for preparing the lithium manganese iron phosphate positive electrode material satisfies at least one of the following conditions:

[0131] (1) The solid particles D50 in the slurry A and the slurry B are each independently 0.1 μm to 0.5 μm;

[0132] Optionally, the solid particles D50 in slurry A and slurry B can be independently 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm or any value between 0.1 μm and 0.5 μm;

[0133] It should be noted that the main components of slurry A and slurry B are ferromanganese phosphate precursors. If the particle size of slurry A and slurry B is too large, it will easily affect the reaction uniformity and rate. A larger D50 means that the ferromanganese phosphate particles are larger. When mixed with other reactants such as lithium sources, the contact area between the particles is relatively small, which will lead to insufficient contact between the reactants, so that the reaction can only proceed on the surface of the particles or in local areas. The uniformity of the reaction is affected, and it is difficult to obtain a lithium ferromanganese phosphate product with uniform composition and performance. At the same time, in the solid-phase reaction, ions need to be transported between particles by diffusion to complete the reaction. If the D50 of the ferromanganese 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 grinding particle size is too small, more energy and longer time will be consumed, which will not only increase production costs, but also reduce production efficiency. Long-term grinding may also lead to increased equipment wear. The manganese iron phosphate precursor with too small D50 has a high surface energy and is in an energy unstable state, which makes it easy to agglomerate. When the agglomerated particles are subsequently mixed with substances such as lithium sources, the uniformity of the mixture will be affected, resulting in uneven reaction. During the high-temperature sintering process, the surface energy is too high, which can also lead to excessive crystal growth or unstable crystal structure, resulting in defects in the crystal structure of lithium manganese iron phosphate, affecting its electrochemical performance.

[0134] (2) D50 of the slurry C is 0.5 μm-0.7 μm;

[0135] Optionally, the D50 of slurry C may be 0.5 μm, 0.6 μm, 0.7 μm, or any value between 0.5 μm and 0.7 μm;

[0136] It should be noted that the main component of slurry C is lithium iron manganese phosphate intermediate A, and the particle size of slurry C is affected by the primary particle size of lithium iron manganese phosphate intermediate A itself, and is also affected by the grinding parameters; the manganese iron phosphate precursor with a lower (Fe+Mn) / P reacts in the first sintering step to generate lithium iron manganese phosphate intermediate A. Since the (Fe+Mn) / P is low, the more sufficient phosphorus source allows the generated lithium iron manganese phosphate crystal nuclei to have more time and space to grow, which is conducive to the formation of larger particles. Therefore, the lithium iron manganese phosphate intermediate A is composed of primary particles with larger particle size; by controlling the grinding parameters, a slurry C with Dv50=0.5-0.7μm is obtained; the primary particles in the slurry C form large particles in the lithium iron manganese phosphate finished product with large and small particle grading in the subsequent second sintering and pulverization steps, providing a supporting skeleton;

[0137] It should also be noted that if the particle size of the solid particles in slurry C is too large, the large particles of the final lithium manganese iron phosphate product will be too large, which is not conducive to the diffusion of lithium ions during the charge and discharge process, and the charge and discharge rate performance of the battery will be reduced. The contact resistance between the large-sized lithium manganese iron phosphate particles is large, and the conduction path of electrons between the particles becomes longer, which hinders the transmission of electrons and increases 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 manganese iron phosphate product will be too large, affecting the electrode preparation process.

[0138] (3) The D50 of the slurry D is 0.1 μm-0.3 μm.

[0139] Optionally, D50 of slurry D may be 0.1 μm, 0.2 μm, 0.3 μm, or any value between 0.1 μm and 0.3 μm.

