Preparation method of lithium manganese iron phosphate positive electrode material

Abstract

This invention relates to a method for preparing lithium manganese iron phosphate cathode material, comprising: (1) mixing a lithium source, a phosphorus source and pure water, controlling the particle size of the slurry within a certain range to obtain slurry A; (2) adding a carbon source and a dopant to slurry A to obtain slurry B; (3) adding an iron source to slurry B, controlling the particle size of the slurry within a certain range to obtain slurry C; (4) adding a manganese source to slurry C, controlling the particle size of the slurry within a certain range to obtain slurry D; (5) grinding slurry D, controlling the particle size of the slurry within a certain range to obtain slurry E; (6) spray drying slurry E to obtain precursor F; (7) sintering precursor F, then pulverizing, sieving and demagnetizing to obtain lithium manganese iron phosphate cathode material.

Description

Technical Field

[0001] This invention belongs to the field of new energy materials technology, and relates to lithium-ion batteries, specifically to a method for preparing lithium manganese iron phosphate cathode material and a lithium-ion battery using the lithium manganese iron phosphate cathode material. Background Technology

[0002] Lithium iron phosphate (LiFePO4), a cathode material for lithium-ion batteries, exhibits superior cycle performance and safety compared to traditional lithium-ion battery cathode materials due to its stable olivine-type structure. Furthermore, its abundant resources for synthesis offer a significant cost advantage, leading to its widespread adoption in previous research and practical applications. However, its inherently low potential of 3.4V (vs. Li / Li) remains a concern. + This limits the energy density of batteries, and the development of lithium iron phosphate has reached saturation, failing to meet the demand for high-energy power batteries.

[0003] Meanwhile, lithium manganese iron phosphate, which also possesses an olivine-type structure, has a similar theoretical capacity to lithium iron phosphate (LiFePO4), but it has a lower capacity of 4.1V (vs. Li / Li). + The voltage platform of lithium manganese iron phosphate (LMP) is much higher than that of lithium iron phosphate (LFP). Under the same capacity conditions, LMP offers a greater advantage in battery energy density. Therefore, LMP is a novel high-energy-density cathode material for power lithium-ion batteries that can perfectly replace LFP, and its widespread adoption in industrial production is beneficial.

[0004] Currently reported technologies for preparing lithium manganese iron phosphate include: a liquid-phase synthesis route, which involves simultaneously mixing lithium, manganese, iron, and phosphorus sources through a liquid-phase hydrothermal reaction, followed by sintering to obtain lithium manganese iron phosphate cathode material; however, this process is difficult to industrialize, and the primary grain size of the material is nanoscale, resulting in insufficient compaction density; a combination of solid-phase and liquid-phase methods, which involves co-precipitating manganese and iron sources, then adding lithium and phosphorus sources and sintering to obtain lithium manganese iron phosphate cathode material; this process is cumbersome, has high industrial production costs, and the resulting product also has low compaction density; another method uses a pure solid-phase synthesis route, which involves batch mixing of iron, manganese, lithium, phosphorus, and carbon sources, followed by sintering of slurries with different particle sizes to obtain lithium manganese iron phosphate cathode material; this process is complex and cannot achieve stable production; and a technique involves two carbon coating processes involving iron, manganese, lithium, phosphorus, carbon sources, and dopants; this process is cumbersome, has high industrial production costs, and the resulting product also has low compaction density. Summary of the Invention

[0005] This invention proposes a method for preparing lithium manganese iron phosphate cathode material, which clarifies the pure solid-phase synthesis process route for preparing lithium manganese iron phosphate, greatly simplifies the process, and ensures the compaction density and electrochemical performance of the material, making it suitable for industrial-scale production.

[0006] On one hand, the present invention provides a method for preparing lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0007] (1) Mix lithium source, phosphorus source and pure water, and control the particle size of slurry within a certain range to obtain slurry A;

[0008] (2) Add carbon source and dopant to slurry A to obtain slurry B;

[0009] (3) Add the iron source to slurry B and control the slurry particle size within a certain range to obtain slurry C;

[0010] (4) Add the manganese source to slurry C and control the slurry particle size within a certain range to obtain slurry D;

[0011] (5) Grind slurry D to control the particle size of the slurry within a certain range to obtain slurry E;

[0012] (6) Spray dry slurry E to obtain lithium manganese iron phosphate precursor F;

[0013] (7) The precursor F of lithium manganese iron phosphate is sintered, then crushed, sieved and demagnetized to obtain lithium manganese iron phosphate cathode material.

[0014] According to some embodiments of the present invention, the preparation method of the lithium manganese iron phosphate cathode material provided by the present invention may further include the following auxiliary technical features:

[0015] In some embodiments, the chemical formula of the lithium manganese iron phosphate cathode material is LiMn. x Fe 1-x PO4, where 0.5 ≤ x ≤ 0.9.

[0016] In some embodiments, the molar ratio of lithium to iron and manganese in the added raw materials is (0.95-1.1):1.

