Lithium manganese iron phosphate positive electrode active material and preparation method therefor, positive electrode sheet, lithium secondary battery, and electric device

Abstract

The present application provides a lithium manganese iron phosphate positive electrode active material and a preparation method therefor, a positive electrode sheet, a lithium secondary battery, and an electric device. The lithium manganese iron phosphate positive electrode active material is secondary particles formed by the aggregation of primary particles, the secondary particles have pores inside, and the porosity of the secondary particles is 20-55%. The molar ratio of P to the metal elements other than Li on the surface of the primary particles is m1, the molar ratio of P to the metal elements other than Li in the bulk phase of the primary particles is m2, and same satisfy 1.01≤m1 / m2≤1.07.

Description

Lithium manganese iron phosphate cathode active materials and their preparation methods, cathode sheets, lithium secondary batteries, and electrical equipment.

[0001] Priority information

[0002] This application claims priority and benefits to patent application No. 202510125142.9, filed with the China National Intellectual Property Administration on January 26, 2025, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of batteries, specifically to lithium manganese iron phosphate cathode active materials and their preparation methods, cathode sheets, lithium secondary batteries, and electrical equipment. Background Technology

[0004] Olivine-type phosphate cathode active materials, such as LiMPO4 (M = Fe, Mn), have gradually gained market share in the lithium-ion battery cathode active material market due to their excellent structural stability, safety, and low cost. Currently, LiFePO4 is the most commercially available material, but its relatively low plateau voltage limits its energy density to only around 580 Wh / kg, restricting its development and application. LiMnPO4 materials have a higher plateau voltage and theoretical energy density (701 Wh / kg), but their low conductivity (<10) hinders their development. -9 S / cm), low lithium-ion diffusion coefficient (<10) -13 The instability of the crystal structure caused by the S / cm and the Jahn-Teller effect of trivalent manganese leads to a significant deterioration in actual electrochemical performance. The synthesis of LiMn1-xFexPO4 material by combining LiFePO4 and LiMnPO4 can combine the advantages of both materials; however, its cycle performance and fast-charging performance still need improvement.

[0005] Public content

[0006] This application aims to at least partially address one of the technical problems in the related art.

[0007] This application provides a lithium manganese iron phosphate (LFP) cathode active material. The LFP cathode active material is composed of secondary particles formed by the aggregation of primary particles. The secondary particles have internal pores with a porosity of 20%-55%. The molar ratio of phosphorus (P) to other metal elements (excluding Li) on the surface of the primary particles is m1, and the molar ratio of P to other metal elements (excluding Li) in the bulk phase of the primary particles is m2, satisfying 1.01 ≤ m1 / m2 ≤ 1.07, preferably 1.02 ≤ m1 / m2 ≤ 1.05. Therefore, by controlling the porosity of the secondary particles and the phosphorus enrichment (m1 / m2) on the surface of the primary particles, this application can improve the wetting effect of the electrolyte on the cathode active material while reducing the corrosion of the bulk phase of the cathode active material by a small amount of HF in the electrolyte, and reducing the amount of Mn. 3+ Side reactions with the electrolyte create a stable solid-liquid interface, ensuring the structural stability and capacity of the positive electrode active material under long-term cycling and high-temperature conditions, resulting in a battery with high energy density, excellent cycle performance, and fast-charging performance.

[0008] According to some embodiments of this application, the average particle size of the primary particles is 50nm-150nm, optionally 70nm-120nm. This shortens the lithium-ion transport path, increases the solid-liquid ion exchange interface, and improves the kinetic performance of the positive electrode active material.

[0009] According to some embodiments of this application, the pores include a first pore and a second pore, the pore diameter of the second pore being smaller than that of the first pore, the pore diameter of the first pore being 100nm-300nm, the total pore volume of the first pore and the second pore in the secondary particles being V, and the total pore volume of the first pore in the secondary particles being V1, satisfying: 20% ≤ V1 / V ≤ 50%, optionally, 30% ≤ V1 / V ≤ 50%. Thus, by controlling the volume ratio of macropores, while ensuring sufficient and rapid wetting of the positive electrode active material by the electrolyte, sufficient contact between primary particles is guaranteed, thereby improving the spherical strength of the secondary particles and increasing electron transport efficiency.

[0010] According to some embodiments of this application, 0.02cm 3 / g≤V1≤0.1cm 3 / g, can be selected as 0.02cm 3 / g≤V1≤0.05cm 3 / g, this part has a larger pore structure, which facilitates the rapid entry of electrolyte and its diffusion into the smaller pore structure. This ensures that the electrolyte can fully enter the active material and that the primary particles can be stably stacked, thereby improving the spherical strength of the secondary particles.

[0011] According to some embodiments of this application, the pore size of the second pore is 1.7 nm to 100 nm. Thus, the smaller second pore is mostly composed of micropores generated by the decomposition of carbon source and mesopores formed by the dense packing of primary particles. It can accommodate electrolyte, form sufficient contact with the active material inside the secondary particles, and provide a stable solid-liquid interface for lithium-ion exchange.

[0012] According to some embodiments of this application, the specific surface area of ​​the lithium manganese iron phosphate cathode active material is 10 m². 2 / g-30m 2 / g, optional 15m 2 / g-25m 2 / g. This increases the lithium-ion exchange rate, reduces the risk of material moisture absorption, and improves the processing capabilities of processes such as homogenization, coating, and rolling during electrode fabrication.