[0140] 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 primary particle size of lithium iron manganese phosphate intermediate B itself, and is also affected by the grinding parameters. The iron manganese phosphate precursor with a higher (Fe+Mn) / P reacts in the first sintering step to generate lithium iron manganese phosphate intermediate B. Since the nucleation rate is greater than the crystal growth rate when the (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 in the subsequent growth process, thereby limiting the growth size of each crystal particle, and ultimately resulting in smaller particles of the prepared lithium iron phosphate material. Therefore, the lithium iron manganese phosphate intermediate B is composed of primary particles with smaller particle size. By controlling the grinding parameters, a slurry D with Dv50=0.1-0.3μm is obtained. The primary particles in the slurry D form small particles in the lithium iron manganese phosphate finished product with large and small particle grading in the second sintering and pulverization step, filling the gaps in the large particles to increase the compaction density.

[0141] It should also be noted that if the particle size of the solid particles in slurry D is too large, the small particles of the final lithium manganese iron phosphate product will be too large, and the gaps between the large particles will not be able to effectively fill, resulting in a decrease in the overall packing density, thereby reducing the energy density of the battery; if the particle size of the solid particles in slurry D is too small, the small particles of the final lithium manganese iron phosphate product will be too small. When the small particles are mixed with large particles, separation is likely to occur due to the large difference in particle size, and it is difficult to achieve uniform grading. During the vibration or compaction process, it is difficult to form a tight stacking with the large particles, resulting in a decrease in the tap density of the material, thereby affecting the energy density of the battery.

[0142] In some embodiments, the solid content of the slurry A, the slurry B, the slurry C, the slurry D, and the slurry E is independently 25%-55%;

[0143] A third aspect of the present application provides a lithium-ion battery, comprising the lithium iron manganese phosphate positive electrode material or the lithium iron manganese phosphate positive electrode material prepared by the method for preparing the lithium iron manganese phosphate positive electrode material.

[0144] The embodiments of the present application will be described in detail below in conjunction with specific examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if specific conditions are not specified, the conditions are carried out according to conventional conditions or manufacturer recommendations. The reagents or instruments used are not specified by the manufacturer and are conventional products that can be purchased commercially.

[0145] Example 1

[0146] This embodiment provides a lithium manganese iron phosphate positive electrode material and a preparation method thereof. The specific preparation steps are as follows:

[0147] S1: Manganese ferrophosphate precursor A (Fe 0.39 Mn 0.58 PO4, (Fe+Mn) / P is 0.97), a lithium source (lithium carbonate), a carbon source (sucrose), and a fluorine source (NH4F) are added to water and homogenized in a dispersion tank to obtain a slurry with a solid content of 40%, wherein the mass ratio of ferromanganese phosphate precursor A to the carbon source is 100:5.5, the molar ratio of Li in the lithium source to P in the ferromanganese phosphate precursor A is 1.025:1, and the molar ratio of P in the ferromanganese phosphate precursor A, the lithium in the lithium source, and the element F in the fluorine source is 1:1.025:0.005. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.3 μm to obtain slurry A;

[0148] S2: Manganese ferrophosphate precursor B (Fe 0.4 Mn 0.59PO4, (Fe+Mn) / P is 0.99), a lithium source (lithium carbonate), a carbon source (sucrose), and a fluorine source (NH4F) are added to water and homogenized in a dispersion tank to obtain a slurry with a solid content of 40%, wherein the mass ratio of the ferromanganese phosphate precursor B to the carbon source is 100:5.5, and the molar ratio of P in the ferromanganese phosphate precursor B, the lithium in the lithium source, and the element F in the fluorine source is 1:1.025:0.005. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.3 μm to obtain slurry B;

[0149] S3: Slurry A and slurry B were spray dried (inlet air temperature of 240°C, outlet air temperature of 100°C) to obtain solid powder A and solid powder B. Solid powder A and solid powder B were sintered under nitrogen atmosphere (sintering temperature of 600°C, constant temperature time of 7h) to obtain lithium manganese iron phosphate intermediate A and lithium manganese iron phosphate intermediate B, respectively. The SEM of lithium manganese iron phosphate intermediate A is as follows: Figure 1 , SEM of lithium manganese iron phosphate intermediate B is as follows Figure 2 ;