[0017] Specifically, the molar ratio of lithium to iron and manganese in the added raw materials is any value within the range of 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1:1, 1.01:1, 1.03:1, 1.06:1, 1.1:1 or (0.95-1.1):1.

[0018] In this invention, when the added lithium source is too small (the molar ratio of lithium to iron and manganese is less than 0.95:1), the added lithium source is insufficient to completely generate lithium manganese iron phosphate, and some of it exists in the form of iron manganese phosphate, affecting the final yield of lithium manganese iron phosphate. When the added lithium source is too large (the molar ratio of lithium to iron and manganese is greater than 1.1:1), some lithium will exist on the surface of lithium manganese iron phosphate in the form of free lithium, lithium phosphate, or lithium hydroxide, which will cause lithium deposition on the surface of lithium manganese iron phosphate, both of which will affect the compaction density and electrochemical performance of lithium manganese iron phosphate.

[0019] In this invention, the raw materials include lithium source, phosphorus source, carbon source, dopant, iron source and manganese source added in steps (1) to (7).

[0020] In some embodiments, the molar ratio of the sum of iron and manganese to phosphorus in the added raw materials is (0.95-1.06):1.

[0021] Specifically, the molar ratio of the sum of iron and manganese to phosphorus in the added raw materials is any value within the range of 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1:1, 1.01:1, 1.04:1, 1.06:1 or (0.95-1.06):1.

[0022] In this invention, when too little phosphorus source is added (the molar ratio of the sum of iron and manganese to phosphorus is greater than 1.06:1), lattice defects will occur, which will have a significant impact on the electrochemical performance of lithium manganese iron phosphate; when too much phosphorus source is added (the molar ratio of the sum of iron and manganese to phosphorus is less than 0.95:1), some phosphorus will exist on the surface of lithium manganese iron phosphate in the form of lithium phosphate, resulting in larger particle size after sintering, which will have a significant impact on the compaction performance of lithium manganese iron phosphate.

[0023] In some embodiments, the amount of carbon source added is 5%-20% of the total mass of the added raw materials.

[0024] Preferably, the amount of carbon source added is 8%-12% of the total mass of the added raw materials.

[0025] Specifically, the mass of the added carbon source is any value within the range of 8%, 9%, 10%, 11%, 12% or 8%-12% of the total mass of the added raw materials.

[0026] In this invention, the main functions of the carbon source include: 1. carbon coating to improve electronic conductivity; 2. participating in the reduction reaction as a reducing agent. If the amount of carbon source added is too low, it will lead to uneven coating and decreased material conductivity; if the amount of carbon source added is too high, the coating layer will be too thick, affecting lithium-ion insertion and extraction, inhibiting the growth of lithium manganese iron phosphate crystals, resulting in excessively small particles, large specific surface area, low compaction density, decreased capacity retention, and reduced material electrical performance.

[0027] In some embodiments, the carbon content in the lithium manganese iron phosphate cathode material is 1.5%-5%.

[0028] Preferably, the carbon content in the lithium manganese iron phosphate cathode material is 1.5%-2.5%.

[0029] Specifically, the carbon content in the lithium manganese iron phosphate cathode material is any value within the range of 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.3%, 2.5%, or 1.5%-2.5%.

[0030] In some embodiments, the amount of the dopant added is 0.2%-2% of the total mass of the added raw materials.

[0031] Preferably, the amount of the dopant added is 0.5%-1.3% of the total mass of the added raw materials.

[0032] Specifically, the amount of the dopant added is any value within the range of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.3%, or 0.5%-1.3% of the total mass of the added raw materials.

[0033] In this invention, the main purpose of doping is to change the crystal lattice structure. Undoped lithium manganese iron phosphate is a one-dimensional lithium-ion migration channel. Doping is necessary to change its crystal lattice structure and alter its ion transport limitations. After doping, the lithium-ion migration rate can be significantly increased, thereby improving the material's capacity. When too little dopant is added, the ionic conductivity is too low, resulting in poor electrical performance.

[0034] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, or lithium acetate.

[0035] In this invention, the lithium source can be used alone or in combination. When multiple lithium sources are used in combination, there are no particular restrictions on the proportion of each lithium source.

[0036] In some embodiments, the phosphorus source includes at least one of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, iron phosphate, or manganese phosphate.

[0037] In this invention, the phosphorus source can be used alone or in combination. When multiple phosphorus sources are used in combination, there are no particular restrictions on the proportion of each phosphorus source.

[0038] In some embodiments, the carbon source includes at least one of toluene, xylene, cyclohexane, citric acid, polyethylene glycol, sucrose, or carbon nanotubes.

[0039] In this invention, the carbon source can be used alone or in combination. When multiple carbon sources are used in combination, there are no particular restrictions on the proportion of each carbon source.

[0040] In some embodiments, the dopant includes at least one of ammonium fluoride, ammonium metavanadate, magnesium oxide, niobium pentoxide, titanate coupling agent, or zirconium oxide.

[0041] In this invention, the dopant can be used alone or in combination. When multiple dopants are used in combination, there is no particular limitation on the proportion of each dopant.