[0013] According to some embodiments of this application, the D of the secondary particles 10 D 50 D = satisfies: k 90 =(D 90 -D 10 ) / D 50 1.5≤k 90 ≤2.5. As a result, the secondary particles have a narrower and more uniform particle size distribution, good consistency, and are easy to process.

[0014] According to some embodiments of this application, the D of the secondary particles 50 Satisfying: 5μm≤D 50 ≤25μm, optionally, 7μm≤D 50 ≤15μm. This reduces the processing difficulty of the electrode fabrication process.

[0015] According to some embodiments of this application, at least a portion of the surface of the lithium manganese iron phosphate cathode active material has a carbon coating layer, and the lithium manganese iron phosphate cathode active material comprises a compound represented by Formula I: Li 1+a Mn x Fe y M' z (PO4) 1+b / C

[0016] Wherein, -0.1≤a≤0.2, 0.3≤x<1, 0<y≤0.7, 0<z≤0.05, and x+y+z=1, 0≤b≤0.2, and M' includes at least one of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W.

[0017] The second aspect of this application provides a method for preparing the lithium manganese iron phosphate cathode active material provided in the first aspect of this application. The method includes: mixing an iron source, a first phosphorus source, a manganese source, a lithium source, a first M' source, a first carbon source, and a first solvent to form a first dispersion; grinding and drying the mixture, followed by a first sintering to obtain an intermediate material; mixing the intermediate material, a second phosphorus source, a second M' source, a second carbon source, and a second solvent to form a second dispersion, wherein the sum of the molar amounts of the metal elements other than lithium in the intermediate material and the second M' source is n1, the molar amount of P in the second phosphorus source is n2, and the conditions are met: 0 < n2 / n1 ≤ 0.025; grinding and drying the mixture, followed by a second sintering at a temperature higher than that of the first sintering to obtain the lithium manganese iron phosphate cathode active material. Therefore, the method for preparing lithium manganese iron phosphate cathode active materials proposed in this application, by adding the phosphorus source twice during the first and second sintering processes, can control the phosphorus enrichment of the primary particle bulk phase and surface, while simultaneously controlling the porosity of the secondary particles. This improves the wetting effect of the electrolyte on the cathode active material, while reducing the corrosion of the cathode active material bulk phase by a small amount of HF in the electrolyte, and reducing the amount of Mn. 3+ Side reactions with the electrolyte create a stable solid-liquid interface, ensuring the structural stability and capacity of the positive electrode active material under long-term cycling and high-temperature conditions, resulting in a battery with high energy density, excellent cycle performance, and fast-charging performance.

[0018] According to some embodiments of this application, the solid content of the second dispersion is 20%-50%. Therefore, by controlling the solid content of the second dispersion before secondary granulation, the porous structure inside the secondary particles is optimized, and the volume ratio of macropores is controlled. This ensures that the electrolyte fully and quickly wets the positive electrode active material while guaranteeing sufficient contact between the primary particles, thereby improving the spherical strength of the secondary particles and increasing electron transport efficiency.

[0019] According to some embodiments of this application, the method satisfies at least one of the following conditions: the temperature of the first sintering is 400℃-700℃, and the time of the first sintering is 3h-10h; the temperature of the second sintering is 600℃-800℃, and the time of the second sintering is 6h-12h. Thus, efficient production capacity matching can be achieved between the first and second sintering processes.

[0020] According to some embodiments of this application, the method satisfies at least one of the following conditions: the iron source includes at least one of iron-containing oxides, iron-containing carbonates, iron-containing hydroxides, and iron-containing phosphates; the manganese source includes at least one of manganese-containing oxides, manganese-containing carbonates, manganese-containing hydroxides, and manganese-containing phosphates; the lithium source includes at least one of lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium formate, lithium acetate, lithium oxalate, lithium carbonate, lithium phosphate, lithium hydrogen phosphate, lithium dihydrogen phosphate, and lithium citrate; the first phosphorus source and the second phosphorus source each independently include at least one of phosphoric acid and phosphates; the first M'

[0021] The first carbon source and the second carbon source each independently include at least one of the following: an oxide containing M', a hydroxide containing M', a hydroxy oxide containing M', a carbonate containing M', an oxalate containing M', a sulfate containing M', an acetate containing M', and a nitrate containing M'; the first carbon source and the second carbon source each independently include at least one of the following: glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol, and dopamine.

[0022] The third aspect of this application provides a positive electrode sheet, which includes the lithium manganese iron phosphate positive electrode active material provided in the first aspect of this application or the lithium manganese iron phosphate positive electrode active material prepared by the method provided in the second aspect of this application.

[0023] The fourth aspect of this application provides a lithium secondary battery, including the positive electrode provided in the third aspect of this application.

[0024] The fifth aspect of this application provides an electrical device, including the lithium secondary battery provided in the fourth aspect of this application. Attached Figure Description

[0025] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0026] Figure 1 shows a schematic flowchart of a method for preparing lithium manganese iron phosphate cathode active material according to an embodiment of this application.

[0027] Figure 2 shows a SEM image of the secondary particles of lithium manganese iron phosphate cathode active material prepared in Example 1.

[0028] Figure 3 shows a cross-sectional SEM image of the secondary particles of lithium manganese iron phosphate cathode active material prepared in Example 1. Detailed Implementation

[0029] The embodiments of this application are described in detail below. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0030] The first aspect of this application provides a lithium manganese iron phosphate cathode active material, wherein the lithium manganese iron phosphate cathode active material is formed by the aggregation of primary particles into secondary particles, the secondary particles having pores inside, the porosity of the secondary particles being 20%-55%, the molar ratio of P element to metal element other than Li element on the surface of the primary particles being m1, the molar ratio of P element to metal element other than Li element in the bulk phase of the primary particles being m2, and satisfying 1.01≤m1 / m2≤1.07.