[0150] S4: adding lithium iron manganese phosphate intermediate A, carbon source (polyethylene glycol), and alkali metal source (Na2CO3) to water, and homogenizing and stirring in a dispersion tank to obtain a slurry with a solid content of 40%, wherein 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 carbon source is 100:2. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.6 μm to obtain slurry C;

[0151] S5: Add the lithium iron manganese phosphate intermediate B, the carbon source (polyethylene glycol), and the alkali metal source (Na2CO3) to water, and stir homogeneously in a dispersion tank to obtain a slurry with a solid content of 40%, wherein the molar ratio of element Na in the alkali metal source to P in the lithium iron manganese phosphate intermediate B is 0.005:1, and the mass ratio of the lithium iron manganese phosphate intermediate B to the carbon source is 100:2. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.2 μm to obtain slurry D;

[0152] S6: Pour slurry C and slurry D into a dispersion tank according to the mass ratio of lithium manganese iron phosphate intermediate A in slurry C to lithium manganese iron phosphate intermediate B in slurry D of 4:6, and stir and mix to obtain slurry E;

[0153] S7: The slurry E is spray-dried (the air inlet temperature is 240°C and the air outlet temperature is 100°C) to obtain a solid product, and then the solid product is subjected to a second sintering under an inert atmosphere (the temperature is 740°C and the constant temperature time is 10.5h). After sintering and cooling to room temperature, the sintered secondary spheres are dissociated into primary particles by air flow milling to obtain a lithium manganese iron phosphate positive electrode material with a particle size D50 of 0.45μm and D100 of 1.28μm.

[0154] The lithium manganese iron phosphate positive electrode material includes primary particles A and primary particles B with a mass ratio of 4:6. The primary particles A include a core (chemical formula: 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 arranged on the surface of the core, the 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 0.2μm) and the coating layer is set on the surface of the core. The SEM of the lithium manganese iron phosphate positive electrode material is as follows Figure 3 shown.

[0155] Example 2

[0156] This embodiment provides a lithium manganese iron phosphate positive electrode material and a preparation method thereof. The specific preparation steps are as follows:

[0157] S1: Manganese ferrophosphate 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) are added to water and homogenized in a dispersion tank to obtain a slurry with a solid content of 40%, wherein the mass ratio of the ferromanganese phosphate precursor A to the carbon source is 100:4, and the molar ratio of P in the ferromanganese phosphate precursor A, the lithium in the lithium source, and the element F in the fluorine source is 1:1:0.001. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.1 μm to obtain slurry A;

[0158] S2: Manganese ferrophosphate precursor B (Fe 0.2 Mn 0.8PO4, (Fe+Mn) / P is 1), a lithium source (lithium dihydrogen phosphate), a carbon source (asphalt), and a fluorine source (LiPF6) are added to water and homogenized in a dispersion tank to obtain a slurry with a solid content of 40%, wherein the mass ratio of the ferromanganese phosphate precursor B to the carbon source is 100:4, and the molar ratio of P in the ferromanganese phosphate precursor B, the lithium in the lithium source, and the element F in the fluorine source is 1:1:0.001. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.1μm to obtain slurry B;

[0159] S3: Slurry A and slurry B are spray-dried (inlet air temperature of 240°C, outlet air temperature of 100°C) to obtain solid powders A and solid powders B, which are then sintered under a nitrogen atmosphere (sintering temperature of 550°C, constant temperature time of 6 hours) to obtain lithium manganese iron phosphate intermediates A and lithium manganese iron phosphate intermediates B, respectively;

[0160] S4: adding lithium iron manganese phosphate intermediate A, carbon source (polyvinyl pyrrolidone), and alkali metal source (CH3COOK) to water, and homogenizing and stirring in a dispersion tank to obtain a slurry with a solid content of 40%, wherein the molar ratio of element Na in the alkali metal source to P in lithium iron manganese phosphate intermediate A is 0.001:1, and the mass ratio of lithium iron manganese phosphate intermediate A to carbon source is 100:1. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.5 μm to obtain slurry C;