[0042] In some embodiments, the iron source includes at least one of ferric phosphate, ferric nitrate, ferric oxide, ferrous oxalate, or ferric oxide.

[0043] In this invention, the iron source can be used alone or in combination. When multiple iron sources are used in combination, there is no particular restriction on the proportion of each iron source.

[0044] In some embodiments, the manganese source includes at least one of manganese carbonate, manganese tetroxide, manganese oxalate, manganese phosphate, ammonium manganese phosphate, or manganese phosphate.

[0045] In this invention, the manganese source can be used alone or in combination. When multiple manganese sources are used in combination, there is no particular limitation on the proportion of each manganese source.

[0046] In this invention, the order in which the iron source and manganese source are added cannot be changed. If the manganese source is added first, the particle size of the slurry will be too large, failing to meet the particle size range required by this invention, resulting in a significant difference in product performance compared to the lithium manganese iron phosphate prepared by the method described in this invention.

[0047] In some embodiments, in step (1), after mixing the lithium source, phosphorus source and pure water, the slurry is sheared at room temperature for 1.5-3 hours.

[0048] In this invention, in step (1), when the shearing time is less than 1.5h, the lithium source and phosphorus source cannot react completely and bubbles are still generated; there is no particular limit to the upper limit of the shearing time, but from the perspective of improving efficiency, it can be controlled at 1.5-3h.

[0049] Specifically, in step (1), after mixing the lithium source, phosphorus source and pure water, the slurry is sheared at room temperature for any value within the range of 1.5h, 2h, 2.5h, 3h or 1.5-3h.

[0050] In some embodiments, after adding the carbon source and dopant in step (2), the slurry is sheared at room temperature for 0.5-2 hours.

[0051] In this invention, in step (2), when the shearing time is less than 0.5h, some carbon sources will settle to the bottom in the form of large particles, affecting subsequent experimental steps; there is no particular limit to the upper limit of the shearing time, but from the perspective of improving efficiency, it can be controlled within 0.5-2h.

[0052] Specifically, in step (2), after adding the carbon source and dopant, the slurry is sheared at room temperature for any value within the range of 0.5h, 0.7h, 1h, 1.2h, 1.5h, 2h or 0.5-2h.

[0053] In some embodiments, in step (3), after adding the iron source, the slurry is sheared at room temperature for 1-2 hours.

[0054] In this invention, after adding the iron source in step (3), the shearing time is at least 1 hour in order to shear the slurry to a D50 range of 100-200nm; there is no particular limit to the upper limit of the shearing time, but from the perspective of improving efficiency, it can be controlled within 1-2 hours.

[0055] Specifically, in step (3), after adding the iron source, the slurry is sheared at room temperature for any value within the range of 1h, 1.2h, 1.5h, 1.8h, 2h or 1-2h.

[0056] In some embodiments, in step (4), after adding the manganese source, the slurry is sheared at room temperature for 0-60 minutes.

[0057] Specifically, in step (4), after adding the manganese source, the slurry is sheared at room temperature for any value within the range of 10 min, 20 min, 45 min, 60 min or 0-60 min.

[0058] Specifically, the shearing described in this invention refers to shearing using a high-speed shearing machine.

[0059] In some embodiments, in step (5), the material is ground for 1-3 hours at room temperature using a sand mill.

[0060] In this invention, in step (5), the grinding time is at least 1 hour in order to grind the slurry to a D50 of 200-300 nm; there is no particular upper limit to the grinding time, but from the perspective of improving efficiency, it can be controlled within 1-3 hours.

[0061] Specifically, in step (5), the grinding is performed at room temperature using a sand mill for any value within the range of 1h, 1.5h, 2h, 2.5h, 3h or 1-3h.

[0062] In some embodiments, in step (1), the particle size D50 of slurry A is 50-500 nm.

[0063] Preferably, the particle size D50 of slurry A is 100-150 nm.

[0064] Specifically, the particle size D50 of slurry A is any value within the range of 100nm, 126nm, 130nm, 134nm, 140nm, 144nm, 150nm, or 100-150nm.

[0065] In this invention, the carbon source and dopant added in step (2) have little effect on the particle size of the slurry. The slurry is sheared at room temperature, which mainly serves to disperse it evenly and avoid agglomeration.

[0066] In some embodiments, in step (3), the particle size D50 of slurry C is 100-200 nm.

[0067] Specifically, the particle size D50 of slurry C is any value within the range of 150nm, 162nm, 172nm, 177nm, 184nm, 190nm, or 100-200nm.

[0068] In some embodiments, in step (4), the particle size D50 of slurry D is 300-800 nm.

[0069] Specifically, the particle size D50 of slurry D is any value within the range of 550nm, 600nm, 634nm, 668nm, 766nm, 780nm, or 300-800nm.

[0070] In some implementations, in step (5), the particle size D50 of slurry E is 200-300 nm.