[0031] The lithium manganese iron phosphate cathode active material (hereinafter referred to as cathode active material) proposed in this application can improve the wetting effect of the electrolyte on the cathode active material by controlling the porosity of the secondary particles, thereby increasing the diffusion rate of lithium ions and thus improving the rate performance of the battery. Simultaneously, under long-cycle and high-temperature conditions, by controlling the slightly phosphorus-rich content on the surface of the primary particles, a chemically stable phosphate phase thin layer with a certain lithium-ion conductivity is generated on the surface of the primary particles. This blocks the direct contact between the electrolyte and the cathode active material, reduces the corrosion of the cathode active material bulk phase by the small amount of HF in the electrolyte, and reduces the amount of Mn... 3+ Side reactions with the electrolyte create a stable solid-liquid interface, ensuring the structural stability and capacity of the positive electrode active material under long-term cycling and high-temperature conditions, resulting in a battery with high energy density, excellent cycle performance, and fast-charging performance.

[0032] If the porosity of the secondary particles is too small, meaning the primary particles are too tightly packed, the electrolyte can only penetrate a small amount into the secondary particles. Lithium-ion exchange between the electrolyte and the positive electrode active material occurs primarily on the surface of the secondary particles. Due to the low bulk ion diffusion capacity of lithium manganese iron phosphate material, it takes a considerable amount of time for lithium ions to diffuse from the interior to the surface of the secondary particles, overcoming a high energy barrier. This results in lower capacity and higher electrochemical polarization in battery testing. Conversely, if the porosity of the secondary particles is too large, the primary particles are too loosely packed. Although the electrolyte can fully wet the particles, the reduced contact points between them decrease the electron conduction pathways within the active material, leading to increased ohmic impedance, increased polarization in battery testing, and weakened structural strength of the secondary particles. This makes them prone to breakage and pulverization, and some primary particles may lose activity due to detachment from contact.

[0033] If m1 / m2 < 1.01, the primary particles lack a phosphorus-rich protective layer, making the positive electrode active material susceptible to corrosion from substances like HF in the electrolyte. Metal elements such as Mn and Fe in the surface crystal structure are easily corroded and dissolved into the electrolyte, forming electrochemically inert byproducts on the surface of the positive electrode active material. This hinders ion diffusion and increases interfacial impedance. The dissolved metal ions diffuse to the negative electrode, damaging the SEI film and exacerbating active lithium loss, thus impairing battery cycle performance. If m1 / m2 > 1.07, excessive phosphate ions and the phosphate-rich interfacial phase, acting as non-electrochemically active materials, significantly affect lithium-ion interfacial exchange, inhibiting the capacity utilization of the positive electrode active material, especially the lithium insertion / extraction rate at high rates.

[0034] The porosity of the lithium iron phosphate cathode active material proposed in this application can be obtained by testing with a specific surface area and pore size analyzer. Before the sample is tested, it is degassed at 300℃ and then subjected to adsorption-desorption test with nitrogen. The porosity of the material is obtained based on the BJH adsorption model.

[0035] In this application, when testing the phosphorus enrichment of the positive electrode active material, the secondary particles are ion-beam cut, and at least n primary particles (n≥10) are randomly selected from their cross-sections. The molar ratio m of the content of P element to the content of metal elements other than Li on the surface of the primary particles is measured by energy dispersive spectroscopy (EDS). n1 And the ratio m of the molar ratio of P to metal elements other than Li in the primary particulate phase. n2 Calculate m n1 / m n2 m of n primary particles n1 / m n2 The summation of these values ​​represents the slight phosphorus enrichment of the secondary particles. This is measured using energy dispersive spectroscopy (EDS). n1 At this time, five points can be randomly selected on the surface of the particle for measurement, and the average value is taken as m. n1 Test m n2 At this time, five points can be randomly selected in a single particulate phase for measurement, and the average value is taken as m. n2 .

[0036] Specifically, the primary particle surface refers to the region within a distance of ≤10nm from the tangent to the primary particle surface, while the primary particle bulk phase refers to the region located at the center of the primary particle and within a minimum distance of >10nm from the primary particle surface.

[0037] As an example, the porosity of the secondary particles can be 20%, 30%, 40%, 50%, 55%, etc., or can be a range of any of the above values.

[0038] As an example, m1 / m2 can be 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, etc., or a range of any of the above values. According to some specific embodiments of this application, 1.02 ≤ m1 / m2 ≤ 1.05.

[0039] According to some embodiments of this application, the average particle size of the primary particles can be 50nm-150nm, for example, 50nm, 70nm, 90nm, 110nm, 130nm, 150nm, etc., or any range of the above values. This shortens the lithium-ion transport path in the solid phase, increases the solid-liquid ion exchange interface, improves the kinetic performance of the positive electrode active material, and enhances the fast-charging performance of the battery. According to some specific embodiments of this application, the average particle size of the primary particles can be 70nm-120nm.

[0040] In this application, the average particle size of the primary particles can be tested by scanning electron microscopy (SEM). Specifically, the image magnification is 30K, 100 primary particles are randomly selected from the image, the lengths of the longest and shortest diagonals of each primary particle are measured, and the average value is taken to obtain the size of the primary particle. SEM testing requires random sampling of primary particles and random selection of areas. The SEM image obtained from the test can represent the average particle size of the primary particles of the positive electrode active material.