[0161] S5: Add the lithium iron manganese phosphate intermediate B, the carbon source (polyethylene glycol), and the alkali metal source (Na2CO3) to water and stir homogenously in a dispersion tank to obtain a slurry with a solid content of 40%, wherein 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. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.1 μm to obtain slurry D;

[0162] S6: Pour slurry C and slurry D into a dispersion tank according to the mass ratio of lithium manganese iron phosphate intermediate A in slurry C to lithium manganese iron phosphate intermediate B in slurry D of 3:7, stir and mix to obtain slurry E;

[0163] S7: The slurry E is spray-dried (the air inlet temperature is 240°C and the air outlet temperature is 100°C) to obtain a solid product, and then the solid product is subjected to a second sintering under an inert atmosphere (the temperature is 690°C and the constant temperature time is 9 hours). After sintering and cooling to room temperature, the sintered secondary spheres are dissociated into primary particles by air jet milling to obtain lithium manganese iron phosphate positive electrode material.

[0164] The lithium manganese iron phosphate positive electrode material includes primary particles A and primary particles B in a mass ratio of 3:7.

[0165] Example 3

[0166] This embodiment provides a lithium manganese iron phosphate positive electrode material and a preparation method thereof. The specific preparation steps are as follows:

[0167] S1: Manganese ferrophosphate 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) are added to water and homogenized in a dispersion tank to obtain a slurry with a solid content of 40%, wherein the mass ratio of the ferromanganese phosphate precursor A to the carbon source is 100:7, and the molar ratio of P in the ferromanganese phosphate precursor A, the lithium in the lithium source, and the element F in the fluorine source is 1:1.05:0.01. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.5 μm to obtain slurry A;

[0168] S2: Manganese ferrophosphate 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) are added to water and homogenized in a dispersion tank to obtain a slurry with a solid content of 40%, wherein the mass ratio of the ferromanganese phosphate precursor B to the carbon source is 100:7, and the molar ratio of P in the ferromanganese phosphate precursor B, the lithium in the lithium source, and the element F in the fluorine source is 1:1.05:0.01. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.5μm to obtain slurry B;

[0169] S3: Slurry A and slurry B are spray-dried (inlet air temperature of 240°C, outlet air temperature of 100°C) to obtain solid powders A and solid powders B, which are then sintered under a nitrogen atmosphere (sintering temperature of 650°C, constant temperature time of 8 hours) to obtain lithium manganese iron phosphate intermediates A and lithium manganese iron phosphate intermediates B, respectively;

[0170] S4: Add lithium iron manganese phosphate intermediate A, carbon source (polydopamine), and alkali metal source (CH3COOK) to water and homogenize and stir in a dispersion tank to obtain a slurry with a solid content of 40%, wherein 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. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.7 μm to obtain slurry C;

[0171] S5: Add the lithium iron manganese phosphate intermediate B, the carbon source (polydopamine), and the alkali metal source (K2CO3) to water and homogenize and stir them in a dispersion tank to obtain a slurry with a solid content of 40%, wherein the molar ratio of the element K in the alkali metal source to the P in the lithium iron manganese phosphate intermediate B is 0.01:1, and the mass ratio of the lithium iron manganese phosphate intermediate B to the carbon source is 100:3. The slurry is transferred to a sand mill, and the average solid particle size of the slurry after sand milling is maintained at 0.3 μm to obtain slurry D;

[0172] S6: Pour slurry C and slurry D into a dispersion tank according to the mass ratio of lithium manganese iron phosphate intermediate A in slurry C to lithium manganese iron phosphate intermediate B in slurry D of 5:5, stir and mix to obtain slurry E;

[0173] S7: The slurry E is spray-dried (the air inlet temperature is 240°C and the air outlet temperature is 100°C) to obtain a solid product, and then the solid product is subjected to a second sintering under an inert atmosphere (the temperature is 790°C and the constant temperature time is 12 hours). After sintering and cooling to room temperature, the sintered secondary spheres are dissociated into primary particles by air jet milling to obtain lithium manganese iron phosphate positive electrode material.