[0071] Specifically, the particle size D50 of slurry E is any value within the range of 200nm, 212nm, 238nm, 264nm, 300nm, or 200-300nm.

[0072] In this invention, if the high-speed shearing machine is used for shearing after step (4), the slurry will agglomerate and the particle size will become larger and larger, making it impossible to control the slurry particle size D50 within the range of 200-300nm. Therefore, after step (4), the slurry needs to be transferred to a sand mill for grinding.

[0073] In some embodiments, in step (4), the solid content of slurry D is 25-40%.

[0074] Preferably, in step (4), the solid content of slurry D is 25-30%.

[0075] Specifically, the solid content of slurry D is any value within the range of 25%, 26%, 27%, 28%, 29%, 30%, or 25-30%.

[0076] In this invention, if the solid content is too low, it will not meet the requirements of subsequent spray drying, and even if spray drying can be carried out, there will be a problem of high energy consumption; if the solid content is too high, the slurry viscosity will be too high, and the particle size cannot be accurately measured.

[0077] In some embodiments, in step (6), the inlet air temperature of the spray dryer is 200-220°C and the outlet air temperature is 100-130°C.

[0078] Specifically, the inlet air temperature for the spray drying is any value within the range of 200℃, 203℃, 205℃, 208℃, 210℃, 215℃, 220℃, or 200-220℃.

[0079] Specifically, the exhaust temperature of the spray dryer is any value within the range of 100℃, 108℃, 110℃, 115℃, 121℃, 130℃ or 100-130℃.

[0080] In some embodiments, in step (7), the sintering is carried out in an inert or reducing atmosphere to prevent material oxidation.

[0081] Specifically, the inert atmosphere is a nitrogen atmosphere, a helium atmosphere, a neon atmosphere, or an argon atmosphere.

[0082] Specifically, the reducing atmosphere is a CO atmosphere or an H2 atmosphere.

[0083] In some embodiments, the sintering temperature in step (7) is 500-800°C.

[0084] Preferably, in step (7), the sintering temperature is 680-730℃.

[0085] Specifically, in step (7), the sintering temperature is any value within the range of 680℃, 684℃, 690℃, 696℃, 700℃, 708℃, 713℃, 726℃, 730℃ or 680-730℃.

[0086] In this application, in step (7), if the sintering temperature is too low, the reaction will be incomplete, the material crystal form will be incomplete and it will contain impurities; if the sintering temperature is too high, the carbon reduction effect is stronger under high temperature environment, which will reduce the material lithium manganese iron phosphate and easily form impurities; at the same time, some materials will decompose, resulting in a reduction of active materials, which will cause a decrease in capacity and capacity retention rate.

[0087] In some embodiments, in step (7), the heating rate during the sintering process is 2-5 °C / min.

[0088] Specifically, in step (7), the heating rate during the sintering process is any value within the range of 2.1℃ / min, 2.3℃ / min, 2.5℃ / min, 2.8℃ / min, 3.3℃ / min, 3.7℃ / min, 4.6℃ / min or 2-5℃ / min.

[0089] In this application, if the heating rate is too high, the carbon cracking rate will be too fast, the distribution will be uneven, the carbon coating effect will be poor, and the electrical performance will be affected; if the heating rate is too low, the sintering time will be long and the energy consumption will be high; at the same time, since the carbon melts first and then solidifies, if the time is too long, the surface coating may be uneven due to the fluidity after melting.

[0090] In some implementations, the sintering time in step (7) is 10-20 hours.

[0091] Specifically, in step (7), the sintering time is any value within the range of 10h, 12h, 14h, 16h, 18h, 20h or 10-20h.

[0092] In this application, in step (7), if the sintering time is greater than 20h, the crystal will continue to grow, the particles will become too large, and the electrical conductivity will decrease.

[0093] On the other hand, the present invention provides a lithium manganese iron phosphate cathode material prepared by the method described in the present invention.

[0094] On the other hand, the present invention provides a lithium battery comprising the lithium manganese iron phosphate cathode material described in the present invention.

[0095] The beneficial effects of this invention are as follows:

[0096] 1. This invention employs the solid-state method, most commonly used in industrial production, to prepare lithium manganese iron phosphate cathode materials. This method allows for a smooth transition from lithium iron phosphate production, ensuring the complete reaction of the lithium and phosphorus sources first, thus reducing the impact of subsequent bubble generation on performance. Shearing and mixing the carbon source and dopant results in more uniform dispersion of the coated carbon and dopant, improving both the conductivity and uniformity of the material. Stepwise shearing and mixing of the iron and manganese sources, while controlling their respective particle sizes, improves the compaction and conductivity of the resulting lithium manganese iron phosphate cathode material.

[0097] 2. Preliminary shearing and mixing reduces the time spent on sand milling. Then, the slurry D undergoes sand milling, spray drying, sintering, and other operations, which greatly simplifies the process, reduces energy consumption and equipment costs, and improves production efficiency.