[0041] According to some embodiments of this application, the pores include a first pore and a second pore, the pore diameter of the second pore is smaller than the pore diameter of the first pore, the pore diameter of the first pore is 100nm-300nm, the total pore volume of the first pore and the second pore in the secondary particle is V, the total pore volume of the first pore in the secondary particle is V1, and satisfies: 20% ≤ V1 / V ≤ 50%.

[0042] Specifically, the pore size of the second pore is smaller than that of the first pore. The smaller second pore is formed by the accumulation of primary particles and can contain electrolyte, providing a stable solid-liquid interface for ion exchange. The larger first pore serves only as a channel for electrolyte to enter the interior of the secondary particles. Too many first pores are not conducive to the contact between primary particles and are a break in the electron transport path.

[0043] Therefore, by controlling the volume ratio of the first pore, this application ensures sufficient and rapid wetting of the positive electrode active material by the electrolyte, while also ensuring full contact between the primary particles, thereby improving electron transport efficiency, increasing the spherical strength of the secondary particles, and further improving the electrode load.

[0044] In this application, the test method for the total pore volume V1 of the first pore in the secondary particles and the total pore volume V of the first pore and the second pore in the secondary particles is as follows: The positive electrode active material is placed in a nitrogen purging environment, dehydrated at 300°C for 30 min, and then placed in a surface area analyzer test tube for nitrogen adsorption-desorption test. Based on the BJH adsorption model, the pore volume corresponding to the pore structure with different pore sizes in the range of 1.7 nm to 300 nm is analyzed. The total pore volume in the range of 100 nm to 300 nm is the total pore volume V1 of the first pore in the secondary particles, and the total pore volume in the range of 1.7 nm to 300 nm is the total pore volume V of the first pore and the second pore in the secondary particles.

[0045] As an example, V1 / V can be 20%, 30%, 40%, 50%, etc., or a range of any of the above values. According to some specific embodiments of this application, 30% ≤ V1 / V ≤ 50%.

[0046] According to some embodiments of this application, 0.02cm 3 / g≤V1≤0.10cm 3 / g, for example, can be 0.02cm 3 / g, 0.05cm 3 / g, 0.07cm 3 / g, 0.09cm 3 / g, 0.1cm 3 / g, etc., or can be any range of the above values. Therefore, by controlling the pore volume of the larger first pore within the above range, sufficient contact between primary particles is ensured, thereby improving the spherical strength of secondary particles and increasing electron transport efficiency. According to some specific embodiments of this application, 0.02cm 3 / g≤V1≤0.10cm 3 / g.

[0047] According to some embodiments of this application, the pore size of the second pore can be 1.7 nm to 100 nm, for example, it can be 1.7 nm, 10 nm, 30 nm, 50 nm, 70 nm, 90 nm, 100 nm, etc., or it can be any range of the above values. Thus, a smaller second pore can accommodate the electrolyte, providing a stable solid-liquid interface for lithium-ion exchange.

[0048] According to some embodiments of this application, the specific surface area of ​​the lithium manganese iron phosphate cathode active material can be 10 m². 2 / g-30m 2 / g, for example, can be 10m 2 / g, 15m 2 / g、20m 2 / g、25m2 / g、30m 2 / g, etc., or can be any range of the above values. This improves the lithium-ion exchange rate of the material, reduces the risk of moisture absorption, and enhances the processing capabilities of steps such as slurry preparation, coating, and rolling during electrode fabrication. According to some specific embodiments of this application, the specific surface area of ​​the lithium manganese iron phosphate cathode active material can be 15m². 2 / g-25m 2 / g.

[0049] In this application, the specific surface area of ​​the positive electrode active material can be obtained by measuring the specific surface area and pore size. Specifically, the sample is degassed at 300°C before testing, and then adsorption-desorption tests are performed with nitrogen. The specific surface area of ​​the material is obtained based on the BJH adsorption model.

[0050] According to some embodiments of this application, the D of the secondary particles 10 D 50 D 90 Satisfy: k 90 =(D 90 -D 10 ) / D 50 1.5≤k 90 ≤2.5. As a result, the secondary particles have a narrower and more uniform particle size distribution, good consistency, and are easy to process.

[0051] According to some embodiments of this application, the D of the secondary particles 50 Satisfying: 5μm≤D 50 ≤25μm, for example, can be 5μm, 10μm, 15μm, 20μm, 25μm, etc., or can be any range of the above values. This reduces the processing difficulty of the material electrode fabrication process. According to some specific embodiments of this application, 7μm≤D 50 ≤15μm.

[0052] According to some embodiments of this application, at least a portion of the surface of the lithium manganese iron phosphate cathode active material has a carbon coating layer, and the lithium manganese iron phosphate cathode active material comprises a compound represented by Formula I: Li 1+a Mn x Fe y M' z (PO4) 1+b / C

[0053] Wherein, -0.1≤a≤0.2, 0.3≤x<1, 0<y≤0.7, 0≤z≤0.05, and x+y+z=1, 0≤b≤0.2, and M' includes at least one of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W.

[0054] As an example, 'a' can be -0.1, 0, 0.1, 0.2, etc., or a range of any of the above values.

[0055] It should be noted that, due to the formation and cycling processes, lithium ions are consumed, which may result in the measured lithium content (1+a) in the positive electrode active material being less than 1. However, if lithium replenishment agents are used on both the positive and negative electrode sheets, the measured lithium content (1+a) in the positive electrode active material may be greater than 1 after the battery has undergone formation and cycling.

[0056] As an example, x can be 0.3, 0.5, 0.7, 0.9, 0.99, etc., or a range of any of the above values.