[0174] The lithium manganese iron phosphate positive electrode material includes primary particles A and primary particles B in a mass ratio of 5:5.

[0175] Example 4

[0176] The difference from Example 1 is that in step S1, the molar ratio of P in the manganese iron phosphate precursor A to the element F in the fluorine source is 1:0.001, and in step S5, the molar ratio of the element Na in the alkali metal source to P in the lithium manganese iron phosphate intermediate B is 0.01:1.

[0177] Example 5

[0178] The difference from Example 1 is that in step S1, the molar ratio of P in the manganese iron phosphate precursor A to the element F in the fluorine source is 1:0.01, and in step S5, the molar ratio of the element Na in the alkali metal source to P in the lithium manganese iron phosphate intermediate B is 0.001:1.

[0179] Example 6

[0180] 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.

[0181] Example 7

[0182] The difference from Example 1 is that in step S4, the average solid particle size of slurry C is 0.7 μm, and in step S5, the average solid particle size of slurry D is 0.1 μm.

[0183] Comparative Example 1

[0184] The difference from Example 1 is that no fluorine source is added in steps S1 and S2.

[0185] Comparative Example 2

[0186] The difference from Example 1 is that no alkali metal source is added in steps S4 and S5.

[0187] Comparative Example 3

[0188] The difference from Example 1 is that no fluorine source is added in steps S1 and S2, and no alkali metal source is added in steps S4 and S5.

[0189] Comparative Example 4

[0190] The difference from Example 1 is that the ferromanganese phosphate precursor B in step S2 is replaced by the ferromanganese phosphate precursor A.

[0191] Comparative Example 5

[0192] The difference from Example 1 is that no carbon source is added in steps S1 and S2.

[0193] Comparative Example 6

[0194] The difference from Example 1 is that no carbon source is added in steps S4 and S5.

[0195] Comparative Example 7

[0196] The difference from Example 1 is that no alkali metal source is added in steps S4 and S5, and the alkali metal source in steps S4 and S5 is added in steps S1 and S2 respectively, that is, the fluorine source and the alkali metal source are added in the same step.

[0197] Comparative Example 8

[0198] The difference from Example 1 is that the sintering temperature in step S3 is 740° C. and the constant temperature time is 10.5.

[0199] Comparative Example 9

[0200] The difference from Example 1 is that no sintering is performed in step S3.

[0201] Comparative Example 10

[0202] The difference from Example 1 is that the mass ratio of primary particles A to primary particles B is 1:9.

[0203] Comparative Example 11

[0204] The difference from Example 1 is that the mass ratio of primary particles A to primary particles B is 9:1.

[0205] The relevant parameters of the lithium manganese iron phosphate positive electrode materials prepared in Examples 1-7 are shown in Table 1, where the compaction density refers to the compaction density at 150 MPa.

[0206]

[0207] The relevant parameters of the lithium manganese iron phosphate positive electrode materials prepared in the above comparative examples 1-11 are shown in Table 2.

[0208]

[0209] The lithium iron phosphate positive electrode materials prepared in the above examples and comparative examples were used to prepare button batteries, wherein the mass ratio of the lithium iron phosphate active material, the conductive agent (carbon black KS-6), and the binder (polyvinylidene fluoride (PVDF)) was 90:5:5. The relevant physical and chemical properties and electrochemical properties were tested in accordance with the national standard "Carbon Composite Lithium Iron Phosphate Cathode Materials for Lithium Ion Batteries" GB / T 30835-2014. Specifically, the 0.1C and 1C discharge specific capacities and the capacity retention rate after 100 1C discharge cycles were tested. The relevant test results are shown in Table 3.