[0098] 3. Compared with the prior art, the one-time sintering process of the present invention simplifies the process flow and, by controlling a certain feeding sequence, ratio and precise parameter requirements, especially by accurately controlling the complete reaction of raw materials, the particle size of slurry, the inlet and outlet temperatures of spray drying, and the temperature and time of sintering, can ensure the morphology of the product and the different carbon sources and dopants, thus producing products with excellent performance, high compaction density and meeting market demands. Attached Figure Description

[0099] Figure 1 This is a SEM image of the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention;

[0100] Figure 2 This is a 0.2C charge-discharge curve of the lithium manganese iron phosphate cathode material obtained in Example 1 of the present invention;

[0101] Figure 3 This is a 0.2C charge-discharge curve of the lithium iron phosphate cathode material obtained in Example 2 of the present invention;

[0102] Figure 4 This is a 0.2C charge-discharge curve of the lithium manganese iron phosphate cathode material obtained in Example 3 of the present invention;

[0103] Figure 5 The image shows the 0.2C charge-discharge curve of the lithium iron phosphate cathode material obtained in Comparative Example 1 of this invention.

[0104] Figure 6 The image shows the 0.2C charge-discharge curve of the lithium iron phosphate cathode material obtained in Comparative Example 2 of this invention.

[0105] Figure 7 This is a 0.2C charge-discharge curve of the lithium manganese iron phosphate cathode material obtained in Comparative Example 3 of the present invention.

[0106] Specific Implementation Scheme

[0107] The following description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0108] The preparation method of the lithium manganese iron phosphate cathode material of the present invention specifically includes the following steps:

[0109] (1) Mix lithium source, phosphorus source and pure water, and shear with a high-speed shearing machine to control the particle size of the slurry within a certain range to obtain slurry A;

[0110] The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, or lithium acetate.

[0111] The phosphorus source includes at least one of diammonium hydrogen phosphate, lithium dihydrogen phosphate, iron phosphate, or manganese phosphate.

[0112] The slurry was sheared at room temperature for 1.5-3 hours;

[0113] The particle size D50 of slurry A is 50-500 nm, preferably 100-150 nm;

[0114] (2) Add carbon source and dopant to slurry A, and shear it with a high-speed shearing machine to obtain slurry B;

[0115] The carbon source includes at least one of toluene, xylene, cyclohexane, citric acid, polyethylene glycol, sucrose, or carbon nanotubes;

[0116] The dopant includes at least one of ammonium fluoride, ammonium metavanadate, magnesium oxide, niobium pentoxide, titanate coupling agent or zirconium oxide;

[0117] After adding the carbon source and dopant, the slurry is sheared at room temperature for 0.5-2 hours.

[0118] (3) Add the iron source to slurry B and shear it with a high-speed shearing machine to control the particle size of the slurry within a certain range to obtain slurry C;

[0119] The iron source includes at least one of ferric phosphate, ferric nitrate, ferric oxide, ferrous oxalate, or ferric oxide.

[0120] After adding the iron source, the slurry is sheared at room temperature for 1-2 hours;

[0121] The particle size D50 of slurry C is 100-200 nm;

[0122] (4) Add the manganese source to slurry C, and shear it with a high-speed shearing machine to control the particle size of the slurry within a certain range to obtain slurry D;

[0123] The manganese source includes at least one of manganese carbonate, manganese tetroxide, manganese oxalate, manganese phosphate, ammonium manganese phosphate, or manganese phosphate.

[0124] After adding the manganese source, the slurry was sheared at room temperature for 0-60 minutes.

[0125] The particle size D50 of slurry D is 300-800 nm;

[0126] The solid content of slurry D is 25-40%; preferably 25-30%.

[0127] (5) Transfer slurry D into a sand mill for grinding, and control the particle size of the slurry within a certain range to obtain slurry E;

[0128] Grind in a sand mill for 1-3 hours at room temperature;

[0129] The particle size D50 of slurry E is 200-300 nm;

[0130] (6) Spray dry slurry E to obtain lithium manganese iron phosphate precursor F;

[0131] The inlet air temperature of the spray dryer is 200-220℃, and the exhaust air temperature is 100-130℃.

[0132] (7) The precursor F of lithium manganese iron phosphate is sintered, then crushed, sieved and demagnetized to obtain lithium manganese iron phosphate cathode material.

[0133] The sintering was carried out under a nitrogen atmosphere;

[0134] The sintering temperature is 500-800℃; preferably 680-730℃.

[0135] The heating rate during sintering is 2-5℃ / min; the sintering time is 10-20h.