[0057] As an example, y can be 0.1, 0.3, 0.5, 0.7, etc., or a range of any of the above values.

[0058] As an example, z can be 0, 0.01, 0.03, 0.05, etc., or a range of any of the above values.

[0059] As an example, b can be 0, 0.05, 0.1, 0.2, etc., or a range of any of the above values.

[0060] The second aspect of this application provides a method for preparing the lithium manganese iron phosphate cathode active material provided in the first aspect of this application. The method includes: mixing an iron source, a first phosphorus source, a manganese source, a lithium source, a first M' source, a first carbon source, and a first solvent to form a first dispersion; grinding and drying the mixture, followed by a first sintering to obtain an intermediate material; mixing the intermediate material, a second phosphorus source, a second carbon source, a second M' source, and a second solvent to form a second dispersion, wherein the sum of the molar amounts of the metal elements other than lithium in the intermediate material and the second M' source is n1, the molar amount of P in the second phosphorus source is n2, and the conditions are met: 0 < n2 / n1 ≤ 0.025; grinding and drying the mixture, followed by a second sintering at a temperature higher than that of the first sintering to obtain the lithium manganese iron phosphate cathode active material. Therefore, the method for preparing lithium manganese iron phosphate cathode active materials proposed in this application, by adding the phosphorus source twice during the first and second sintering processes, can control the phosphorus enrichment of the primary particle bulk phase and surface, while simultaneously controlling the porosity of the secondary particles. This improves the wetting effect of the electrolyte on the cathode active material, while reducing the corrosion of the cathode active material bulk phase by a small amount of HF in the electrolyte, and reducing the amount of Mn. 3+Side reactions with the electrolyte create a stable solid-liquid interface, ensuring the structural stability and capacity of the positive electrode active material under long-term cycling and high-temperature conditions, resulting in a battery with high energy density, excellent cycle performance, and fast-charging performance.

[0061] The method for preparing lithium manganese iron phosphate cathode active materials proposed in this application is described in detail below. Referring to Figure 1, the method includes:

[0062] S10: Iron source, first phosphorus source, manganese source, lithium source, first M' source, first carbon source, and first solvent are mixed to form a first dispersion. After grinding and drying, a first sintering is performed to obtain an intermediate material.

[0063] According to some embodiments of this application, an iron source, a first phosphorus source, a manganese source, a lithium source, a first M' source, and a first carbon source are weighed in a certain metric ratio, dispersed in a first solvent, ground, and atomized and dried before being transferred to a protective atmosphere furnace for a first sintering to obtain an intermediate material. This allows for precise control of the amount of raw materials added and ensures that they are fully dispersed and uniformly mixed in the first solvent.

[0064] According to some embodiments of this application, the temperature of the first sintering can be 400℃-700℃, and the time of the first sintering can be 3h-10h.

[0065] As an example, the temperature of the first sintering can be 400℃, 500℃, 600℃, 700℃, etc., or can be a range of any of the above values.

[0066] As an example, the first sintering time can be 3h, 5h, 8h, 10h, etc., or a range of any of the above values.

[0067] Therefore, by keeping the temperature and time of the first sintering within the above range, the raw materials can react fully to generate lithium manganese iron phosphate crystals, and the primary particle size can be controlled to prevent excessive growth.

[0068] According to some embodiments of this application, the iron source includes at least one of iron-containing oxides, iron-containing carbonates, iron-containing hydroxides, and iron-containing phosphates.

[0069] According to some embodiments of this application, the manganese source includes at least one of manganese-containing oxides, manganese-containing carbonates, manganese-containing hydroxides, and manganese-containing phosphates.

[0070] According to some embodiments of this application, the lithium source includes at least one of lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium formate, lithium acetate, lithium oxalate, lithium carbonate, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, and lithium citrate.

[0071] According to some embodiments of this application, the first phosphorus source includes at least one of phosphoric acid and phosphate.

[0072] According to some embodiments of this application, the first M' source includes at least one of the following: an oxide containing M', a hydroxide containing M', a hydroxy oxide containing M', a carbonate containing M', an oxalate containing M', a sulfate containing M', an acetate containing M', and a nitrate containing M'.

[0073] According to some embodiments of this application, the first carbon source includes at least one selected from glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol, and dopamine.

[0074] S20: The intermediate material, the second phosphorus source, the second carbon source, the second M' source, and the second solvent are mixed to form a second dispersion. After grinding and drying, a second sintering is performed. The temperature of the second sintering is higher than that of the first sintering to obtain the lithium manganese iron phosphate cathode active material.

[0075] Therefore, by selecting and using the second carbon source, the second phosphorus source, and the second M' source, the internal pore structure of the secondary particles and the micro-phosphorus enrichment on the surface of the primary particles can be further controlled within a wider range, thereby improving the compatibility of the materials in this application with different application scenarios and usage requirements.

[0076] According to some embodiments of this application, the solid content of the second dispersion is 20%-50%. For example, it can be 20%, 30%, 40%, 50%, or any range of the above values. Therefore, by controlling the solid content of the second dispersion before secondary granulation, the degree of sphere shrinkage, carbon source distribution, and residual pore structure during water evaporation in the secondary granulation process can be adjusted, optimizing the porous structure inside the secondary particles and controlling the volume ratio of macropores. This ensures sufficient and rapid wetting of the positive electrode active material by the electrolyte while maintaining adequate contact between primary particles, thereby improving the spherical strength of the secondary particles and increasing electron transport efficiency.