[0210] analyze:

[0211] From the above results, it can be seen that the present application adopts ferromanganese phosphate precursors with different (Fe+Mn) / P ratios to form two lithium iron manganese phosphate intermediates with different primary particle sizes through one sintering. The two intermediates are subjected to secondary grinding, mixing, secondary spraying, secondary sintering, and crushing steps to form lithium iron manganese phosphate finished products with large and small particle grading, which effectively improves the compaction density of the material; through dual doping of lithium sites and oxygen sites, the rate and cycle performance of the material are improved.

[0212] 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

[0213] Furthermore, those skilled in the art will appreciate that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and to form distinct embodiments. For example, in the claims above, any of the claimed embodiments may be used in any combination. The information disclosed in this background section is intended solely to enhance understanding of the overall background of this application and should not be construed as an admission or any implication that such information constitutes prior art known to those skilled in the art.

Claims

1. A lithium manganese iron phosphate positive electrode material, characterized in that: Comprising primary particles A and primary particles B; the primary particles A comprising a first core and a first coating layer disposed on the surface of the first core, and the primary particles B comprising 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 independently [Li a-x M x ]Fe y Mn b-y PO 4-0.5z F z ; wherein 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, 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 the particle size of the primary particles B; The method for preparing the lithium manganese iron phosphate positive electrode material comprises: The ferromanganese phosphate precursor A, the first lithium source, the first carbon source, the first fluorine source and the solvent are mixed and wet-ground to obtain slurry A; the ferromanganese phosphate precursor B, the second lithium source, the second carbon source, the second fluorine source and the solvent are mixed and wet-ground to obtain slurry B; The slurry A and the slurry B are respectively subjected to a first spray drying to obtain solid powder A and solid powder B, and the solid powder A and the solid powder B are respectively subjected to a first sintering under an inert atmosphere to obtain lithium iron manganese phosphate intermediate A and lithium iron manganese phosphate intermediate B; Mixing lithium manganese iron phosphate intermediate A, a first alkali metal source, a third carbon source and a solvent, and wet-grinding to obtain slurry C; mixing lithium manganese iron phosphate intermediate B, a second alkali metal source, a fourth carbon source and a solvent, and wet-grinding to obtain slurry D; mixing the slurry C and the slurry D to obtain slurry E; The slurry E is subjected to a second spray drying and a second sintering; The (Fe+Mn) / P ratio of the ferromanganese phosphate precursor A is smaller than the (Fe+Mn) / P ratio of the ferromanganese phosphate precursor B.

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

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4. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that At least one of the following conditions is met: (1) The chemical formula of the ferromanganese 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 ferromanganese 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.

5. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that At least one of the following conditions is met: (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, and lithium oxalate; (2) the first carbon source and the second carbon source each independently include one or more of sucrose, glucose, starch, maltodextrin, and asphalt; (3) the first fluorine source and the second fluorine source each independently include one or more of LiF, NH4F, and LiPF6; (4) The mass ratio of the first carbon source to the ferromanganese phosphate precursor A is 4-7:100; (5) The mass ratio of the second carbon source to the ferromanganese phosphate precursor B is 4-7:100; (6) The molar ratio of P in the manganese iron phosphate precursor A, the lithium in the first lithium source, and the 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.

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6. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that At least one of the following conditions is met: (1) The first alkali metal source and the second alkali metal source 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, polyvinyl pyrrolidone, 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.

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7. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that At least one of the following conditions is met: (1) The end temperature of the first sintering is 550°C-650°C, and the constant temperature time is 6h-8h; (2) The final temperature of the second sintering is 690°C-790°C, and the constant temperature time is 9h-12h.

8. The lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 7, characterized in that At least one of the following conditions is met: (1) The solid particles D50 in the slurry A and the slurry B are each independently 0.1 μm to 0.5 μm; (2) 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.

9. A lithium-ion battery, characterized in that: The lithium manganese iron phosphate positive electrode material comprises the lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 8.

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