[0136] The embodiments of the present invention will be described in detail below with reference to specific examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific conditions are not specified in the examples, the conditions described in the instruction manual, conventional conditions, or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0137] Example 1:

[0138] Add 132.4g of lithium dihydrogen phosphate, 30.4g of lithium carbonate, and pure water to a beaker. Shear the mixture at room temperature using a high-speed shear press for 2 hours until no more bubbles are generated and the slurry particle size D50 is 126nm. Add 20.3g of toluene, 20.3g of carbon nanotubes, 10.2g of citric acid, and 4.51g of ammonium fluoride. Shear the mixture at room temperature using a high-speed shear press for 1 hour. Add 122.9g of ferric phosphate. Shear the mixture at room temperature using a high-speed shear press for 1 hour. The slurry particle size D50 is 172nm. Finally, add 150g of manganese carbonate. Shear the mixture at room temperature using a high-speed shear press. After 10 minutes, the slurry particle size D50 was 634 nm, and the solid content of the slurry was controlled at 30%. The obtained slurry was transferred to a sand mill, and the speed of the sand mill was adjusted to 2500 rpm / min. It was ground at room temperature for 2 hours, and the slurry particle size D50 was 238 nm. The ground slurry was then transferred to a spray dryer with an inlet air temperature of 208℃ and an outlet air temperature of 110℃ for spray granulation. The lithium manganese iron phosphate precursor obtained after spray granulation was transferred to a muffle furnace and sintered at 730℃ for 12 hours under a nitrogen atmosphere, with a heating rate of 2.5℃ / min. After automatic cooling, it was further crushed, sieved, and demagnetized to obtain the lithium manganese iron phosphate cathode material. The carbon content in the lithium manganese iron phosphate cathode material was determined to be 2.3% by infrared carbon-sulfur analyzer.

[0139] Example 2

[0140] Add 196.4g ammonium dihydrogen phosphate, 63.4g lithium carbonate, and pure water to a beaker. Shear the mixture at room temperature using a high-speed shear press for 2 hours until no more bubbles are generated and the slurry particle size D50 is 134nm. Add 22.4g xylene, 20.3g carbon nanotubes, 10.2g citric acid, 3.2g titanate coupling agent, and 1.6g niobium pentoxide. Shear the mixture at room temperature using a high-speed shear press for 1 hour. Add 162.6g ferric nitrate. Shear the mixture at room temperature using a high-speed shear press for 1 hour, and the slurry particle size D50 is 184nm. Finally, add 85g manganese tetroxide. Shear the mixture at room temperature using a high-speed shear press. The slurry was sheared for 10 minutes using a high-speed shear mill, resulting in a particle size D50 of 766 nm and a solid content of 28%. The resulting slurry was then transferred to a sand mill, with the mill speed adjusted to 2500 rpm / min, and ground at room temperature for 2 hours, resulting in a particle size D50 of 212 nm. The ground slurry was then transferred to a spray dryer with an inlet air temperature of 203°C and an outlet air temperature of 108°C for spray granulation. The resulting lithium manganese iron phosphate precursor was transferred to a muffle furnace and sintered at 700°C for 10 hours under a nitrogen atmosphere, with a heating rate of 2.3°C / min. After automatic cooling, the material underwent further crushing, sieving, and demagnetization to obtain the lithium manganese iron phosphate cathode material. The carbon content of the lithium manganese iron phosphate cathode material was determined to be 1.8% using an infrared carbon-sulfur analyzer.

[0141] Example 3

[0142] Add 116.8g of diammonium hydrogen phosphate, 77.4g of lithium carbonate, and pure water to a beaker. Shear the mixture at room temperature using a high-speed shear press for 2 hours until no more bubbles are generated and the slurry particle size D50 is 144nm. Add 21.8g of cyclohexane, 20.3g of carbon nanotubes, 10.2g of citric acid, and 3.8g of zirconium oxide. Shear the mixture at room temperature using a high-speed shear press for 1 hour. Add 112.8g of ferrous oxalate. Shear the mixture at room temperature using a high-speed shear press for 1 hour. The slurry particle size D50 is 177nm. Finally, add 180g of manganese phosphate. Shear the mixture at room temperature using a high-speed shear press. After 10 minutes of grinding, the slurry particle size D50 was 668 nm, and the solid content of the slurry was controlled at 25%. The resulting slurry was transferred to a sand mill, and the mill speed was adjusted to 2500 rpm / min. Grinding was carried out at room temperature for 2 hours, resulting in a slurry particle size D50 of 264 nm. The ground slurry was then transferred to a spray dryer with an inlet air temperature of 215℃ and an outlet air temperature of 121℃ for spray granulation. The resulting lithium manganese iron phosphate precursor was transferred to a muffle furnace and sintered at 680℃ for 14 hours under a nitrogen atmosphere, with a heating rate of 2.8℃ / min. After automatic cooling, the precursor was further pulverized, sieved, and demagnetized to obtain the lithium manganese iron phosphate cathode material. The carbon content in the lithium manganese iron phosphate cathode material was determined to be 2.1% using an infrared carbon-sulfur analyzer.