[0077] By controlling the solid content of the second dispersion before drying, the porosity of the positive electrode active material can be adjusted. The choice and amount of the second solvent determine the redispersion of soluble phosphorus compounds remaining in the intermediate material after sintering. These compounds are enriched on the surface of the primary particles during the second sintering process, forming a slightly phosphorus-rich environment. This improves the wetting effect of the electrolyte on the positive electrode active material while reducing the corrosion of the bulk phase of the positive electrode active material by a small amount of HF in the electrolyte, and reducing the amount of Mn. 3+Side reactions with the electrolyte create a stable solid-liquid interface, ensuring the structural stability and capacity of the positive electrode active material under long-term cycling and high-temperature conditions, resulting in a battery with high energy density, excellent cycle performance, and fast-charging performance.

[0078] According to some embodiments of this application, the sum of the molar amounts of the intermediate material and the metal elements other than lithium in the second M' source is n1, and the molar amount of P in the second phosphorus source is n2, satisfying 0 < n2 / n1 ≤ 0.025. For example, it can be 0.001, 0.005, 0.01, 0.015, 0.02, 0.025, etc., or a range of any of the above values. Thus, the phosphorus enrichment on the surface of the primary particles can be adjusted, generating a chemically stable phosphate phase thin layer with a certain lithium-ion conductivity on the surface of the primary particles, blocking the direct contact between the electrolyte and the positive electrode active material, reducing the corrosion of the bulk phase of the positive electrode active material by a small amount of HF in the electrolyte, and reducing Mn. 3+ Side reactions with the electrolyte create a stable solid-liquid interface, ensuring the structural stability and capacity of the positive electrode active material under long-term cycling and high-temperature conditions, resulting in a battery with high energy density, excellent cycle performance, and fast-charging performance.

[0079] According to some embodiments of this application, the temperature of the second sintering is 600℃-800℃, and the time of the second sintering is 6h-12h.

[0080] As an example, the temperature of the second sintering can be 600℃, 650℃, 700℃, 750℃, 800℃, etc., or can be a range of any of the above values.

[0081] As an example, the second sintering time can be 6h, 8h, 10h, 12h, etc., or a range of any of the above values.

[0082] Therefore, by keeping the temperature and time of the second sintering within the above range, it is possible to ensure that the carbon source is fully coated and carbonized on the surface of the lithium manganese iron phosphate material, thereby improving the conductivity of the material, controlling the formation of the micro-phosphorus-rich phase on the surface of the primary particles, and preventing the micro-phosphorus-rich phase from diffusing into the interior of the primary particles.

[0083] According to some embodiments of this application, the second phosphorus source includes at least one of phosphoric acid and phosphate.

[0084] According to some embodiments of this application, the second M' source includes at least one of the following: an oxide containing M', a hydroxide containing M', a hydroxy oxide containing M', a carbonate containing M', an oxalate containing M', a sulfate containing M', an acetate containing M', and a nitrate containing M'.

[0085] According to some embodiments of this application, the second carbon source includes at least one selected from glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol, and dopamine.

[0086] The third aspect of this application provides a positive electrode sheet, which includes the lithium manganese iron phosphate positive electrode active material provided in the first aspect of this application or the lithium manganese iron phosphate positive electrode active material prepared by the method provided in the second aspect of this application.

[0087] The fourth aspect of this application provides a lithium secondary battery, including the positive electrode provided in the third aspect of this application.

[0088] The fifth aspect of this application provides an electrical device, including the lithium secondary battery provided in the fourth aspect of this application.

[0089] The embodiments of this application are described in detail below. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0090] Example 1

[0091] 1. Preparation of lithium manganese iron phosphate-based cathode active materials

[0092] (1) Lithium carbonate, ferromanganese phosphate, and titanium dioxide were weighed and dispersed in pure water at a molar ratio of Li:Mn:Fe:Ti:P = 1.04:0.74:0.25:0.01:1.01. The first carbon source was glucose, which was added at 5% of the mass of ferromanganese phosphate. After the first grinding, the mixture was spray-dried and then transferred to a nitrogen atmosphere furnace for the first sintering at 600°C for 8 hours to obtain the first lithium manganese iron phosphate material.

[0093] (2) Weigh the first lithium manganese iron phosphate material and the second carbon source starch at a weight ratio of 100:10, disperse them in pure water, add ammonium dihydrogen phosphate at 1% based on the molar amount of non-lithium metal elements in the first lithium manganese iron phosphate material, control the solid content of the second dispersion to 40%, perform a second grinding, then spray dry and transfer to a nitrogen atmosphere furnace for a second sintering at 650℃ for 10 hours. After crushing and sieving, the finished lithium manganese iron phosphate material Li is obtained. 1.04 Mn 0.74 Fe 0.25 Ti 0.01 (PO4) 1.02 / C, Li 1.04 Mn 0.74 Fe 0.25 Ti0.01 (PO4) 1.02 The SEM images and cross-sectional SEM images of / C are shown in Figures 2 and 3.

[0094] 2. Preparation of positive electrode sheet

[0095] Li, the positive electrode active material 1.04 Mn 0.74 Fe 0.25 Ti 0.01 (PO4) 1.02 / C, conductive carbon black, and binder polyvinylidene fluoride (PVDF) are placed in a mixing tank at a mass ratio of 90:5:5. N-methylpyrrolidone (NMP) is added dropwise to adjust the consistency. NMP is added and stirred to form a uniform slurry. The slurry is coated onto aluminum foil, leveled, dried, and then rolled flat. It is then stamped at a pressure of 100 MPa into a positive electrode sheet with a diameter of 12 mm and a thickness of 120 μm. Finally, it is placed in a vacuum oven and dried at 120 °C for 12 h.

[0096] 3. Preparation of negative electrode sheet

[0097] The negative electrode uses a purchased Li metal sheet with a diameter of 17mm and a thickness of 1mm.

[0098] 4. Electrolyte

[0099] The electrolyte used is a carbonate electrolyte with 1 mol / L LiPF6 as the electrolyte and a mixture of ethyl oxalate, dicarboximide and acetylimide in a volume ratio of 1:1:1 as the solvent.

[0100] 5. Diaphragm

[0101] The diaphragm is 25μm thick, manufactured by Celgard, and model number 2325.

[0102] 6. Assemble the battery

[0103] The button cell assembly process was carried out in an Ar gas-protected glove box, where the water and oxygen contents were both less than 5 ppm. The positive electrode battery case, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode lithium sheet, spring gasket, and negative electrode case were placed in sequence, sealed tightly, and tested after standing for 6 hours.

[0104] Performance testing

[0105] 1. First-week charge / discharge capacity, initial efficiency, and rate capability testing:

[0106] The assembled button cells were placed in a 25°C constant temperature chamber and connected to the Xinwei Battery Test Cabinet with a test fixture. They were charged at a constant current of 0.1C (1C = 140mA / g) to the cutoff voltage of 4.3V, and then charged at a constant voltage for 30 minutes. Subsequently, they were discharged at a constant current of 0.1C to the cutoff voltage of 2.5V. The capacity of this charge and discharge cycle was taken as the first week's charge and discharge capacity of the battery, and the ratio of the discharge capacity to the charge capacity was taken as the first efficiency.

[0107] After the first charge and discharge, the above charge and discharge process was repeated sequentially with currents of 0.2C, 0.33C, 0.5C, 1C, and 2C, respectively. The ratio of the 1C discharge capacity to the 0.1C discharge capacity was used as the evaluation index of the rate performance.

[0108] 2.80-week capacity retention rate:

[0109] The assembled button cell was placed in a 45°C constant temperature chamber and connected to the Xinwei Battery Test Cabinet with a test fixture. Two charge-discharge cycles were completed at a current of 0.1C according to the above charge-discharge procedure. Then, 80 cycles of charge-discharge were completed at a current of 1C according to the above charge-discharge procedure. The ratio of the discharge capacity at 80 weeks to the discharge capacity at 1C was used as the evaluation index of the 80-week capacity retention rate.

[0110] Examples 2-9 and Comparative Examples 1-2

[0111] The positive electrode active material was prepared according to the method of Example 1. The material composition and specific process conditions were varied as shown in Table 1. Examples 2-9 and Comparative Examples 1-2 were carried out respectively. The physicochemical properties of the samples obtained are shown in Table 2, and the electrical properties were tested as shown in Table 3.

[0112] Table 1

[0113] Table 2

[0114] Table 3

[0115] In Comparative Example 1, phosphorus source was added only during the first sintering process in the preparation of lithium manganese iron phosphate cathode active material. The resulting cathode active material had the same P content on the surface of the primary particles and in the bulk phase of the primary particles. P was not enriched on the surface of the primary particles, resulting in poor cycle performance of the battery.

[0116] In Comparative Example 2, phosphorus source was added twice during the preparation of lithium manganese iron phosphate cathode active material. However, the amount of phosphorus source added in the second step was too large, resulting in an excessive amount of P element enriched on the surface of the primary particles, which would reduce the battery capacity and rate performance.

[0117] As can be seen from the comparison between Examples 1-9 and Comparative Examples 1 and 2, by controlling the porosity of the secondary particles and the m1 / m2 ratio, the battery can have a higher charge / discharge capacity and cycle capacity retention rate. This indicates that by controlling the porosity of the secondary particles, the wetting effect of the electrolyte on the positive electrode active material can be guaranteed, and the diffusion rate of lithium ions can be improved. By controlling the slightly phosphorus-rich content on the surface of the primary particles, a thin layer of phosphate phase can be formed on the surface of the primary particles, blocking the direct contact between the electrolyte and the positive electrode active material, reducing the corrosion of the positive electrode active material bulk phase by a small amount of HF in the electrolyte, improving the cycle performance of the battery, and thus obtaining a battery with good rate performance, high capacity utilization, and excellent cycle performance.

[0118] As can be seen from the comparison of Examples 1-4, by adjusting the amount of the second phosphorus source added, the enrichment degree of P element on the surface of the primary particles, i.e., the ratio of m1 / m2, can be adjusted, thereby improving the charge-discharge capacity while also improving the cycle capacity retention rate of the battery.

[0119] As can be seen from the comparison between Examples 1 and Examples 5-7, when the amount of the first phosphorus source and the second phosphorus source added is the same, the m1 / m2 ratio of the positive electrode active material is the same. By changing the solid content of the second dispersion, the total volume V1 of the first pore in the secondary particles and the total pore volume V of the first pore and the second pore in the secondary particles can be adjusted, thereby adjusting the V1 / V ratio, improving the diffusion rate of lithium ions, thereby improving the battery capacity, and improving the rate performance and cycle performance of the battery.

[0120] As can be seen from Examples 8 and 9, by selecting different types of first carbon source, second phosphorus source, and second carbon source, and by adjusting the temperature and time of the first and second sintering, lithium manganese iron phosphate cathode active materials with stable structure, good cycle performance, and good rate performance can be obtained.