[0143] Comparative Example 1

[0144] 132.4g lithium dihydrogen phosphate, 30.4g lithium carbonate, 150g manganese carbonate, 122.9g iron phosphate, 20.3g toluene, 20.3g carbon nanotubes, 10.2g citric acid, and 4.51g ammonium metavanadate, along with pure water, were added to a beaker. The solid content of the slurry was controlled at 30%, and the mixture was stirred at room temperature for 2 hours. The resulting slurry was then transferred to a sand mill, and the mill speed was adjusted to 2500 rpm / min. The mixture was ground at room temperature for 2 hours, resulting in a particle size D50 of 297nm. The ground slurry was then transferred to a spray dryer with an inlet air temperature of 208℃ and an outlet air temperature of 110℃ for spray granulation. The resulting lithium manganese iron phosphate precursor was then transferred to a muffle furnace and sintered at 730℃ for 12 hours under a nitrogen atmosphere, with a heating rate of 2.5℃ / min. After automatic cooling, the material is then crushed, sieved, and demagnetized to obtain lithium manganese iron phosphate cathode material. The carbon content in the lithium manganese iron phosphate cathode material is determined to be 1.9% by infrared carbon-sulfur analyzer.

[0145] Comparative Example 2

[0146] Add 196.4g ammonium dihydrogen phosphate, 63.4g lithium carbonate, 22.4g xylene, 20.3g carbon nanotubes, 10.2g citric acid, 3.2g titanate coupling agent, 1.6g niobium pentoxide, and pure water to a beaker. Shear the mixture at room temperature using a high-speed shear press for 0.5h, resulting in a slurry particle size D50 of 633nm. Add 162.6g ferric nitrate and shear the mixture at room temperature using a high-speed shear press for 0.5h, resulting in a slurry particle size D50 of 554nm. Finally, add 85g manganese tetroxide and shear the mixture at room temperature using a high-speed shear press for 10min. The slurry particle size D50 was 936 nm, and the solid content was controlled at 35%. The resulting slurry was transferred to a sand mill, and the mill speed was adjusted to 2500 rpm / min. Grinding was carried out at room temperature for 2 hours, resulting in a particle size D50 of 356 nm. The ground slurry was then transferred to a spray dryer with an inlet air temperature of 203℃ and an outlet air temperature of 108℃ for spray granulation. The resulting lithium manganese iron phosphate precursor was transferred to a muffle furnace and sintered at 700℃ for 10 hours under a nitrogen atmosphere, with a heating rate of 2.3℃ / min. After automatic cooling, the precursor was further processed by crushing, sieving, and demagnetization to obtain the lithium manganese iron phosphate cathode material. The carbon content in the lithium manganese iron phosphate cathode material was determined to be 1.6% using an infrared carbon-sulfur analyzer.

[0147] Comparative Example 3

[0148] Add 116.8g of diammonium hydrogen phosphate, 77.4g of lithium carbonate, and pure water to a beaker. Shear the mixture at room temperature using a high-speed shear press for 0.2h, resulting in a slurry particle size D50 of 637nm. Add 21.8g of cyclohexane, 20.3g of carbon nanotubes, 10.2g of citric acid, and 3.8g of zirconium oxide. Shear the mixture at room temperature using a high-speed shear press for 0.5h. Add 112.8g of ferrous oxalate. Shear the mixture at room temperature using a high-speed shear press for 0.5h, resulting in a slurry particle size D50 of 472nm. Finally, add 180g of manganese phosphate. Shear the mixture at room temperature using a high-speed shear press. After shearing for 10 min, the slurry particle size D50 was 845 nm, and the solid content of the slurry was controlled at 25%. The resulting slurry was transferred to a sand mill, and the mill speed was adjusted to 2500 rpm / min. Grinding was carried out at room temperature for 2 h, resulting in a slurry particle size D50 of 377 nm. The ground slurry was then transferred to a spray dryer with an inlet air temperature of 215℃ and an outlet air temperature of 121℃ for spray granulation. The spray-granulated lithium manganese iron phosphate precursor was transferred to a muffle furnace and sintered at 680℃ for 14 h under a nitrogen atmosphere, with a heating rate of 2.8℃ / min. After automatic cooling, it underwent further crushing, sieving, and demagnetization to obtain the lithium manganese iron phosphate cathode material. The carbon content in the lithium manganese iron phosphate cathode material was determined to be 1.9% using an infrared carbon-sulfur analyzer.

[0149] Performance testing

[0150] The lithium manganese iron phosphate positive electrode material, binder solution (PVDF:NMP mass ratio of 1:39), and conductive agent (Super-P) prepared in the examples and comparative examples were weighed at a mass ratio of 92:4:4. After being ball-milled and dispersed evenly in a degassing mixer, the mixture was coated onto aluminum foil and vacuum-dried at 110°C. After rolling and punching, the positive electrode sheet was obtained. The electrolyte was 1 mol / L LiPF6, with a solvent volume ratio of EC:DMC:EMC = 1:1:1 (volume ratio). The separator was Celgard polypropylene membrane, and the negative electrode was lithium metal sheet. A coin cell was assembled together in a vacuum glove box. Electrochemical tests were performed on the coin cell using a Blue Battery testing system, with a test voltage range of 2V-4.5V.