[0121] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A lithium manganese iron phosphate-based positive electrode active material, wherein, is Secondary particles are formed by the aggregation of primary particles. The secondary particles have pores inside and the porosity of the secondary particles is 20%-55%. The molar ratio of P element to metal element other than Li element on the surface of the primary particles is m1, and the molar ratio of P element to metal element other than Li element in the bulk phase of the primary particles is m2, and satisfies 1.01≤m1 / m2≤1.

07.

2. The lithium iron phosphate cathode active material according to claim 1, wherein, 1.02≤m1 / m2≤1.

05.

3. The lithium iron phosphate cathode active material according to claim 1 or 2, wherein, The average particle size of the primary particles is 50nm-150nm.

4. The lithium iron phosphate cathode active material according to any one of claims 1-3, wherein, The average particle size of the primary particles is 70nm-120nm.

5. The lithium iron phosphate cathode active material according to any one of claims 1-4, wherein, The pores include a first pore and a second pore, the diameter of the second pore is smaller than the diameter of the first pore, the diameter of the first pore is 100nm-300nm, the total pore volume of the first pore and the second pore in the secondary particle is V, the total pore volume of the first pore in the secondary particle is V1, and satisfies: 20%V1 / V≤50%.

6. The lithium iron phosphate cathode active material according to any one of claims 1-5, wherein, 30% ≤ V1 / V ≤ 50%.

7. The lithium iron phosphate cathode active material according to any one of claims 1-6, wherein, 0.02cm 3 / g≤V1≤0.1cm 3 / g。 8. The lithium iron phosphate cathode active material according to any one of claims 1-7, wherein, 0.02cm 3 / g≤V1≤0.05cm 3 / g。 9. The lithium iron phosphate cathode active material according to any one of claims 1-8, wherein, The pore size of the second pore is 1.7nm-100nm.

10. The lithium iron phosphate cathode active material according to any one of claims 1-9, wherein, The specific surface area of ​​the lithium manganese iron phosphate cathode active material is 10 m². 2 / g-30m 2 / g.

11. The lithium iron phosphate cathode active material according to any one of claims 1-10, wherein, The specific surface area of ​​the lithium manganese iron phosphate cathode active material is 15m². 2 / g-25m 2 / g.

12. The lithium iron phosphate cathode active material according to any one of claims 1-11, wherein, The secondary particles' D 10 D 50 D 90 Satisfy: k 90 =(D 90 -D 10 ) / D 50 1.5≤k 90 ≤2.

5.

13. The lithium iron phosphate cathode active material according to any one of claims 1-12, wherein, The secondary particles' D 50 Satisfying: 5μm≤D 50 ≤25μm.

14. The lithium iron phosphate cathode active material according to any one of claims 1-13, wherein, 7μm≤D 50 ≤15μm。 15. The lithium iron phosphate cathode active material according to any one of claims 1-14, wherein, The lithium manganese iron phosphate cathode active material has at least a carbon coating layer on its surface, and the lithium manganese iron phosphate cathode active material includes a compound represented by Formula I: Li 1+a Mn x Fe y M' z (PO4) 1+b / C Wherein, -0.1≤a≤0.2, 0.3≤x<1, 0<y≤0.7, 0<z≤0.05, and x+y+z=1, 0≤b≤0.2, and M' includes at least one of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W.

16. A method for preparing the lithium manganese iron phosphate cathode active material according to any one of claims 1-15, wherein, include: The iron source, the first phosphorus source, the manganese source, the lithium source, the first M' source, the first carbon source, and the first solvent are mixed to form a first dispersion. After grinding and drying, the first sintering is carried out to obtain an intermediate material. The intermediate material, the second phosphorus source, the second carbon source, the second M' source, and the second solvent are mixed to form a second dispersion, wherein the sum of the molar amounts of the intermediate material and the second M' source, excluding lithium, is n1, and the molar amount of P in the second phosphorus source is n2, and 0 < n2 / n1 ≤ 0.025 is satisfied. After grinding and drying, a second sintering is performed, and the temperature of the second sintering is higher than that of the first sintering, to obtain the lithium manganese iron phosphate cathode active material.

17. The method according to claim 16, wherein, The solid content of the second dispersion is 20%-50%.

18. The method according to claim 16 or 17, wherein, At least one of the following conditions must be met: The temperature of the first sintering is 400℃-700℃, and the time of the first sintering is 3h-10h; The temperature of the second sintering is 600℃-800℃, and the time of the second sintering is 6h-12h.

19. The method according to any one of claims 16-18, wherein, At least one of the following conditions must be met: The iron source includes at least one of iron oxides, iron carbonates, iron hydroxides, and iron phosphates. The manganese source includes at least one of manganese oxides, manganese carbonates, manganese hydroxides, and manganese phosphates. The lithium source includes at least one of lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium formate, lithium acetate, lithium oxalate, lithium carbonate, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, and lithium citrate. The first phosphorus source and the second phosphorus source each independently include at least one of phosphoric acid and phosphate; The first M' source and the second M' source each independently include at least one of the following: an oxide containing M', a hydroxide containing M', a hydroxy oxide containing M', a carbonate containing M', an oxalate containing M', a sulfate containing M', an acetate containing M', and a nitrate containing M'. The first carbon source and the second carbon source each independently include at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol and dopamine.

20. A positive electrode plate, wherein, This includes the lithium manganese iron phosphate cathode active material according to any one of claims 1-15 or the lithium manganese iron phosphate cathode active material prepared by the method according to any one of claims 16-19.

21. A lithium secondary battery, wherein, Includes the positive electrode sheet as described in claim 20.

22. An electrical appliance, wherein, Including the lithium secondary battery as described in claim 21.

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