[0151] Table 1: Test data of lithium manganese iron phosphate cathode materials prepared in each embodiment and comparative example

[0152]

[0153]

[0154] Examples 1-3 are high-capacity, high-density lithium manganese iron phosphate cathode materials prepared by the method described in this invention. Comparative Example 1 is a lithium manganese iron phosphate cathode material prepared by a conventional method. Comparative Examples 2-3 are lithium manganese iron phosphate cathode materials prepared by the method described in this invention, but with changes in the order of raw material addition and particle size during the preparation process. Data from Table 1 shows that the high-capacity, high-density lithium manganese iron phosphate cathode material prepared by the method described in this invention has higher initial coulombic efficiency, 0.2C discharge specific capacity, and compaction density than the lithium manganese iron phosphate cathode materials prepared by the comparative examples. Figure 1 It can be seen that the high-capacity, high-density lithium manganese iron phosphate crystals prepared by the method of the present invention have obvious interfaces, and the particles of different sizes are mixed after crushing. Small particles increase the high capacity, and the mixture of particles of different sizes provides high density.

[0155] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0156] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that, The preparation method includes the following steps: (1) Mix lithium source, phosphorus source and pure water, and shear at room temperature using a high-speed shearing machine for 1.5-3 h to control the slurry particle size D50 in the range of 50-500 nm to obtain slurry A; (2) Add carbon source and dopant to slurry A, and shear it at room temperature using a high-speed shearing machine for 0.5-2 h to obtain slurry B; (3) Add the iron source to slurry B and shear it at room temperature using a high-speed shearing machine for 1-2 h to control the slurry particle size D50 within the range of 100-200 nm to obtain slurry C; (4) Add the manganese source to slurry C and shear it at room temperature for 0-60 min using a high-speed shearing machine to control the slurry particle size D50 within the range of 300-800 nm to obtain slurry D; (5) Grind slurry D at room temperature using a sand mill for 1-3 h, and control the slurry particle size D50 within the range of 200-300 nm to obtain slurry E; (6) Spray dry slurry E to obtain lithium manganese iron phosphate precursor F; (7) The precursor F of lithium manganese iron phosphate is sintered, then crushed, sieved and demagnetized to obtain lithium manganese iron phosphate cathode material.

2. The preparation method according to claim 1, characterized in that, The chemical formula of the lithium manganese iron phosphate cathode material is LiMn. x Fe 1-x PO4, wherein 0.5 ≤ x ≤ 0.9; the molar ratio of lithium to iron and manganese in the added raw materials is (0.95-1.1):1; the molar ratio of iron and manganese to phosphorus in the added raw materials is (0.95-1.06):1; the amount of carbon source added is 5%-20% of the total mass of the added raw materials; the amount of dopant added is 0.2%-2% of the total mass of the added raw materials.

3. The preparation method according to claim 1, characterized in that, The amount of carbon source added is 8%-12% of the total mass of the added raw materials.

4. The preparation method according to claim 1, characterized in that, The amount of the dopant added is 0.5%-1.3% of the total mass of the added raw materials.

5. The preparation method according to claim 1, characterized in that, The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, or lithium acetate. The phosphorus source includes at least one of diammonium hydrogen phosphate, lithium dihydrogen phosphate, iron phosphate, or manganese phosphate. The carbon source includes at least one of toluene, xylene, cyclohexane, citric acid, polyethylene glycol, sucrose, or carbon nanotubes; The dopant includes at least one of ammonium fluoride, ammonium metavanadate, magnesium oxide, niobium pentoxide, titanate coupling agent or zirconium oxide; The iron source includes at least one of ferric phosphate, ferric nitrate, ferric oxide, ferrous oxalate, or ferric oxide. The manganese source includes at least one of manganese carbonate, manganese tetroxide, manganese oxalate, manganese phosphate, ammonium manganese phosphate, or manganese phosphate.

6. The preparation method according to claim 1, characterized in that, In step (1), the particle size D50 of slurry A is 100-150 nm.

7. The preparation method according to claim 1, characterized in that, In step (4), the solid content of slurry D is 25-40%.

8. The preparation method according to claim 1, characterized in that, In step (4), the solid content of slurry D is 25-30%.

9. The preparation method according to claim 1, characterized in that, In step (6), the inlet air temperature of the spray dryer is 200-220℃ and the exhaust air temperature is 100-130℃.

10. The preparation method according to claim 1, characterized in that, In step (7), the sintering is carried out in an inert or reducing atmosphere; the sintering temperature is 500-800℃; the heating rate during the sintering process is 2-5℃ / min; and the sintering time is 10-20 h.

11. The preparation method according to claim 1, characterized in that, In step (7), the sintering is carried out in an inert or reducing atmosphere; the sintering temperature is 680-730℃; the heating rate during the sintering process is 2-5℃ / min; and the sintering time is 10-20 h.

12. A lithium iron phosphate cathode material, characterized in that, The lithium manganese iron phosphate cathode material is prepared by any one of claims 1-11.

13. A lithium battery, characterized in that, The lithium battery comprises lithium manganese iron phosphate cathode material obtained by any one of claims 1-11 or lithium manganese iron phosphate cathode material as described in claim 12.

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